Shaft friction of piles in clay-a simple fundamental approachDr. JOHN BURLAND, Head, Geotechnical Division, Building Research Station

IntroductionMany engineers hold the v iew that the

factors controlling the behaviour o f a pi leand i t s maximum load carrying capaci tyare too complex to study in a fundamentalway and tha t our understanding mus t o fnecessity stem from an empirical approachbased on carrying o u t load tests . H o w -ever, j u s t a s the re are dangers i n t h epurely theoret ical approach s o t o o a r ethere dangers i n empiricism wh ich takesno account o f well-established fundamen-tals. The art o f ground engineering l ies inthe ab i l i t y t o comb ine t h e establ ishedprinciples o f soi l mechanics w i t h experi-ence and judgement.

This paper outlines an approach to thecalculation o f the shaft resistance o f pilesin clay using simple effective stress prin-ciples. A l though t h e method involves anumber o f simplifying assumptions i t ap-pears to account for many of the observedfeatures o f pi le behaviour and may proveuseful f o r estimating shaft resistance andnegative sk in f r ic t ion i n n e w o r unusualground conditions.

Conventional method of analysisThe conventional method o f estimating

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

The ultimate bearing capacity o f the pilebase Out, is given t o a sufficient accuracyby t h e formula:

Q Ab b N - .cc(1)

where A i s the area of the baseN, i s a bearing capacity fac to r

usually taken as 9.0and c u i s the undrained strength of the

clay beneath the base.Although care i s needed i n measuring

cu, particularly in st i ff fissured clays (Bur-land, Butler and Dunican (1966)) , i ts useappears t o b e justif ied f o r t w o reasons.Firstly, failure usually takes place throughthe soi l some distance beneath the baseand disturbance during installation o f thepile w i l l usua l ly n o t great ly a f fec t t h emajor p a r t o f t h e c l ay involved i n t h eshearing process. This i s particularly truefor large diameter bored pi les where thebase resistance forms a substantial p ro -portion o f the total resistance o f the pile.Secondly, in the long-term the soil beneaththe base w i l l normally experience an i n -crease in effective stress and a consequentincrease in strength. Hence the undrainedbearing capacity represents a safe lowerlimit.

It i s customary t o relate t h e averageshaft adhesion c,, t o the mean undrainedstrength down the shaft E by an empiricalcoefficient a = c/5. The value o f a canvary from as low as 0,3 t o as high as 1.5depending o n t h e so i l a n d t h e t y p e o fpile. Even f o r a given s e t o f condit ionsa can have a wide range o f values. More-over, i t is often no easy matter to choosea va lue o f E f rom a p l o t o f undrained30

strength aga ins t d e p t h because o f t h escatter o f the results. On the basis o f alarge number of tests i t has been possibleto assign ranges o f a values t o particulartypes o f pile in various ground conditions(see f o r example Tomlinson (1963) a n d(1971)).

Whereas the use o f undrained strengthfor calculating t h e e n d bearing capaci tyof a p i l e appears just i f ied the re seemslittle fundamental justif ication f o r relatingshaft adhesion t o undrained strength f o rthe following reasons:(1) t h e major shear distortion is confined

to a relatively th in zone around thepile shaft (Cooke and Price (1973)),Drainage either to or from this narrowzone w i l l therefore take place rapidlyduring loading;

(2) t h e instal lat ion o f a p i l e , whe the rdriven or cast-in situ, inevitably mustdisturb and remould t h e ground ad-jacent to the pile shaft;

(3) qu i te a p a r t f r o m t h e d is turbancecaused by the pile there is no simplerelationship be tween t h e undrainedstrength and drained strength o f theground.

There can b e n o doub t about the im-portanoe i n design o f empirical relation-ships be tween c a n d E provided theyare appl ied t o t h e same p i l e t y p e a n dsimilar ground condit ions f o r which they

1.0

08

06

02

15 20

were establ ished. H o w e v e r, t h e r e a r edangers in extrapolating them to new anduntried situations. In these circumstancesan understanding o f the underlying pr in -ciples is essential and requires a treatmentof p i l e behaviour i n t e r m s o f effect ivestresses. T h e effect ive s t ress approachoutlined here i s b y n o means t h e o n l ypossible one but i t has the virtue o f beingvery simple.

Principle of effective stressIn a paper dealing w i t h t h e effect ive

stress behaviour o f piles i t i s as we l l t obe quite clear what is meant by "effectivestress". Soil may be visualised as a com-pressible skeleton o f so l id part icles e n -closing v o i d s wh ich , i n t h e case o f afully saturated soi l , are f i l led w i t h water.Shear stress T can only be carried by theskeleton, However, the total normal stress,r on any plane is the sum o f t w o com-ponentsthe pressure in the pore water uand the stress carried by the solid particlesand termed t h e effective stress T h eeffective stress is given b y the differencebetween a n d u i.e.

0 .1=cr_a ( 2 )The shear strength o f so i ls i s largely

Fig. 1. Relationship between 13 ( = K . tan0) and 0 , for a normally consolidatedclay

25 3 0 35 40

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x .0

40o

Timber - DrammenConcrete

clay

Timber Port Khorramshahr c lay30 0 Steel

0

20 0

10

0

F1=0 4

0

D 0 . 2 5

10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0Effective overburden p ressure - KN/m2

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 b y the frictional forces arisingduring s l i p a t t h e contac t between t h esoil part icles, These a re clearly a f unc -tion o f the normal stress transmitted b ythe so i l skeleton rather than o f the tota lnormal stress. T h e maximum shear res-istance 1-, o n any plane through the coi lis therefore given by:

Ti = C l+ (cyu) tan 0, (-=-- J r a l tan 95)(3)

where c , i s the effective cohesionand c b ' i s the effective angle of friction.

For present purposes i t i s assumed tha tthe groundwater is static, although this isnot fundamental t o t he theory, and tha tthe pore pressure a t any po in t i s g ivenby t h e d e p t h o f t h e p o i n t b e l o w t h eground water level

Shaft friction in terms ofeffective stress

During installation o f any p i le t he soi limmediately adjacent t o the shaft w i l l bedisturbed and remoulded t o a greater o rlesser extent and excess pore pressures,which may be either positive o r negative,will b e se t u p i n the ground around thepile. I n th is paper the fol lowing assump-tions are made:1. Be fo re loading t he excess pore pres-

sures s e t u p du r i ng instal lat ion a r ecompletely dissipated.

2. Because the zone o f major distort ionaround the shaft is relatively thin load-ing takes place under drained condi-tions.

3. A s a result of remoulding during instal-lation the soi l has n o effective cohe-sion. Hence the shaft friction i s at anypoint is given by

Ts 0 - l b tan 8 ( 4 )where ,T, is t h e horizontal effect ive

stress act ing on the pi leand 3 i s the effective angle of fric-

tion between the clay and32

the pi le shaft.4 T h e fur ther simpli fying assumption i s

made t h a t 0-1 i s proportional t o t h evertical effective overburden pressure*p, i.e.

= K - p ( 5 )Assumption (4 ) is perhaps the most ques-tionable a n d requires c lose examinationand possible refinement. Nevertheless i trepresents a s imp le and logical start ingpoint. Equation ( 4 ) therefore becomes:

p - K t a n 8 ( 6 )Equation ( 6 ) i s n o t n e w and has beenused by Zeevaert (1959), Eide, Hutchinsonand Landva ( 1 9 6 1 ) , Johannessen a n dBjerrum ( 1 9 6 5 ) , Chand le r ( 1 9 6 8 ) a n dothers.The quantity K- tan 8 may be denoted by/3 so that

K - tan 6 ( 7 )

It can b e seen t h a t if,3 i s similar t o t h eempirical f ac to r a , t h e impor tant di ffer-ence being that p is related to the funda-mental effect ive stress parameters K and8.

The magnitude o f t h e ear th pressurecoefficient K depends o n t h e so i l t ype ,the st ress h i s to r y o f t h e so i l a n d t h emethod o f installing the pile. The value of8 depends on the soil type and the prop-erties o f the pi le surface. Evidently p cantake on a wide range of values. Neverthe-less i t is possible to make reasonable esti-mates of K and I and hence 13.

Average values o f 13 can a lso b e o b -tained empi r ica l l y f r o m p i l e t es t s p r o -vided a sufficient length o f t ime has beenallowed a f t e r instal lat ion a n d t h e tes tshave been carried o u t sufficiently s lowly.In these cases:

P =

*p = yw-h) where 7 , i s the bulkdensity o f the soil, d is the depth belowground level, 7,, i s the density o f waterand h is the depth below the water table.

where 2 , i s t h e average s h a f t f r i c t ionand t h e effect ive overburden pres-

sure.Thus i t is possible to make estimates of

ja based o n fundamental so i l mechanicsparameters. I t i s recognised t h a t these )estimates may require modification i n thelight o f empirical evidence. This approachwill be illustrated by applying the methodto t h e t w o extreme condi t ions o f s o f tnormally conso l ida ted c l a y a n d s t i f fheavily overconsolidated clay.

Shaft friction for piles in soft clayIt i s assumed tha t fai lure takes place

in t h e remoulded so i l c lose t o t he shaf tsurface (Tomlinson (1971)) so that 8 =0 ,where 0 i s the remoulded drained angleof fr ict ion o f the soil . Before the p i le i sinstalled the earth pressure coefficient Kis equal t o K . For a driven p i le K mightbe expected to be somewhat greater than

so that sett ing K = K . should give alower l imi t t o the shaft fr ict ion. For nor-mally consol idated c l a y K e h a s b e e nfound t o be related t o 0 b y the expres-sion K,, = 1 sin od.Substituting f o r 8 and K i n equation ( 7 )gives:

/3 = 1 - sin od) tan od ( 8 )

as a l ower l imi t f o r dr iven pi les i n nor -mally consolidated clay. Values o f od w i l lnormally l i e somewhere i n t h e range o f20 deg t o 3 0 deg and i t is interesting t onote that over this w ide range the valueof p o n l y var ies f r o m 0 .24 t o 0 .29 a sshown i n fig. 1 . This rather surprising re-sult implies t ha t f o r sof t clays p i s n o tvery sensit ive t o c lay t ype and tha t f o rall s o f t c lays there should b e a fa i r l yunique relationship between 7 a n d p.

This prediction can be checked by com-paring the values of shaft friction obtainedfrom load ing tes ts o n p i les d r i ven i n t otwo very different so f t clays. Hutchinsonand Jensen (1968) present the results o floading tes ts o n a number o f concrete,steel a n d t imber pi les dr iven i n t o deepestuarine c l a y i n t h e p o r t o f Khorram-shahr, Iran. The average l iquid and plasticlimits f o r t he c lay are 4 8 p e r cen t and23 p e r c e n t respectively a n d i t has a

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sensitivity o f between 2.5 and 3.0. Valuesof a ranged from 0.43 to 0.79. In fig. 2 theresults o f Hutchinson a n d Jensen havebeen plotted o n a graph o f T, against 5and are shown by the op-en points. Valuesof 17 (=-Fs/15) are also shown and i t can

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

Fig. 3

2

12

14

Average s h a f t Fr ic t ion - KN/m210 2 0 3 0 4 01

0

0*A0V

A

be seen t h a t t h e resul ts l i e be tweenp = 0.25 and 0.4 w i th an average o f ap-proximately 0.32.

Eide, Hutchinson a n d Landva (1951)have presented the results o f some testson timber piles driven into Drammen clayThe average l iquid and plast ic l imi ts areabout 35 per cent and 15 per cent respec-tively and t h e c lay has a sensit iv i ty o fbetween 4 and 8. The value of a obtained

SteelConcrete Toml inson (1957)TimberH.R.B. (1961)Sharman (1961)Brand (1971)Fellenius (1971)Eide e t a l (1961)Concrete Hutchinson a n d

Jensen (1968)TimberSteel

=

DAP

/ 0 = 0 25

50 6 0

3 0 4 0

from t h e test was 1.6. In fig. 2 the resultof this test is shown as a fu l l po int cor-responding t o p 0 . 3 2 . Insp i te o f t h eclay at the two sites having very differentproperties and the values of a for the twosites having extreme u p p e r a n d l o w e rlimits fo r soft clay, the average values o fp are t h e same and on ly s l ight ly largerthan t h e predicted l ower l im i t value o f0.29.

At each o f the two sites mentioned thedensity of the clay and the position of thewater table had been accurately measuredso tha t the values o f (.3 could b e calcu-lated. A number of results of pile tests onsoft c lays have been publ ished w i thou tthis information. The density o f soft c laydoes not vary a great deal and the watertable w i l l usually be close to the surface.Therefore i f p i s approximately indepen-dent of clay type, the results o f pile testson s o f t c l ay should show o n l y a smal lscatter when p lo t ted o n a graph o fagainst average depth. In fig. 3 are plottedthe results of a large number of pile testscarried o u t o n a w i d e var ie ty o f clays.Bearing in mind the possible variations indensity a n d groundwater condi t ions i tcan b e seen tha t t he scatter i s remark-ably small.

The l ines showing values o f p havebeen constructed b y making the assump-tion tha t t he soi l has a bu lk densi ty o f1 SOO kg/m" and that the water table is, onaverage, one metre be low ground level .Most o f the results l ie between = 0.25and 0.40 which represents a ve r y muchsmaller spread than the equivalent a valueswhich l ie between 0.5 and 1.6. I t appearsthat the simple effective stress theory forthe l o w e r l i m i t o f s h a f t f r ic t ion i s i nvery c lose agreement w i t h t he observa-tions and tha t the correct value o f K i sslightly higher than K O n the basis of theresults i t would appear that a reasonablevalue o f 13 t o use i n design w o u l d b eabout 0.3.

Negative skin frictionNegative skin f r ict ion o r "drag d o w n

can d e v e l o p w h e n p i l e s a r e d r i v e nthrough s o f t so i l s i n t o s t i f f underly ingstrata o r when a superimposed loading,usually in the form of fill, is applied to theground surface. Negative skin fr ict ion re-sults f rom t h e consolidation o f the c layand usually takes a long period of t ime todevelop ful ly.

As i n t h e case o f sha f t f r ic t ion d e -veloped during loading i t seems that nega-tive skin friction is best accounted fo r interms o f effect ive stress a n d equations(4) t o ( 8 ) apply. During consolidation o fthe clay the pore pressures wi l l be signifi-cantly greater than hydrostat ic and w i l lgradually decrease as consolidation p r o -ceeds. Hence t h e effect ive overburdenpressure p ( = u, u) w i l l gradually i n -crease causing a corresponding increasein negat ive sk in f r i c t ion un t i l t h e p o r epressures become hydrostatic.

Fellenius (1972) h a s presented s o m edetailed measurements o f t h e bu i l d u pof negative sk in f r ic t ion o n t w o instru-mented precast piles driven through 40 mof soft clay into a firm underlying stratum..The results s h o w a s teady increase o fnegative friction wi th t ime. The shaft fric-tion was far f rom fu l ly developed a t thetermination o f the test, bu t a t this stagethere w a s a l inear increase w i t h dep thwith r , / p (-= ,d) equal to 0.095.

Johannessen and Eljerrum (1965) d e s -cribe t h e results o f tes ts o n t w o s tee l

Ground Engineering 3 7

Jerome St-Michel

Fig. 4

18

20

22

24

38

1 2Ko

piles d r i ven t o bedrock th rough abou t50 m o f soft clay. A f te r installation, a f i l lwith a height o f about 10 m was placedover t h e g round surface. Equat ion ( 6 )was used t o compute the distribution o fnegative sk in fr ict ion along the p i le andhence i t s change o f length w i t h depth.The computed resul ts w e r e t h e n c o m -pared w i th measurements o f pile shorten-ing a t various depths. I t was found tha ta v a l u e o f K tan a ( = g) e q u a l t o 0 . 2gave t h e m o s t sat isfactory agreementwith the observations.

Equation (8 ) predicts values o f /3 lyingbetween 0.24 and 0.29. These values maybe on the high s ide since orientation o fthe c l a y par t ic les du r ing consol idat ionpast the shaft surface may result in a be-ing less then O n the basis o f the re-sults available to date and the predictionsof the simple effective stress theory i t ap-pears that a value of 13 = 0.25 representsa reasonable upper l imit for negative skinfriction in soft clay.

Shaft friction for piles in stiff clayAn effect ive stress approach t o shaf t

friction i n s t i f f c lays i s m o r e complexthan f o r s o f t c lays. The approach des -cribed here is similar t o that outlined b yChandler (1966 and 1968). Equation ( 6 )is sti l l assumed to apply although its vali-dity f o r driven piles i n st i ff c lay may beopen t o quest ion d u e t o t h e complexstress condi t ions se t u p dur ing dr iv ing.The central problem i s t o est imate t h evalue o f K. I n t he undisturbed state thevalue o f K ( = Ko) f o r heavi ly overcon-solidated c lay varies w i th depth and canhave values as high as 3 near the surfacedecreasing to less than 1 a t great depth.

As f o r sof t clays a logical f i rst step isto estimate the shaft fr ict ion correspond-

Fig. 5. Relationship between average shaftfriction and average depth in clay fordriven piles in London Clay

Fig. 4. The variation of K 0 with depth forLondon Clay

ing to K = K. This may be thought of asthe shaft friction o f an " ideal" pi le whichhas b e e n instal led w i t h o u t al ter ing t h einitial s t ress condi t ions i n t h e ground.Since the value o f K varies w i t h depththe total shaft resistance R is given by:

p. K tan a A 1where d i s t h e d iameter and 1 i s t h elength o f the pile T h e mean shaft frictionT. i s therefore*

7, -(9)

Values o f K a t various depths in Lon-don Clay have been deduced from labora-tory tests b y Skempton (1961) a n d Bis-hop, W e b b and Lewin (1965). Their re-sults are plotted i n f ig. 4 and ( fo l lowingChandler, 1966) a mean cu rve w i l l b e*Equation ( 9 ) g i v e s resul ts w h i c h c a ndiffer appreciably f rom the approximateequation i = K,,.5 tan a used by Chand-ler (1966).

Fig. 5

0

2

4

6

10

12

13 1p. K tan 8 A 1

ci. 1 1

Average s h a f t f r ic t ion - Kl\l/m2

2,0 4 0 6 0

used for estimating 7s. A s before i t is as-sumed tha t failure takes place i n the re-moulded so i l c lose t o t h e shaf t surfaceso t h a t a i s equa l t o t h e remouldeddrained angle of friction which for LondonClay i s taken a s 21.5 deg. i n f ig . 5 thefull l i ne represents t h e relationship b e -tween the mean " ideal" shaft friction andaverage depth i n London Clay assumingthat the water table is a t the surface andthe bulk density of the clay is 2 000 kg/ma,

The presence o f overlying f i l l o r gravelis n o t thought t o influence the values o fshaft f r i c t ion great ly. T h e overlying de-posits result in a reapplication of pressureto the c lay surface fol lowing earlier ero-sion and under these circumstances theclay wi l l behave approximately elastically,at least below the top few metres. I f 4,1 isthe effective vertical pressure due t o theoverlying deposits the resulting additionalshaft friction t , Ts is given by:

A T s = c r l X x t a n a1

where ,,, = Poisson's r a t i o o f t h e s o i lskeleton.

For London C l a y v l i s l e s s t h a n 0 . 2(Wroth, 1972) and therefore Z\ 1-, i s ap-

80 1 0 0 1 2 0 1 4 0

Wembley W h i t a k e r a n dTension C o o k e (1966)MoorfieldsBarbican B u r l a n d , ButlerHayesSt.GilesKensal greenFinsburyMillbank

(3-=0 8 \

and Dunican (1966)

Skempton (1959)

\10m overburden

equation (9)

Jerome St-Michel

'01

Fit. A v e r a g e shaf t Friction - KN/m220 4 0 6 0 8 0 1 0 0

2

8

10

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

proximately equal t o OA T h e brokenline in fig. 5 corresponds to 10 m of over-lying deposits having a density o f 1 760kg/m3 w i t h t h e water table s t i l l a t t h eclay surface. The influence i s n o t greatand is even smaller w i th a higher watertable. T h e - i d e a l " relationship be tween,-, and average depth may b e comparedwith the results o f p i le tests i n LondonClay. Bored piles wi l l be considered sepa-rately from driven piles.

Bored pi les: T h e process o f boring ashaft i n c lay causes lateral y ie ld o f theground around t h e borehole due t o t h eremoval o f s t ress a t t h e wa l l s o f t h eshaft. A f t e r installation o f t h e p i l e t h estresses w i l l gradually bui ld u p and thefield values wi l l depend on the degree o fsoftening t h a t takes p lace i n t h e c l ayaround the shaft prior to and during con-creting. Even w i t h per fect conditions i t

0

0

0

X

X

0

120

equation (9)

D r i v e n through sand and gravelo D r i v e n through sof t clays or si l ts N o overlying strata

seems doubtful i f the initial a t rest hori-zontal stresses c a n e v e r b e f u l l y r e -established a t t h e shaf t face. Thus t h e"ideal" curve in fig. 5 should represent anupper 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 o f Ts from Wembley and S t .Giles Circus w e r e obtained d i rec t ly b ymeans o f load cells installed a t the baseof the piles. For the remaining tests theshaft friction was deduced b y estimatingthe bearing capacity o f the base and sub-tracting t h i s f r o m t h e measured t o t a lfailure load.

It can be seen that the majority o f theresults fa l l be l ow t h e " ideal " curve b u tare surprisingly close t o i t . A s predictedthe results f rom sites overlain b y an ap-preciable thickness of fill and gravel (Mi l l -bank 9.5 m, St. Giles 6.6 m) d o no t differsignificantly from the pattern o f results. Ingeneral the scatter o f the results in fig. 5is n o greater t han w h e n T-s i s p lo t ted

against the average undrained strength c(see Skempton (1959), fig. 5) . Moreover,values o f E are themselves obtained b yaveraging l a b o r a t o r y s t r e n g t h r e s u l t swhich fequently have a very wide scatter.

On the basis o f the results p lot ted i nfig. 5 i t appears that the chain dotted line(given b y 7-s/F5 -= 0.8) could be used as areasonably conservative prel iminary d e -sign curve fo r London. Figure 5 can alsobe used as a check fo r design values o f7-, and values falling above the observedlimits should be used wi th caution.

Driven pi les: For bored pi les equation(9) appears t o g ive a reasonable upperlimit f o r shaf t f r ict ion. W h e n a p i l e i sdriven i n t o t h e ground t h e equil ibr iumhorizontal stresses adjacent t o t h e shaft

w i l l b e g rea te r t h a n t h e undis turbedvalues over most o f the length o f the piledue to compaction o f the ground. Henceequation ( 9 ) wou ld be expected t o givea lower limit for values of Fs.

Tomlinson (1971) has quoted t h e re -sults o f a number of tests on piles driveninto London Clay and the results for pilesgreater than 4 m i n length w e plotted infig. 6. I t can be seen that the scatter isvery large but the -ideal" curve does givea lower limit.

The reasons f o r t h e scat te r include:variable ground condit ions near the sur-face ( in particular the ground water level),overlying material being drawn down dur-ing installation thereby affecting the valueof 8 , and variations i n t he depth o f thegap o f ten found between p i l e a n d s o i lnear t h e g r o u n d su r f ace ( To m l i n s o n ,1971). These fac tors predominate n e a rground surface and it is perhaps significantthat t h e results i n f ig. 6 a re f o r muchshorter p i les than t h e equivalent boredpiles results i n f ig. 5 which s h o w muchless scatter. The " ideal " curve i s basedon the assumptions that the water tableis a t the surface of the clay and that 8 isequal t o the remoulded drained angle o ffriction f o r London Clay. Both these as-sumptions are likely to be on the conser-vative side.

Without attempting t o explain t he de-tailed behaviour o f dr iven p i les i n s t i f fclay the existence o f a theoretical lowerlimit t o shaf t f r ic t ion m a y prove usefulparticularly when results o f pile tests arevery variable.

Concluding remarksThe object o f th is paper has been t o

demonstrate t h a t m a n y o f t h e featuresof the behaviour o f piles i n c lay can beaccounted f o r b y adopting a simple ap -proach in terms of effective stresses. Theapproach has been particularly rewardingfor pi les i n so f t clays where i t has beendemonstrated that the rat io between theaverage shaft friction and the main effec-tive overburden pressure Ts/15 ( = /3) l iesbetween about 0.25 and 0 .4 irrespectiveof c l a y t ype . A n example i s g iven i nwhich t h e results o f p i le tes ts o n t w osoft c lays both g i ve values o f p 0.32 while the values o f o f o r the t w o claysare 0.6 and 1.6 respectively.

The approach can be used to estimatenegative f r ic t ion a n d i s essential ly t h esame as the method adopted b y Johan-nessen and Bjerrum (1965). On the basisof the limited amount of field data availableit appears that a value of /3 equal to about0.25 gives an upper l imit for negative skinfriction on piles in soft clay.

For s t i f f c lays t h e s i tuat ion i s m o r ecomplex a n d t h e main diff iculty l i e s i n

Ground Engineering 4 1

Jerome St-Michel

estimating the value o f the coefficient o fearth pressure a t r e s t l c a t va r iousdepths. I t i s t o b e hoped tha t direct i nsitu methods o f measuring t h e a t - res thorizontal effect ive pressures w i l l s o o nbe available (Wro th and Hughes, 1973).For London Clay values o f k have beenestimated from laboratory tests and usedto obtain the relationship between Fs andthe mean depth fo r an " ideal- pile, i.e. apile that i s installed w i thout altering thein situ effective stresses. Comparison withpile t e s t s appears t o conf i rm t h a t t h i sideal relationship gives an upper l imi t o f

for bored pi les and a l ower l im i t f o rdriving piles.

The s imple approach outl ined here i snot intended to replace the traditional em-pirical method o f estimating shaft friction,particularly in t he st i ffer materials. How-ever, i t may well be useful for preliminarydesign purposes o r a check particularlyin unusual or untried conditions. I ts mainpractical va lue might b e i n providing asimple model which enables the engineerto understand some o f t he fundamentalprinciples governing pile behaviour

From a research p o i n t o f v i e w t h emethod i s clearly t o o simple t o accountfor t h e detailed behaviour o f pi les. Th ismust await the results o f careful and ex-pensive research in to the distribution o fboth normal and shear stresses along theshafts of various types of pile for a varietyof ground conditions. The simple approachis sufficiently promising t o j us t i f y suchresearch.

For pi le tests t o b e o f greatest valuethe fol lowing points should b e borne i nmind:

1. Suff icient t ime should b e allowed be-fore test ing fo r the excess pore pres-sures s e t u p du r ing instal lat ion t odissipate.

2. T h e tes t should b e carried o u t suffi-ciently s l ow l y f o r drained condit ionsadjacent to the shaft to develop.

3. T h e position of the ground water tableshould be measured.

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

5 Add i t iona l information i n t he form o ftriaxial and oedometer tests on undis-turbed samples a n d tr iaxial tes ts o nremoulded samples i s desirable.

AcknowledgementThis paper i s published w i t h the per-

mission o f t h e Director o f t he BuildingResearel Establishment.

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Jerome St-Michel

Jerome St-Michel