< ~a '". 'ric",iona':.i =sine ay-a sirriple fundarriental approachDr. JOHN BURLAND, Head, Geotechnical Division, Building Research Station

IntroductionMany engineers hold the view that the

factors controlling the behaviour of a pileand its maximum load carrying capacityare too complex to study in a fundamentalway and that our understanding must ofnecessity stem from an empirical approachbased on carrying out load tests. How-ever, just as there are dangers in thepurely theoretical approach so too arethere dangers in empiricism which takesno account of well-established fundamen-tals. The art of ground engineering lies in

the ability to combine the establishedprinciples of soil mechanics with experi-ence and judgement.

This paper outlines an approach to thecalculation of the shaft resistance of pilesin clay using simple effective stress prin-ciples. Although the method involves anumber of simplifying assumptions it ap-pears to account for many of the observedfeatures of pile behaviour and may proveuseful for estimating shaft resistance andnegative skin friction in new or unusualground conditions.

Conventional method of analysisThe conventional method of estimating

the load carrying capacity of a pile makesuse of the undrained strength of the clayin the calculation of both the end bearingcapacity and the shaft bearing capacity.

The ultimate bearing capacity of the pilebase Q„b is given to a sufficient accuracyby the formula:

Q =A -N -c

strength against depth because of thescatter of the results. On the basis of alarge number of tests it has been possibleto assign ranges of n values to particulartypes of pile in various ground conditions(see for example Tomlinson (1963) and(1971)).

Whereas the use of undrained strengthfor calculating the end bearing capacityof a pile appears justified there seemslittle fundamental justification for relatingshaft adhesion to undrained strength forthe following reasons:(1) the major shear distortion is confined

to a relatively thin zone around thepile shaft (Cooke and Price (1973)).Drainage either to or from this narrowzone will therefore take place rapidlyduring loading;

(2) the installation of a pile, whetherdriven or cast-in situ, inevitably mustdisturb and remould the ground ad-jacent to the pile shaft;

(3) quite apart from the disturbancecaused by the pile there is no simplerelationship between the undrainedstrength and drained strength of theground.

There can be no doubt about the im-portance in design of empirical relation-ships between c„and c provided theyare applied to the same pile type andsimilar ground conditions for which they

1.0

were established. However, there aredangers in extrapolating them to new anduntried situations. In these circumstancesan understanding of the underlying prin-ciples is essential and requires a treatmentof pile behaviour in terms of effectivestresses. The effective stress approachoutlined here is by no means the onlypossible one but it has the virtue of beingvery simple.

Principle of effective stressIn a paper dealing with the effective

stress behaviour of piles it is as well tobe quite clear what is meant by "effectivestress". Soil may be visualised as a com-pressible skeleton of solid particles en-closing voids which, in the case of afully saturated soil, are filled with water.Shear stress r can only be carried by theskeleton. However, the total normal stresso on any plane is the sum of two com-ponents —the pressure in the pore water uand the stress carried by the solid particlesand termed the effective stress ~'. Theeffective stress is given by the differencebetween o and u i.e. —u (2)

The shear strength of soils is largely

Fig. 1. Relationship between P (=K. tanB,) and 8, for a normally consolidatedclay

Fig. t.

where A is the area of the baseb

N,, is a bearing capacity factorusually taken as 9.0

and c is the undrained strength of thell

clay beneath the base.Although care is needed in measuring

c„, particularly in stiff fissured clays (Bur-land, Butler and Dunican (1966)), its useappears to be justified for two reasons.Firstly, failure usually takes place throughthe soil some distance beneath the baseand disturbance during installation of thepile will usually not greatly affect themajor part of the clay involved in theshearing process. This is particularly truefor large diameter bored piles where thebase resistance forms a substantial pro-portion of the total resistance of the pile.Secondly, in the long-term the soil beneaththe base will normally experience an in-crease in effective stress and a consequentincrease in strength. Hence the undrainedbearing capacity represents a safe lowerlimit.

It is customary to relate the averageshaft adhesion c„ to the mean undrainedstrength down the shaft c by an empiricalcoefficient n = c„/c. The value of a canvary from as low as 0.3 to as high as 1.5depending on the soil and the type ofpile. Even for a given set of conditionsa can have a wide range of values. More-over, it is often no easy matter to choosea value of c from a plot of undrained

30

08

0.6

0.4

0-2

015 20 25 30 35 40

I8 = (1-Sin fd j X tan gd

40

E

30-Z

I

C00 20-

10—4ICIIII

0

Fig. 1

Ci ~I:I

i

10I

20I

30I

40 50 60 70 80Effective overburden pressure —KN/m 2

~ Timber —Drammen clayo Concretex Timber

,

'Port Khorramshahr clayo Steel

I I

90 100

Fig. 2. Comparison between results of piletest of Port Khorramshahr clay. (LL 48;P1 23; Sensitivity 2.5-3.0;a=0.43-0.79)and Drammen Clay (LL 39; PL 20;Sensitivity 4-8; a=1.6)

determined by the frictional forces arisingduring slip at the contact between thesoil particles. These are clearly a func-tion of the normal stress transmitted bythe soil skeleton rather than of the totalnormal stress. The maximum shear res-istance T, on any plane through the coilis therefore given by:

T~ c + (o') tan 41'= c'+o'an y')(3)

where c's the effective cohesionand 4's the effective angle of friction.

For present purposes it is assumed thatthe groundwater is static, although this isnot fundamental to the theory, and thatthe pore pressure at any point is givenby the depth of the point below theground water level.

Shaft friction in terms ofeffective stress

During installation of any pile the soilimmediately adjacent to the shaft will bedisturbed and remoulded to a greater orlesser extent and excess pore pressures,which may be either positive or negative,will be set up in the ground around thepile. In this paper the following assump-tions are made:1. Before loading the excess pore pres-

sures set up during installation arecompletely dissipated.

2. Because the zone of major distortionaround the shaft is relatively thin load-ing takes place under drained condi-tions.

3. As a result of remoulding during instal-lation the soil has no effective cohe-sion. Hence the shaft friction T, at anypoint is given by

T, = o'„ tan b (4)where o'„ is the horizontal effective

stress acting on the pileand b is the effective angle of fric-

tion between the clay and

32

the pile shaft.4. The further simplifying assumption is

made that o'„ is proportional to thevertical effective overburden pressure"p, i.e.

=K-p (5)Assumption (4) is perhaps the most ques-tionable and requires close examinationand possible refinement. Nevertheless itrepresents a simple and logical startingpoint. Equation (4) therefore becomes:

T,=p- K-tang (6)Equation (6) is not new and has beenused by Zeevaert (1959), Eide, Hutchinsonand Landva (1961), Johannessen andBjerrum (1965), Chandler (1968) andothers.The quantity K- tan b may be denoted byp so that

"P = (y,-d —y„.-h) where y, is the bulkdensity of the soil, d is the depth belowground level, y„. is the density of waterand h is the depth below the water table.

p = —= K-tang (7)P

It can be seen that p is similar to theempirical factor a, the important differ-ence being that p is related to the funda-mental effective stress parameters K andb.

The magnitude of the earth pressurecoefficient K depends on the soil type,the stress history of the soil and themethod of installing the pile. The value ofb depends on the soil type and the prop-erties of the pile surface. Evidently p cantake on a wide range of values. Neverthe-less it is possible to make reasonable esti-mates of K and 4 and hence p.

Average values of p can also be ob-tained empirically from pile tests pro-vided a sufficlent length of time has beenallowed after installation and the testshave been carried out sufficiently slowly.In these cases:

TSp=—P

where T,. is the average shaft frictionand p the effective overburden pres-

sure.Thus it is possible to make estimates of

p based on fundamental soil mechanicsparameters. It is recognised that theseestimates may require modification in thelight of empirical evidence. This approachwill be illustrated by applying the methodto the two extreme conditions of softnormally consolidated clay and stiffheavily overconsolidated clay.

Shaft friction for piles in soft clayIt is assumed that failure takes place

in the remoulded soil close to the shaftsurface (Tomlinson (1971)) so that 5 =4,,where 4, is the remoulded drained angleof friction of the soil. Before the pile isinstalled the earth pressure coefficient Kis equal to K„. For a driven pile K mightbe expected to be somewhat greater thanK, so that setting K = K„should give alower limit to the shaft friction. For nor-mally consolidated clay K, has beenfound to be related to 4„by the expres-sion K, = 1 —sin 4,.Substituting for b and K in equation (7)gives:

p = (1 —sin4 ) tan4 (8)

as a lower limit for driven piles in nor-mally consolidated clay. Values of 4d willnormally lie somewhere in the range of20deg to 30deg and it is interesting tonote that over this wide range the valueof p only varies from 0.24 to 0.29 asshown in fig. 1. This rather surprising re-sult implies that for soft clays p is notvery sensitive to clay type and that forall soft clays there should be a fairlyunique relationship between T,, and p.

This prediction can be checked by com-paring the values of shaft friction obtainedfrom loading tests on piles driven intotwo very different soft clays. Hutchinsonand Jensen (1968) present the results ofloading tests on a number of concrete,steel and timber piles driven into deepestuarine clay in the port of Khorram-shahr, Iran. The average liquid and plasticlimits for the clay are 48 per cent and23 per cent respectively and it has a

sensitivity of between 2.5 and 3.0. Valuesof n ranged from 0.43 to 0.79. In fig. 2 theresults of Hutchinson and Jensen havebeen plotted on a graph of ~, against pand are shown by the open points. Valoesof /f (=~,./p) are also shown and it can

Fig. 3. Relationship between average shaftfriction ~,. and average depth for drivenpiles in soft clay

be seen that the results lie betweenP = 0.25 and 0.4 with an average of ap-proximately 0.32.

Eide, Hutchinson and Landva (1961)have presented the results of some testson timber piles driven into Drammen clayThe average liquid and plastic limits areabout 35 per cent and 15 per cent respec-tively and the clay has a sensitivity ofbetween 4 and 8. The value of a obtained

Fig. 3

10I

20I

30 40 50 60

on (1957)

4-

0EI

8Utg

6rh

CIC

0CD

o 8-0

son and(1968)

CL43

IDCDtl3 10-43

12-

I9-0.40

14—

Average shaft Friction —KN/m 2

from the test was 1.6. In fig. 2 the resultof this test is shown as a full point cor-responding to P = 0.32. Inspite of theclay at the two sites having very differentproperties and the values of n for the twosites having extreme upper and lowerlimits for soft clay, the average values ofP are the same and only slightly large!than the predicted lower limit value of0.29.

At each of the two sites mentioned thedensity of the clay and the position of thewater table had been accurately measuredso that the values of p could be calcu-lated. A number of results of pile tests onsoft clays have been published withoutthis information. The density of soft claydoes not vary a great deal and the watertable will usually be close to the surface.Therefore if P is approximately indepen-dent of clay type, the results of pile testson soft clay should show only a smallscatter when plotted on a graph of r.,against average depth. In fig. 3 are plottedthe results of a large number of pile testscarried out on a wide variety of clays.Bearing in mind the possible variations indensity and groundwater conditions itcan be seen that the scatter is remark-ably small.

The lines showing values of P havebeen constructed by making the assump-tion that the soil has a bulk density of1 EOO kg/m'nd that the water table is, onaverage, one metre below ground level.Most of the results lie between P = 0.25and 0.40 which represents a very muchsmaller spread than the equivalent n valueswhich lie between 0.5 and 1.6. It appearsthat the simple effective stress theory forthe lower limit of shaft friction is invery close agreement with the observa-tions and that the correct value of K isslightly higher than K„. On the basis of theresults it would appear that a reasonablevalue of p to use in design would beabout 0.3.

Negative skin frictionNegative skin friction or "drag down"

can develop when piles are driventhrough soft soils into stiff underlyingstrata or when a superimposed loading,usually in the form of fill, is applied to theground surface. Negative skin friction re-sults from the consolidation of the clayand usually takes a long period of time todevelop fully.

As in the case of shaft friction de-veloped during loading it seems that nega-tive skin friction is best accounted for interms of effective stress and equations(4) to (8) apply. During consolidation ofthe clay the pore pressures will be signifi-cantly greater than hydrostatic and willgradually decrease as consolidation pro-ceeds. Hence the effective overburdenpressure p (= tr„—u) will gradually in-crease causing a corresponding increasein negative skin friction until the porepressures become hydrostatic.

Fellenius (1972) has presented somedetailed measurements of the build upof negative skin friction on two instru-mented precast piles driven through 40mof soft clay into a firm underlying stratumThe results show a steady increase ofnegative friction with time. The shaft fric-tion was far from fully developed at thetermination of the test, but at this stagethere was a linear increase with depthwith r,/p (= P) equal to 0.095.

Johannessen and Bjerrum (1965) des-cribe the results of tests on two steel

Ground Engineering 37

0

rOIIIi

E 4I

Iti

V

CLol 6giQiC

10

20I

Average shaft Friction —KNlm2

40 60I

80 100 120

equation (9)

~ Driven through sand and gravel

o Driven through soft clays or silts

x No overlying strata

against the average undrained strength c(see Skempton (1959), fig. 5). Moreover,values of c are themselves obtained byaveraging laboratory strength resultswhich fequently have a very wide scatter.

On the basis of the results plotted in

fig. 5 it appears that the chain dotted line

(given by T.,/p = 0.8) could be used as a

reasonably conservative preliminary de-

sign curve for London. Figure 5 can alsobe used as a check for design values ofF, and values falling above the observedlimits should be used with caution.

Driven piles: For bored piles equation

(9) appears to give a reasonable upperlimit for shaft friction. When a pile isdriven into the ground the equilibrium

horizontal stresses adjacent to the shaftwill be greater than the undisturbedvalues over most of the length of the pile

due to compaction of the ground. Henceequation (9) would be expected to givea lower limit for values of r,.

Tomlinson (1971) has quoted the re-

sults of a number of tests on piles driven

into London Clay and the results for pilesgreater than 4m in length are plotted in

fig. 6. It can be seen that the scatter isvery large but the "ideal" curve does givea lower limit.

The reasons for the scatter include:variable ground conditions near the sur-face (in particular the ground water level),overlying material being drawn down dur-

ing installation thereby affecting the valueof s, and variations in the depth of thegap often found between pile and soilnear the ground surface (Tomlinson,1971). These factors predominate nearground surface and it is perhaps significantthat the results in fig. 6 are for muchshorter piles than the equivalent boredpiles results in fig. 5 which show muchless scatter. The "ideal" curve is basedon the assumptions that the water tableis at the surface of the clay and that s isequal to the remoulded drained angle offriction for London Clay. Both these as-sumptions are likely to be on the conser-vative side.

Without attempting to explain the de-tailed behaviour of driven piles in stiffclay the existence of a theoretical lowerlimit to shaft friction may prove usefulparticularly when results of pile tests arevery variable.

Fig. 6. Relationship between average shaftfriction and average depth in clay forlarge diameter bored piles in London Clay

proximately equal to 0.1 <'. The brokenline in fig. 5 corresponds to 10m of over-

lying deposits having a density of 1 760kg/m'ith the water table still at theclay surface. The influence is not greatand is even smaller with a higher watertable. The "ideal" relationship between~, and average depth may be comparedwith the results of pile tests in London

Clay. Bored piles will be considered sepa-rately from driven piles.

Bored piles: The process of boring ashaft in clay causes lateral yield of theground around the borehole due to theremoval of stress at the walls of theshaft. After installation of the pile thestresses will gradually build up and thefield values will depend on the degree ofsoftening that takes place in the clayaround the shaft prior to and during con-creting. Even with perfect conditions it

seems doubtful if the initial at rest hori-zontal stresses can ever be fully re-established at the shaft face. Thus the"ideal" curve in fig. 5 should represent an

upper limit for values of 7.„. for bored pilesin London Clay.

In fig. 5 the results of a number of testson large diameter bored piles are plotted.The values of r, from Wembley and St.Giles Circus were obtained directly bymeans of load cells installed at the baseof the piles. For the remaining tests theshaft friction was deduced by estimatingthe bearing capacity of the base and sub-tracting this from the measured totalfailure load.

It can be seen that the majority of theresults fall below the "ideal" curve butare surprisingly close to it. As predictedthe results from sites overlain by an ap-preciable thickness of fill and gravel (Mill-bank 9.5 m, St. Giles 6.6 m) do not differsignificantly from the pattern of results. In

general the scatter of the results in fig. 5is no greater than when F, is plotted

Concluding remarksThe object of this paper has been to

demonstrate that many of the featuresof the behaviour of piles in clay can beaccounted for by adopting a simple ap-proach in terms of effective stresses. Theapproach has been particularly rewardingfor piles in soft clays where it has beendemonstrated that the ratio between theaverage shaft friction and the main effec-tive overburden pressure ~,/p (= p) liesbetween about 0.25 and 0.4 irrespectiveof clay type. An example is given in

which the results of pile tests on twosoft clays both give values of p = 0.32while the values of a for the two claysare 0.6 and 1.6 respectively.

The approach can be used to estimatenegative friction and is essentially thesame as the method adopted by Johan-nessen and Bjerrum (1965). On the basisof the limited amount of field data availableit appears that a value of p equal to about0.25 gives an upper limit for negative skinfriction on piles in soft clay.

For stiff clays the situation is morecomplex and the main difficulty lies in

Ground Engineerinq 41

estimating the value of the coefficient ofearth pressure at rest K, at variousdepths. It is to be hcped that direct insitu methods of measuring the at-resthorizontal effective pressures will soonbe available (Wroth and Hughes, 1973).For London Clay values of K„have beenestimated from laboratory tests and usedto obtain the relationship between =., andthe mean depth for an "ideal" pile, i.e. apile that is installed without altering thein situ effective stresses. Comparison withpile tests appears to confirm that thisideal relationship gives an upper limit ofF, for bored piles and a lower limit fordriving piles.

The simple approach outlined hero isnot intended to replace the traditional em-pirical method of estimating shaft friction,particularly in the stiffer materials. How-ever, it may well be useful for preliminarydesign purposes or a check particularlyin unusual or untried conditions. Its mainpractical value might be in providing asimple model which enables the engineerto understand some of the fundamentalprinciples governing pile behaviour.

From a research point of view themethod is clearly too simple to accountfor the detailed behaviour of piles. Thismust await the results of careful and ex-pensive research into the distribution ofboth normal and shear stresses along theshafts of various types of pile for a varietyof ground conditions. The simple approachis sufficiently promising to justify suchresearch.

For pile tests to be of greatest valuethe following points should be borne inmin d'.

1. Sufficient time should be allowed be-fore testing for the excess pore pres-sures set up during installation todissipate.

2. The test should be carried out suffi-ciently slowly for drained conditionsadjacent to the shaft to develop.

3. Tho position of the ground water tableshould be measured.

4. A detailed description of the soil pro-file including index properties shouldbe given.

5. Additional information in the form oftriaxial and oedometer tests on undis-turbed samples and triaxial tests onremoulded samples is desirable.

AcknowledgementThis papor is published with the per-

mission of the Director of the BuildingResearch Establishment.

References1 Bishop, A. W., Webb, D. C. and Lewin, P. I(1965): Undisturbed samples of London Clay

from Ashford Common shaft: strength/effectivestress relationships. Geotechnique, 15, 1-31.

2 Brand, E. W (1971): Discussion. Behaviour ofPiles. Instn. Civ. Engrs., London 1971, 42-44.

3 Borland, J. B., Butler, F. G. and Dunican, P(1966): The behaviour and design of large dia-meter bored piles in stiff clay. Large BoredPiles. Instn. Civ. Engrs., London, 51-71.

4 Chandler, R. J (f966): Discussion. Large BoredPiles. Instn. Civ. Engrs., London 1966, 95-97.

5 Chandler, R. J (1968): The shaft friction ofp les in cohes'.ve soils in terms of effectivstress. Civ. Eng. and Pub. Wks. Rev., 63,48-51.

6 Cooke, R. W. and Price. G (1973): Strains anddisplacements around friction piles. Proc. 8thInt. Conf. Soil Mech, and Found. Eng.,Moscow.

7 Eide, O., Hutchinson, J. N. and Land vs, A(1961): Short and long-term test loading of afriction pile in clay. Proc. 5th Int. Conf. SoilMech. and Found. Eng.. Vol. II, paper 38/8.

8 Felfenius, B. H (1971): Discussion. Behaviourof Piles. Instn. Civ. Engrs., London 1971, 44-45.

9 Fellenius, B. H (1972): Down-drag on piles inclay due to negative skin friction. Proc. Can.Geotechnical J., 9, 4, 323-337.

10 Highway Research Board (1961): Records ofload tests on friction piles. HRS, Special Re-port 67.

11 Hurchinson. J. N. and Jensen, E. V ( 1968):Loading tests on piles driven into estuarineclays at Port of Khorramshahr, and observa-tions on the effect of bitumen coatings onshaft bearing capacity. Pub. 78, NGI, Oslo.

12 Johannessen, I. J. and Bierrum, L ( 1965):Measurements of the compression of a steelpile to rock due to settlement of the surround-ing clay. Proc. 6th Int. Conf. Soil Mech. andFound. Eng., II, 261-264.

13 Sherman, F. A (1961):The anticipated and ob-served penetration resistance of some fnctionpiles entirely in clay. Proc. 5th Int. Conf. SoilMech. and Found. Eng., 2, 135-141.

14 Skempton, A. W (1959): Cast in situ boredpiles in London Clay. Geotechnique, 9, 4, 153-173.

15 Skempton, A. W (1961): Horizontal stresses inoverconsolidated Eocene clay. Proc. 5th Int.Conf. Soil Mech. and Found. Eng., 1961, 351-357.

16 Tomlinson, M. J (1957): The adhesion of pilesdriven in clay soils. Proc. 4th Int. Conf. SoilMech. and Found. Eng., 2, 66-71.

17 Tomlinson, M. J ( 1963): Foundation Designand Construction. Pitman, London 1963.

18 Tomfinson, M. J (1971): Some effects of piledriving on skin friction. Behaviour of Pi 4s.Instn. Civ. Engrs., London, 107-114.

19 Whitaker, T. and Cooke, R. W (1966): An in-vestigation of the shaft and base resistance oflarge bored piles in London Clay. Large BoredPiles. Instn. Civ. Engrs., London 1966, 7-49.

20 Wroth, C. P ( 1972): Some aspects of theelastic behaviour of overconsolidated clay.Proc. Roscoe Memorial Symp.. 347-361. Foulis,1972.

21 Wroth, C. P. and Hughes, J. M. 0 (1973): Aninstrument for the in situ measurement of theproperties of soft clay. Proc. 8th Int. Conf.Soil Mech. and Found. Eng., Moscow.

22 Zeevaert, L (1959): Reduction of point-bearingcapacity of piles because of negative friction.Proc. 1st Panamerican Conf. Soil Mech. andFound Eng., Vol. III, 1145-1152.