Ge 260104

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Compressi on and t ensi on est s on dri ven pi l es i n chal k

N. B. HOBBS* and P. ROBINS?

A programme of test pil i ng w as ecent l y carri ed out by the GLC in connexion w i t h he Thames

Flood Prevent i on Scheme at Nort h Woolw i ch, London, compri si ng l oading and pull i ng tests on a

number of closed-end st eel t ube and H-secti on pi l es dri ven var yi ng di stances int o gravel and chal k.

The ground condit i ons, soft al l uvi um, gravel and chal k ar e descri bed. M aint ained load tests

immediately ol l ow ed by CRP t ests w ere carri ed out 5 to 40 days aft er dri vi ng, and ollowing a

fur t her i nt erv al of 24 t o 50 days the pi l es w ere subjected t o pul l i ng t ests. The ski n ri ct i on in the

chalk w as assessed by deducti ng rom t he measured pul l -out resi stance the ski n ri cti on i n gravel

and al l uvi um based on pull i ng t est s n pi l es terminat i ng in he gravel . I t w s possi bl e t o det ermi ne

t he end beari ng capaci t y of one steel t ube and t hree H- pi l es di rectl y by deducti ng t he measured

pull -out r esi stance from t he ul t imat e beari ng capaci t y i n di rect l oading. The rel at i onship, q,JN

in kN /m2, betw een the end resistance and t he SPT N val ue w as ound t o vary betw een 200 and 280

for t he H-pi l es, and t o have a val ue of 230 for t he t est on the steel t ube pi l e. The dynamic

resi st ance usi ng t he Hi l ey ormul a w as ound t o overestimat e the t otal st at i c ul t imat e resi st ance by

about 30%. The result s of t he compressi on t ests are compared w i t h hose obtai ned on simi l ar

t ypes of pi l e driv en i nto chalk at Chat ham, Erit h and New bury. Final l y t he perfo rmances of t he

pi l es are compared i n t erms of efi ciency and pri ce per t on of l oad carri ed.

Un programme d’essai de pi eux a re’cemment e’t e’ ntrepri s par l e GLC a propos du proj et de l a

prevent i on d’i nondat i on de l a Tami se, h Nort h Woolw i ch, Londres, comprenant des essai s de

chargement et de t ract i on sur di fl erent s pi eux creux en acier ermd h a poi nt e et de secti on H , qui

ont e’t e’ at t us a des distances diverses dans du gravi er ou du cal cair e. Les caracteri sti ques du sol ,

depot al l uvi onnair e t endre, gravi er et crai e sont d& ri t es. Des essais de chargement de l ongue

duree immedia t ement sui v i spar des essai s CRP, ont et 6 executes de 5 a 40 ours apres bat t age, et

d la suit e dun nouvel i nt erval l e de 24 a 50 ours, l espieux ont et& oumi s h des essai s de t ract i on.

Le rot t ement l at eral dans a craie a e’t t val ud part i r des essai s de racti on et e rot t ement l ateraldans Ie gravi er et l e depot al l uvi onnai re det ermi ne d part i r des essai s de tr acti on sur des pi eux se

t ermi nant dans e gravi er. I I a et epossi ble de det ermi ner di rectement l a orce portante depointe

d’un t ube en aci er et de t roi s pi eux H en dedui sant l a tract i on mesure’e de l a resistance, ul t ime

tot al e dans e chargement di rect. On a const at eque l a el at i on, qJN en kN lm2, ent re l a esistance

depoi nt e et a val eur N du SPT, var i e ent re 200 et 280pour l espieux H , et qu’el l e a une val eur de

230 pour l ’essai sur e pi eu h ube en aci er. On a const at equ’en calculant a esistance dynami que

en uti l i sant l a ormul e Hi l ey on surest imai t l a resi stance tot al e st ati que l imi te d’envi ron 30%.

Les resul t at s des essai s de chargement sont compares avec ceux obt enus sur des t ypes de pi eux

sembl abl es bat t us dans de la craie h Chatham, Eri t h et New bury . Final ement l es perf ormances

des pi eux sont comparees au poi nt de vue d’e& acit e’ et pri x par t onne de charge port & e.

* Soil Mechanics Limited.

t Department of Public Health Engineering, GLC.

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34 N B. HOBBS AND P ROBINS

loose to medium

Group I testfr

UBP254xZS4xSkg 1,2,3.5.12

3 5xllOkg 4

6.7

Fig. 1. Layout of test piles and boreboles

A series of loading tests on driven piles was carried out in 1974 by the Greater London Council

in connexion with the Thames Flood Prevention Scheme. The site selected for these tests, the

Harland and Wolff premises, lies on the north bank of the Thames close to Gallions Point,

North Woolwich, where the geology consists successively of made ground, alluvium, gravel

and chalk. The tests were carried out in three groups, group 1 comprising tests on piles

driven into gravel and chalk; group 2 tests on piles driven into gravel at a position about 300 m

NE of group 1; and group 3, tests on sheet piles. This Paper presents the results of the tests

on the piles of group 1 driven into chalk. In analysing the results, account has been taken ofthe shaft resistance data obtained from the tests on piles terminating in the gravel from groups

1 and 2.

The ground conditions at the site of the group 1 tests (Fig. 1) comprise varying thicknesses of

made ground (clay with clinker, gravel and brick fragments) overlying soft to firm peaty silty

clay, peat, Thames sand and gravel, and the Upper Chalk. Piezometers installed in the sand

and gravel in all four boreholes showed that the groundwater fluctuated with the water level in

the river (some few metres away), but within a smaller range. The range of groundwater

fluctuation is indicated in Fig. 1. The results of standard penetration tests in the sand and

gravel and chalk in all four boreholes are plotted against depth in the stratum in Fig. 2. .These

show the sand and gravel to be loose to medium dense with no clear trend of increasing relativedensity. The N values in the chalk do however appear to increase steadily with depth, the

chalk, weak at its surface, becoming moderately strong with depth, apart from BH 1 where low

N values persist to about 6 m depth in the chalk. Occasionally flints were also encountered.

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PILES IN WEAK ROCK 35

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Fig. 2. Standard penetration test results

TEST PILES

The test piles comprised 364 mm o.d. steel tube piles driven with a closed end (BSP cased

piles), and H-section universal bearing (UB) piles in two sizes, 254 x 254 mm x 85 kg/m and305 x 305 x 110 kg/m. The BSP piles were driven with a 2 t internal drop hammer with a 1 m

drop, and the UB piles with a DE40 Mackiernan Terry diesel hammer, with the exception of

pile 12, the last in the series, which was driven with a D12 Delmag diesel hammer. (Relevant

details of the two hammers are given in Table 3.) The piles were pitched into prebored holes

through the made ground to the depths shown in Fig. 1, following which the driving resistances

were continuously recorded as the number of blows to cause 300 mm penetration of the pile.

The sets for the final 900 mm penetration are given in Table 1.

The piles were driven in a line in the positions and to the depths shown in Fig. 1. Two of

the UB piles, 4 and 5, were extended and redriven after load testing and were subsequently

retested. An attempt was made to redrive the BSP pile, 6, but without success, the casingbuckling under the severe stresses caused by top driving.

PILE TESTING

Load was applied to the piles by jacking against a kentledge supported on cribbages set up on

either side of the line of test piles. These cribbages were so dimensioned that the pressure

applied to the ground prior to loading did not exceed 100 kN/m2. The pile heads were fitted

with flat horizontal steel plate cappings to accommodate the hydraulic jack and load cell.

The test programme comprised maintained load (ML) tests followed by constant rate of

penetration (CRP) tests at 2 mm/min. and then, after a long delay tension tests under a constant

rate of extraction (CRE) at 1 mm/min. In the ML tests the load was generally applied in200 kN increments and held until the rate of settlement fell below 0.1 mm in 20 min. Similar

requirements were observed when unloading, and on removal of the load, observations were

continued until all movement ceased.

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38 N. B. HOBBS AND P. ROBINS

Load : kN

600

- - - RP comprerrlo” tests

. . CRP fenlion tests

@ Interval between driving

or redriving and

testing days)

Pile 4 comprerslon tests

Load : kN

400 WO l2W 1600

1‘Bare rerlstance

Load : kN

200 400 600 800

Pile 4 a) redriven)

Compresrlon test

also rhowlng bare resistance curve

using the recqnd tenrion test)

Tension tests

6) (c)

Fig. 3. Piles 4 and 4a: compression and tension tests

TEST RESULTS

The load/settlement curves for the compression tests and load/displacement curves for the

tension tests are given in Figs 3-6, and the ultimate loads for the various types of test are set

out in Tables 1 and 2. The figures in circles on the curves refer to the interval in days between

driving, or redriving in the case of piles 4(a) and 5(a), and load testing. Where a CRP test

has the same number as the ML test the CRP test always followed the ML test. It will be

seen that in general fairly lengthy intervals elapsed between driving and testing.

A point of interest in these results is the marked difference in the shape of the load/settlement

curves between the ML and CRP tests carried out on the same day, the CRP tests exhibiting

considerably higher coefficients of reaction (load/settlement) and somewhat greater maximum

resistances than the ML tests. Discrepancies of this sort are commonly observed when ML

tests are followed by CRP tests on piles, in the London Clay for instance. They are not nearly

so marked as in the chalk, however, a circumstance due to the greater rate of consolidation in

chalk than in clay as well as to its more frictional nature, the former factor tending to steepen

the slope of the ML test curve and the latter factor tending to flatten the slope of the CRP curve.

The effects of work hardening have also lo be considered. In the analysis which follows it hasbeen necessary to use the results of both types of test in compression and CRE tests in extrac-

tion and the results should therefore be considered in the light of these discrepancies.

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Load : kN

- MaintaIned Icedcompression ests

- - -- CRP comprerslon tests

-- -- CRP tenslollests

Pile 5 compression ests

4

Load:k

Pile S a) redriven)

lb>

Compression tests also showing base

resistancecurve deduced rom

CRP test at 12 days and tension test)

Fig. 4. Piles 5 and Sa: compression and tension tests

Load : k

Load : kN

Compression tests

also showing base resistance

cuwes deduced for maintained

load test and CRP test)

Fig. 5. Pile 6: compression and tension tests

Tenrlon test

71 days after initial drive,

but only 33 after abortive

redrive attempt)

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40 N B HOBBS AND P ROBINS

ANALYSIS OF RESULTS

In order to analyse the results it is necessary to determine the contributions to shaft resistance

made by the alluvium, gravel and chalk. As no tension tests were made in the alluvium alone,

the undrained shear strength of the clay was taken as 30 of the effective overburden pressure,

and the effective angle of shearing resistance of the peat as 30” with k,,’ = 0.5. It was furtherassumed in view of the long delay between driving and testing, that the full strength of the

alluvium would be developed as adhesion. The average values of the adhesion arrived at in

this way are given column 3 of Table 2.

Two tension tests were carried out on adjacent piles in the gravel in group 2, a BSP pile and a

UB pile. By deducting the adhesion in the alluvium from the maximum uplift force the mean

values of frictional resistance in the gravel (column 5) were obtained. The values of k tan 6

(column 4) were derived by dividing the mean shaft resistances by the mean effective pressure

in the gravel. While the high value of 0*35 for k tan 6 can be readily understood for the BSP

pile, which has a large displacement volume, the exceptionally low value of 0.112 for the UB

pile can only be accounted for on the assumption of contamination of the gravel by clay drawndownwards into the gravel along the shaft of the pile. These values of tan 6 have been used

to assess the resistance from the gravel along the shafts of the piles in group 1.

Estimates of the unit shaft resistance in the chalk based on the results of the BSP pile 6 and

on the UB piles 4a, 5a and 12, after deducting the assessed adhesion in the alluvium and friction

in the gravel, are given in column 6 of Table 2. The basis for calculating these values is shown

in the table. These results, it must be acknowledged, are confusing. The BSP pile, 6, has

developed only 60 of the unit resistance in chalk of the adjacent UB pile, 4a, a circumstance

which cannot be wholly explained by the greater penetration of the latter pile, particularly in

view of the fact that the N values in the nearby borehole IA appear to be relatively constant

within the depth range concerned. Furthermore, among the three deep UB piles themselvesthere appears to be a falI-off of mean shaft resistance with depth against the trend of increasing

iV value. A possible explanation of these low and apparently anomalous values may lie in

the effects of damage to the chalk, a large displacement pile being more destructive than a UB

pile, and stronger chalk being more susceptible to damage than weaker chalk. Whatever the

reason, these values are considerably lower than those customarily assumed for piles in chalk.

Indeed, if the low deduced resistance of the UB piles in the gravel were to be discounted and

recalculated on the basis of earth pressure at rest conditions in the gravel, the resistance in the

chalk would be found to be zero, which is clearly not possible. It is advisable to approach the

question of skin friction of piles driven into chalk with caution, particularly when the chalk is

strong and the possibility of damage high.The directly derived values of the ultimate end bearing capacity for the piles subjected to

both compression and tension tests are given in italics in column 9 of Table 2, the end bearing

area of the UB piles being taken as the bounding area of the pile section and not the area of

the steel alone. The ultimate end bearing capacities of UB piles 4 and 5 have been derived

using the deduced values of adhesion and friction. The ultimate loads used in all these

calculations have been taken as the greater of the ML value at a settlement of 10 of the pile

size and 90 of the maximum CRP load. The ultimate bearing capacities, apart from piles 4

and 6, are remarkably high and generally accord with the standard penetration test N values,

and on this basis the bearing capacity of the chalk was found to be about half that of the gravel l.

An exceptionally high value of the ratioq,JN,

(column 11) was obtained for UB pile 5, but thiscould be due to the very low value of friction of 8 kN/m2, assumed for the chalk on the basis of

1 The tests in the gravel, it is hoped, will be presented in a later paper.

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42 N B HOBBS AND P ROBI NS

chalk alone, and is considered to be due to the very different end actions between a simple

ended BSP pile and a complex ended UB pile, where the support is given not only by the steel

section but also by the compacted chalk lodged within the troughs of the section. Owing to

its greater rigidity the steel will exert higher stresses on the chalk beneath the point than will

the chalk enclosed within the troughs of the UB piles and will tend to cut into the chalk, thus re-sulting in a higher base settlement and a lower modulus.

These very crude modulus results are plotted in Fig. 7 against N values after Wakeling

(1970) and Hobbs (1974), and lie well above the modulus results reported by Lake and Simons

(1970) for small diameter plate tests in deep boreholes, and for various other plate test results

presented by Kee and Clapham (1971). The exceptionally high modulus value for the chalk

beneath BSP pile 6, is not understood and should be disregarded. That the modulus values

for the Woolwich test piles are not higher is no doubt due to the damage done to the chalk

immediately beneath the pile bases by the driving energy.

If the apparently low modulus values derived for the UB piles are, in fact, due to the complex

toe action with the steel section imposing high stresses on the chalk, then it will be necessaryto consider the effects of creep on the long-term performance of such piles. At present this

possibility can best be dealt with by using conservative working loads in those cases where only

small settlements can be tolerated.

DYNAMIC RESISTANCE

Blow diagrams were taken during the setting of UB piles 5 (a) and 12, from which the

temporary compressions and sets were obtained. These measurements plus an allowance of

1.3 mm for the temporary compression of the driving head were introduced into the Hiley

formula to give values of R,, the ultimate dynamic resistance. These results, together with

the static resistances, are given in Table 3, from which it will be seen that the Hiley formulaover-estimates the static bearing capacity, the discrepancy depending on the definition of the

static bearing capacity and also on the magnitude of the measured temporary compression.

It seems from these tests that there is better agreement between the dynamic and static resist-

ance when the temporary compression is small in relation to the set, that is to say when the

driving energy is usefully spent. A high temporary compression, however, compared with the

set implies a high resistance t driving provided the pile is not excessively long. It is worth

recording that a 510 mm BSP cased pile at Newbury, driven easily in chalk to 17 m depth

where resistance was suddenly encountered with the temporary compression more than three

times the set, had a calculated Hiley bearing capacity of 2400 kN. The static ultimate resist-

ance determined by a maintained load test after a delay of only 5 days was 2350 kN. (Theratio of qu/N for this pile worked out at 250 with N= 30). The Hiley formula appears to be

well suited to the estimation of static resistance in those cases where the shaft resistance com-

ponent of the total resistance is small and when the temporary compression at set is measured

and not assumed. An indication of the relative importance of the shaft resistance can be

obtained from the driving log.

The lack of agreement between dynamic and static bearing capacity for the piles at Woolwich

may be due not only to the relatively high proportion of shaft to total resistance, but also to the

type of pile, in which during driving intense stresses are communicated by the steel point in

direct contact with the chalk, whereas in static bearing the steel is assisted by the compacted

chalk lodged in the troughs. Notwithstanding this, the customary factor of safety of 3 on thedynamic resistances would have resulted in adequately safe piles.

Cornfield (1971) has suggested that the empirical expression R= WH) 51 (044-s), where

WH is the gross hammer energy in ft tons and s is the final set per blow in inches, may be used

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PIL ES IN WEAK ROCK 43

IO0 8 IO 15 20 25 30 35 40 45 50 55 60

Chalk grade and SPT S value

Fig. 7 Deduced field modulus against chalk grade and degree of settlement

as a guide to the ultimate static resistance of steel bearing piles. On this basis the ultimate

static bearing capacities are 1030 and 720 kN for piles 5a and 12 respectively compared with,

say, 1190 and 1100. Applying a factor of safety of 3 to the values deduced by Cornfield’s

expression would result in over-conservative safe working loads.

OVERALL PILE PERFORMANCE

The contributions of the shaft and end resistances to the ultimate bearing capacity of the

piles are set out in columns 12-20 of Table 2. It will be seen that while there is a markeddifference between the ultimate bearing capacities of the two shallow piles, 4 and 6, compared

with the remaining deeper piles, there appears to be virtually no increase in bearing capacity

with depth at the greater penetrations of the deeper small UB piles. The 305 x 305 mm UB

pile has a greater bearing capacity than the 254 x 254 mm piles, the increase being entirely due

to the shaft resistance, since the end bearing capacity of this pile is little different from that of

the smaller piles. The ratios of shaft resistance to total ultimate load are given in columns

18-20. These results indicate, contrary to expectation, that the shaft resistance plays a smaller

part in the longer thinner piles than in the shorter thicker piles. As far as the UB piles are

concerned this is no doubt due not only to the lower pull-out resistances of the deeper UB

piles, 5a and 12, but also to the steady increase of N value with depth which is reflected in thehigher end bearing resistances of the deeper piles. The shallower pile 5, however, appears to

have the highest end bearing resistance of all the piles tested; but it should be pointed out

that this pile was not subjected to a direct pulling test, the shaft resistance used in arriving at

the end resistance being assessed and not measured. The value given for the end bearing

resistance of this pile should be regarded with caution.

The lower pull-out resistances of the longer piles could be due to the problems of getting

true alignment when splicing on new lengths of pile. On resuming driving, considerable

eccentricity can occur within the section of the pile below ground causing whip of the pile and

trumpeting and loosening of the ground; certainly, the performance of UB pile 5 was not

improved by its extension. Such an occurrence makes a lottery of the estimation of the shaftresistance in any lower strata by the process of deduction using the results of tests on shorter

unspliced piles to determine the shaft resistance of the upper strata. Clearly if this behaviour

is typical of long slender spliced UB piles these tests will have served a useful purpose in

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44 N B HOBBS AND P ROBINS

demonstrating that pull-out resistance does not necessarily increase pari passu with increasing

depth, but may even diminish. To get the best results with long piles in chalk it seems that

certain practical problems have to be overcome, and certainly the straighter and stiffer the

piles the better the results are likely to be. It should not be assumed from these observations

that low shaft resistances will only result with spliced piles: very long slender piles driven inone length are also likely to have low shaft resistances. The high dynamic resistances of the

UB piles may well be due to the above cause, in that energy lost in the lateral vibration of

the pile due to lack of alignment is not taken into account in the Hiley formula. This being so,

the formula will inevitably lead to over-estimates of the static resistance.

Meigh (197lb) compared the performance of a variety of piles by means of the settlement

at a load on the pile equivalent to 5.15 MN/m2 irrespective of the type of pile, the bounded

area being used in the case of UB piles. The results of such a procedure are given in Table 4,

based on the ML test curves, and show quite clearly the superiority of shorter, larger piles over

longer, thinner ones. Apart from UB piles 4 and 5, the settlements at a working stress of

5 MN/m2 are all comparable. The working load on pile 5 at the same settlement as the otherpiles is also given in the table, and this corresponds to a working stress of 9.2 MN/m2, leading

to a considerably higher efficiency. Of all the piles in this group, only the BSP cased pile 6, is

being worked at maximum efficiency, that is to say the applied working load is the maximum

possible from considerations of the allowable stress on the material of the shaft. The maxi-

mum possible working loads on the 305 mm and 254 mm UB piles in mild steel are 1700 and

1300 kN respectively, with corresponding efficiencies of 27.5 and 23 . The results presented

by Meigh (197lb) for BSP cased piles at Chatham and a 16 in. x 12 in. x 100 lb UB pile at

Erith are also given in Table 4, from which it will be seen that the performances of the BSP

piles are comparable, with the Woolwich BSP pile having a somewhat better coefficient of

reaction QJs at a smaller penetration into the chalk than the smaller Chatham pile. If pile 4,which penetrated the chalk at Woolwich for only 3.3 m, is ignored, the 305 mm UB pile 4a

Table 4. Comparison of pile performances and prices

Pile No.

-

Type

4, UB6, BSP5 UB

4a UB12 UB5a UB

::;7.8

1i.z12.0

47052030055ot470300300

-

Chatham At 48 ton/fta (5.15 MN/m2)

BSP 16 in.BSP 20 in.

Erith

HlZxl6in.

--

-6.1 640 8.4 76 26.5 Williams (1971)

Penetrationinto chalk,

m load

E?

At 5 MN/ma stress on end area of the pile

Settlement

s,m

Qa pile efficiency-9

kN;mm c Qa

-- -_

-.

-.

Price

perkN

carried,f

0.870.67l-430.781.161.551.68

Price

perpile,

L

415350430430545465505

Reference

Meigh (1971b)

* Q,,, is the maximum working capacity of the pile shaft.t Ptle 5 with the load at 550 kN to give comparable settlement with the other piles.

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Estimated price per pile : Lsterling

I

IS-

20.

E.

5tn 2s’

30.

--y--x- UBP305x305xllOk~--_-

Fig 8 Prices of various piles

appears to have a comparable performance and efficiency with the Erith pile, which was

driven to a somewhat smaller penetration into the chalk. Apart from the very long slender

spliced piles the performances of the piles in the Woolwich test series do not appear to be very

different from the performance of similar types of pile further down the Thames.

The performance of a pile must also be judged in relation to its price. Curves of estimated

price (including on-costs and margin) against length of pile are given in Fig. 8 for the three

types of pile used at Woolwich. The prices for the test piles, regarded as working piles driven

on land in a large scheme, are given in Table 4 together with the price per ton carried. Havingregard to the coefficient of reaction QJs, the efficiency and the price, it is clear that the BSP

pile is superior to the UB piles, and that long slender spliced piles UB piles are not economical

in comparison, although their low displacement may be the governing reason for their selection.

The conclusion that the UB piles develop considerable end bearing resistance suggests a

device which could increase this capacity by up to 200 ; that is, to weld on short wings of the

same section as the pile to both column flanges, thus effectively trebling the end area. The

loss of shaft resistance will be of little account in these circumstances, but can be recovered, at

additional cost, by making the wing sections long enough to provide such shaft resistance as is

necessary. By such a device the effects of poor alignment during splicing are eliminated. If

necessary the gap around the shaft left by the wing sections may be grouted up or filled withsand. Trebling of the end bearing capacity of the UB piles at Woolwich would not only make

the steel piles almost as efficient as the BSP pile, but more economical in price per ton carried.

The coefficient of pile reaction would however be lower.

CONCLUSIONS

From the results of the Woolwich and other pile tests in chalk the following conchtsions

may be drawn :

(a) The ultimate end bearing capacity of universal bearing columns and displacement

type piles in chalk may be estimated from standard penetration tests.

(b) The ultimate resistance of displacement piles may also be estimated by means of theHiley formula, provided the shaft resistance is not predominant and the temporary

compression is measured when taking the set. This method is not particularly accurate

with very long slender UB piles.

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46 N. B. HOBBS AND P. ROBINS

c) The shaft resistance of long slender UB piles in chalk cannot be reliably forecast.

(d) Shorter stouter piles are likely to be more effective and more economical than longer

thinner piles, and in general displacement type piles are to be preferred to UB piles,

although it is recognized that there may be special circumstances when UB piles are

essential.

ACKNOWLEDGEMENTS

Acknowledgement is made to the Greater London Council and to Mr N. D. Ayres, Director

of Public Health Engineering, for the use of the material included in this Paper and the permis-

sion to publish it. The tests were carried out by Soil Mechanics Limited.

REFERENCES

Cornfield, G. M. (1971). Steel bearingpiles. London: Constrado.Hobbs, N. B. (1974). Factors affecting the prediction of settlement of structures on rock. Proc. Conf.

Settlement of Structur es. Cambridge: Pentech Press.

Lake, L. M. & Simons, N. E. (1970). Investigations into the engineering properties of chalk at Welford Theale,Berkshire. Proc. Conf. I n situ investigations in soils and rocks. London: British Geotechnical Society.

Meigh, A. C. (1971a). Some driving and loading tests on piles in chalk. Proc. Conf. Behaviour of Pil es.

London: Institution of Civil Engineers.Meigh, A. C. (1971b). Discussion: Some driving and loading tests on piles in chalk. Proc. Co Behaviour

of Pi les. London: Institution of Civil Engineers.

Wakeling, T. R. M. (1970). A comparison of the results of standard site investigation methods with the resultsof a detailed geotechnical investigation in Middle Chalk at Mundford, Norfolk. Proc. Conf. In situ investi-

gations in soil s and rocks. London: British Geotechnical Society.Wiliams, J. A. Discussion: Some driving and loading tests on piles in chalk. Proc. Conf. Behaviour of Piles.

London: Institution of Civil Engineers.

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