earthworks j in soft chalk a study of the factors ... · to be not less than a stated percentage of...

ROAD RESEARCH LABORATORY

Ministry of Transport

RRL Report LRl12

E A R T H W O R K S j IN S O F T C H A L K A S T U D Y OF T H E F A C T O R S AFFECTING C O N S T R U C T I O N

by

A . W . Parsons

CROVVTHORNE

ROAD RESEARCH LABORATORY

1967

A b s t r a c t

1.

2.

o

4.

CONTENTS

.

o

7.

o

9.

Introduction

Sites of the investigations

2.1 High Wycombe By-pass (Buckinghamshire)

2.2 Baldock By-pass (Hertfordshire)

Chalk earthwork specifications in use on the sites

State of compaction achieved in embankments

4.1 Experimental procedure

4.2 Discussion of results

4.2.1 Determination of air content

4.2.2 Determination of "relative compaction"

4.2.3 Comparison of air content and "relative compaction" results

4.2.4 Variation of results

Determination of the state of compaction of the natural chalk in- situ



Instability of chalk fill

Specifications for chalk earthworks

7.1 Compaction

7.2 Moisture content

7.3 Grading

Acknowledgements

References

P a g e 1

2

3

3

3

4

4

4

10

10

10

11

13

15

15

16

18

19

19

20

20

20

2 1

CROWN COPYRIGHT 1967

Extracts from the text may be reproduced provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

f j

EARTHWORKS IN SOFT CHALK

A STUDY OF SOME OF THE FACTORS AFFECTING CONSTRUCTION

ABSTRACT

This Report describes an investigation into the problems associated with the construction of earthworks in soft chalk. During the construction of chalk earthworks on two major road projects, measurements were made of the state of compaction achieved in the compacted fill and of the dry density and moisture content of the natural chalk, and observations were made to determine possible causes of instability in the compacted fill.

It was found on the sites investigated that the dry density of the

natural chalk was less than that of the compacted chalk fill. It is con- cluded that, where shallow cuttings of up to i0 ft in depth are excavated, reductions in volume of the order of I0 to 13 per cent between cut and fill might be expected where similar plant operations and specifications are employed.

I n s t a b i l i t y o f t e n o c c u r r e d w i t h i n t h e u p p e r l a y e r s o f c o m p a c t e d f i l l , c a u s i n g d e l a y s to c o n s t r u c t i o n o p e r a t i o n s . T h e o n s e t o f u n s t a b l e c o n - d i t i o n s c o u l d n o t b e r e l a t e d to t h e m o i s t u r e c o n t e n t o f t h e c h a l k a n d i t i s c o n c l u d e d , f r o m o b s e r v a t i o n s m a d e o n t h e s i t e s , t h a t t h e i n s t a b i l i t y w a s c a u s e d m a i n l y b y w a t e r r e l e a s e d f r o m t h e c h a l k l u m p s w h e n t h e m a t e r i a l w a s c r u s h e d d u r i n g e x c a v a t i o n . A l t e r n a t i v e m e t h o d s o f e x c a v - a t i o n a r e s u g g e s t e d w h i c h m i g h t r e d u c e t h e c r u s h i n g , w h i c h w a s p a r t i c u l a r l y e x c e s s i v e w i t h t h e u s e o f s c r a p e r s .

Comparisons made between two types of end-product specification for controlling compaction indicate that specification in terms of air voids is less likely to produce errors when flints are present in the chalk, and would almost certainly require less effort, than the relative- compaction specification which was in use on the sites investigated.

A l a r g e n u m b e r o f t e s t s a r e r e q u i r e d , h o w e v e r , i f c o m p a c t i o n c o n t r o l by a n e n d - p r o d u c t s p e c i f i c a t i o n i s t o b e e f f e c t i v e . I n a d d i t i o n , t h e m e a s u r e m e n t o f t h e s t a t e o f c o m p a c t i o n b e c o m e s m o r e d i f f i c u l t a s t h e d e g r e e o f c r u s h i n g o f t h e c h a l k i s r e d u c e d . T h e s e a r e r e g a r d e d a s s t r o n g r e a s o n s f o r c o n s i d e r i n g t h e u s e o f a m e t h o d s p e c i f i c a t i o n f o r c o m p a c t i o n o f t h e f i l l , in w h i c h t h e m i n i m u m n u m b e r o f p a s s e s o f t h e c o m p a c t i o n p l a n t a n d t h e m a x i m u m d e p t h o f l a y e r a r e s p e c i f i e d f o r v a r i o u s t y p e s o f e q u i p m e n t .

I. INTRODUCTION

This Report describes an investigation into some of the more important problems associated with the construction of earthworks in soft chalk.

A high proportion of the area of southern and eastern England has a chalk sub-soil (Fig. I) and the construction of large quantities of chalk earthworks will be required as the network of new motorways and trunk roads is developed. It was considered, therefore, that further information on the problems which are peculiar to construction with chalk would be of value to engineers.

Chalk is a soft limestone consisting principally of the remains of marine organisms. It has a rigid porous structure, and a hardness which varies considerably. The hardness can be arbitrarily related to the porosity, which in turn can be expressed in terms of the saturation moisture content. For a very hard lower chalk the saturation moisture content is between 5 and I0 per cent, whilst values of 35 to 40 per cent are found in the case of the softest varieties of upper chalk I. In earthworks associated with road construction it is usually the softer types of chalk which are encountered. Crushing of chalk produces a material consisting mainly of silt-sized particles, and much of the water held within the pores of the chalk lump is released, creating the so-called "putty chalk".

Prior to this investigation the Laboratory had only a limited experience with th.e construction of mass earthworks using chalk. An earlier study of the compaction of chalk with a 10-ton smooth-wheeled roller has been described elsewhere 2, but the results gave little guidance as to the methods of specification for compaction or of control of moisture content.

The opportunity for carrying out this type of investigation arose with the commencement of construction on two major road projects. These were the High Wycornbe By-pass (Buckinghamshire), in 1964, and Baldock By-pass (Hertfordshire), in 1965. A team from the Laboratory spent a number of weeks on each site early in the construction pro- gramme, making in-situ determinations of dry density and moisture content in the compacted fill and the natural chalk and, in addition, taking samples which were subsequently tested at the Laboratory. Tests carried out on these samples included laboratory compaction tests and determinations of specific gravity.

Conclusions were reached on the following problems:-

(I) Means of specifying the compaction of the chalk fill,

(2) Volume changes when chalk in its natural condition is excavated and compacted.

(3) Loss of stability of the chalk fill during compaction.

2. SITES OF THE INVESTIGATIONS

2.1 High Wycornbe By-pass (Buckinghamshire)

Earthworks for Contract No. 1 of the High Wycombe By-pass commenced in the late summer of 1964. This contract was about 8 miles in length, extending from Stokenchurch in the west (where the By-pass joined the existing A.40 trunk road) to a junction with the A.404 (High Wycombe to IV[arlow) road in the east. The earthworks were predominantly of upper chalk in the eastern half of the site, with clay and sand in the west. The chalk, which contained bands of flints, had an overburden of clay with flints varying in depth up to a maximum of about I0 ft.

Excavation and eartbmoving were by motorized scrapers and compaction of the fill was carried out mainly by towed heavy smooth-wheeled rollers and a grid roller.

2.2 Baldock By-pass (Hertfordshire)

Earthworks commenced in late summer 1965. The site was about 7miles

long, extending from the northern end of the Stevenage By-pass to the junction with the existing A.I trunk road, about 2 miles north of Baldock. The soil encountered was predominantly chalk, reported to be middle and lower chalk, which had an overburden of clay which deepened considerably in the highest areas of the site. The chalk was free from stone, and was white in colour at the south end of the site. To the north,

• however, the chalk contained a brown discolouration.

Excavation and earthmoving were carried out by motorized and towed scrapers, the latter type of machine being used where the length of haul was smallest. Towed heavy smooth-wheeled rollers were used for compaction of the fill.

3. CHALK EARTHWORK SPECIFICATIONS IN USE ON THE SITES

The specification used for control of compaction of the chalk on both sites was of the end-result type, using a form of relative compaction as the criterion. That is, the dry density of the compacted fill was required to be not less than a stated percentage of the dry density obtained by compacting samples from the in-situ density tests, at their existing moisture content, in accordance with the procedure for the British Standard laboratory compaction test using a 10-1b rammer (Test ii, B.S. 1377) 3 . The specified minimum percentages were 96 per cent for the High Wycombe By-pass and 95 per cent for the Baldock By-pass, with the requirement that at least 9 results in I0 should comply. It should be noted that this form of relative compaction is different from the relative compaction specifications normally used throughout the world, where the dry density achieved in the compacted fill is expressed as a percentage of the maximum dry density achieved in a series of laboratory compaction tests over a range of moisture contents.

No moisture-content limits within which the chalk fill would be regarded as suitable were given in the specification, but it was required that a limiting value should be laid down by the engineer in the light of experience with the material. Chalk which, on excavation, had a moisture content exceeding this limiting value was required to be spread out

in loose layers and left to dry.

The specifications anticipated that sponginess and heaving of the compacted chalk might occur under the action of construction plant, it was required that, in areas where this occurred, work should be suspended until stability had been regained.

and

Because of the anticipated difficulty of working with soft, wet chalk the construction period was limited to the "summer" period. That is, mid-March to mid-November in the case of the High Wycombe By-pass and April to October inclusive in the case of the Baldock By-pass.

4. STATE OF COMPACTION ACHIEVED IN EMBANKMENTS


To determine the state of compaction of the chalk fill, determinations of dry density were made by the sand-replacement method, using 8-in- diameter samples (Test 12 (B), B.S. 1377) 3 . The depth of layer being compacted was usually about 6 in and, therefore, the depth of sample taken was 6 in in the majority of tests; however, at High Wycombe some 8-in-deep samples were also taken. The tests were made in areas which

4

were regarded as representative of the chalk fill generally and on which the compaction had been completed.

After taking a sample of the material from the excavated density hole for the determination of moisture content, the remainder of the material, in the majority of tests, was screened through a ¼-in B.S. sieve and the finer fraction subjected to the laboratory compaction test procedure using a 10-1b rammer, (Test ii, B.S. 1377) 3 . (In this test the sample is com-

pacted, in five layers, in a 1/30 ft 3 mould. Each layer is subjected to 25 blows of a 10-1b rammer falling through 18in). A second sample for determination of moisture content was taken from the compacted material and the dry density achieved in the mould calculated.

Subsequently, values of specific gravity of selected samples were obtained, using the pycnometer method (Test 5(B), B.S. 1377) 3 .

From the results obtained in all these tests both the percentage "relative compaction", (field dry density expressed as a percentage of the dry density achieved by laboratory compaction), the criterion in use in the site specifications (see 3.), and the air voids of the compacted fill were calculated.

At High Wycombe By-pass only a few tests were made on pure chalk, the chalk fill being polluted by the overburden of clay with flints and by layers of flints in the chalk itself. At Baldock the majority of tests were unaffected by pollution of the chalk by other materials.

The results of specific gravity determinations are given in Table I, and the individual results of tests in which both air voids and "relative compaction" were determined are given in Tables 2 and 3. The results of all sand-replacement tests carried out in compacted fill are shown in Figs. 2 and 3.

TABLE I Results of specific-gravity determinations for samples from

Sample No.

compacted chalk fill Specific gravity

High Wycombe By-pass

2.69 2.66 2.63 2.63 2.67 2.68 2.70 2.71

Baldock By-pass

2.67 2.68 2.67 2.68 2.66 2.71 2.72

Mean 2.67 2.68 Standard deviation +0.04 -+0.03 Coefficient of

variation -+i. 5% -+I. 1%

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4.2.1 Determination of air content. The specific gravity of the fill

material (Table i) varied between 2.63 and 2.72 with mean values of

2.67 and 2.68 for the High Wycombe By-pass and the Baldock By-pass

respectively. Reasons given for using a form of "relative compaction"

specification on these sites were that the air-voids method would be

difficult to apply because of the likelihood of variations in the specific

gravity of the chalk and inaccuracies in its measurement. The results

in Table 1 are so close to the specific gravity of pure calcite (2.71)

that the possibility of a systematic error in the determinations can be

assumed to be negligible. It is also apparent that little error would occur in the determination of air voids of the compacted fill from

variation of the specific gravity if the mean values given in Table 1 were used. The maximum error likely to be introduced by using a mean value would be _+I per cent of air for the range of values of specific

gravity shown in Table I. In any case, some of the variations in

specific gravity were probably caused by experimental error in the individual measurements and the actual range was probably somewhat

less. Thus the maximum error involved in determining the air content may, in fact, be even less that +-i per cent.

The results of sand-replacement tests carried out on the compacted

fill (Figs. 2 and 3) follow the pattern shown by other types of soil compacted in mass earthworks, with a spectrum of air voids ranging

generally between zero and I0 per cent. In each figure the air-void

lines are calculated from the appropriate mean specific gravity.

The results lying significantly above the zero air voids line in Figs.

2 and 3 are clearly inaccurate. The probable cause for these is the

"slump" effect in the sand-replacement test, i.e the sides of the density hole slump inwards while it is being excavated in spongy, unstable

chalk, resulting in a smaller volume being measured and causing a

positive error in the measured value of dry density. In this respect the

chalk fill behaves similarly to non-cohesive granular soils 4.

It appears, therefore, that there would be no problems in using an

air-void specification for the chalk, other than those normally encount-

ered with soils in general. As with earthworks generally, a value of

I0 per cent air voids would appear to have been suitable as the minimum allowable state of compaction for at least 9 results in I0. One advantage

of using an air-void specification, as shown in Figs. 2 and 3, is that it

highlights the results that are inaccurate and lie above the zero-air-void

line as a result of the slump effect.

4.2.2 Determination of "relative compaction". The results of sand-

replacement tests and laboratory 10-1b rammer compaction tests, and

the calculated values of "relative compaction" and air content, are

i0

given in Tables 2 and 3. The results obtained in the 10-1b rammer compaction test are also shown in Figs. 4 and 5, in which the dry density achieved has been plotted against the moisture content of each compacted specimen. The results all lie roughly along a constant air-void line, the

mean air content of the results being 2.1 per cent for High Wycombe By- pass and 2.3 per cent for Baldock By-pass.

To establish the reason for the results of the laboratory compaction tests lying on a constant air-void line, samples of the compacted fill were taken and subsequently air-dried in the laboratory. Sub-samples of the material passing the 1½-in B.S. sieve were taken and mixed with varying quantities of water to produce a range of moisture contents. After storing in sealed tins for a period of at least 7 days to ensure uniform distribution of moisture, each sub-sample was compacted in a 6-in diameter CBR (California Bearing Ratio) mould, using the procedure for compaction by the 10-1b rammer method (Test II, B.S. 1377) 3 . Mean relations between dry density and moisture content so obtained are shown in Fig. 6. The average optimum moisture contents are about 15 per cent for High Wyeombe By-pass and about 17 per cent for Baldock By-pass, the lower

value of the optimum moisture content at High Wycombe probably being caused by the flints in the fill material on that site. The materials encountered in-situ (Figs. 4 and 5) were, however, generally at moisture contents in excess of the optimum moisture contents shown in Fig. 6, and the results for the laboratory 10-1b rammer compaction test would be expected, therefore, to lie on a line parallel to the zero-air-void line. It is eiear that, once the mean air content achieved in the laboratory compaction tests has been established, the rate of carrying out laboratory tests may be reduced to save the efforts of the site control team.

It is apparent from these findings that on these sites the specifications, which required the dry density of the fill to be a percentage of the dry density achieved in the laboratory compaction test at the in-situ moisture content, were similar to specifying the air content of the material. In fact, using the average air contents given in Figs. 4 and 5 for the laboratory compacti0n-test results, the specifications in use at High Wycombe By-pass (96 per cent "relative compaction") and Baldock By- pass (95 per cent "relative compaction") were equivalent to specifying minimum states of compaction of about 6 and 7 per cent air voids respectively. As 9 results in I0 were required to comply, it is apparent that the states of compaction to be achieved were extremely high. The effect of these high states of compaction on the stability of the fill is discussed later in this Report.

4.2.3 Comparison of air content and "relative compaction" results. To compare the relative merits of the air voids and "relative compaction" methods of compaction control with chalk fill, the relations between the values of "relative compaction" and air voids measured in each test are

Ii

shown in Fig. 7, together with the theoretical relations, in which the "relative compaction" is based upon the average air content of the laboratory compaction-test results and the individual measured values of the air content of the compacted fill. The results for Baldock By-pass follow the theoretical relation fairly closely, but those for High Wycombe By-pass diverge considerably from the theoretical line. In general, those results which diverge most from the line are those for samples containing flints, and it is thought that the main cause of the differences is the removal of the flints exceeding ¼in. in size from the sample used in the laboratory compaction test.

The removal of flints from the sample would result in an apparent increase in moisture content of the remaining material, and the difference in moisture content of the laboratory compaction test specimen as compared with that of the sand-replacement test sample would, therefore, provide an indication of the proportion of flints removed on the ¼-in. sieve. The difference between the measured "relative compaction" and the theoretical "relative compaction" (based on the measured air content) for High Wycombe By-pass, has been plotted against this difference in moisture content in Fig. 8. The relation obtained shows an increase in the divergence between the two values of "relative compaction" with increase in difference between the moisture contents of the samples, thus confirming that the presence of flints was introducing significant errors. These errors cannot be eliminated simply by making a correction to the moisture content of the laboratory compaction-test sample, as the dry density of the removed flints will also have an important influence on the results.

It appears, therefore, that when the chalk contains flints, as was the case at High Wycombe, the "relative compaction" method, as specified, is likely to produce serious errors if the in-situ dry density is expressed as a percentage of the dry density achieved in an individual laboratory compaction test on the fraction passing ¼-in. of the same material. These errors cause an over-estimation of the state of compaction of the chalk.

The "relative compaction" method is likely to be accurate only if the laboratory compaction dry density is "read-off" from an established relation between dry density and moisture content (similar to that shown in Fig. 4), using the in-situ moisture content obtained in the sand- replacement test. Alternatively, a compaction mould could be used which is large enough to allow the use of all the flints, although the effort involved in carrying out the test in this way is likely to be excessive.

Thus, if it is considered that an end-product type of specification is required, the air content method is less likely to produce errors, and would almost certainly involve less effort, than the "relative compaction" method as used on the sites investigated.

12

4.2.4 Variation of results. It has already been shown 4 that large numbers of tests are often required to control adequately the state of compaction of eaf'thworks. To show the variation of results obtained in chalk earthworks, the mean air content measured in each successive i000 yd 3 of compacted fill and the standard deviations of the results are given in Table 4. The number of tests required to give a mean air content within an accuracy of 1 and 2 per cent of air, with a 90 per cent probability, has also been calculated, together with the limits of error for the mean air content if only 5 and i0 tests had been made per 1000 yd 3.

The standard deviation of the results, as might be expected, tended to increase as the mean air content increased, the average standard deviation, for the results obtained, being +-3.4 per cent of air. The number of results required to give a mean air content with an accuracy of +i per cent of air increased from 9 to 76 with the increase in standard deviation, with an average of about 35.

The number of tests required on any given vdlume of fill to achieve an accuracy within +i per cent of air in the measurement o'f the mean air content is likely to increase, therefore, as the state of compaction is reduced until, in poorly compacted conditions, large numbers of tests would be required to show, with sufficient confidence, that the material wasnot compacted satisfactorily.

If the 35 tests required to achieve an accuracy of +_i per cent of air (average conditions on the sites investigated)were spread over a volume of I000 yd 3, it is estimated that the cost of testing (at £i. 5s. per test) would be about 10½d. per yd 3 (if the tests were spread over 5000 yd 3, the cost would be about 2d. per yd3). This compares with the cost of compaction in mass earthworks, which is estimated to be somewhat less than 6d. per yd 3. The cost of controlling the state of compaction adequately is, therefore, excessive compared with the cost of actually compacting the material.

These results show that chalk earthworks require a large testing effort and comparatively high testing costs if compaction control by end-product specification is to be effective. The question of alternative specifications for compaction, involving the method by which compaction is to be carried out, is discussed further in 7.1.

13

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. D E T E R M I N A T I O N OF THE S T A T E OF C O M P A C T I O N OF T H E

N A T U R A L CHALK I N - S I T U


M e a s u r e m e n t s of the i n - s i t u d r y d e n s i t y of t he n a t u r a l c h a l k w e r e m a d e in a r e a s of t he c u t t i n g s w h e r e d i s t u r b a n c e f r o m e x c a v a t i o n p l a n t w a s c o n s i d e r e d to be n e g l i g i b l e . At High W y c o m b e c h a l k w a s e x c a v a t e d b y hand to a d e p t h of about 6 in b e l o w t h e e x p o s e d s u r f a c e b e f o r e c o m m e n c - ing the t e s t , o r a l t e r n a t i v e l y , t he t e s t w a s c a r r i e d out in a s m o o t h a r e a f r e s l ~ y e x p o s e d by the cu t t i ng b l a d e of t h e s c r a p e r . At B a l d o c k t h e t e s t s w e r e m a d e n e a r the foot of t he s i d e - s l o p e s of t h e c u t t i n g s , wi th b e n c h e s cut in the s l o p e s to f o r m a h o r i z o n t a l s u r f a c e f o r t he t e s t s .

The m e a s u r e m e n t s w e r e r n a d e u s i n g t h e s a n d - r e p l a c e m e n t m e t h o d , wi th s a m p l e s 8 - i n . in d i a m e t e r by 6 in. d e e p ( T e s t 12(B), B.S. 1377) 3

• At High W y c o m b e c a r e w a s t a k e n to a v o i d a r e a s w h e r e f l i n t s m i g h t h a v e o c c u r r e d in the t e s t s a m p l e .

L a b o r a t o r y c o m p a c t i o n t e s t s , s p e c i f i c g r a v i t y t e s t s ( p y c n o m e t e r m e t h o d ) and , in the c a s e of H igh W y c o m b e B y - p a s s , m e a s u r e m e n t s of the d r y d e n s i t y of the c h a l k l u m p s , w e r e m a d e on s a m p l e s of t he n a t u r a l cha lk . T h e d r y d e n s i t y of the c h a l k l u m p s ( a v e r a g e v o l u m e about 500 c m 3) was d e t e r m i h e d by c o a t i n g e a c h l u m p w i t h w a x and d e t e r m i n i n g i t s v o l u m e f r o m the l o s s in w e i g h t on s u s p e n s i o n in w a t e r . T h e l u m p w a s s u b s e q u e n t - ly o v e n - d r i e d to d e t e r m i n e i t s d r y w e i g h t .

The r e s u l t s of the s a n d - r e p l a c e m e n t t e s t s a r e s h o w n in F i g s . 9 and 10 and the a v e r a g e r e s u l t s of t he l a b o r a t o r y c o m p a c t i o n t e s t s in F i g . 11. The r e s u l t s of s p e c i f i c g r a v i t y t e s t s and t h e d r y d e n s i t y d e t e r m i n a t i o n s on cha lk l u m p s a r e g iven in T a b l e s 5 and 6 r e s p e c t i v e l y .

.~ T A B L E 5

R e s u l t s of s p e c i f i c - g r a v i t y d e t e r m i n a t i o n s f o r s a m p l e s of n a t u r a l c h a l k

S a m p l e No.

1 2 3 4

.5 6

S p e c i f i c g r a v i t y High W y c o m b e B y - p a s s B a l d o c k B y - p a s s

2.73 2.71 2.69 2.69 2.66 2.67

Mean 2.69 Standard deviation +_0.03 Coefficient of

v a r i a t i o n ± 1.1%

2.67 2.69 2.68 2.71 2.69 2.69 2.69

+0.01

+0.4%

15

TABLE 6

Results of dry-density determinations on lumps of natural chalk

Depth below original surface level at

which sample was taken

(f t)

4.4

9.0

11.2

12

(t :igh W y c o m b e By-pass)

D r y d e n s i t y ( l b / f t 3)

L u m p No. 1

105

98

97

108

L u m p No. 2

104

99

96

102

L u m p No. 3

I01

i01

i01

103

L u m p No. 4

99

102

100

105 , ,

Mean

102

i00

99

105


T h e m e a n s p e c i f i c g r a v i t y of t h e n a t u r a l cha lk (Tab l e 5) was s i m i l a r on b o t h s i t e s (2.69) and l i t t l e v a r i a t i o n o c c u r r e d b e t w e e n the r e s u l t s of i n d i v i d u a l t e s t s . C o m p a r i s o n w i t h t h e r e s u l t s of s p e c i f i c - g r a v i t y t e s t s m a d e on s a m p l e s of t he c o m p a c t e d f i l l (Tab le 1) s h o w s f a i r l y good a g r e e m e n t . T h i s i s to be e x p e c t e d , h o w e v e r , as the on ly l i k e l y c a u s e of d i f f e r e n c e s b e t w e e n the s p e c i f i c g r a v i t i e s of t he f i l l m a t e r i a l and t h o s e of t h e n a t u r a l c h a l k w o u l d be t he c l a y and f l i n t s w h i c h w e r e p r e s e n t in m a n y of t he s a m p l e s of f i l l m a t e r i a l o b t a i n e d at High W y c o m b e .

The results of measurements of dry density of lumps of chalk (Table 6) show an appreciable variation in individual results, with no clearly defined relation between dry density and the depth below ground level at which the lumps were obtained. From the dry density of the chalk lumps (Table 6) and the specific gravity (Table 5) the mean saturation moisture content, for the lumps tested, is calculated to have been about 25 per cent, with a range, based on the individual results given in

Table 6, of 21 to 28 per cent.

The results of the sand-replacement tests carried out on the natural chalk (Fig. 9) show a large variation in air content, with the air voids decreasing with increase in depth below the original ground level. This is most clearly defined in the results obtained at High Wycombe. For High Wycombe, the only site at which the information was obtained, the moisture contents measured in the sand-replacement tests were very similar to the calculated values for the saturation moisture content (see above). This indicates thai, despite the values of air content measured, the in-situ lumps of chalk were probably fully saturated.

16

To illustrate the variation in the condition of the chalk with depth below the original ground level, the relations between in- situ dry density and depth at which the measurements were made are shown in Fig. i0. At High Wyeombe there was a clearly defined increase in the dry density of the chalk as the depth below the surface was increased to 15 ft, whereas at Baldock the dry density remained fairly uniform to a depth of about 13 ft, below which there was a considerable increase in dry density, possibly caused by a" change in the type of chalk. Each point shown in Fig. I0 is the mean of from two tosix results.

Comparison of the in-situ dry density values measured at High Wycombe (Fig. i0) with the dry density values obtained on lumps of chalk from the same site (Table 6) shows that the dry density of the chalk lumps was, generally, much higher than the measured in-situ dry density. This difference was caused by joints and fissures between the chalk lumps in the undisturbed material. This would be expected to be most pronounced near the surface as a result of frost action and other weathering effects, and lower values of in-situ dry density would occur, therefore, near the surface, as shown in Fig. I0. In fact, during the tests the joints could be easily observed in the chalk, especially at High Wycombe, with the size of lump between the joints appearing to

increase with increase in depth below ground level.

The mean value of dry density measured in the compacted fill on each site is also included in Fig. I0. (This is the mean result of tests in which the material sampled was chalk only). The dry density achieved in the compacted fill was generally higher than the in-situ dry density of the natural chalk over the range of depths which were tested. It is apparent, therefore, that a substantial decrease in volume will occur upon recompaction of chalk of this type when methods of excavation and compaction are similar to those used on the two sites. This decrease in volume will be a maximum on sites where comparatively shallow cuttings are excavated, say to a maximum depth of I0 ft, where the differences between values of natural and recompacted dry density are likely to be-at a maximum. From the results shown in Fig. I0, reductions in volume of the order of i0 to 13 per cent might be expected under such

condition s.

The results are shown in Fig. II of laboratory compaction tests (10-1b rammer method, Test ii, B.S. 1377) 3 , employing a CBR mould to allow the use of particles of chalk up to a maximum size of 1½in. Very similar results were obtained on samples of natural chalk from both High Wycombe and Baldock, despite the reported geological differences in the material (see 2). The similarity of these results confirms that the diff-

erence, shown in Fig. 6, between the results of laboratory compaction tests on samples of fill material from the two sites, was caused by the clay and flints in the fill material at High Wycombe. The lower values of maximum dry density of the natural chalk from Baldock compared with

the fill material from the same site is probably caused by the better

17

particle-size distribution of the chalk in the fill which, during excavation and recompaction, would have been subjected to a high degree of crushing.

6. INSTABILITY OF CHALK FILL

D u r i n g t h e c o u r s e of t h e i n v e s t i g a t i o n s i n s t a b i l i t y o f ten o c c u r r e d withi r : t h e u p p e r l a y e r s of c o m p a c t e d f i l l . T h e p a s s a g e of h e a v i l y l o a d e d s c r a p e r s o v e r t h e a r e a c a u s e d s e v e r e r u t t i n g of t h e l a y e r and t h e s e " s p o n g y " c o n d i t i o n s p r e v a i l e d f o r s u b s t a n t i a l p e r i o d s . Of ten w o r k w a s b r o u g h t to a c o m p l e t e s t a n d s t i l l and a n u m b e r of w e e k s e l a p s e d b e f o r e t he s t a b i l i t y r e t u r n e d to s u c h a l e v e l t h a t w o r k c o u l d c o n t i n u e .

Attempts to relate the onset of the unstable conditions to a unique value of moisture content were unsuccessful, but close observation of the earthwork operations indicated that the major cause of the unstable conditions was the crushing of the chalk lumps, resulting in the release • of water. During good drying weather this released water evaporated and the chalk remained in a condition in which it compacted into a firm and stable layer. However, when sufficient drying did not occur, this free water caused positive pore-water pressures to be produced during compaction of the fill, resulting in spongy conditions under traffic. The pore-water pressures so produced usually dissipated over a period of three to four weeks.

O n e of t h e m a j o r c a u s e s of e x c e s s i v e b r e a k i n g - d o w n of the c h a l k l u m p s w a s t h e e x c a v a t i o n by s c r a p e r s . B e c a u s e of the n a t u r a l s t r e n g t h of t h e m a t e r i a l , t h e s c r a p e r s , e v e n w i t h a p u s h - l o a d e r , w e r e c a p a b l e of e x c a v a t i n g o n l y f a i r l y s h a l l o w d e p t h s , u s u a l l y of abou t 6 i n c h e s . T h u s a v e r y h i g h p r o p o r t i o n of t h e c h a l k w a s e x p o s e d to t he c u t t i n g b l a d e of the s c r a p e r , w h i c h c a u s e d a h igh d e g r e e of c r u s h i n g . A l t e r n a t i v e m e t h o d s w h i c h m i g h t e f f e c t i v e l y r e d u c e t h e p r o d u c t i o n of f i n e s d u r i n g e x c a v a t i o n of c h a l k w o u l d be t h e u s e of r i p p e r s , e n a b l i n g t h e s c r a p e r s to l o a d f r o m m u c h d e e p e r l a y e r s , o r t he u s e of f a c e s h o v e l s in a v e r t i c a l - f a c e e x c a v a t i o n .

Further crushing of the chalk occurred upon compaction and trafficking by the earthmoving plant, and it is considered that the use of comparatively light-weight equipment, reducing the stresses to which the chalk is subjected, would also tend to reduce the degree of crushing of the chalk.

Specifications which require very high states of compaction, such as those in use at High Wycombe and Baldock, also give rise to the spongy conditions. It is often necessary to crush the chalk lumps to attain the specified state of compaction.

18

L i t t l e q u a n t i t a t i v e i n f o r m a t i o n is a v a i l a b l e on t h e p e r f o r m a n c e of c h a l k e m b a n k m e n t s i n r o a d c o n s t r u c t i o n in w h i c h s t a t e s of c o m p a c t i o n l o w e r t h a n t h o s e n o r m a l l y e x p e c t e d ( i 0 p e r c e n t a i r v o i d s ) h a v e b e e n a c h i e v e d . H o w e v e r , i f t h e l u m p s t r u c t u r e of t he c h a l k i s p r e s e r v e d to the m a x i m u m p o s s i b l e e x t e n t , t h e n u n d o u b t e d l y t h e s t a t e of c o m p a c t i o n a c h i e v e d in t h e f i l l w i l l be f a i r l y low b e c a u s e of t h e a b s e n c e of f i n e s to f i l l the i n t e r s t i c e s b e t w e e n the l u m p s . M o r e r e s e a r c h i n t o t h e p e r f o r - m a n c e of c h a l k e m b a n k m e n t s w h e n c o n s t r u c t e d f r o m c h ~ k in t h e l u m p s t a t e i s r e q u i r e d , t h e r e f o r e , b e f o r e a n y d e f i n i t e r e c o m m e n d a t i o n s c a n be m a d e on t h e m o s t e f f i c i e n t m e t h o d s f o r e a r t h w o r k c o n s t r u c t i o n in cha lk : H o w e v e r , at t h i s s t a g e i t i s s u g g e s t e d t h a t , to r e d u c e t he l i k e l i - hood of s p o n g y c o n d i t i o n s and , t h e r e f o r e , to m a i n t a i n m a x i m u m c o n t i n - u i t y in t h e e a r t h w o r k o p e r a t i o n s , m e t h o d s of e x c a v a t i o n s h o u l d be u s e d w h i c h p r e s e r v e , as f a r as p o s s i b l e , t he r o c k s t r u c t u r e of t he c h a l k .

E v e n w i t h m a x i m u m p r e s e r v a t i o n of t h e r o c k s t r u c t u r e of t h e c h a l k , h o w e v e r , i t m a y be p o s s i b l e to w o r k c o n t i n u o u s l y o n l y w h e n r a t e s of e v a p o r a t i o n a r e h igh e n o u g h to c a u s e t h e d i s s i p a t i o n of a n y w a t e r r e l e a s e d on e x c a v a t i o n . T h u s , i t m a y s t i l l be n e c e s s a r y to c o n f i n e t h e c o n s t r u c t i o n p e r i o d f o r c h a l k e a r t h w o r k s to t h e s u m m e r m o n t h s .

7. SPECIFICATIONS FOR CHALK EARTHWORKS

7.1 C o m p a c t i o n

A c o m p a r i s o n h a s a l r e a d y b e e n m a d e in 4. of t w o a l t e r n a t i v e f o r m s of e n d - r e s u l t s p e c i f i c a t i o n , in t e r m s of a i r v o i d s and of r e l a t i v e c o m p a c t i o n at i n - s i t u m o i s t u r e c o n t e n t . W i t h e i t h e r f o r m , h o w e v e r , l a r g e n u m b e r s of t e s t r e s u l t s a r e r e q u i r e d to d e t e r m i n e the a v e r a g e s t a t e of c o m p a c t i o n of any g i v e n v o l u m e of f i l l w i t h s u f f i c i e n t a c c u r a c y f o r a d e q u a t e c o n t r o l ; t he cos t of s u c h t e s t i n g i s of t he s a m e o r d e r as t h e c o s t of c o m p a c t i o n i t s e l f . In a d d i t i o n , the s t a t e of c o m p a c t i o n to be a c h i e v e d in an e n d - r e s u l t t y p e s p e c i f i c a t i o n m a y h a v e to b e a m e n d e d if t h e l u m p s t r u c t u r e of the c h a l k is p r e s e r v e d to a g r e a t e r e x t e n t t h a n a t p r e s e n t . T h e d e t e r - m i n a t i o n of the s t a t e of c o m p a c t i o n a l s o b e c o m e s m o r e d i f f i c u l t a n d l e s s a c c u r a t e a s the l u m p c o n t e n t of the c o m P a c t e d f i l l i n c r e a s e s . T h e s e a r e , t h e r e f o r e , s t r o n g r e a s o n s f o r c o n s i d e r i n g the u s e of a m e t h o d s p e c i f i c - a t ion fo r t h e c o m p a c t i o n of the f i l l , in w h i c h the m i n i m u m n u m b e r of p a s s e s of t h e c o m p a c t i o n p l a n t and the m a x i m u m d e p t h of l a y e r a r e s p e c i f i e d f o r v a r i o u s t y p e s of e q u i p m e n t . Such a s p e c i f i c a t i o n f o r g e n e r a l e a r t h w o r k c o n s t r u c t i o n i s l i k e l y to be i n t r o d u c e d b y the M i n i s t r y of T r a n s p o r t in the n e a r f u t u r e .

19

7 .2 M o i s t u r e c o n t e n t

As stated previously the onset of instability in the fill could not be related to the moisture content of the chalk. The methods required by the current Ministry of Transport Specification for Road and Bridge Works for the control of moisture content of either plastic or non- plastic materials would not, therefore, be applicable.

The information contained in this Report does not give any guidance as to the maximum permissible moisture content of the chalk, which would largely depend upon the degree of crushing during excavation and compaction. Further research might usefully be carried out into the relations between moisture content, grading and suitability as fill material.

7.3 Grading

The grading of the chalk indicates the degree of crushing which has occurred, and it is likely that the suitability of the chalk might be related to this parameter. However, the grading of the fill material will also affect the state of compaction which can be achieved. Further research would be useful, therefore, into the inter-relation of grading, state of compaction achieved, and performance in an embankment.

8. ACKNOWLEDGEMENTS

Thanks are due to the County Surveyors of Buckinghamshire and Hertfordshire and to their Resident Engineers and site staffs for their co-operation in providing the sites for the investigation.

The investigation was carried out in the Earthworks & Foundations Section under the direction of W.A. Lewis, and the author was assisted in the experimental work by T.O. Odubanjo, B, Pimley, and A.F. Toombs.

2O I

9. REFERENCES

i. HIGGINBOTTOM, I.E. The engineering geology of chalk.

Proceedings of Symposium on chalk in earthworks and foundations.

London, 1966 (Institution of Civil Engineers).

. DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH,

ROAD RESEARCH LABORATORY. Chalk embankments and

subgrades. Soil Mechanics for Road Engineers. London, 1952.

(H. M. Stationery Office). Ch. 7.

. BRITISH STANDARDS INSTITUTION. British Standard No. 1377:1961.

Methods of testing soils for civil engineering purposes. London,

1961. (British Standards Institution).

. LEWIS, W.A. and A.W. PARSONS. An analysis of the states of

compaction measured during the construction of embankments on

six major road schemes. Department of Scientific and Industrial

Research, Road Research Laboratory Note No. LN/399/WAL. AWP.

(Unpublished).

21

ScQrborough

York •

• Leeds

Chalk

• Monches te r • She f f i e ld

• L incoln

• Derby

• Le ices te r

• B i r m i n g h a m

• Peterborm

• Worces te r Bedford •

• Bristol

Oxford •

S a l i s b u r y

. . . . . . . ipto

~ ,LONDON

Reading

g

Winches te r

/ /Co l ches te r •

. .j Che tms fo rd . ,

,_z.-~ Maidstone

• HorshQm

Fig.1. AREAS OF ENGLAND IN WHICH CHALK IS LIKELY TO BE ENCOUNTERED IN EARTHWORKS

R.112. I.

A

z., o

125

120

115

110

10S

100

95

90

85

Fig. 2.

I

\ \

grav i t y )

0 Cha lk • Cha lk and, • C h a l k and • C h a l k , c l a y n Cha l k and • F l i n t

c l a y f l i n t and f l i n t c lay , w i t h sanC

10°/o a i r voids

AA ~

O

i _ m HIGH WYCOMBE B Y - PASS \

10

• L \

5 20 25 30 35

M o i s t u r e c o n t e n t ( p e r c e n t )

2-0

1-9

1-8

1:. '7

1-6

1"5

1.4

RESULTS OF SAND-REPLACEMENT TESTS MADE IN COMPACTEO FILL DURING CONSTRUCTION OF H[GH WYCOMBE BY-PASS

E

C (P "D

i.. n:

R.112.2.

130

125

120

115

~a 110

4-*

t -

105

s , .

Q

100

95

9 0

8 5

Fig. 3.

\ \ \ \ \ \

\ 10°/o o i r v o i d s

B A L D O C K B Y - PASS

o B r o w n c h o l k • Wh i te cha l k & B r o w n cha l k ond c loy • Wh i t e c h a l k end c l a y - - D B r o w n chalk and f l i n t

q r a v i t y 2"681

\

O

\ \

o ~ o \ _\

\ ~ • ~ , o

\ o . _ '

x~\ Ao

c~X°

\ \ A \

\ \

\ 15 20 25

M o i s t u r e c o n t e n t ( ' pe r c e n t )

2"0

- 1"9

- 1 . 8 _

E

- 1 " 7 c qO

- 1"6

- 1 . 5

5 10 30 35

RESULTS OF SAND-REPLACEMENT TESTS MADE IN COMPACTED FILL DURING CONSTRUCTION OF 8ALOOCK BY-PASS

1.4

R.112.3.

.o

E '10

L--

o

120

115

110

105

100

95

9 0 -

\ II

\ \

\

\ \

\ - / \

10°/o a i r voids

8 5 5 10 15 20

0 Chalk • Chalk and c lay

Chalk and f l i n t • Chalk, c lay and f l i n t 13 Chalk and c lay ,w i th sand • F l i n t

I I I

- 1 - 9

HIGH WYCOMBE BY-PASS

~j Zero a i r v o i d s (Spec i f ic g rav i t y 2-67)

\

- 1 " 8

- - 1 ' ?

1"6

1'5

MeQn a i r \ ' , content of \

r e s u l t s " ~ ~ 2 l p e r c e n t ) ~ _ 1. 4

\ 25 30 35

M o i s t u r e content (per cent)

E . i - , ,

..,m*

(,-

I . . -

Q

Fig.t,. RESULTS OF LABORATORY COMPACTION TESTS(IO-LB RAMMER) ON SAMPLES OF FILL MATERIAL AT IN-SITU MOISTURE CONTENT

R.112.4,

115

\ \

110

- - 105

e--

' - 1 0 0 a

95

90

8 5 5

Fig. 5.

\ \

1 0 % o i r j vo ids

• lk

~ 'BALDOCK Y- PASS

12o \

-k \

\

[ I o B r o w n cha lk • Whi te chalk & Brown chalk and c l oy • Wh i te chalk and c l ay

Zero Qir vo ids Spec i f ic g rav i ty 2.68)

\

\ Mean ai r content of

r e s u l t s

~ (2"3 per cent)

10 15 20 25 30 35

M o i s t u r e c o n t e n t (pe r c e n t )

1-9

1.8

E 1'7 ""

¢-

0J " 0

> ,

1.6 Q

1"5

1"/,

RESULTS OF LABORATORY COMPACTION TESTS (IO-LBRAMMER) ON SAMPLES OF FILL MATERIAL AT IN-SITU

MOISTURE CONTENT

R.112.5.

125

120

115 A-"

>, 110

e,-

100

95 S

\\ 10"/. air

voids

High Wycol by -pass

Batdock by-pass

1 ,Zero air voids (Speci f ic gravi ty 2-68)

2-0

1-9

e,)

E

1,8

"10

L -

D 1'7

1-6

10 15 20 25 30 Moisture content (per cent)

Fig. 6. TYPICAL RELATIONS BETWEEN DRY DENSITY AND NOISTURE" CONTENT FOR SAMPLES OF FILL MATERIAL. USING THE

BRITISH STANDARD LABORATORY COMPACTION TEST (IO'LB RAMMER METHOD)

R.112.6.

110 f I. I

e--

U

105 a . .

o ow

u o

E 100 0 u

G, 9 5

9 0

O••O '~ •

- - - ' - . L

O Cha l k • C h a l k and c lay & Cha l k and f l i n t • Cha lk , c lay and f l i n t O Cha lk and clay, w i th sand m F l i n t

& & &

A

HIGH WYCOMBE B Y - P A S S

Theore t i ca l r e l a t i o n /

0 2 /, 6 8 10 12

A i r v o i d s ( p e r cen t )

C

U

O o - -

u

o

E O u

o - -

O

K~

110

105

100

95

9 0

O

BA

I O B r o w n cha lk • Whi te cha l k /1 B r o w n c h a l k and c lay • White cha l k and c lay

LDOCK B Y - P A S S

• Theore t i ca l r e l a t i o n

/

0 2 /, 6 8 0 12

A i r v o i d s ( pe r cen t )

Fig. 7. RELATIONS BETWEEN MEASURED VALUES OF RELATIVE COMPACTION ANO AIR VOIOS OF COMPACTEO FILL

R.112.7.

15

o:

5

~ o

c -5 . ~

I , -

ILl

-10

Fig.8.

• S

J

A Cha l k and f l i n t • Cha lk . c | a y a n d f l i n t

m Cha lk a n d c t a y , w i t h s a n d • F l i n t

0 1 2 3 4 5 6

Di f ference between moisture con ten t s of s a n d - r e p l a c e m e n t s a m p l e and labora tory compact ion tes t s p e c i m e n (per cen t )

EFFECT OF REMOVAL OF FLINTS ON 3/4INCH SIEVE (ILLUSTRATED BY DIFFERENCES IN MOISTURE CONTENT) UPON THE ACCURACY OF MEASURING RELATIVE COMPACTION

(HIGH WYCOMBE BY-PASS)

R.112.~.

o ~

C

105

100

95

90

85

80 18

\

10% a i r vo ids

0

C

0

A

A A

p o •

0

[ L m I HIGH WYCOMBE BY-PASq

I Zero a i r vo ids

/ ( S p e c i f i c g r a v i t y _ ~,,~ ~ 2.69)

Z Z~Z~

Depth be low su r face

O 0 - 5 f t • 6 - l O f t & 11-15ft

I 1

1'6

Z" E

1,5 >,

C

" 0

I , . .

- 1'4

1.3

20 22 24 26 28 30 M o i s t u r e con ten t (per cent )

r~

v

. t

U1 C

"I0

I - -

, '7

110

105

100

95

90

85 16

Fig.9.

\

__10°/o a i r vo ids

\ I [ ~ I BALDOCK B Y - P A S S ] I

Zero a i r vo ids z~ ~ e c i f i c g r a v i t y

> ° E

~ - 1 .6 ~-

g~ D~ ¢-

"U

0 - - " " z~ - 1 - 5 &

Depth be low s u r f a c e

0 0 - 5 f t l ~ _ •

• 6 - lOf t 11-15ft

2"69)

18 20 22 24 26 28 M o i s t u r e con ten t (pe r cent)

1"7

1-4

RESULTS OF SAND-REPLACEMENT TESTS MADE IN NATURAL CHALK IN CUTTINGS

R.112.9,

U o

3 (/I

E

L. O

o

t~

80 0

10

15 ̧

8 5

Dry dens i t y ( l b / f t 3) 90 95 100

HIGH WYCOMBE BY-PASS

I I

I i

Mean dry densi ty of compacted

chalk f i l l " % t

0 O.

105 0

- 1

- 2

- 3

- 4

A E

v

U El

I/1

t-

O

0

JE

1:3

0 0 E v w

U o

5

o E O~

10 0

.D

I-

® 15 a

BALDOCK BY-PASS

I

1'3

J I Mean dry dens i ty

of compacted

o/' i °

I I

1"4 1"5

chalk t i l t " ~

o i

I

1"6

Dry densi ty ( t /m 3 )

i

0

>

U o

2 _ o ¢,- ¢;n

.m t,=

3 o 3= 0

4 "' r- Q. t.-,t

Fig lO RELATIONS BETWEEN DRY DENSITY AND DEPTH BELOW ORIGINAL SURFACE LEVEL OBTAINEO BY SAND-REPLACEMENT TESTS MADE IN NATURAL

CHALK IN CUTTINGS

R.112.10.

125

120

115 g-

v

110 ¢ -

" U

o

105

100

\ \

\

Fig.11.

J 1 0 % air

voids \ \

\ \

Batdock by-pass\ _,. ~ ~ . ~

High Wycombe by-pass ~ \

Zero air voids ecific gravity

\ \

2 . 6 9 )

9 5 5 10 15 2 0 25 3 0

-- 2"0

-- 1"9

E - 1 ' 8 "

v

,c"

" 0

6

- 1.7 o

1-6

Moisture content (per cent)

RELATIONS BETWEEN DRY DENSITY AND MOISTURE CONTENT FOR SAMPLES OF NATURAL CHALK,USING THE BRITISH STANDARD LABORATORY

COMPACTION TEST (lO-LB RAMMER METHOD)

R.112.11

earthworks j in soft chalk a study of the factors ... · to be not less than a stated percentage of...

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