settlement of foundations sand gravel 1985

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Proc. Instn Ciu. Engrs, Part 1, 1985,78, Dec., 1325-1381

8917 GROUND ENGINEERING GROUP

Settlement of foundations on sand and gravel

J. B. BURLAND, PhD, DSc(Eng), FEng, FICE, MIStructE

M. C . BURBIDGE, BSc, MSc, DIC, FGSt

The Paper describes the analysis of over 200 records of settlement of foundations, tanks and embankments on sands and gravels. A remarkably simple picture has emerged relating the settlement to the bearing pressure, the breadth of loaded area and the average SPT blow count or cone resistance over the depth of influence. The influence of a number of factors such as shape and depth of foundation, depth of water table, grain size and time have been investigated. The Paper first briefly describes the application of the results to the prediction of settlement with particular emphasis on the limits of accuracy. Paragraphs 6 2 4 are self contained and may be used on their own for design purposes. The Paper follows this with a detailed account of the analysis of the case records.

Notation radius of loaded area foundation subgrade compressibility (ApJAq‘), mm/(kN/m2) most probable value of a, width of loaded area, m depth of founding level effective Young’s modulus correction factor for thickness of sand layer correction factor for shape correction factor for time thickness of sand layer depth of water table below founding level index of compressibility (ar/BO”) rate of increase of Young’s modulus with depth length of loaded area volume compressibility from oedometer test average SPT blow count over the depth of influence corrected value of SPT blow count average bearing pressure, kN/m2 cone resistance, MN/m2 time-dependent settlement (expressed as a proportion of pi) occurring during first 3 years after construction time-dependent settlement (expressed as a proportion of pi) occurring each log cycle of time after 3 years defined in equation (14) most probable value of T

Ordinary meeting, 5.30 pm., 25 February 1986. Written discussion closes 14 March 1986. For further details see p. (ii). *Imperial College of Science and Technology. ?E. J. Wilson, Consulting Engineering Geologist.

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B U R L A N D A N D B U R B I D G E

t ZI V‘

Pr Pi PI U

time in years depth of influence of loaded area effective Poisson’s ratio final measurement of settlement settlement at the end of construction or completion of loading settlement at time t after completion of loading standard deviation maximum previous effective overburden pressure, kN/mZ

Introduction Numerous methods of predicting settlement of foundations on sands and gravels have been published-many more methods than for clays. The reason lies in the extreme difliculty of obtaining undisturbed samples for the laboratory determination of compressibility under appropriate conditions of stress and stress history. Hence resort has been made to the interpretation of field in situ tests such as the standard penetration test (SPT), cone penetration test and plate loading test, and much of the literature has been devoted to such interpretations. This extensive literature will not be reviewed here as it has been adequately covered by Suther- land,’ Simons and Menzies’ and N i ~ o n . ~

2. The practical importance of the problem was perhaps put in perspective by Terzaghi4 when he stated that all buildings resting on sand which were known to him had settled less than 75 mm (3 in) whereas the settlement of buildings on clay foundations quite often exceeded 500mm (20 in). This statement provided the impetus for the study described in the present Paper in which a large number of case records of settlement on sands and gravels have been assembled by Burbidge’.

3. The essential details of most of these case records are tabulated in Appendix 1 of the present Paper and the associated references are given in Appendix 2. The case record numbering used by Burbidge has been retained for ease of reference. 4. The prime objective of the study was to check whether the above statement

of Terzaghi’s still held true and reference to Appendix 1 shows that, with a few exceptions, it does for buildings. However, settlements well in excess of 75mm have been recorded for tanks and embankments on very loose sands. In view of the small settlements usually experienced with sands and gravels the second objective of the study was to analyse the data on actual observations of settlement employ- ing a minimum of interpretation to see if a simple and useful picture emerged. A preliminary study of this type was undertaken by Burland et aL6 and a similar but more detailed approach is described here.

5 . The picture that has emerged from the statistical analysis of over 200 cases is remarkably simple and gives a range of settlements which is generally less than the range of predictions offered by the current commonly accepted methods.’ A brief description of the method and its application is given first, followed by a detailed account of the analysis of the settlement records.

Settlement prediction 6. The outcome of the analysis of the large number of settlement records

summarized in Appendix 1 is presented first, in the form of a simple direct method of settlement prediction. Paragraphs 6 2 4 are self contained and can be used on their own for design purposes. However, frequent cross-references are made to the work described later, so that the basis of the various assumptions can be studied.

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SETTLEMENT OF FOUNDATIONS ON SAND AND GRAVEL

Determination of the foundation subgrade compressibility 7. The nub of the method is the empirical relationship which has been estab-

lished between the slope of the pressure-settlement relationship for the foundation (ApJAq'), the breadth of the foundation B and the average SPT blow count over the depth of influence of the foundation. The quantity ApJAq' is the foundation subgrade compressibility, denoted by a , , and the units are mm/(kN/m*). The relationship is shown in Fig. 1, where u , / B " ~ is plotted against N on double log axes. The quantity u, /B"~ is denoted as I , , the compressibility index. The full line in Fig. 1 has been derived from a regression analysis of over 200 settlement records on sand and gravel. The chain dotted lines approximate to two standard deviations above and below the mean line. Mathematically the regression line is given by

with a coefficient of correlation of 0.848. 8. The following features should be noted about Fig. 1.

(a) a , is the subgrade compressibility for a normally consolidated sand or gravel. In 8 6 4 7 2 it is shown that the relationship between bearing pressure and settlement is approximately linear for normally consolidated granular materials for factors of safety against bearing capacity failure of 3 or more. When the material is overconsolidated or loaded at the base of an excavation, the values of a, and I , are reduced by a factor of 3 for pressure changes below the effective preconsolidation pressure

(b) The SPT blow count is not corrected for effective overburden pressure and the horizontal axis is therefore not strictly a measure of relative density. Instead a new classification is proposed in which ranges of uncorrected N values are assigned to compressibility grades. The correlation between N and compressibility grade is given in Table 1 and in Fig. 1. The concept of compressibility grades proved particularly valuable in the analysis described in Q 26 et seq.

(c) Although the N values are not corrected for overburden pressure it is necessary to make certain other corrections. It is shown in Section 8.5 that for very fine and silty sand below the water table the correction proposed by Terzaghi and Peck' gives improved results, i.e. when N is greater than 15

4 0 '

N' = 15 + 0.5(N - 15) (2)

where N' is the corrected value of N . When the material consists of gravel or sandy gravel it is shown in $9 103-106 that a correction should be applied such that

N' = 1.25 X N (3)

(d ) The results of cone penetration tests may be converted to equivalent N values using Fig. 2, where q,/N is related to grain size (qc is in MN/m2).

(e) The results of plate loading tests may be related to compressibility grade using the methods described in 47-50. Care must be used in the application of equation ( l ) in conjunction with plate loading test results

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BURLAND AND BURBIDGE

AP

W a, = 4 mm/(kN/m2)

B in metres

Compresslbllity grades

0.1 ‘1 I l I I I I I I I I I I I I I I I I

10 100 SPT

Fig . 1. Relationship between compressibility ( I , ) and mean SPT blow count (m) over depth of influence. Chain dotted lines show upper and lower limits (see Figs 22 and 23)

Table 1. Classijcation of compressibility of normally tonsolidated sands and gravels with SPT blow count

Compressibility grade

IV 111 I1 r

No. of blows N*

<4 4-8

1 C 2 5 9-15

2 6 4 0 41-60 > 60

Interval

3 5 7

10 15 20

~

* Uncorrected for overburden pressure.

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SETTLEMENT O F FOUNDATIONS ON SAND AND GRAVEL

/ 20- /-

0 - 10- X :

E - - W -

/- /- ’ o h 2

I I I I

0.06 0.2 0.6 2 0 6.0 Partlcle sue: mm

Medium] Coarse I Fine I Medlum I Coarse I Fme IMediurn Silt I Sand Gravel

Fig. 2. Relationship between q J N and grain size. Values of N are not correctedfor overburden pressure

since as B increases the value of N will often increase as well due to the associated increase in the depth of influence.

Depth of influence and the derivation of I? 9. An important feature of the method is the assessment of the depth of influ-

ence z, of the foundation. This is discussed in detail in & 51-63 where it is shown that, when N increases with depth, the relative depth of influence (z/B), decreases significantly as the breadth of the foundation increases. Although the depth of influence depends on many factors, for present purposes it is assumed to be given by the full line in Fig. 3 for cases where N increases or is constant with depth. Where N shows a consistent decrease with depth the depth of influence is taken as 2B or the bottom of the soft layer, whichever is the lesser. The value of N for use in Fig. 1 or equation (1) is given by the arithmetic mean of the measured N values over the depth of influence.

Calculation of settlement 10. For a normally consolidated sand the immediate average settlement pi at

the end of construction, corresponding to the average effective foundation pressure q’, is given by

pi = q’ X B’” X I , (4)

where pi is in mm, q’ in kN/m2 and B in metres. Values of I , corresponding to the best estimate and the upper and lower limits are obtained from Fig. 1.

11. For an over consolidated sand, or for loading at the base of an excavation, for which the maximum previous effective overburden pressure is U:, , the average end of construction settlement pi corresponding to the average gross effective pressure q’ (where q’ > aka) is made up of two components as follows

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’ O f

B: m

Fig . 3. Relationship between breadth of loaded area B and depth of influence z, (within which 75% of the settlement takes place)

= (q‘ - &,)B0’7 X l , mm ( 5 4

When q’ is less than oto the above expression becomes

pi = q’ X B’” X - 1, mm 3 (5b)

Corrections for depth offounding, depth of water table, shape and thickness of layer 12. In # 91-106 a statistical analysis of the influence of the above factors is

described for foundations with depth ratios D / B < 3. It is shown that, within the limits of accuracy of the analysis, there is no obvious correlation between D/B and settlement. This result agrees with the results of DAppolonia et al.’ who found from the analysis of a number of results on one site that only a 12% reduction in settlement occurred when D / B increased from 0.5 to 1.0.

13. It is also concluded that the level of the water table beneath the founding level does not have a statistically significant influence on the settlement. This result appears to support Meyerhofsg view that the effect of the water table is reflected in the measured blow count. Thus water table changes subsequent to the determination of I? may have some influence on settlement.

14. The statistical analysis indicates that there is a significant correlation

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SETTLEMENT OF FOUNDATIONS O N SAND AND GRAVEL

between settlement and LIB (the length-to-breadth ratio of the foundation). The correction factor is quite small and can be expressed as

where pi (L/B > 1) =f, X pi(L/B = 1). It can be seen thatf, tends to 1.56 as LIB tends to infinity.

15. There were insufficient data to study the influence of the thickness of the sand or gravel layer beneath the foundation (H,) but it is recommended that when H , is less than z, (the depth of influence) a correctionf, should be applied such that

ZI

Time-dependent settlement 16. The case records referred to in $9 107-115 indicate quite clearly that foun-

dations on sands and gravels exhibit time-dependent settlement. However, no distinct pattern emerges. In some cases the time-dependent process appears to be ,more or less continuous, with the settlement following an approximately linear log time relationship (after an initial transition period). In other cases the process appears to be stepwise with quiescent periods of up to 3 years interspersed with periods of significant rates of settlement.

17. The records show very clearly that foundations subject to fluctuating loads such as tall chimneys, bridges, silos and turbines exhibit much larger time- dependent settlements than those subject only to static loads.

18. The results suggest that the time correction factor for the settlement (p,) at any time t , when t is 3 years or more after the end of construction, is given by

Pi

where f, is the correction factor for time, t > 3 years, R , is the time-dependent settlement (expressed as a proportion of pi) that takes place during the first 3 years after construction and R, is the time-dependent settlement (expressed as a proportion of pi) that takes place each log cycle of time after 3 years.

19. For static loads conservative values of R , and R, are 0.3 and 0.2 respectively. Thus at t = 30 years,f, = 1.5. For fluctuating loads conservative values of R , and R, are 0.7 and 0.8 respectively so that at t = 30 years,f, = 2.5.

Summary 20. In summary the average settlement of a foundation at the end of construc-

tion and then at any time t , 3 or more years after the end of construction, may be expressed by the following equations:

pi =f, x f i X [(q’ - fo:,) X B’” X I,] mm ( 9 4

and

where q’ is the average gross effective applied pressure (kN/mZ), cr:, is the

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maximum previous effective overburden pressure (kN/m2), B is the breadth in metres, I, is the compressibility index obtained from Fig. 1 or equation (l),f, is a shape correction factor given by equation (6), f , is a correction factor for the thickness of the sand layer given by equation (7) andf; is a time factor given by equation (8).

21. The probable limits of accuracy of equation (9a) can be assessed from the upper and lower limits of I, given in Fig. 1 and it may be necessary to take these into account in the design.

22. It must be emphasized that the factor of safety against bearing capacity failure should always be checked in addition to the settlement. If the factor of safety is less than about 3 the pressure settlement curve may be non-linear and the method will underestimate the settlement.

23. Furthermore, the method has been based on case studies with quartzitic sand and gravel deposits. Sites where coral (calcite) or other mineralogically unusual sand and gravel deposits are encountered should not be analysed by this method unless the deformation properties of these deposits can be demonstrated to be similar to quartzitic deposits.

24. The method is well suited for routine design purpases. However, it is suggested that, for major projects, or those where the proposed structure has strict permissible total or differential settlements, other well-established methods of estimating the settlement are also used as a check. On such projects it may prove valuable to refer to the case studies listed in Appendix 2 in which similar structures or ground conditions are involved. In general it seems unlikely that the limits of accuracy can be significantly improved unless resort is made to the direct determination of in situ compressibility.

25. In conclusion it is appropriate to bear in mind the following remarks by Sutherland

‘Before a designer becomes entangled in the details of predicting settlement (in sand) he must satisfy himself whether a real problem actually exists and ascertain what advantages and economies can result from refinements in settlement prediction.’

Analysis of case records of settlement on sands and gravels 26. The object of the study described in this section of the Paper was to

assemble as much data as possible on actual field observations of settlement with a minimum of interpretation to see if a simple picture emerged. The most important factors controlling settlement p are the effective bearing pressure q‘, the breadth B of the loaded area and the compressibility of the ground within the depth of influence of the loaded area. There are many other factors influencing settlement such as depth of founding, geometry of the loaded area, depth of water table, time etc. These factors were felt to be secondary compared with the above three principal factors and could be examined separately after the main trends had been established.

27. For any case record the quantities p, q’ and B are well defined. Thus, in their preliminary study Burland er al.6 chose to correlate p / q with B. The compressibility characteristics of the ground are much more difficult to define and Burland et al. only distinguished between three categories of granular material: loose, medium dense and dense. In the present study the same basic approach is adopted but a more refined method of classifying the compressibility of sands and

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gravels has been found to be justified. Moreover, it has proved necessary to consider in some detail the depth within which the compressibility significantly influences the settlement (i.e. the depth of influence z,) and also the validity of the assumption of a linear pressuresettlement relationship. These matters are discussed in the following paragraphs as a preliminary to the presentation of the analysis of the case records.

The standard penetration test ( S P T ) as a measure of compressibility 28. For the majority of the case records assembled for this study the ground

conditions were investigated using the standard penetration test (SPT). For this reason, and because it is a test which is widely used, it was decided to use the SPT blow count as a measure of the compressibility of granular soils. Nevertheless, it is of the utmost importance to appreciate the limitations both of the test itself and the correlation of its results with compressibility.”

29. The standard penetration test. At present the two most widely used standards are BS 1377: 197511 and ASTM D1586-67.” The testing procedures are broadly similar, and outside the UK and the USA one of these two standards is normally used. An important exception to the general SPT procedure is in Brazil where the Mohr-Geotecnica sampler is extensively used.

30. There are numerous details of the test and its operation which are not standard.13 For example, there are considerable differences in the dimensions and lengths of drilling rod used in the test. Also, the driving technique can vary significantly. The British and European standards specify the use of a trip hammer whereas American practice is to operate the driving weight manually using a cathead. Other factors which can influence the N value are the diameter of casing, the condition of the driving shoe, the type of boring rig and the method of cleaning the base of the borehole. According to Schmertmann14 almost all samplers used in the USA have enlarged internal diameters to hold a liner. However, they are frequently used without a liner, which leads to a significant reduction in the N value. Over and above all these factors the crucial importance of maintaining an adequate level of water in the borehole must of course be emphasized.

31. It has always been recognized that the SPT is an empirical test. It is a test which will have to become completely standardized if its use as a yardstick for judging in situ properties, such as compressibility, is to be enhanced. The need for standardization has been emphasized by Nixon3 who calls for the international use of the 1977 ISSMFE ‘Report of the Sub-Committee on Penetration Test for use in Europe’.15 Any future changes or standardization in the test that do take place should not deviate significantly from present procedures, so that experience already gained from the test is not lost.

32. Influence ofgrain size. The effects of grain properties, such as angularity and uniformity coefficient, on SPT resistance have not been adequately studied. Holubec and D’Appolonia16 suggest that the SPT is influenced by the angularity of granular soil. Gibbs and Holtz” found that the grain size had some influence. Tests on dry loose sands showed that the N value for coarse sand was marginally higher than for fine sand at the same relative density and overburden pressure. However, for dense sand there was no appreciable difference between fine and coarse sands. D’Appolonia and D’Appolonia’’ concluded that the particle size does not appear to have a major influence provided gravel sizes are not present.

33. Influence of submergence. Schultze and M e n ~ e n b a c h ‘ ~ and Bazaraa” have shown that the SPT resistance for coarse sand and gravel is not affected by

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submergence. Terzaghi and Peck’ recommend that for dense ( N > 15), fine or silty sands beneath the water table, the measured N values should be reduced, and put forward the following procedure

N’ = 15 + 0.5(N - 15) (2)

34. This proposal appears to be contradicted by the results of some laboratory tests of Gibbs and Holtz” and Schultz and Melzer.*’ However, Bazaraa” concluded from analysis of a large number of results of SPT tests within l m above and below the water table that the effect of submergence on penetration resistance on very fine or silty sand is generally to increase the blow count. On the basis of his results he suggested that the measured N values should be corrected by the formula

N = 0.6 X N ( 10)

35. InJuence of overburden pressure. Although SPT resistance for a granular soil is likely to be dependent on a number of factors it appears that the two most important ones are the relative density and the effective overburden pressure. Thus, in order to assess the relative density, numerous methods have been proposed for correcting the SPT blow count to a standard overburden pressure (e.g. those of Gibbs and Holtz”, Bazaraa2’ and Thorburn2*).

36. Turning now to compressibility, laboratory tests by D a r a m ~ l a ~ ~ show that, for a given K O stress history, the two most important factors influencing the vertical compressibility are relative density and stress level-the same as for SPT resistance.

37. It therefore appears that, in attempting to correlate compressibility with SPT blow count, the effect of overburden pressure should not be eliminated since it has an important influence on both. Hence no correction for overburden pressure was used in this study. However, it is recognized that the SPT blow count does not, on its own, reflect the previous consolidation history of a deposit to any significant extent and the effect of this has to be accounted for separately.

38. Cornpressibility grade in terms of S P T . Terzaghi’sZ4 descriptive correlation between the ‘relative density’ and N value was originally based on the Terzaghi and Peck allowable bearing pressure chart and the terms were therefore originally used as qualitative measures of compressibility. Since their original introduction the influence of overburden pressure on blow count has been recognized, as discussed under the previous sub-heading. Moreover, when an attempt was made to correlate foundation compressibility a, from the case records given in Appendix 1 with ‘ relative density ’, it was found that the range of compressibilities associated with each density zone was very uneven. It will become apparent that any descriptive classification of compressibility based on SPT requires a scale in which the range of N values associated with each ‘zone’ or ‘grade’ increases approximately exponentially to give an even spread of a, values.

39. A new descriptive correlation between SPT and compressibility of normally consolidated granular materials has been introduced and is given in Table 1. The opportunity has been taken to dispense with the terminology of ‘relative density’ and replace it with a number of ‘ compressibility grades ’ which, since they relate to uncorrected blow count, are a function both of relative density and overburden pressure.

40. It must be emphasized that the SPT blow count can never be anything more than a crude indicator of compressibility, even when restricted to normally

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consolidated sands (as in Table 1) and when the procedures are perfectly standardized.

The relationship between SPT and other tests 41. As discussed in Q$ 2840, the SPT resistance is used as a measure of com-

pressibility in the present study. However, for many of the case records given in Appendix 1, no standard penetration tests were carried out. In order to make use of these case records it has been necessary to attempt to correlate SPT blow count with other tests, the three main ones being cone penetration tests, oedometer tests and plate loading tests. It is recognized that the correlations are only approximate. Nevertheless, it is important to relate these widely used tests to the ‘compressibility grade’ if the results of the present study are to be generally useful.

42. Cone penetration tests. MeyerhoP5 investigated the relationship between N value and static cone resistance qc for a number of sites, mainly for fine and silty sands and suggested that

q, = 4.4N

where qc is in kgf/cm2. This relationship was found to be independent of density. 43. Meigh and Nixon,26 Rodin” and Sutherland” have shown that the above

relationship is restricted to fine and silty sands and that the ratio qc/N increases with grain size. Burbidge’ collected together the original data used by the above workers, together with other results, including those associated with the case records referred to in this Paper. Following the work of T h ~ r b u r n , ’ ~ these data have been correlated with average grain size and were found to be within the zones shown in Fig. 2. For the case records from Brazil the measured blow counts were reduced by a factor of 0.7.30*31 The results confirm that the density has little influence on q J N although there is a slight trend for loose sands to lie towards the upper limit of the scatter of the results.

44. Oedometer tests. The oedometer test is the most commonly used laboratory test for estimating settlements on sands. In Poland and Russia it is fairly frequently used but outside these countries it has been less popular.

45. The major difficulty with oedometer tests, as with other laboratory tests on granular soil, is obtaining undisturbed samples. It has been found that fine sands are generally not as prone to mechanical disturbance as coarse sand and where representative samples have been carefully hand cut from excavations oedometer tests have been reasonably successful.

46. About a quarter of the case studies found in the literature contain oedometer results. The opportunity was taken to compare m, values from such tests with N values for sites where the two tests were made. Average N values were obtained over a depth range of 5-15 m and were compared with values ofm, evaluated for a loading intensity of 100 kN/m2. Fig. 4 shows the results of the comparison for eight sites. The number against each point refers to the appropriate case number given in Appendix 1. Seven of the cases are for fine sand. Many more data are required before any firm conclusions can be drawn about the relationship between m, and N . For the purposes of this study the full line in Fig. 4 was used to assess the compressibility grade from oedometer results.

47. Plate loading tests. Terzaghi and Peck7 published a diagram showing a collective pressure-settlement chart for 0.3 m ( 1 ft) square plates bearing on loose to very dense sand strata above the groundwater table. At the time when the chart was constructed only limited plate bearing test data were available and, moreover,

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O°F

0 Fine sand 8Medlum sand

I I I I I , 1 1 1 , I I I 1 I l l , , I I I I I

0 5 0.1 0-05 0 01 0.005 0,001 m, mz/MN (at U"' = 100 kN/mz)

Fig. 4. Relationship of N with m, (numbers refer to case records in Appendix I )

the corresponding SPT values were probably from two different sized spoons. It was therefore decided to construct an updated version of the chart using the compressibility grading classification. Data were collected from the case records examined by B ~ r b i d g e , ~ Bazaraa," Meigh and Nixon26 and R ~ d i n . ~ ~

48. The resulting charts are shown in Fig. 5 and they relate to three size ranges of plate: 0.254.4 m, 04-0.7 m and 0.7-1.2 m. The scatter of results on which these charts are based is large but tends to decrease with larger plates. In compil- ing the charts no difference was apparent between tests on dry and moist sands. A few test results were available for which the water table was at a depth of less than B and these showed considerably larger settlements than for dry sand. The influence of the depth of water table is discussed in $9 98-101 where it is shown that the results of plate tests with H J B > 1 correlate well with the data for larger loaded areas and high water tables.

49. Despite the approximate nature of Fig. 5 several interesting features emerge.

(a) The larger the plates the greater the linear range and the lower the curvature of the pressure-settlement curves.

(b) For any given pressure and compressibility grade, settlement increases with plate size.

(c) The initial tangent slopes to the pressure-settlement curves become more distinct with increasing plate size. Thus, for the smallest size of plate, bedding errors and minor density variations would lead to serious errors in interpretation.

50. Meigh32 has suggested that the grain size and grading of sands could be important factors influencing their compressibility under a test plate. While constructing the charts in Fig. 5 no discernible relationship with grain size could be found, a conclusion also reached by Terzaghi and Peck.7 In the case of gravels and gravelly sands the data were found to be only in the zones for grade IV and better with little correlation with SPT blow count.

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S E T T L E M E N T O F FOUNDATIONS ON SAND AND G R A V E L

Bearlng pressure: kN/m2

B = 0.25 m-0 4 m

(a)

E E

1

(C)

Fig. 5. Charts for assessing the compressibility grade of sand from plate loading tests carried out at shallow depth or in the base of wide excavations

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Depth of influence 51. Guidance varies on the depth of influence beneath a loaded area on sand.

Terzaghi and Peck' recommend taking the average blow count over a depth equal to the breadth B. Parry33 takes the depth of influence equal to 2B but places greater emphasis on the N values immediately below the foundation.

52. S ~ h r n e r t m a n n ~ ~ also takes the depth of influence equal to 2B and uses a simple influence diagram to obtain the distribution of vertical strain. In their statistical analysis of a number of settlement observations Schultze and Sherir5 took the depth of influence equal to 2B.

53. For a uniformly distributed circular load on an isotropic elastic half space the depth of influence is usually taken as 2B. The settlement at this depth is about 25% of the surface settlement. Hence, for practical purposes, the depth of influence may be assumed to be the depth at which the settlement is 25% of the surface settlement and is denoted by zI (or the relative depth of influence (z/B),).

54. There are not many experimental data for assessing the depth of influence for foundations on sand and much of the data are from model tests. Morgan and G e ~ ~ a r d ~ ~ plot the distributions of vertical displacement with depth for a number of tests on model footings ranging from 0.2 m to 0.9 m in diameter. The 25% settlement points correspond to ( z /B ) , varying from 1.8 to 1-13. Breth et d 3 '

measured the settlement distributions beneath 1.0 m dia. footings on carefully prepared beds of very loose medium to coarse dried sand. An approximately linear distribution of settlement with depth for all the tests was observed with 25% of the surface settlement occurring at (z/B), equal to about 1.5.

55. Turning now to the field measurements, Shvets and Ku lch i t~k i i~~ measured the settlement distribution beneath l m square plates on two alluvial soils-a slightly silty sandy gravel and a very silty slightly gravelly sand. The results are given in Fig. 6 and it can be seen that the 25% settlement point occurs between ( z /B ) , equal to 0.8 and 0.6.

56. Figure 7 shows the normalized distribution of measured settlement with z / B beneath five buildings on deep layers of sand. The results from Nikitin et al.39 are from a 61 m dia. ring foundation for a television tower (Case 63). Within the main ring were footings for a service tower along with a second foundation ring. The whole foundation complex occupied most of the area and may be treated as a single entity. The soil profile consisted of 20 m of dense to very dense sand, 15 m of stiff to very stiff clay, followed by rock. Reference points were located at depths of 6 m, 12 m and 25 m below the foundation. The maximum observed settlement of the foundation was 37.8 mm, of which at least 19.5 mm took place in the clay. Eighty-five per cent of the compression of the sand took place in the top 12 m, i.e. for z / B equal to 0.2. The results plotted in Fig. 7 relate only to the immediate compression of the sand.

57. The results from Breth and Chambosse4' are for a reactor building at Biblis, Germany (Case 27). The settlement distribution was measured down a borehole 1.8 m to one side of the 60 m dia. circular raft foundation. The ground conditions beneath the foundations consisted of 7 m of dense gravelly sand, 48 m of dense to very dense fine and medium sand, followed by a great depth of very stiff clayey silt. When the settlement of the raft had reached 40 mm the surface settlement of the instrument was 1 3 3 mm. Near the surface the settlement decreased very rapidly with depth, decreasing to 5 mm at z / B equal to 0.25. Thereafter it decreased more slowly, becoming about 2.5mm at a depth of 57 m at the top of the silt layer. The normalized settlement distribution shown by the curve labelled

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SETTLEMENT O F FOUNDATIONS O N SAND AND G R A V E L

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(27) in Fig. 7 is for the sand layer only. Although no measurements were made beneath the centre of the raft it is clear that most of the settlement takes place above zjB equal to 0.25.

58. The results presented by Dunn41 are for a nuclear power station founded on a raft 55 m wide and 101 m long (Case 32). The underlying ground consists of 31 m of very dense fine beach sand overlying stiff silty clays and dense silt. Five settlement plates were located at various levels in a borehole beneath the raft, with the deepest plate being at a depth of about 12 m. Curve (32) shows the observed settlement distribution. Undoubtedly some settlement will have taken place in the underlying clays and silts, in which case the settlements in the sand would diminish more rapidly than shown by the curve.

59. Curves A and B in Fig. 7 show the settlement distribution beneath two buildings in Berlin Kriegel and W e i ~ n e r ~ ~ . The detailed normalized distribution of settlement varied with the magnitude of the loading. The points show the extreme values and the curves have been drawn through the mean values. It must be emphasized that very little information is given by Kriegel and Weisner about the ground conditions for these two buildings except that the sand is 4&50 m deep and is of medium density. Clay layers are frequently encountered in Berlin and the possibility of a deep clay layer at this site should not be ruled out thereby reducing the values of (zjB), .

60. In Fig. 8 the values of ( Z / B ) ~ corresponding to p/po = 25% have been plotted against breadth for the various model studies and field measurements. It can be seen that, although the scatter is large, there is a marked tendency for (z /B) , to decrease as the breadth increases.

61. It should be emphasized that the depth of influence corresponding to a given value of B will not be unique and will depend on the variation of stiffness with depth. Nevertheless the results given in Fig. 8 indicate a trend which is

0.1 B : m

1 10 01

100

I *Case 27 .Case 63

Shvets and

Kriegel and Weisner4*

Non-homogeneous elastic

Breth er a / 37

Melbourne Series II

2-OL

Fig. 8 . Relationship between measured depth ofinJuence z, andfoundation breadth. Full line is taken from Fig. 9 and isjitted at B = 0-2 m assuming that Ebjak = 10

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broadly in accordance with theory for a non-homogeneous layer having an increasing Young's modulus with depth. Fig. 9 shows the normalized distribution of settlement with z/2a beneath the centre of a rigid rough circular load of radius a on a Gibson solid for various values of Eb/ak (the results were obtained by means of a finite element analysis). For a given value of EL and k it is clear that as a increases the relative depth of influence (z/2a), decreases. The full line in Fig. 8 was

PlPO' %

0.

m 1.

1.

2.

a

E

1,' = 1/3

Fig. 9. Distribution oj'settlement with depth for a circular rough rigid foundation resting on an isotropic non-homogeneous elastic soil

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derived from Fig. 9 and was fitted at a breadth of 0.2 m corresponding to a value of Eb/ak equal to 10. Garga and Q ~ i n ~ ~ give a similar relationship for the depth of strain influence for a non-homogeneous layer.

62. Many more measurements are needed of the distribution of settlement with depth beneath foundations on granular soils both from the point of view of establishing the depth of influence and, of more importance, for studying the in situ deformation properties. For the purposes of this study the full line in Fig. 8 was used as a rough guide to the depth of influence when N is constant or increases with depth. In a very few cases N decreased with depth and in these instances the best fit to the general trends of the data was obtained by taking the depth of influence equal to 2B.

63. The arithmetic mean of the SPT blow count ( N ) over the depth of influence was used to obtain the compressibility grade of the foundation subgrade. The full line in Fig. 8 when plotted as B against zI on double log axes forms a straight line as given in Fig. 3.

Pressure-settlement relationship 64. Most of the current methods of settlement prediction on sands assume that

the relationship between bearing pressure and settlement is linear over the working range of stresses. S ~ h u l t z e ~ ~ and Shultze and SheriP5 conclude from the study of a number of case records that the pressuresettlement relationship is linear over the period of construction. I t has already been noted from Fig. 5 that as the size of test plates is increased the initial portion of the pressure-settlement curve becomes more linear.

65. A number of the case records collected by Burbidge’ contain complete pressuresettlement data and make possible a study covering a range of ground conditions, foundation dimensions and bearing pressures. Five examples will be given.

66. Case 27 is a nuclear reactor founded at a depth of 5 m on a 3 m thick 60 m dia. raft. The underlying ground consists of 60m of dense sand and gravel, assessed as grade 11, overlying Tertiary sands and clays. The net pressure against average settlement relationship for the reactor is shown in Fig. 10 and is, for all practical purposes, linear.

67. Case 51 consists of two 12 storey towers each founded on four 5 m deep footings 4 m wide and 7 m long. The underlying ground consists of 7 m of dense sandy gravel over weathered sandstone. SPT tests on the gravel indicate that it is of grade 111. The net pressuresettlement curves for the four outermost footings are shown in Fig. 11. Settlement observations only began once the pressure had reached 134 kN/mZ. Three of the footings exhibited little settlement up to a bearing pressure of 223 kN/mZ but thereafter the relationships are again linear for all practical purposes. This case is of interest since the bearing pressures exceed 500 kN/mZ.

68. Case 60A is an 18 storey reinforced concrete building founded on a 1-2 m thick raft. The raft is 22.9 m X 32.6 m and is founded at a depth of 3 m. The underlying ground consists of fine to medium sands to great depth with the top 7 m compacted by vibroflotation to grade IV. Settlement observations were started at a gross pressure of 55 kN/m2. It can be seen from Fig. 12 that the initial portion of the pressure-settlement curve is linear but at higher pressures the curve steepens. This is thought to be due to time dependent settlements occurring during the slow application of load near the end of construction.

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SETTLEMENT O F FOUNDATIONS O N SAND AND GRAVEL

0

F 10-

20 -

30 -

40 -

50 -

100 200 300 400 500 Net bearing pressure: kN/m2

Fig. I O . Case 27. Nuclear reactor founded at 5 m depth on dense sand and gravel assessed as grade I1 (N = 41-60)

Net bearing pressure: kN/m2

'B--& U & rm - B-: 1 4 m

I

I m- - P : F o o t l n g s F H

Fig. II. Case 51. Two tower blocks each founded on four 5 m deep footings resting on sandy gravel assessed as grade 111 ( N = 2 6 4 0 )

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Gross bearlng pressure: kN/mz

\-:2*.9 " A

Fig . 12. Case 60A. An 18 storey building on a raft founded at a depth of 3 m onjine to medium sands assessed on grade IV ( N = 16-25)

69. Case 41 is a 10 storey building founded on a raft at the centre surrounded by pad footings. The raft is 11.0 m wide and 33.4 m long, and is founded at a depth of 5 m. The footings are also founded at a depth of 5 m. The underlying ground consists of 12.4 m of sand, assessed from SPT values as grade IV, over stiff clay. Settlement observations were begun when the gross pressure on the raft was 38 kN/m2. The pressure-settlement relationship for the raft is shown by the full line in Fig. 13. Once the gross pressure exceeds the initial vertical effective pressure a:, the curve becomes significantly steeper. The broken line is for a 4.1 m square footing adjacent to the raft. Settlement readings only began when the gross bearing pressure was larger than the effective overburden pressure, and the pressure-settlement relationship can be seen to be linear.

70. Case 69 is for a building in north-west Berlin which is founded on a 5.5 m X 6.5 m raft on sand of grade V. Although the precise depth of founding is not known it is presumed to be between 2 and 3 m. The pressure-settlement relationship is plotted in Fig. 14 and it is evident that there is a marked change of curvature over the initial portion of the pressuresettlement curve, after which it is linear.

71. The cases discussed in $5 64-72 include sands and gravels with grades ranging from I1 to V, foundation widths ranging from 4 m to 60 m and bearing pressures up to 500 kN/m2. All the deposits are believed to be normally consolidated. It can be concluded that for pressures in excess of the initial effective overburden pressure the pressuresettlement relationship is, for practical purposes, linear. For pressures less than the initial effective overburden pressure the compressibility is reduced by a factor of 2 to 4. Observations presented by Dunn4' on the settlement of the Dungeness B nuclear power station (Case 32) are in agreement with this conclusion. DAppolonia et al.45 deduced that the modulus

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Gross bearlng pressure. k N / m 2

18 l 5

0 50 100 150 200 I

2 - \

4 - \

\ \

E 8 -

g 1 0 -

m \

F m 12- ‘ 1 4 -

\ \

E \

\

\ \ \ \

16- \ \ \

1 8 - Rafl

Footing b

l -77.4 m+

E ‘ I w i v 33.4 m X 11-0 m m ’ I I

4.1 m square Footlng

Fig. 13. Case 41. A 10 storey building founded at a depth of 5 m, partly on a raji and partly on footings, on sand assessed as grade IV ( m = 16-25)

[ M = E’/(l - v’’)] for a preloaded sand was approximately twice that of a normally consolidated sand.

72. The conclusion that compressibility is reduced at pressures below the maximum previous overburden pressure is at variance with the conclusions of S ~ h u l t z e , ~ ~ Sherif,46 and Schultze and Sherif3* who conclude that the pressure- settlement curve is uninfluenced by the removal of overburden pressure. A study of the observations presented by S c h ~ l t z e ~ ~ . ~ ’ reveals the following. First, rather large time corrections have been applied to the settlement observations, and sec- ondly, in many instances excavation for the raft foundations took place below the water table. If the uncorrected settlement observations are used and the gross total pressures towards the end of construction are reduced by the hydrostatic uplift of the groundwater then the pressure-settlement relationships reveal small but discernible preconsolidation pressures. For example, the results for Case 83 are plotted in Fig. 15 and a kink in the vicinity of the effective overburden pressure is apparent, giving a change of slope of about 2.

Relationship between foundation subgrade compressibility and breadth 73. In $8 6 4 7 2 it was shown that the slope of the pressure-settlement curve

Ap/Aq‘ (equal to a,, the foundation subgrade compressibility) is approximately

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Bearing pressure: kN/mz

0 100 200 I

L 5 . 5 m-+ 10-

E E C m

+ .-. 3 m m

2 3 0 -

40-

Fig. 14. Case 69. Building founded on raft at a depth of 2-3 m on sand of grade V (m = 9-15)

Gross bearmg pressure. kN/m* 0 ,.l 00 200

I I

E E C

= 10- E I

m

m W

m E

k

20 -

Fig. 1.5. Case 83. Building founded on a 1716 m X 84.0 m raft at a depth of 10.7 m in sand and gravel assessed as grade I V (N = 16-25). Water table at 8.5 m depth below ground level

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constant for normally consolidated sands and gravels. In Appendix 1 the important details of each of the case records referred to in the present Paper are summarized. Where the detailed pressure-settlement relationship was available the virgin portion of the curve was used to determine Ap/Aq’. Where the pressure- settlement relationship was not available and only the immediate settlement and the gross effective pressures are given the assumption was made that the reloading curve up to the effective overburden pressure a:, has a slope equal to one-third of the virgin curve. Thus the value of Ap/Aq’ is given by

74. In many cases (e.g. for most footings) only the net bearing pressure and immediate settlement were known, in which case values of pJqh,, are given.

75. Relationship between a, and B. As stated in 26 and 27, the approach to analysing the case records is similar to that adopted by Burland et namely, to correlate the values of foundation subgrade compressibility a, given in Appendix 1 with the breadth B for each compressibility grade.

76. In Figs 16 to 20 the measured values of foundation subgrade compressibility a, (mm/(kN/m2) are plotted against B (metres) on double log axes for compressibility grades I1 to VI. It can be seen that for each grade the majority of the observations give a well-defined linear correlation between log a, and log B. A few of the cases lie outside the general spread of the results. In Fig. 16 (grade 11) Cases 29 and 32 lie well above the scatter of the results. Case 29 is the 93 m dia. oil storage tank in the Ekofisk Field of the North Sea. The soil profile shows that the sand is underlain at a depth of 26 m by a 50 m thick stratum of hard clay. It seems very probable that significant settlements took place within this clay stratum.

77. Case 32 is the Dungeness B nuclear power station in Kent, England, which is founded on fine sand. It will be shown later that there is some evidence from the present study to suggest that SPT blow counts on submerged fine sand give N values which are too high and should be reduced in accordance with the recommendations of Terzaghi and Peck (see 33 and 34). On this basis the value of N for Case 32 decreases from 60 to 36; this falls within compressibility grade 111 and is plotted in Fig. 17, where it lies within the spread of the results.

78. In Fig. 19 (compressibility grade V) Case 85 lies well below the spread of the results. This case is a chimney for a power station at Cologne, Germany, reported by S c h ~ l t z e . ~ ~ . ~ ’ The SPT results were consistently less than 10 below the water table and, in view of the small settlement of the structure, it can only be concluded that the SPT results are unreliable. Parry33 also noted the anomalous results from this case.

79. In Fig. 20 (compressibility grade VI) Case 79B lies above the spread of the results. Cases 79A and B consist of two preload embankments next to each other, being 8 m and 11 m high respectively. A study of the pressure-settlement curves for each embankment shows that Case 79B had a steeper pressure-settlement curve from the start of loading and that the much larger value of a, cannot therefore be attributed to local yielding under the higher embankment. It appears that the compressibility of the soil for Case 79B corresponds to grade VII, which is consistent with some of the cone test results on the site. 80. In Figs 1620 the broken lines drawn through the points have been fitted

using linear regression of log a, on log B. The cases discussed previously which lie

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Regresslon line of log a, on log B

Compressiblllty grade II (m = 41-60) ,

Breadth: m

Fig. 16. Relationship between a, and B for compressibility grade I I (S = 4 1 6 0 )

outside the spread of the results have not been included in the analysis. For the grade I1 results in Fig. 16 the regression line is heavily weighted by the relatively large number of cases for B less than 3 m and the parallel chain dotted line is felt to be more realistic.

81. Table 2 lists the slope, correlation coeficient and standard error for each regression line in Figs 1&20. In all cases the correlation coefficient exceeds 0.8. The standard error for a, varies from x1.46 to x1.9 with a tendency to increase as the compressibility increases.

82. A particularly significant feature to note in Table 2 is the similarity in the slopes of the regression lines. The weighted average of the slopes is 0.704 (the weighting takes account of the correlation coefficient and the number of cases

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_ _ _ Regression lme of log a, on log B

Compressibllity grade 1 1 1 (m= 26-40)

01 ' ' l ;

I I 1 I I I I I I I I I I > / I

10 100 Breadth: m

Fig. 17. Relationship between a, and B for compressibility grade 111 (m = 2640)

associated with each regression line) and the greatest deviations from this are +20% and - 14%. It therefore appears that the slope is independent of the compressibility grade-an observation which leads to considerable simplifications in the subsequent analysis.

83. In Fig. 21 the regression lines for each compressibility grade are shown as broken lines. The full lines all have slopes of 0.7 and their locations have been fixed by a least-squares analysis of the deviations of log a,. They are termed ' adjusted mean lines '.

84. Comparison of the adjusted mean lines in Fig. 21 with the observations reveals an interesting result. It transpires that the adjusted mean line for any given grade forms a reasonable upper bound for the next grade up and a reasonable

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- -- Regression line of log a, on log B

Compressiblllty grade IV (r = 16-25)

0.l1 7 10 100

Breadth: m

Fig. 18. Relationship between a, and B for compressibility grade I V (N = 16-25)

lower bound for the next grade down. This is illustrated in Fig. 17 for grade 111 compressibility. The adjusted mean lines for grades IV and I1 are shown chain dotted and are seen to form very reasonable upper and lower limits to the spread of the results. Upper and lower limit lines for the other compressibility grades have been obtained in the same way and are shown by chain dotted lines in the appropriate figures. In general the limit lines shown in Figs 16-20 correspond to a spread of rather less than plus or minus two standard errors from the regression line for each grade. Very few of the results lie above the upper limit lines whereas rather more lie below the lower limit lines, particularly for values of B less than about 3 m. Thus Fig. 21 forms a convenient summary of the data and could be

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-- - Regression line of log a, on log E Cornpressibility grade V ( N = 9-1 5)

O 1 d 1 10 Breadth: m

100

Fig. 19. Relationship between a, and B for compressibility grade V (N = 9-15)

Table 2. Analysis of r

Grade

I1 111 IV V VI

No. of cases

19 45 68 39 27

,egression lines in Figs 16 to 20

Slope m

0.669 0.710 0.592 0.833 0.805

Weighted average = 0.704 I

Correlation coefficient

0.89 0.91 0.82 0.84 0.86

X 1.60 X 1.46

0.620 X 10-’

14.656 X 10-* X 1.79 5.585 X 10-’ X 1.90 2.168 X 10-’ X 1.60 1.279 X 10-’

* at is in mm/(kN/m’); B is in metres.

used for design purposes. 85. It is of the utmost importance to recognize that the regression lines or

adjusted mean lines cannot be used for extrapolating the settlement for a small footing to a larger one. By increasing the breadth of the footing the depth of influence is increased and this may well result in an upgrading of the compressibility grade.

86. Relationship between a , , B and N. The equation for the adjusted mean lines

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Fig .

1 ooop L

179'B ,,/

/ / /

-

1 -

- Compressibility grade VI ( N = 4-8) _ - - Regression line of log a, on log B

-

-

O 1 ' l ;

I I I I , 1 1 1 1 I I 1 1 I I l l 1

10 100 Breadth: m

20. Relationship between a, and B for compressibility grade VI ( N = 4 3 )

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1 I I 1 8 8 ,

10 100 Breadth. m

Fig. 21. Relationship between a , , B and compressibility grade showing mean lines and upper and lower limits

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in Fig. 21 is given by

log a, = m X log B + C(#)

where the slope m = 0.7. The term C(N) is a function of the compressibility grade and hence of the mean SPT blow count N . The value of C corresponding to a particular compressibility grade is given by the intercept of the appropriate adjusted mean line with the a, axis (i.e. when B = 1 m). Hence

C(N) = log - = log p (= log I,) a, a, B"

where I, is termed the compressibility index. 87. In Table 2 the values of u,/B"~( = I , ) for each compressibility grade are

listed and in Fig. 22 they are plotted as open points against on double log axes. The upper and lower limits for each compressibility grade are also shown. It cay be seen that there is an approximately linear relationship between l_og I, and log N and the reason for having a progressively increasing interval in N for successive compressibility grades now becomes apparent. The spread of I , between the upper and lower limits increases from a factor of about four for grade I1 to about eight for grade VI.

88. In view of the apparently linear relationship between log I, and log R an independent regression analysis was carried out on all the cases in Appendix 1 for which SPT or cone test data are available. The results of the analysis are given in Fig. 23. The regression line for log ( u J B " ~ ) on log is shown as a full line. It has a slope of - 1.43 and an intercept on the N = 1 axis of I , = 1.7. The coefficient of correlation is 04348.

89. The regression line in Fig. 23 has been plotted in Fig. 22 and is seen to agree well with the points for each compressibility grade. Mean upper and lower limit lines have also been drawn in as chain dotted lines and are reproduced in Fig. 23. It can be seen that most of the individual cases lie between these limit lines and the majority of those that do not are for B less than 3 m, for which the scatter is somewhat larger.

90. Thus Fig. 22 is a more compact form of Fig. 21 and can be used for design purposes-see @ 7 and 8.

The influence of various factors on settlement 91. The collection of a relatively large set of data such as is given in Appendix

1 makes it possible to study statistically the influence of various factors on the immediate settlement of loaded areas on granular materials.

92. The regression line in Fig. 23 can be represented by the expression

where the figure in brackets represents one standard error. Denoting IO~(N"~ /B ' '~ ) X a, X 10' as T , the value of T for each case given in Appendix 1 may be calculated. Fig. 24 shows a histogram of T for the complete data set which can be seen to be approximately normally distributed. The mean value of T is denoted by and is equal to 2.232. For any given foundation the best estimate of the foundation compressibility ii, is given by

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t Cornpressibility grades

1 I 1 I , I , ,

1 I I 1 I 1 I l l ,

SPT N 10- 100

Fig. 22. Relationship between compressibility index I , and compressibility grade- derivedfrom Fig, 21

94. The deviation of any measured value of a, from the best estimate 71, may be expressed as aJ2, or G,/a,. The values of aJ2, and 2,/a, corresponding to one

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lOOr

t

--- Upper and lower limlt llnes from Fig. 22

D B 3 3 m o B < 3 r n

0

0.1 I I I I I I , I , I I I I I I I l l 1 10 100

SPT N

Fig. 23. Relationship between compressibility index I , and N for all cases in which SPT or cone tests were carried out

standard deviation from ii, are both 1.82. Hence any statistical analysis will only detect major influences on a, and care must be taken not to read too much into minor trends.

95. It is assumed that the major factors influencing a, (apart from N and B) are the length L, the depth D, the depth of the water table H , and the thickness of the

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30r

Fig. 24. Frequency distribution of the settlement observations for all the cases

sand layer beneath the foundation H , . Each of these parameters will be studied separately.

96. Influence of LIB. In Fig. 25 values of a,/ii, have been plotted for various values of LIB for those cases in which DIB < 0.25, H,/B < 0.2 and HJB > 2. The chain dotted lines correspond to one standard deviation either side of the mean. It can be seen that the measured values of a, are larger than 2, for the majority of cases, indicating a positive correlation between LIB and a , . However, the influence of LIB is not large and it would appear that the average value of a,/ii, is unlikely to exceed about 1.5 at LIB equal to 5 . Although there are very few observations for larger values of LIB the results do not point to any further increase in the average value of a,/ii, beyond about 1.6. For comparison the relationship for homogeneous elastic theory is shown as a broken line and is seen to give significantly larger average values of a,& than observed. The full line in Fig. 25 is given by the empirical expression

which appears to represent the observed trend reasonably well and tends to 1.56 as LIB tends to infinity.

97. Influence of DIB. In Fig. 26 observed values of a,& have been plotted against DIB for the cases in which LIB < 1.5, H,/B < 0 . 2 and HJB > 2. There are a number of observations for D / B < 0.3 but relatively few for larger values. It appears that there is no obvious correlation between a,/ii, and DIB for DIB 3. In their analysis of a large number of observations on one site DAppolonia et aL8 report only a 12% reduction in settlement as DIB increases from 0.5 to 1.0. Such a variation is too small to detect in relation to the scatter of the results presented

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a,/%,

t

2 c -

3 ,I' Z,/a,

Fig. 25. Influence of LIB on foundation subgrade compressibility a,

0

0 .-

0

I O B 2 3 m

Zf/al

Fig. 26. Influence of DIB on foundation subgrade compressibility af

here and supports the conclusion that the influence of DIB is small. 98. InJluence of depth of water table H, . There are differing opinions about the

influence of the depth of the water table on settlement. For example Terzaghi and Peck' assume that for a deep water table the settlement of a foundation is half that for a water table at founding level. However, MeyerhoP recommends that the presence of the groundwater table should be ignored on the basis that its effect is already reflected in the SPT blow count.

99. In the following analysis a distinction is drawn between results from building foundations and results from plate loading tests. It can be seen from Appendix

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G R A V E L

30-

H,/B > 1

0s 20- i;

($af) mean = 1.34 U

I T(mean = 2.103 I No of values = 24

c a, 3

U = 0.215

10-

I l I L

1.0 2.0 ‘ 3- 0

T = log - X a, X 10’ n1.4

~ 0 . 7

(b)

Fig. 27. ( a ) Frequency distribution of settlement observations for buildings with H , greater than 5 m; (b ) frequency distribution of settlement observations for plate loading tests with H,/B greater than unity

1 that, for the majority of building foundations, embankments and tanks, the water table is close to founding level. Hence it is possible to analyse the results of those cases where the water table is deep (taken as greater than 5 m) and compare them with the whole data set. There are 15 cases which fall into this category and Fig. 27(a) shows the frequency distribution of T . The mean value of T for this data set (given by the full line) is 2.176 and the standard deviation 0 is 0.187. These values may be compared with the corresponding values for the complete data set, which are = 2.232 (shown by the broken line) and U = 0263. The average value of aJa, = 1.13, i.e. the settlements of the foundations with deep water tables are, on average, only 13% less than the best estimates from the whole data set. It must

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be concluded from the above analysis that the level of the water table has no significant influence on the value of a, for building foundations.

100. Almost all the plate loading tests listed in Appendix 1 have H J B > 1. Fig. 27(b) shows the frequency distribution of T for 24 plate tests with H,/B > 1. The mean value of T for this data set is 2.103 and the standard deviation is 0.215. The average value of ii,/a, is 1.34. Thus the settlements of the plates are, on average, 25% less than the best estimates. However, the loading of the plates was carried out in less than a day, whereas the loading of the building foundations usually took place over a year or more. It will become evident later in the Paper that significant time-dependent settlements occur on sand. Hence the fact that a, from the plate tests is less than ii, is not surprising and can be attributed principally to time effects.

101. It appears from this study that the position of the water table has only a small influence on the value of T . It is important to emphasize that this conclusion must not be taken to imply that the position of the water table does not influence the settlement. What it does do is to confirm Meyerhofs view that the effect of the water table is probably reflected in the value of N. If a water table changes subsequent to the determination of the N values the settlements may differ appre- ciably from the predicted values.

102. Influence of thickness of sand layer. A few of the cases given in Appendix 1 have a thickness of sand layer H , beneath the foundation of less than 2B. As might be expected there is a tendency for the values of a, to lie below iir but there are insufficient cases for any useful trend to emerge. It is suggested that, for design purposes, the predicted value of a, should be reduced by the factor

when H, z , where zI is the depth of influence given by Fig. 3. 103. Influence ofgrain size on N. It is generally accepted that grain size does

not have a major influence on the number of blows in an SPT. Two soil types where uncertainties exist are fine sands or silty sands below the water table, and gravelly soils, as discussed in @ 33 and 34, and 9 32 respectively.

104. Fine sands and silty sands. In Fig. 28 the black points refer to the observed settlements of foundations on fine sands and silty sands for which N was evaluated from SPT tests below the water table. In §$ 33 and 34, two methods of correcting for submergence for fine sands were mentioned, Terzaghi and Peck' and Bazaraa." The open points in Fig. 28 refer to corrected blow counts N' where N' = 15 + 0.5(N - 15), as proposed by Terzaghi and Peck. It can be seen that only five cases are affected and only two of them (32 and 64) significantly so. The effect of the correction is to bring these two cases closer to the mean regression line taken from Fig. 23. Application of the Bazaraa correction (W = 0.6N) translates all the points significantly to the left, which results in a poorer overall correlation. Therefore on the basis of the limited evidence available it appears that the SPT correction proposed by Terzaghi and Peck for submerged fine or silty sands results in an improved assessment of compressibility.

105. Gravel and gravelly sands. Fig. 29 shows a plot of I , against N for all the cases involving gravel, sandy gravel and gravel/sand. By inspection it can be seen that the mean of the points tends to lie to the left of the mean regression line for the complete data set. This is confirmed by a statistical analysis of the results, which

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.N O N' = 15 + % (N-15)

\ B I 1 1 1 I l , , I I I I I I I I J

10 SPT i? and

100

Fig. 28. Relationship between compressibility index I , and N for submergedfine and silty sands

gives a mean value of T = 2.085 and U = 0.246 compared with = 2.232 and 3 = 0.263 for the whole data set. It is a simple matter to show that T',,,, can be made equal to by correcting the N values such that N' = 1.25 X N . This is a fairly small correction and in many cases could perhaps be neglected. It is worth noting that the lowest value of N for the case records involving gravel is 13 and more data are required for lower values of N .

106. Inherent variability. Natural deposits of granular soils are inherently vari- able, both laterally and vertically: the variability will differ from one site to

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t 0 6 2 3 m o 6 < 3 m

t \

0.1 1 10 ? 00

SPT

Fig. 29. Relationship between compressibility index I , and A for gravels and sandy gravels

another. Effects of different foundation geometries and loadings have made it difficult to isolate the effects of inherent variability on the settlement of separate foundations at a given site. The present study indicates that the influence of geometry and load can be largely eliminated by expressing the measurements from a given site as the quantity T . The variation in T for a given site is then a measure of the inherent variability of the settlement characteristics of the site. Fig. 30 shows a plot of TIT mean for all the cases where more than one foundation was observed at a given site. The measured settlements generally lie between about 50% of the average, although on some sites it is less than f20%. Therefore, given perfect

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Fig. 30. Investigations into the inherent variability of settlement characteristics at a number of sites

methods of measuring the compression characteristics of granular deposits and predicting settlement, one could normally expect differences of up to a factor of about 3 in the actual settlements, depending on the site conditions. The method of correlating foundation compressibility with compressibility grade given in Fig. 22 gives differences varying between factors of 4 to 8. Thus there is still room for considerable improvement in predictive methods but the limitations of inherent variability should always be borne in mind.

Time-dependent settlement 107. As pointed out by S ~ h m e r t m a n n , ~ ~ it is not common to consider the

time-dependent settlement of sand. However, all the case records reported here which have measurements subsequent to completion of construction show time- dependent settlement, as can be seen from Appendix 1. However, of the 27 cases given in Appendix 1, 14 have to be treated with caution owing to the presence of clay or silt layers beneath the loaded area or owing to the fact that the sand has been recently placed. Moreover, for a number of the remaining cases the periods over which time-dependent settlement measurements have been measured are relatively short and do not give a clear pattern of behaviour.

108. Fortunately, a very complete set of settlement observations has been published by Bolenski4* for ten structures founded on sand in Warsaw (Cases 16

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to 25). Bolenski, who was not in fact an engineer, while working for the Polish Building Research Institute, and later, as a hobby, collected settlement data on structures over a period of about 20 years. In view of the length of the records (some over 16 years) they are invaluable for studying the time-dependent settlement of foundations on sand. 100. In Fig. 31 the measurements on four buildings in Warsaw (Cases 22 to 25)

are plotted in terms of pJpi against log time after completion of construction (pi is the settlement at completion of construction and pt is the settlement at time t after construction). In spite of the length of the settlement records no clear pattern emerges. Cases 22 and 25 appear to show continuing settlement which is approximately linear with the logarithm of time. However, Cases 23 and 24 show stepwise behaviour with long periods of little settlement followed by sudden downward movement.

110. Bolenski also presented long case records of settlement of chimneys on sand and these show markedly more time-dependent behaviour than buildings. In Fig. 32 the results for three chimneys (Cases 16 and 19) are plotted as pJpi against log time. Case 19 shows linear settlement with the logarithm of time but cases 16A and B again show stepwise behaviour. By comparing Figs 3 1 and 32 it can be seen that the chimneys reach much larger values of pJpi than the buildings and also show a more rapid rate of settlement. The reason for this is thought to be due to the action of wind inducing fluctuating bearing pressures on the sand. Bolenski also reports some records of settlement of turbine foundations which show similar characteristics to the chimneys and which were undoubtedly subjected to fluctuating loads. The stepwise nature of the timesettlement characteristic of some of the structures may be due to perturbations in loading (possibly minor seismic events) triggering grain slip within the mass of the sand.

1 1 1. If it is assumed that the settlement pI at times greater than 3 years after construction increases linearly with log time then

P I = Pi + A P ~ + Apt log(t/3)

Time after completion of construction: days 100

1 .o - 500 1000 5000

I , l I ,

1.3 i Fig. 31. Timesettlement characteristics of four buildings in Warsaw observed by Bolenski4'

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where t is the time in years after completion of construction and is 3, A p , is the increase in settlement during the first 3 years and Apt is the increase in settlement per log cycle of time after 3 years. Dividing by pi

- = 1 + R , + R, log(t/3) P I

Pi

where R , is the propprtional increase in settlement during the first 3 years, and R, is the proportional increase in settlement per log cycle of time after 3 years.

112. In Table 3 the values of R , and R, are listed for the relevant cases. There is no obvious correlation with soil type. Case 51 consists of eight large footings on gravel and it can be seen that the values of R , and R, have a wide range even on the same site.

Time after completion of construction: days

1 1000 10 000 I I I I 1 1 1 1 1 I I 1 l I I I l l

\

Fig. 32. Time-settlement characteristics of three chimneys in Warsaw observed by Bolenski4*

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113. A conservative interpretation of Table 3 leads to the following expression for buildings

_ - p' - 1.3 + 0.2 log(t/3) Pi

and for chimneys

= 1.7 + 0.8 log(t/3) Pi

114. The latter expression may be appropriate for other foundations subject to fluctuating loads such as bridge abutments and silos. Another way of interpreting the above expressions is that after 30 years p' = 1 . 5 ~ ~ for buildings and p' = 2 . 5 ~ ~ for chimneys.

115. The above expressions have been derived from limited data which are mainly restricted to grade 111 sand and gravel. Clearly, there is a need for more post-construction settlement observations over a period of years.

Discussion and conclusions 116. No attempt is made here to compare the results of the correlations

derived in this Paper with predictions of other methods. To do so would require a case-by-case comparison. The confidence limits of the correlation summarized in Fig. 1 are large and are believed principally to reflect the limitations of the SPT, cone penetration test and other indirect methods for assessing the compressibility of granular materials. Most other methods of settlement prediction, although more analytically based, rely on such indirect methods of assessing compressibility and will therefore also have wide confidence limits. Recognition of this and of the variability inherent on any site, is important in the design process.

Table 3. Time-dependent settlement offoundations on sand and gravel*

Case Grade ~~

I Buildings

22 111 23

V 84 IV 83 I1 I 51 I1 I 25 111 24 111

Chimneys

19

~

Principal soil type

Fine/medium sand Clayey silty sand Silty fine sand Fine sand Gravel Sand/gravel Sand-gravel

Medium sand Medium sand Fine silty sand

ti R3 (loading period in

days)

751

0.05 488 0.13 822 0.37t 880 0.14 894 0.26 355 0.30 334 0.1 1

1208

1 .oo 61 0.35 542 0.55

R,

0.23 0 0

0.21 0.17f 0.13 0.07

0.85 0.53 0.67

* Note: pJpj = 1 + R , + R, log(@) with t in years-see equation (8). t mean of range O.OM.62. 1 Mean of range 0.02-0.4.

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117. Schultze and carried out a similar correlation to the one described here using a multi-correlation technique. Their data base was very much more limited than in this study and was largely dominated by cases with N equal to 20 and N equal to 30. Nevertheless it is of interest to compare the results of their correlation with the one derived here. For a square footing resting on the surface of a deep sand layer Schultze and Sherifs results may be expressed as

0.364 X El’’ a, = N0.87

which may be compared with equation (1 5)

1.706 X a, = N1.4

118. The two equations are compared in Fig. 33 for E = 3 m and E = 30 m and agree reasonably well for footings up to about 5 m wide. For larger foundations equation (15) gives significantly larger settlements. The difference in slope between the two relationships is probably due to the fact that in equation (18) is determined over a much greater depth (2E) than for equation (15). It should also be noted that Schultze and Sherif arrived at much larger shape and depth correction factors than were obtained in the present study.

119. The following conclusions can be drawn from the study described in this Paper.

(a) The results of a statistical analysis of over 200 case records of settlement on sands and gravels has resulted in a simple correlation between a, (the foundation subgrade compressibility), E and A, the average SPT blow count over the depth of influence. The standard error of a, varies from about (x/+)1.5 for N greater than 25 to (x/+)1.8 for N less than about 10. Thus the accuracy of the correlation is not particularly high but, in view of the small settlements that are usually involved, it is good enough for most practical purposes. However, it is recommended that other widely accepted methods are also used as a check.

(b) If more precise predictions of settlement on granular soils are required they must be based on direct methods of determining in situ compressibility and not on indirect methods such as the cone and SPT. It is hoped that the results of this study will serve to stimulate the development of such methods while at the same time providing a simple approach for routine design purposes.

(c) The available experimental evidence suggests that the relative depth of influence ( z / E ) , beneath a foundation decreases significantly as the value of B increases. There is an urgent need for field measurements of settlement at various depths beneath loaded areas to establish not only the depth of influence but also the in situ compressibility of granular soils with depth.

( d ) For normally consolidated sands the relationship between the effective foundation pressure and settlement is approximately linear up to about one-third of the bearing capacity. The effect of overconsolidation and loading at the base of excavations is to reduce the foundation subgrade compressibility for bearing pressures below the effective preconsolidation pressure.

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t

‘ \ \

- Equation (1 5)

- - - Schultze and Sherif35

1- - - -

I 1 I l l 1 1 I I I I l l 1 1

10 100 - N

Fig. 33. Comparison between the correlation between a, and N derived in the Paper with that obtained by Schultze and Sherif3’

( e ) Using the complete data set as a basis for comparison it appears that for DIE 3 the depth of founding and the level of the water table do not have a significant influence on a, . However, the effect of increasing L/B is to increase a, by up to about 50%.

cf) It has been shown that the Terzaghi and Peck recommendations for correcting the SPT blow count for submerged dense fine sands and silty sands give an improved correlation. Similarly, an analysis of all the case records involving gravels and sandy gravels indicates that the SPT blow count should be increased by a factor of about 1.25 for the purpose of assessing the compressibility.

( g ) An analysis of the results from a number of sites where the settlement of two or more foundations has been observed indicates that the inherent

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variability of the ground frequently gives rise to settlements which differ from the mean by a factor of 1.5 or more. This finding gives support to Terzaghi's4 statement that the settlements of uniformly loaded areas on natural sand strata vary erratically.

(h) The field measurements show that time-dependent settlements take place on sands and gravels. For static loads this additional settlement is fairly small and may reach 50% of the end of construction settlement after about 30 years. For fluctuating loads the time-dependent settlements are much larger.

Appendix 1. Details of case records and measurements 120. Table 4 gives details of the various case records referred to in the Paper. For ease of

reference the numbering of the records is the same as that used by B~rbidge,~ although for various reasons not all his cases have been used. An explanation of some of the columns is given below: Column 3 R, mean values of N over the depth of influence (see 51-63). Column 4 grade, see Table 1. Column 5 method, SPT, standard penetration test; C, static cone penetration

Columns 6 8 B = breadth; L = length; D = depth of founding. Column 9 H , , depth of water table beneath founding level. Column 10 H , , thickness of sand or gravel stratum. Columns 11-13 qsross is the gross bearing pressure at founding level; qbe, is the net

effective bearing pressure at founding level; A&, is the known change

Column 14 p i , observed average settlement at the end of construction.* Column 15 Api , observed increase in average settlement due to Aqbe,. Column 17 ApJAq', obtained from columns 13 and 15 or from slope of pressure-

Column 18 t i , length of construction or loading period. Columns 19 and 20 total final settlement p, and corresponding time t , since start of con-

struction or loading.

test; Oed, oedometer; P, plate loading test.

in Q b d '

settlement curve or from equation (1 1).

*When only the edge settlement of a tank has been measured a factor of 1.1 has been applied for tanks up to 40 m in diameter and a factor of 1.2 for diameters above 40 m.5

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Table 4. Details of case records

I

2 3lA

61P 318

6lR 71A 71p 8lB 81P

9 6 l21A 13/A I3jB I 3jc 14 15," 1517 15/17 l5/8-l8 I6/A I6jB 19 20;A 20/B 21 22 23 24 25 27 29 3011-7

9 1 ~

3018 30p- I5

30/16-18

30119-30

30/3 l -32 3013343 30144 30145 30146 30/4748 30/49-50 3o/c 3I/A 3118 3 I/C 3 I/D 32 33lA 3318 33ic 34 35!A 35/B 36 37

38 39/0 39JC 39iP

39/P

(2) Principal soil

type

Fine 10 coarse

Fine sand sand

Sand Sand Sand Sand Sand Sand Silty sand Silty sand Sand Sand Silty sand Sdty fine sand Silty fine sand Silty fine sand Fme sand

Sand Sand

Sand Sand Medium sand Medium sand

Finelmedium sand Fine silty sand

Finetmedium sand Fine/medium sand

Clayey silty sand Finelmedium sand

Silty fine sand

Gravelly sand Fme sand

Finelmedium sand Finelrnedium sand

Fine/medlum sand Fine/medium sand

Finelmedium sand

Fine/medium sand

Fine/medium sand Fine/mediurn sand Finelmedium sand Finclmedium sand Finelmcdium sand Finelmedium sand Fine/medlum sand FlncJmedium sand Fme/mcdiurn sand

Fine/medium sand Flneimedlum sand

F!ne/medium sand Flne sand Fme sand Fine sand

Sandigravel Flnc/mcd!um sand

Medium sand Medium sand Sand Gravelly sand

Medium sand Medlum sand Medium sand Medlum sand

Medium sand

- ( g N

-

28 17 8 8

30 30 35 38 10 10 60 60 17 15 15 I5 7 6 6 6 6

~

- - - - - ~

- - - 47

20

20 20

20

20

20 20 20 20 20 21 22

21 19 17 20 60

-

-

- - - - I I I I 25 60

I2 21 21 16

16

Grade 14)

111 IV VI VI 111 111 111 111 V V I1 11 IV V V V VI

VI VI

VI VI 111 111 111 111 111 111 111 111

111 111

11 11 IV

IV I V

IV

IV

IV IV IV IV IV IV IV I 1 IV IV v IV I 1 IV IV IV 111 V V IV I

V IV IV IV

IV

-

Method ( 5 )

SPT SPT C C C C C C C C C C C C C C C C C C C Oed Oed

Ocd OCd

Oed OCd Oed OCd

OCd Oed

C

SPT

SFT S PT

S PT

SPT

SFT S P T SPT S PT S P T SPT S PT P S PT S PT S PT S PT S PT P P P P 5 PT S PT FPT SPT

-

SPT S P T SPT j PT

$ P T

T B L D

0 I .2 I I 2.8 3.6 2.85 2.85 2.5 2.0 3.0 3.0 2.6 0 0 0 0 I .o I .o I .o

1-2.6 4.0 3.7 6 2 2

4.0 2.3

2.7 3-0-5.0

2-8 2.2

5.2 0 1.5

1-7 1.8

2.0

2. I

2.3 2-5 2.6 3.0 3 2 3.4

0.6 3.5

0 0 0 0 9.7 5.0 4 5 4-0

0 0 0.1

20.9

5.0 1.5

5.0 0

0

1.5 7.3 I .6 I -6

- 1.5 - 2.3 - 1.6 - 1.6

0.5 0 -

- 0.5 I .o I .o I .o 0 2 2 I .6 2

- 0.7 - 1.0

0.7 0.5

0.6 2.2

- 1.7

0.7 3.2

2.6 - 3.7

0 4

4 4

4

4

4 4 4 4 4 4 4

S 6 LL6 0-6 0-6

- 7.2 0 0.5 10

0 6 0 6

-9 2 3.7

-

-~

0 10.0 10-0 10.0

10 0

-

T ( 1 1 ) (1.2) ( I ! Foundatlon press.

_. q,roll

150

196

245 196

151 151 245 140 I34

I80 I47

417

289 294 147 186

500

254

245

-

kN/m'

4.d

193 I30 52 52

162 I62

93 1 4 0

147 93

284 284

121 80 60

I 6 4 78

74 64 75

70-86 I l 8

I43 I26

I l4 I12 I77 77 66

I32 I10

23 I

247 139-290

97-225

102-161

113-166 97-199 I39 161 I50 113 I77 215 I66 I56 I54 24 I

209 70

I l 8 490 I82 I58 I80

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S E T T L E M E N T OF F O U N D A T I O N S O N S A N D A N D G R A V E L

(14)

P , * mm

18 22 20 35 10.5 11.0 6.5 3.0 8

12 I 3 6-12

52 80 7

143 74

50-90 75

84121 7.2

10.1 5.4

17.3

21.1 14.5

10.1 3.3

12.5 8-6

45

8 1

12.2 10-4

7 5

7. I

5- 1-8. I 8.7

10.2 9.4

14.5 4-1-5.1 7.6-8.5 I .7

(mean)

(mean)

(mean)

(mean)

80

1 0 0 90

131 65 4s 24 25 14

232 I96 39 4 15

(mean)

(mean)

11.0

9 9

9.3 16.9

67.3 38.5

6. 5 S. I 7.0 2.1 8.6

0.35 8.2

0.88 5.0-16.5

77.5 I33

9 87.2

1 0 0

123 78-141

112-173

3.4 mean)

4.6 4-9

mean) 5. I

mean) 4.9

mean) 4.54.9 6. I 6.8 6.3 9.7 3.6-5.1 4.3-5.0 0.8

40.2

w.9 57.7

54

I28 I24

2.9

21.9

man) 4.5

4-0 nean)

- (18)

I, , days -

l o o 0 loo0 m m

1800 I800 500 500

1 1 0 0 I 1 0 0 300 400

I 120

-

- - -

1208 542 61

522 500

1 4 0 0

334 751

355 894 880

6 1460

I460 I460

I460

I460

I460 I460 1460 I460 I460 1 4 6 0 1460

I

-

~

- -

2821 1 5 0 0 I820 I820

52 30

-

761

I

I

Remarks

Centre settlement Edge settlement X 1 . 1

Bridge piers founded in base of 3-8 m deep cuttmg Bridge piers founded in base of 3.8 m deep cuttlng

Measured edge settlement 130mm Poorly graded sand, some thin layers of clay and silt Poorly graded sand, some thin layers of clay and silt

Poorly graded sand, some thin layers of clay and silt Poorly graded sand, some thin layers of clay and silt

AplAq' (corrected for 0.b. pressure) ApplAq' (corrected for 0.b. pressure)

ApplAq' (corrected for 0.b. pressure) ApplAq' (corrected for 0.b. pressure), clay underlying sand ApjAq' (corrected for 0.b. pressure), clay underlying sand AplAq' (corrected for 0.b. pressure), clay underlying sand

ApplAq' (corrected for 0.b. pressure) ApplAq' (corrected for 0.b. pressure)

AplAq'(correc1ed for 0.b. pressure), clayey silly sand below 12.1 m

ApjAq' from slope of pressure-settlement curve. Stiff to very stiff sandy clay below 60m Hard clay below 26m Range of p, = 6.61 I .2mm; p/q' = 2 . 8 4 7

W A q '

Occasional clay bands Occasional clay bands Occasional clay bands Occasional clay bands Stiff silly clays below 21.3 m. ApplAq' from slope of q' v. p curve Clay ll-14m. mar1 below 14m. AplAq' corrected for 0.b. pressure clay below 1 I m. AplAq' corrected for 0.b. pressure ( p , = 3 mm at depth = 14.5 m) Clay below 12.5m. AplAq' corrected for 0.b. pressure.

9.6-1 1 . 1 m limestone; l I 1-16.1 m very stiffsilty clay 9 . 6 1 1 . 1 m Imestone: 11.1-16.1 m very st~ffsilty clay 7.&9.1 m stiBclay. Below 9.1 m Gneiss bedrock hpJAq' from pressure-seltlemenl curve

Boreholes 3 and 6 used for SPT Boreholes 3 and 6 used for SPT I2 plate tests: p = 6.3-20.5mm: plq' = 2.68.4

i plate tests: p = 7.0-14-Omm: pplq' = 2 9-57

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Table 4. Continued

Case ( 1 )

no.

40 41 43/A 4318 44/PI

44/MI 44/P2

44/M 3 45/A 45jB 45JC 47/A 4718 47/c 48 49 5O/A 50/B 51/A to H

52/H 52lC

52/A3 52/D3 52/J 53 56jB 5610 57

58/A 5818 %/C 59/A

59/B

59jC

59/D

59lE

59/F

59jG

59lH

5911

5911

59/K

59/M

59/N

59/0

59/P

W Q

59/R

W A

6018

W C

61/A 61lB

61/CI

61jC2

63

65 66/A 66/B 69lA 69/B 70

W C

Princlpal soil tYF

(2)

Fine sand Medium sand

Sand Sand

Medium sand

Medium sand Medium sand

Medium sand Fine to coarse sand Fine to coarse sand Fine to coarse sand

Sand with gravel Sand with gravel

Sand wlth gravel Medium sand

Silty fine sand Medium sand

Silty fine sand Gravel

Sand/gravcl Sandlgravel

Sandlgravcl Sand/gravel Sand/graveI Sllty sand Fine sand Fine sand Fine sand

Sandy gravel Sandy gravel Sandy gravel Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine to

Fine 10

Fine to

Fine to

Fine to

Fine sand Compacted

moist sand Compacted

moist sand Compacted

moist sand Sand and loam Fine sand Sandlgravel Fine sand Fine sand Coarse sand Coarse sand Fine to

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

medium sand

- ( >) N

- 12 22

- -

35

28 50

45 18 I8 I8

26 29

18 30 6

20

37 20

50 50

20 30

20 I 2 - - 6

13 13 13

35

25

25

25

35

35

25

25

25

25

25

40

40

40

40

40

40

30

30

25 34

15

15

$5

23 25 I 2 I 2

-

- -

-

Grade (4)

V IV 11 11 111 11 111 11 IV IV IV

111 111

V 111

IV VI

IV 111

11 11 111 IV IV V V V VI

V V V

Ill

IV

I V

I V

111

111

1v

IV

IV

I V

I V

111

111

111

111

111

111

IV

I V

IV 111

11

V

V I1 IV 111 V V VI VI

V

( 5 ) Method T C S PT Oed Oed S PT

SPT SPT

S PT CISPT c / s P T CjSPT

SPT S PT

S PT S PT C S PT

SPT SPT

S PT S PT

S PT S PT SPT C Oed Oed C

S PT S PT SPT

C

C

C

C

C

C

C

>

IPT ;PT ;PT ;PT , , >

T B

T - Ysru...

220

193 193 193

270

I67

R I

I65

I48

2 0 0 1 4 0

- q"d -

144 l44

I50 I50 I50 I50

2 1 5 215 215

75 41 33 SI8

300 300 300

300 300

91 171 171 I23

78 77 77

230

230

284

195

226

2 50

250

!94

206

!94

1 0 4

!04

1 0 4

1 0 4

I 0 4

96 !20

64

l39

184

'20 68 88 44 41

37 -

Aq' -

5 5 7

I60

1372


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(18)

l , ,

days -

600

488 580 488

I 1 1

7 30 15

207 207 880

l I 1 1 I

l o o 0

125 125 I25

1

304

366

I 1

I

1

1 1220 520

l 47 47

I

(20)

l r 3

days -

853 853 853

1838

2020

257 257

Remarks

Stiff clay below 12.5 m Firm silly clay below 26.7111. Grade determmed over (:/BJl = I

Treated by vibroflotation to 7.5m depth. Limestone below 15.2m Treated by vibroflotation to 7.5m depth. Limestone below l5 2 m

Compacted by vibroflotation

Compacted by vibroflotatlon AplAq' from slope of q' v. p curve. Firm clay below 19.2 m AplAq' from slope of q' v. p curve. Flrm clay below 19.2 m AplAq' lrom slope 01 q' v. p curve. Flrm clay below 19.2 m

Ap/Aq' corrected for 0.b. pressure

Low values of SPT below W.T. due to boiling Moraine below 8.7 m

AplAq' from slope of q v. p curve. ApIAq' = 1 . 5 4 2

Sandstone below 7.1 m

Pressure Increased to 91 kNlm' without lurther settlement Pressure increased to 107kN/m2 without further settlement

ApIAq' corrected for o.b. pressure

AplAq' corrected for 0.b. pressure

AplAq' from slope of pressure-settlement curve

AplAq' from slope of pressuresettlement curve

Model tests Model tests

Model tests

Model tests

Settlement measured over depth of sand layer Model tests


StiR clay bclow 5.3m Stiff clay below 5.3 m

1373


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B U R L A N D AND B U R B I D G E

Table 4. Continued

(1) (2) ( 1 1 ) (12) (13) (10) (9) (6) (7) (8) (5) (4) (!) Case Foundation press, H , 4 Dimensions, m Method Grade N Prlnclpal soil no. type m k N/m' m

B L D 4 ' *:er 4.ro..

71

40 0 C 111 36 Sandy gravel 80

0 27.5 27.5 40 0 0 20.0 27-S c VI 5

c VI 5 Fine sandisill 7918

85 45 - I 3.0 20.0 20.0 Fine sand/silt 79/A

85 32 - I C VI 5 Sdty fine sand 788

3.0 20.0 20.0 1.5

C VI 5 Silly fine sand 78.4 240 12.0 10 - 10.04 SPT I I M) Medlum sand 77

I96 245-295 2 30 -2.5

- 5 6 4.8 111 10.0 65.0 22.5 SPT IV 20 Fine/coarse sand 76

216 S I5 Shallow Fine sand 75

I57 - 3.0 2.8

~B .3 IV

- - 3.0 3.0 - Fine/medium sand 74

187 $ B V P - Fme/medlum sand 73

- 3.5 3.5 ~ 1 2 . 8 Shallow 2.3 Strip 1.5

P v1 P VI

Fine sand 72 78 - Fine sand

41.2 41.2 10.0 8ljC Flne sand 5 VI c 0.9 0.9 0.3 Deep - 81/D Fme sand 6 VI C 0.9 0-9 0.9 Deep --

8I/E Fine sand 7 VI c 81/F Fine sand

I 2 1.2 0.2 Deep -

B3 Sandigravel 20 I V SPT 17.6 84 0 10.7 84 Sand'gravel

-2.2 2 3 7 240

85 Gravelisand 14 V SPT 16.0 43.0 7.3 -I 8 2 2 3 228 10 V SPT 20.5q5 - 3.5

86 Sandigravel 26 111 SPT 14.5 14.5 3.5 2 5 2 2 6 173

B7 Sandigravel 34 111 1.0 10.7 2.6 SPT 111 37 Sandigravel 89/A

216 8.2 -2.5 5.3 - 33.06 SPT ~

91 Sand 27 111 SPT 24.44 - 0 92/A Sand 92iB Sand 50 I I SPT 2.1 2-1 I 5 92iC Sand 92fD Sand 50 I1 SPT 2-1 2.4 3-0 - -

92/E Sand 93/A Sand 5 V1 SPT 8.2 61-0 - -

93/B Sand 5 VI SPT I 8 Strip - ~

94/A Flneicoarse sand 18 IV SPT 30.2 308 2 7 94/B Sandigravel 50 I I SPT 3.8 Strip 7.0 95iA Silty fine sand -

9SjB Sllty fine sand ~ IV Oed 2.5 3.0 3 7 220

V Oed 2.3 3-4 2-5 0.4 10.0 220

96/A Silly fine sand - V Oed 2 3 2.7 2.5

I 6 13.0 I20 96iB Silly fine sand - 1 6 1 3 0 96JC Medlum Qnd - V Oed 2 3 3.4 1.5 2-6 13.0

I10

96/D Medium sand - V Oed 2.8 3 3 1-5 2.6 13.0 I10

7 VI SPT 6.0 6.0 0 v0

97/P Fme sand 0-9 18.0 97/E Flne sand 7 VI SPT 20.0 20-0 0

1 9 0

98:A Fine sand 4 VI c 0.9 18.0 145

98/B Fine sand 4 VI c - 10.0 99 10.0 I42

IV Oed 18.0 26.0 1-5-3.5 10 > 18 140 I00 Fine medlum sand -

- -

- ~-

130 I76

I33 I l3 199 268

-5.5 158 13 34

8 - Deep 0.9 1.2 1.2 c VI

1.5 255 21 5

5. l 293

584 I20

697

584 575

347 35 45

- .-

50 2.4 2.4 2.1 SPT II - - ~ __

50 1.5 2.8 I 8 SPT 11 ~ -

50 3.0 4.1 2-1 SPT I1 - ~

- - 6.5

383 17.0 6.0 386 22 3

IV 10.0 0.4 3-7 3 0 2-5 Oed

2 8 1.0 14.5 3.3 I 0 14.0 -

1374


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9.5

16.2 4.9

5.8 5-0

15-18 7-0

I l6 81 139-368 993-1401

76 6.4

13-0 I ? 7 21.2 179

l 5 5 X0

434 10.9 14 3 4.4 2.3 27 46 1 8

10

19 25 91 6 4.x

15 10 I2 IX 17 17

I20 74

97 37 27 2

S E T T L E M E N T OF

26.2

2.6 10-5-19.7 2.9

5W796 107-283

5.7 57

4.7 6.5

46

3.7 11.9 0.7 0.33 0 47 0.79 0.52

55 6 54.3

10.0

15.5 164

18.9

83 68 37

15.2

10-3 4.7

231 I65

11.8

11.4 7.5

20.5 6. I

25.9

x.9 1.6

7.7

59

20-8

(18)

l , .

days ~

696 700

854 752 12 40

I I I I

822 488 195

532

7 I 1 1 I I

350

I594 790

790 546

5 0 0

39 I 5 0 0

F O U N D A T I O N S O N SAND AND GRAVEL

1225.1355 219487

23.3 18.6

18

22 24

36.9

1 0 0 1 6 0

I462 I l58

I020

1594 I594

Remarks


AplAq' from pressure-settlement curve ApplAq' from pressure-settlement curve

AplAq' corrected for 0.b. pressure Limestone below 12m. Sand compacted to 5 m depth ApjAq' Corrected for 0.h. pressure AplAq' corrected for o.h. pressure

ApjAq' from pressure-settlement curve

AplAq' from pressurC-seltlemenl curve AplAq' from pressure-settlement curve Settlements suggest grade I I . Suspect SPT results ApfAq' from pressure-settlement curve. Stiff clay below 21.5 rn ApjAq' from pressure-settlement curve. Clayey sllt 8-2-15.7 m

AplAq' corrected for 0.b. pressure AplAq' corrected for 0.h. pressure AplAq' from pressuresettlement curve. Hydraulic sand fill AplAq' from pressure-settlement curve. Hydraulic sand fill Hydraullc sand fill

Hydraulic sand fill Hydraulic sand fill

ApjAq' from pressure settlement curve Hydraulic sand fill

Overconsolidaled clay below IOm Overconsol~dated clay below IOm A d A a from Dressure settlement curve. Comoacted sand fill

1375


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Appendix 2. Case recordsstructures, geology and references Case

1 2 3 6 7 8 9

12 13 14 15

16

19 20

21

22

23

24

25

27

29 30 31

32

33

34 35 36 37

38 39

40

41

43 44 45

47

1376

Structure Steel tank Steel tank Bridge Bridge Bridge Bridge Bridge Bridge Embankments Steel tank 14 storey building lOOm high chimneys Chimneys 11 storey buildings 22 storey building 11 storey building 9 storey building 11 storey building 12 storey buildings Nuclear reactor Concrete tank Steel mill Steel tank

Nuclear reactor Chimneys 120/250 m Silo Steel tanks Silo Nuclear reactor Building Steel mill complex 22 storey building 10 storey building Steel tanks Test footings 20 storey buildings Plate tests

Bearing strata Recent and Pleistocene sands Recent and Pleistocene sand Recent and Eocene sands Recent alluvial/Eocene sands Eocene sand Eocene sand Eocene sand Recent river/Eocene sands Recent river sand Recent river sand Quaternary river sand

Recent river sand

Pleistocene lacustrine sands Recent river sands

Pleistocene river sands

Recent river sands


Pleistocene lake sands



Pleistocene sands Recent beachldune sands Recent river sand and gravel Recent beach sand

Quaternary sand

Pleistocene river sand/gravel Quaternary beach sand Quaternary beach sand Paleocene/Cretaceous sand

Quaternary beachldune sand Recent river sand

Recent marine sand

Recent dune sand

Compacted Cainozoic sand Pleistocene river/dune sand Recent beach and lagoon sands

Pleistocene river sand

Reference Baker49 Baker4’ de BeerSo de Beerso de BeerSo de BeerSo de Beer5’ de Beer” Bjerrum” Bjerrum” Bogdanovic et al.”

Bolenski4’

Bolenski4’ Bolenski4’

Bolenski4’

Bolenski4’

Bolenski4’

Bolenski4’

Bolenski4’

Breth and C h a m b ~ s s e ~ ~

Clausen et DAppolonia et aL8 Davisson and Salley5’

Dunn41

Egorov and PopovaS6

Bjerrum and Eggestad5’ Farrent” FarrentSE Fischer et aLS9

Frost6’ Garga and Q ~ i n ~ ~

Geilly et

Glick6’

Greenwood and T a i P Greenwood and T a i P Grimes and C a n t l a ~ ~ ~

Bazaraa”


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48

49 50 51

52

53 56 57

58

59 60

61 63

64 65 66

13 storey buildings Embankment Concrete tank 12 storey buildings Plate tests

Bridge Bridge 10 storey building Factory building Various 18 storey buildings Test footings 533 m tower

Silos Plate test 2 storey building

6%75 Buildings 76

77

78

79 80 81

83

84

85

86

87

89 91 92 93

94

95

~~

25 storey buildings Concrete tank

10 storey buildings Embankments Building Plate tests

30 storey building 20 storey building 120 m chimney

120 m chimney

Nuclear reactor Buildings Steel tank Machine hall 3 storey buildings 5 storey buildings 1 storey frame building

Tertiary sand

Hydraulic sand fill Quaternary marine sands Pleistocene river sand/ gravel Quaternary river sand/ gravel Recent river/Eocene sand Recent river/Eocene sands Recent coastal sands

Pleistocene river gravel

Pleistocene river sand Pleistocene river sand

Pleistocene river sand Pleistocene river/ Jurassic sand Recent marine sand

Pleistocene river sand

Quaternary river sand Tertiary sand

Recent aeolian sand (compacted) Quaternary river sands

Quaternary river sands Recent river gravel Recent coastal/river sand Pleistocene river/ Tertiary sand Pleistocene river/ Tertiary sand Recent/Pleistocene/ Tertiary sand Pleistocene river sand/ gravel Pleistocene river/ Tertiary sand Quaternary deposits Quaternary coastal sand Quaternary coastal sand Recent dune sand

Quaternary deposit

Recent river sand

-

K ~ r n g o l d ~ ~

Lagging and Eresund66 Langfelder and J ~ h n s t o n ~ ~ Levy and Morton6*

Levy and Morton6'

Marivoet6' Marivoet6' Martins et al.70

Meigh and NixonZ6

Muhs and Kah17' Muhs and Weiss7'

Muhs7' Nikitin et al."

N ~ n v e i l e r ~ ~ Oweis7' Bazaraa"

p r e ~ s ~ ~ . ~ ' Rios and Silva7'

Ronan7'

Sanderat et al.''

Sanderat et al." Sanglerat et al."' S~hmertrnann'~

S c h ~ l t z e ~ ~ . ~ ~

S c h ~ l t z e ~ ~ . ~ ~

S ~ h u l t z e ~ ~ . ~ ~

S ~ h u l t z e ~ ~

S ~ h u l t z e ~ ~

Schultze and Sherif" Thorne" Thorne'' Tomlinsona2

Tschebotario@'

V~tjakov'~

1377


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96 1 storey Recent river sand/ frame fill building

embankment 97 Footingi Quaternary marine sand

98 Bridge 100 9 storey

Recent sand Pleistocene river/

building aeolian sands

Votjakov”

Webba6

Wennerstrand” Zakharenkov”

References 1. SUTHERLAND H. B. Granular materials (review paper), Proc. ConJ the Settlement of

2. SIMONS, N. E. and MENZIES B. K. A short course in foundation engineering. IPC Science

3. NIXON I. K. Standard penetration test state-of-the-art-report. 2nd European Symposium

4. TERZAGHI K. Discussion on paper by Skempton and MacDonald: The allowable settle-

5. BURBIDGE, M. A case study review of settlements on granular soil. MSc/Dissertation,

6. BURLAND J. B. et al. Behaviour of foundations and structures: State of Art Report,

7. TERZAGHI K. and PECK R. B. Soil mechanics in engineering practice, lst/2nd ed. John

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91, SM2,21-31.

19-31.

95-120.

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38. SHVETS V. B. and KULCHITSKII G. B. Experimental investigation of the depth of com- pressed soil foundation stratum under a plate. Osnou. fund. mekh. Grunt., 1970, Jan- Feb, No. 1, 1C~12.

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21,7542.

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67. LANCFELDER J. and JOHNSTON D. W. Settlement of two tauks on loose cohesionless soil. Proc. 4th Pan Am. Con$ Soil Mech. Foundation Engng, 2,15-25.

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