Bearing Capacity of Soils
Table 8.3
337
Presumed safe bearing c<pacity, q" values (based on SS 8004: 1986l.
Rocks
(Values based on assumption that fo undation is carried down to unweathered rock)
Hard igneous and gneissic
Hard sandstones and limestones
Schists and slates
Hard shale and mudstones, soft sa~ dstone
Soft shales and mudstones
Hard chalk, soft limestone
10000
4000
3000
2000
1000- 600
600
Cohesionless soils
(Values to be halved if soil submerge:Jl
Compact gravel, sand and gravel
Medium dense gravel, or sand anc gravel
loose gravel, or sand and gravel
Compact sand
Medium dense sand
loose sand
>600
600- 200
<200
>300
300 - 100
<100
Cohesive soils
(Susceptible 10 long-Ierm consolidat on settlement)
Very stiffboul~ c:tays and hard days
Stiff clays
Firm clays
Soft clays and silts
Very soft days and silts
Tiilile 8.4
Undrained shear strength of cohesive soils.
Consistency
cu:kPa)
Field behaviour
Hard
>300
300- 150
, 50-75
7) - 40
40 -20
Brittle
Brittle or very tough
Cannot be moulded in fingers
Ca n just be mou lded in fingers
Easily moulded in fingers
Exudes between fingers if squeezed
Vfl.ry sbff
Stifi
Firm
Soft
Very soft
8.10
600 - 300
300-150
, 50-75
<75
Not applicable
<20
Pile foundations
The use of sheet pil ing, which can be of tim ber, concrete o r steel, fo r earth retaining
slructures has been descri bed ir. Chapter 7. Piled foundations fonn a separate category
and are generall y used:
338
Smith's Elements o£Soil Mechcnics
(i)
(ii)
(iii)
(iv)
8.10.1
10 transmit a foundation load 10 a solid soil st/atum;
10 support a foundation by friction oflhe pileI against the soil;
to resist a horizontal or upliflload;
to compact a loose layer of granular soil.
Classification of piles
Piles can be classified by different criteria, such as :he ir material (e.g. concrete, steel,
limber), their method of install ation (e.g. driven or bored), the degree of soil di splacement during installation , or their size (e.g. large diameter, small diamete r). However,
in term s of pil e de~ i gn, the mos.t appropriate ciassif.cation criteria is the beha viour of
the pile once in stalled (e.g. e nd bearing pile, frictioo pile, combination pile).
End bearing (Fig. 8. 16a)
Derive most of Ihei r carry ing capacity from the penetration resistance of the so il at
the toe of the pile. The pile behaves as an ordinary column and should be designed as
such except that, even in weak so il , a pile will not fail by buckling and thi s effect need
on ly be considered if part of the pil e is unsupported, i.e. it is in e ithe r air or water.
Friction (Fig. 8. 16b)
Carrying capacity is deri ved mainl y from the adhesi)n or fri ction of the soil in contact
with the shaft of the pile.
Combination (Fig. 8. 16c)
Rea lly an extens ion of the end bearing pile when the bearing stratum is not hard ,
such as a firm clay. The pile is drive n far enou gh into the lowe r malerialto develop
adequate fri ctional resistance. A further variation 01 the end bearing pile is piles with
e nlarged bearing areas. Thi s is achieved by forcin~ a bulb of conc rete into the soft
stratum immediately above the firm layer to give anenlarged base. A similar effect is
SoO
500
500
Firm
Hard :" .' ....
(a) End bearing
Fig. 8.1 (,
Classification of piles.
(b) Friction
(c) Combination
l earing Cap.1city o f Soils
Hammer
339
Runners
Pile __
fig.8.17
Pile driving rig.
produced with bored piles b} fonning a large cone or bell at Ihe bottom with a spec ial
Teaming tool.
8.10.2
Driven piles
These aTe prefabricated piles that are installed into the grou nd through the use of
a pile driver as illu strated ir Fig. 8.17. The pi le is hoisted into position on the pi le
driver and aligned against t"le runners so that Ihe pile is drive n inlo the ground at
exactly the requ ired angle, b exact ly lhe required depth . The pi le is driven into the
soi l by striking the top of the pile repeated ly with a pneumatic or percussive hammer
or by driving the pile dowr using a hydraulic ram. Most commonl y the piles are
made from precasl concrele 3lthough timber and steel piles are also ava ilable.
Precast concrete
These are usuall y of square ar octagonal seclion. Reinforcement is necessary within
the pile to he lp wi thstand b:::)th handling and driving stresses. Prestressed concrete
piles are also used and are becoming more popular than ordinary precast as less
reinforcement is required.
Timber
Timber piles have been us=:d from earl iest recorded times and are still used for
permanent work where limter is plentiful. in the UK,lirnber piles are used mainly in
temporary works, due to their lightness and shoc k resistance, but they are also used
for piers and fenders and C!ln have a useful li fe of some 25 years or more if kept
Smith's Elemellts of Soil M echinic5
340
complete ly be low the water tabl e. However, they can dete ri orate rapidly if used in
g round in whi ch the water leve l varies and allovs the upper part to come above
the water surface. Pressure creosoting is the us uaJ method of protection. In tropical
cl imes timber piles above groundwater le ve l are liable to be destroyed by woodeating in sects, some times in a matte r of weeks.
Steel piles: tubular, box or H-section
These are suitable for handling and driving in l on~ lengths. They have a relatively
small cross-section al area and pene tratio n is easier than with other types. The risk
from corrosion is not as great as one might think although tar coating or cathodic
protection can be employed in penn anenl work.
Jetted pile
When dri ving piles in non-cohes ive soil s the perelration res istance can oflen be
considerably reduced by jetting a stream of hi gh-rressured water into the soil just
belo w the pile. There have bee n cases where piles have been installed by jelling
alone. The method requires conside rab le experi Ence, partic ularl y whe n near to
ex isting foundations.
Vibrated pile
As an aitemati ve 10 jetting, vi bration techniques can be used to place piles in granular
soil s. Vibralo rs are not effic ient in clays but can be used if pi les are to be extracted.
Ja cked pile
Generally built up with a series of short sections of precast concre te, this pil e is
jac ked into Ihe ground and progressively inc reased in length by Ihe addition of a pile
sec tion whenever space becomes av ailable . The jacking force is eai' ily measured and
the load to pi le penetration relationship can be obtain.ed as jacking proceeds. Jacked
piles are often used 10 underpin ex isting struc tures .... he re lack of spact: excludes the
lI se of pil e driving hammers.
Screw pile
A scre w pile consists of a steel, or concrete, c ylinde l with helica l blades attached to
il s lower e nd. The pile is made to screw down into tiC soil by rotating the cy linder
with a capstan at the top of thc pitc. ;\ sc rew pile, {he to the large size of its scre w
blades, can offe r large uplift res istance.
8.10.3
Driven alld cas/-ill-place piles
T wo of the main types of this pile, used in Britain, aredescril>ed be low.
leafing Capacity of Soils
,
Reinforce:l
shell
,
'
~
Concrete toe
(1)
Fig. 8.18
(2)
(3)
341
(1) R.C . shells threaded on
mandrel and set in
position
(2) Pile driven 10 req'd sel
(3) Mandrel withdrawn , spare
shells removed and core
reinforcement placed
(4) Core concrete inserted
(4)
West's shell pile.
West's shell pile
Precast, re inforced concrete tubes, about 1 m long. are threaded on to a steel mandrel
and dri ven into the ground (fter a concrete shoe has been placed at the front of the
shells. Once the shells have been driven to spec ifi cation the mandrel is withdrawn
and rei nforced concrete ins(rled in the core . Diameters vary from 325 to 600 mm.
Details of the pile and the method of installation are shown in Fig. 8. 18.
Franki pile
A steel tube is erected verti:ally over the place where the pile is to be driven, and
about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500
to 4000 kg mass, compacts fte aggregate into a solid plug which then penetrates the
soil and takes the steel tubedown with it. When the required set has been achieved
the tube is raised slightl y am the aggregate broken out. Dry concrete is now added
and hammered until a bu lb is fonned. Reinforcement is placed in position and more
dry ~on c rete is placed and rammed until the pile top comes up 10 grou nd level.
The sequence of operations 5 illu strated in Fig. 8.19.
8. 10.4
Bored and casl-in-!ilu piles
'fhese piles are fomled wilhil a drilled borehole. During the dril ling process Ihe sides
of the lx>rehole are supportedto prevent the soi l from collapsing inwards and temporary
section s of stee l cylindrical casing are advanced along wi th the drilling process to
provide thi s required su ppO"t. As the drilling progresses, the soil is removed from
with in the casing and brougll to the surface. Once the full depth of the borehole has
been reached, the cas ing is graduall y withdrawn, the re inforcement cage is placed
Smith's Elements of Soil Mecha,ics
342
(1) Gravel plug compacted
(2) Req'd set obtained
(3) Plug broken out and
concrete bulb formed
(4) Reinforcement placed
(5) Tube withdrawn and
concrete placed
r ·)
(1)
Fig.8.19
(2)
8( •. ')
(3)
(4)
(5)
Installation of a Franki pile.
and the concrete which forms the pile is pumped nto the boreholc. Fur very deep
boreholes the instal lati on of many sections of tempJrary casing can be an ex pen sive
and s low process, and an alternative means of suppocing the sides is through the use of
a ben tonite slurry in the same manne r as for a diaph-agm wall (see Section 7.3.2).
An alternative technique which does not use borelnle s ide-support is the continuous
flight auger (CFA) pile. With this technique a contouous fl ight auger with a hollow
stem is used to create the borehole. The si des of the t orehole are supported by the soil
on [he fli ght s of the auge r and so no cas in g is requi,ed. Once the requhed depth has
been reached , the concrete is pumped down the hollow ste m and the auger is steadily
withdrawn. The steel rei nforcement is placed once he auger is clear of the borehole.
8.10.5
Large diameter bored piles
The drive n or bored and cast-in-place pi les disc1l3sed previously genera ll y have
ma ximum diameters in the order of 0.6 m and are ca pable of working loads round
about 2 MN. With modern buildings column l oad ~ in the order of 20 MN are not
uncom mon. A co lumn carrying such a load wou ld m ed about ten convent ional piles,
placed in a group and capped by a conc rete slab, pro::labl y some 25 m2 in area.
A consequence of this problem has been Ihe increasi ng use of the large diameter
bored pile. This pile has a minimum shafr diamele:- of 0.75 m and may be underreamed to give a large r beari ng area if necessary. S. ch a pile is capable of working
loads in the order of 25 MN and , ifwken down throLgh the so rt to the ha rd material ,
will minimi se sett leme nt problems so Ihat only onc_<;uch pile is required to support
eac h column of the building. Large diameter bon cl piles have been installed in
de pths dow n to 60 m.
8. 10.6
Determination of the bearing capacitylOf a pile by load tests
The load test is the only reaJly reliable mea ns of determinin g a pile's load capacity,
but it is ex pensive, parti ou~a rJ y if the groUl~.d is vari .:lJle and a large number of piles
mu stlhere fo rc be tested.
·
Bearing Capacity
or Soils
.
343
e ntl ed g ~
~
Support C(a)
~
-
Jack
CC- Su pport
(b)
Load
.an
Pile
under
test
Jack
Sa nd s
Clays
(c)
fig. 8.20
(d)
(a)- (c) Methods for testing a pile. (d) Load to settlement relationship.
Full-se.lie pi les shou ld re used-and Ihese should be driven in the same manner as
those placed fo r the pennanenl work.
Fig ure 8.20 gives rough Ind ications of how a test pile may be loaded. A large mass
of dead we ight is placed 01 a platfoml suppo rted by the pile. The load is applied in
inc re ments and the settlement is recorded when the rate of settlement has reduced 10
0.25 mm in an hour, at which stage a furthe r increment can be app li ed (Fi g. 8.203).
The method has the disadvantage that the pl atfonn must be balanced on top of tile
pile and there is always the risk of collapse. An alternat ive, and better, technique is to
jack the pil e aga inst a ke n t l ~d ge using an arrangement similar to Fig. 8.20b.
Sometimes the pi les to re used permanently can be used to test a pile as shown in
Fig.8.2Oc.
The form of load 10 se t1 l ~ men t relalionship o btained from a load in g test is shown
in Fig. 8.20<1 . Loading is <ont inued until fa ilure occurs, except for large diameter
bored piles which, having a working load of some 25 MN , wou ld require mass ive
kentledges if fai lu re loads '.Vere 10 be achieved. General practice has become to test
load these piles to the working load plus 50 per cenl.
Design standards offer mOle Jilllilt!d guidance on stalic.: toad pile test methods.
BS 8004 specifies two I ypc:of t~s l , described below, from which the ult imate load of
a pile can be obtained, and furocode 7 (sce Sec tion 8. 1! ) makes reference to the ASTM
suggested method for the lx ial pi le loading lest, desc ri bed by Smoltczyk (1985) .
Furthermo re , it is likely thGtthe fort hcom ing European standard for pile test ing wilt
adopt the recomme ndatiom and procedures d escr ih~d by De Coc k et (1/. (2003).
(1) The maintained )oa(( test
Here the load is applied 10 the pile in a series of increments, usuall y equal to 25 per
cent of the designated working load for the pile. The ultimate pi le load is taken 10 be
the load that achieves some specified amount of settlement , usua ll y 10 per cent of the
pile's diame ter.
•
Smith's Elements of Soil MechaniG
344
(2) The constant rate or penetration test
In [his test the pi le is jacked downwards at a comtant rate o f penetration. The
ultimate pil e load is considered to be the load at w hic h e ither a shear failure
lakes pl ace within the soi l or the penetration of the pile equal s 10 per cent of its
diameter.
The fi gure of onc tenth is intended for norm al sized piles and , if appli ed to large
diameter bo red piles, could lead to excess ive sertlernmts if a facto r of safety of 2.5
were adopted. Thi s. of course, only applies to Jarge di amctcr piles resting on soft
rocks. fn the case o fa large di ameter bored pile restin gon h;ud rock the ultim ate load
depends upo n the uhim ate SLress in the concretc.
8.10.7
Determination of the bearing capacity of a pile by soil
mechanics
A pile is suppo rted in the soil by the res istance of the 10{: to f Ulher penetration plus
the fri ctional o r adhesive fo rces along its embedded lelglh .
Ultim ate bearing ca pac ity = Ultim ate base res i s l anc~ + Uhimate skin frict io n:
Q,= Q.+ Q,
Cohesive soils
Q b for pil es in cohesive soi ls is based on Meyerho f's eq.tatio n ( 195 1):
w here
Ne = bearing ca pac it y factor, widely accepted as equal 10 9.0
Cb = undi srurbed undrained shear strength of the so il at base of pile.
Os is g iven by the equ ation:
w here
(l'= adhesio n facto r
(\ = a vemge undisturbed undrained shear strength of soil adjo in ing pile
A, = s urface are a of e mbedded length of pile.
Hence
8eair.g Capaciry of Soils
The adhesion factor
345
a
Most of the bearing capaci ty ofl pile in cohesive soi l is derived from its shaft resistance, and the problem of deteminin g the ultimate load resolves into determining
a va lue fo r a. Fo r soft clays acan be equal to or greater th an 1.0 as, after driving,
soft clays tend to increase in stnngth. In overconsolidated clays a has bee n fo und to
vary from 0.3 to 0.6. The usual'lalue assumed for London clay was, for many years,
taken as 0.45 but more recentl y a value of 0.6 fo r this type of soil has become more
accepted.
Cohesio n less soils
The ul timate load of a pile inst.. lled in cohesion less so il is estimated usi ng on ly the
value of the drained parameter, if, and assuming that any contribution due to c' is zero.
where
0": = the effec tive overbur:len pressure at the base of the pile
Nq = the bearing capacity we ffi cient
Ab = the area of the pile bzse.
The se lection of a suitable value for Nq is obv iously a cruc ial part of the design of
the pi le. The values suggested by Berezantzev et af. (196 1) are often used and are
reproduced in Fig, 8.2 1. Note ttat the full value ofNq is used as it is assu med that the
we ight of soil removed or disp llced is equa l to the weight of the pi le that rep laced it.
1000
z, •
600
/
0
13
.!'!
300
/
~
·0
re
~
re
100
/
0
'"
•
/
0
';ij
Ol
30
V
10
25
I I
I I
I I I I
30
I
35
I I
I
I I I I
40
Argte of shearing resistance (degrees)
Fig. 8. 21
Bearing capacity factor Nq (after Berezantzev et al., 1961).
45
Smith's Elements o(5oil Mechanics
346
where
fs = average value of the ultimate skin frict ion over the e m bedded length o f the
pile
As = surface area o f e mbedded length of pile.
Meyerhof ( 1959) s uggested that for the average va lue d the ultimate skin friction:
where
Ks = the coeffic ient o f lateral earth pressure
a: = average effecti ve overburden press ure actin&: along the embedded length
of the pi le shaft
() = angle of frict ion between the p il e and the so il
Hence
and
T ypical va lues for () and Ks were de ri ved by Brorv.s (1966), and are li sted in
Tab le 8.5.
Vesic ( 1973) pointed oul that the value of qb' i.c. a ;Nq• does not increase inde·
fi nite ly but has a limi ting va lue at <t depth of some 20 times the pile dia meter. T here is
therefore a maxim um va lue o f a~Nq that can be used in ihe calcu lations for Qb'
In a s imilar manner there is a li miting va lue that can be .Jsed for the average ultimate
s kin fr iction , fs' This maximum va lue o f fs occ urs whea the pile has an embedded
Table 8.5
Typical values for !Sand Ks suggested b y 8roms ( 1966).
K,
Pile material
Relat vedensity of soil
Steel
20 0
Concrete
U.75tf"
Timber
0.670'
Loose
Dense
0.5
1.0
1.5
1.0
2.0
4.0
•
Bearir...g Capaciry of Soils
34 7
length betwee n 10 and 20 pile d j_meters. Ves ic ( 1970) s uggested that the max imum
value of the average ultimate s kie res istance shou ld be obtained from the form ul a:
f, = 0. 08( 1O)'5(D.)·
0:
whe re Or = the re lati ve de ns ity
the cohes ion less soil .
In practice fs is o fl cn take n as _00 kPa if the fo rmula g ives a greater value.
Unlike piles e mbedded in coles ivc soils, the c nd rcs istances of piles in cohes ionless soil!> are o f cons ide rable s ig.ificance and shorl piles are therefore more efficient
in cohesio nless soils.
8. 10.8 Determination of soil ,..iling parameters from in situ tests
With cohes io n less soils it is pos9ible lo make reasonable estimates of the values of qb
and fs fro m in silll penetration tes:s. Meyerhof (1976) suggests the foll owing fo rmul ae
to be used in conjunction with (oc standard pene tration test.
Driven piles
Sands and grave l
qb =
40ND
8
5 400N (kPa)
Non-plas tic s ilt s
'Ib =
40ND
8
5 300N (kPa)
Boredpiles
Any type o f granul ar soil
Large dia mete r d ri ven piles
A verage diameter driven Ji,les
Bo red piles
14ND
qb= - - kPa
8
f,= 2N kPa
fs "" N kPa
f, =0 .67N kPa
where
N=
N=
0 =
B=
the uncorrected blow count at the pile base
the average llIlcorrecned N value over the e m bedded length of the pile
em bedded le ngth oftae pile in the end bear ing stratum
width . o r di ameter, of pile.
An alternati ve me thod is to ase the result s of the Dutch cone test. T ypical results
from such a test are s hown in F~ . 8.22 and are given in the fo nn of a pl ot show ing the
variation of the cone penetratiCIls res istance w ith de pth .
For the ultimate base res is laoce, Cr. the cone resistance is take n as bei ng the average va lue o f C r ove r the de pth Ld as shown, where d = diameter o f shaft. Then:
Qb = C,Ab
•
Smith's f/emt.·nts of Soil Mechancs
348
C,(kPa)
Estimated
deptll 01pile
,S
£
<>
•
o
Fig.8.22
Typi cal results from a Dutch cone test.
The ultimate skin fri ction, fs' c~n be obtained from oneof the fol lowing:
f
,
=
C, kPa
200
rOI
driven piles in dense sand
f = C' kPa
for d riven piles in loose sand
f = C, kPa
for driven pi les in non ~ p la st i c silts
,
,
400
150
where C, = average cone resistance along the e mbedded le ngth of the pile (De Bee r,
1963).
T he n Q~ ::.: fsAs and , as before, Q u = Q b + Qs'
ExampleB.9
A 5 m thick layer of medium sand overlies a deep deposit of dense g ravel. A
seri.es of l'tandard ptlldf3l i o n tests carneJ ou t througl.lhe depth of the sand has
establ ished that the average blow count , N, is 22. Further tests show that the
grave l has a stand ard penetration value of N -:::: 40 in the region of the inte rface
with the sand. A precast pile of square section 0.25 1<. 0.25 m 2 is to be driven
down through the sand and to penetrate s ufficiently into the gravel (0 g ive good
end beari ng.
Adopting a safety factor of 3.0 determine the allowable load that the pile will
be able to carry.
BeJr;/lg Capacity of Soils
349
Solution
Ultimate bearing capacity of the pi le = Q u =
~
+ Qb
Qb : All end bearing effects " ill occur in the gravel. Now
qb = 40 N
~ kPa or 400 )( N kPa (whichever is the lesser)
i.e.
D
qb = 40 x 40 x =400 x40 = 16000 kPa
0.25
. .
16 000 x 0.25
Penetration mlo grave~ D, =
= 2.5 m
40x40
and
Q b = 16000 x 0.25' = 1000 kN
QJ in sand:
~ in
gravel:
Q, = f,A, = 12 X 5 x 0.25 x 4 = 110 kN
Q, = f,A, = LOx 2.5 x 0.25 x 4 = 100 kN
I.e.
Q. =2 10 + 1000 = 1210kN
1210
Allowable load = - - = 400 kN
3
Example 8.9 illustrates that , as discussed earlier, the end bearing effects are
much greater than those due to side friction. It can be argued that, in order to
develop side friction (shaft f!!sistance) fu lly, a signifi cant downward movement
of the pile is required which cannot occur in this example because of the end
resistance of the gravel. Asa result of this phenomenon, it is common practice to
apply a different factor of safety to the shaft res istance than that appli ed to the
end bearing resistance. T)'?ically a factor of safe ty of around 15 is applied to
shaft res istance, and a factor of safety between 2.5 and 3.0 is applied to the end
beari ng res istance.
Returning to Example 8.9, l nd adopting Fb = 3, Fs = 1.5, the allowable load now
becomes:
1000 + 2 10 = 473kN
3
1.5
Smith'S Elements o(Soil Mechanics
350
Negative skin friction
If a soil subsides or consolidates around a group cf pi les these piles will tend to
support the soil and there can be a considerable increJse in the load 0 11 the piles.
The main causes for this state of affairs are that:
(i) bearing piles have been driven into recently pl aced fi ll;
(ii) fi ll has been placed around the pi les aflerdriving.
1f negative friction effects are like ly to occur l1lel the piles must be des igned to
carry the additional load. In extre me cases the vah.e of negative skin fri ction can
equal the positive skin friction' but, of course, this maximum value cannot act over
the entire bedded length o f the pi le, being virtually zero at the top of the pile and
reach ing some max imum value at its base.
8.11
Designing pile foundations to Eurocode 7
The principles of Eurocode 7, as described in Section 7.4.2, apply to the des ign
of pi le founda ti ons, and the reade r is advised to re:'cr back to that section whil st
studying the following few pages.
The des ign of pi le foundations is covered in Secln n 7 of Eurocode 7. There are
I1 limit states listed that shou ld be considered, though onl y those limit states most
relevant Lo the particular situation wou ld nomlall y be considered in the design. T hese
include the loss of overall stability, bearing resistan::e fai lure of the pile, uplift of
the pile and struc tural fai lure of the pile. in this c hapter we will look only at c hecki ng
aga inst ground res istance failure through the compressive load ing of the pile.
Pile design methods acceptable to Eurocode 7 are fl the main based on the results
of static pile load tests. and the design calculation s IDou ld be validated again st the
test results. When considering the compressive grourd resistance limit state the task
is to demonstrate thallhe des ign axial compression lead on a pile or pile group, F C;d'
is less than or equal 10 the design compress ive gro11l1d resistance, Rc:d' against the
pile or pile group. In the case of pile groups, R C;d is laken as the lesser value of the
design ground resistance of an indi vidu al pile and lha! o f the who le group .
In keeping with the rules of Eurocode 7. the de~ jgn value of the compressive
resistance of the gro und is obtained by dividing the characteristic value by a parti al
fac tor of safety. The characteristic value is obtai ned by one of three approaChes :
from static load teSiS, from ground tests res ults or fron dynamic lests results.
(i) Vllimale c:ompressive
,.e.~ iSlallce from
stalic load 1ests
The characterist ic value of the compressive ground resi~tan ce. Rd' is obtained by combining the measured value from the pile load tests with a correlation factor, ~ (related
10 the number of pi les tested). More expl icitly, Rc:k is .aken as the lesser value of:
and
R
_ (Rc:m)cnin
c;k -
~2
Smith's Elements of So if Mechanics
356
(iii) Ultimate compressive resistance from dynamic t~ts results
Although static load tests and ground tests are the mo.! common method s of determining the compressive res istance o f the pHe, the res slance can also be esti mated
from dy namic tesls provided that the test procedure has been calibrated against static
load tests.
8.12
Pile groups
8.12.1
Action of pile groups
Piles are usually dri ven in groups (see Fig. 8.23).
[n the case of end bearing piles the pressure bu lbs of tle individual piles w ill overlap (if spaci ng < 5d - the usual condition). Provided Ih,H the beari ng strata are firm
thro ughout the affected depth of this combined bulb the:! the beari ng capac ity of the
group w ill be equal to the summation of the individual strengths of the piles. However, if there is a compress ible soil layer beneath the firm layer in w hich the pi les are
found ed , care must be taken to ensure that this weaker layer is not overstressed.
Pile groups in cohesion less soils
Pil e dri ving in sands and gravels com pacts the soi l be:ween the piles. Th is compactive effect can make the bearing capacity of the pile g-oup greate r than the sum of
thc indi vidual pile strengths. Spacing of piles is usually from two 10 three limes the
diameter, or breadth, of the piles.
Pile groups in cohesive soils
A pile group pl aced in a cohes ive soil has a collective strength w.h.ich is considerabl y
less than the summation of the individual pile slrengths "hich compose it.
One characteristic of pile groups in cohesive sents is:be pheoomeoon of ' block
fa il ure'. If the piles are placed very dose together (a common tem ptal ion when
L
I'
'I
0
0
0
0
0
0
0
0
0
8
0
0
0
0
0
0
0
/
0
I
, -~
Ptan
Fig. 8.23
A typical pile group.
'J
.
'
. ,\
\
I
\
Ele. . ation
Bearng Capacity of Soils
deali ng with a limited site areal lhe streng th of the groups may be gove rned by its
strength al block failure. This is ",hen the soil fail s a long the perimeter of the group.
For block failure :
where
D = depth of pile penetralun
L == length of pile group
B == breadth of pi le group
Ne = bearing capacity coeficient (take n generall y as 9.0).
Whitaker ( 1957), in a series of noddlests. showed that block failure will not occur if
the pi les are spaced at nOlless thar 1.5d apart. General practice is to use 2d to 3d spacings.
In suc h cases:
where
E = e ffic iency of pile grrup (0.7 for spacings 2d- 3d)
Q up == ultimate bearing caplc ity of single pile
n = number of piles in goup.
8.12.2
Settlement effects in p;/e groups
Quil e oft en it is the allowable s~ t1l e ment, rather than Ihe safe bearing capac ity. that
decides Ihe working load that a pi le group may calry.
For beari ng piles the total fOUldation load is assumed 10 act althe base of the piles
on a foundation of the same sizeas the plan of the pil e group. With thi s assumption it
becomes a simple matter to exanine settlement e ffects.
With fri ction piles it is viruall y impossible to detennine the leve l at which
the foundati on load is effectivdy transferred to the soi l. An approximate method ,
ofte n used in design , is to assune that the effecti ve transfer level is at a depth of
20 /3 below the (OP of the piles It is also assumed that there is a spre ad of the tOlal
load , one horizonta l to four ve1ical. The settlement of this equi va le nt found at ion
(Fig. 8.24) can the n be delerm ired by lhe nonnal methods.
I
D
Fig.8.24
~
L'_
\
• D
\
3
\
f-- -
-
Transference of load in fri:tion piles.
-
-
-
_ .c, ~'--
,
Smilh 's Elements of Soil Mechanics
358
Exercises
NOfe Where app li cab le the answers quoteo incorporate a fac tor of safety
equal to 3.0.
Exercise 8. 1
A fine sand deposit is saturated throughout with a unit weight of20 kN/m 3 .
Ground water level is at a depth of 1 m below n e surface. A standard penetration test, carried out at a depth of 2 m, gave an N value of 18. U the settlement is to be limited .to not mor<; than 25 mm , detennine an allowable bearing
pressure value for a 2 m square foundation famded at a depth of2 m .
Answers
46012 = 230 kPa (N
~
40)
Exercise 8.2
A strip foot ing 3 m wide is to be founded ala depth of2 m in a saturated
soil of unit weight 19 kN/m 3 . The soil has an lOgle offriclion , 41, of 28° and
a cohesio n, c, of 5 kPa. Groundwater level is 1l a depth of 4 m . Detennine a
va lue for the safe bearing capac ity of the foutdation .
If the groundwater level was to rise to the ~ round surface, dete rmine the
new value of safe bea ring capacity.
Answer
459 kPa ; 249 kPa
Exercise 8. 3
A 2.44 m wide strip fOOl ing is to be found ed in a coarse sand at a depth of
3.05 m . The unit we ight of the sand is 19.3 kN/m 3 and standard penetration
tests at the 3.05 m depth gave an N value of 12.
(i) Determine the sa fe bearing capacity of Ihe foundation if settlement is
of no account.
(ii) Detennine the allowable bearing press ulC if settlement of the foundation is not to exceed 25 mm.
Answers
(i) 1300 kPa,
(ii) 300 kPa
Exercise 8.4
A single test pile, 300 mm diameter, is driven through a depth of 8 m of clay
wh ich has an undrained cohesive strength var!ing from 10 kPu at its surface
to 50 kPa at a depth of 8 m. Estimate the safe load that the pile can carry.
Answer
60kN
Bearng Capacityo{ Soils
Exercise 8 .5
A continuous concrete footing (Ye =24 kN/ml ) of breadth 2.0 m and thickness O.5 m is to be fo unded inaclaysoil (4'u = 0°; cu = 22 kPa; Y = 19 kN/ml)
a1 a depth of 1.0 m. The footing will carry an applied vert ical load of
magnitude 85 kN per :netre run . The load will ac t on the centre-line of the
footing.
Us.ing Eurocode 7 Design Approach I, detenni ne the magnitude of the
over-design factor for both Combination I and Combination 2.
Answer
1.53 (OAI- I); 1.56 (OAI-2)
If you were to include depth factors in the design procedure , what wou ld be
the revised value of the over-design factor for each combina tion'?
Answer
1.79(OAI-I); 1.8 1 (OA I-2)
Note : Adopting der'th fac tors in the design will in variably lead to higher
values of ove r-des ign fac tor.
Exercise 8.6
A rectangular foundation (2.5 m X 6 m X 0.8 m deep) is to be fou nded at
a depth of 1.2 m in a dense sand (c' = 0; 4" = 32°; r = 19.4 kN/m1). The
unit weight of concrete = 24 kN/ml . The foundation wi ll carry a vert ica l
line load 0(250 kN/m at an eccentric ity of 0.4 m.
By following Eurocode 7, Design Approach I estab li sh the proportion of
the ava ilable resistan.::e that wi ll be used.
Answer
16 per cenl (OA I- I); 24 per cenl (OA 1-2)
Note: The proport ion of avai labl e res istance that will be used is determined by taki ng the rec iprocal of Ihe over-des ign factor.