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8. Axial Capacity
of Single Piles
CIV42491998 Dr . J .P. Seid el
Mod ified by J.K . K odik ara, 2001
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Methods
Pile dr ivin g fo rm ulae Static lo ad test Dyn am ic or Statnam ic load tes t Stat ic form ulae
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Pile driving formulae e.g. Hiley for m ula (Energ y balance)
Q = e .W.h .
F (s et + tc / 2) R u = w ork ing lo ad, W=weigh t of th e
hamm er, h= height o f the ham m er dro p(str ok e), F=facto r o f s afety
tc= elast ic (tem po rary) co m press ion e = eff ic iency
F s
tc
R u
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Static Load Test
Plung ing fai lure
Load to speci f iedcon trac t requi rement
What is thefai lure load?
Davissons Method But ler and HoyChins Method Br inch Hansonetc. etc.
What i s the d is t r ibut ionof resis tance?
Approx imate methodsIns t rumenta t ion
Load
Deflect ion
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Dynamic and Statnamic
Testing Methods Rapid al ternat ives to s ta t ic tes t ing Cheaper Separate dyn am ic resis tanc e Correlat ion
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Base ResistanceQ b = A b [c b N c + P ob (N q - 1) + 0.5 gB N g + P ob ]
m inus w eight o f p i le, W p
but W p A b .P ob
and as L >>B , 0.5 gB N g 0, N q - 1 N qQ
b
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Shaft Resistance
Due to co hes ion or f r ic t ion
Cohesive co m po nent : Q sc = A s . a . c s
Fric t ion al com pon ent : Q s f = A s .K P os tan d
P o sK.P os
Q s = Q sc + Q s f = A s [ a .c s + K P os tan d ]
As
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Total Pile Resistance
Q u = Q b + Q s
Q u = Ab [c b N c +P ob N q ] + A s [ a .c s +K P o tan d ]
How do w e com pute Q u w hen sh af t res is tanc ealon g th e pi le is varying ?
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Mobilization
Shaft
2 - 5mm
Base
10 - 20% diam
Total
Sett lement
L o a
d
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Piles in Clay
Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P o tan d ]
Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P o tan d ]
Q u = A b c b N c + A s a .c s
Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P o tan d ]
Q u = A
b P
ob N
q + A
s K P
o tan d
Q u = A b c b N c + A s a .c s
Q u = A b P ob N q + A s K P os tan d
Undrained
Drained / Effectiv e
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Driven Piles in Clay
2.0
1.5
1.0
0.5
0 10 20 30 40 50 60r a
Du
vo
Average curve for sensitiveamarine clay
Average curve for clays of low-medium sensitivity
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Driven Piles in Clay
300
250
200
150
100
50
01 5 10 50 100 500 1000
Time after driving in days
B e a r
i n g c a p a c
i t y
i n
k N 200 x 215mm conrete
(Gothenberg)
300 x 150mm tapered timber (Drammen)
150mm (8 in) steel tube (San Francisco)300 x 125mm I-Beam
(Gothenberg)
30
25
20
15
10
5
B e a r
i n g c a p a c
i t y
i n
t o n s
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Nc Parameter
N c
Compare Skemptons N c for sh al low foun dations
N c = 5(1+0.2B /L)(1+0.2D/ B )
10
9
8
7
6
50 1 2 3 4 5
L/d B
B e n
d i n g c a p a c i
t y f a c t o r
N c
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Adhesion Factor, a
50 100 150 200 250
1000 2000 3000 4000 5000
2.0
1.5
1.0
0.5
0
Figures denote penetration ratio =Depth of penetration in clay
Pile diameter Key:Steel tube pilesPrecast concrete
pilesDesign curve for
penetration ratio >
49 49
49 56 13 15
17 27 33
4010
5815
38
3327 39
44
44 39
1917
19
13
35 44 A d h e s
i o n
f a c t o r
Undrained shear strength (c ) lb/ft 2u
Undrained shear strength (c ) kN/m 2u
20
1.0
0.8
0.6
0.4
0.2
0 100 200
Average Undrained Shear Strength, c , kPau
R e d u c t
i o n
F a c
t o r ,
a
A us t. Piling Cod e,A S159 (1978)
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Bored Piles in Clay
Skemptons recommendations for sideres is tance
=0.45 fo r c u 215 kPa
N c is l im ited to 9. A reduct io n factor is app l ied to accou nt for
lik ely fis su rin g (I.e., Q b = A b c b N c )
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Soil disturbance
samp l ing at tem pts to es tabl ish in-s i tus t rength values
so i l is fai led/remo uld ed b y dr iv ing o rdr i l l ing
pile ins tal lat ion caus es su bs tant iald is turbance
bo red p i les : potent ial loosening driven p i les : p robable densif icat ion
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Scale effects
Laboratory samp les or in -s i tu tes tsinvo lve sm al l volum es o f so i l
Fai lure of so i l aroun d pi les inv olves m uchlarger so i l volum es
If so i l is f iss ured, the sam ple may n ot b erepresentat ive
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Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P os tan d ]
Piles in Sand
Q u = A b [c b N c +P ob N q ] + A s [ a .c s +K P os tan d ]
Q u = A
b P
ob N
q ] + A
s K P
os tan d ]
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Overburden Stress P ob
Q u = A b P ob N q ] + A s K P os tan d ]
Meyerhof Method : P ob = gz
Vesic Method : cr i t ical depth , z c
for z < z c : P ob = gz for z > z c : P ob = gz c
z c /d is a function o f f after installation - see gr aph p. 24
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Critical Depth (z c)
L
zc
vc
W.T.
d
20
15
10
5
028 33 38 43
f
z
/ d
c
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B earing Fac to r, N q
N q i s a func t ion o f : f r ic t ion ang le , fN q i s a func t ion o f :
Q u = A b P ob N q ] + A s K P os tan d ]
Wh at affects f ? In-s i tu dens i ty Part icle p ro pert ies Ins tallat ion pro cedure
N q de term ined f rom graphs app ropr ia te
to each par ticular m ethod
Total end bear ing m ay also b e lim ited:
Meyerho f : Q b < A
b 50N
q tan f
B eware if f is pre- or po st-ins tal lat ion:
Layered s oi ls :N q m ay b e reduc ed i f penetrat ion
ins uffic ient . e .g. Meyerh of (p 21)
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Nq factor (Berezantzevs Method)
1000
100
1025 30 35 40 45
f
Nq
If D/B
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Overburden Stress P os
Q u = A b P ob N q ] + A s K P os tan d ]
Meyerhof Method : P os = gz mid
Vesic Method : cr i t ical depth , z c
for z m id < z c : P ob = gz for z m id > z c : P ob = gz c
z c /d is a function o f f after installation - see gr aph p. 24
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Lateral s tres s param eter, K
A func t ion of K o no rm ally co nso l idated o r ov ercon sol idated -
see Ku lhawy p roper t ies m anual see recom m endations by Das, K ulh awy (p26)
A func t ion o f ins tal lat ion driv en p iles (ful l , part ial d isplacem ent) bo red p i les augercast pi les screwed pi les
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Das (1990) recommends the following values for K / K o :
Pile Type K / K o
Bored or Jetted piles 1
Low-displacement, driven piles 1 to 1.4
High-displacement, driven piles 1 to 1.8
Kulhawy (1984) makes the following similar recommendations:
Pile Type K / K o
Jetted piles 1/2 to 2/3
Drilled shaft, cast-in-place 2/3 to 1Driven pile, small displacement 3/4 to 5/4
Driven pile, large displacement 1 to 2
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K.tan d
The K and tan d values are of ten co m binedinto a s ingle func t ion
see p 28 for Vesic values f rom Poulos andDavis
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Pile-so il fr ic t ion ang le, d
A func t ion of f See values b y B rom s and K ulhaw y (p26) A fu nc t ion o f pi le m ater ial
steel, co nc rete, t im ber A func t ion of p i le roughn ess
precast conc rete Cast-in-place con crete
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Pile-soil friction angleBroms (1966) suggests the following
Pile Material / f '
Steel d
Concrete 0.75
Timber 0.66
Kulhawy (1984)
Pile Material / f ' Typical analogy
Rough concrete 1.0 Cast-in-place
Smooth concrete 0.8 to 1.0 Precast
Rough steel 0.7 to 0.9 Corrugated
Smooth steel 0.5 to 0.7 Coated
Timber 0.8 to 0.9 Pressure-treated
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Example Driven precast co nc rete pi le 350mm squ are Uniform dense sand (f = 40 o ; g = 21kN/m 3)
Water tabl e at 1m Pile leng th 15m Check end bearing with Vesic and Meyerho f Metho ds Pi le is driven on 2m furth er into a very d ense layer f = 44 o ; g = 21.7 kN/m 3 Com pu te m od if ied capaci ty using Meyerhof
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Example Bo red pi le 900mm d iam eter Uniform medium dense sand (f = 35 o ; g = 19.5kN/m 3)
Water tabl e at 1m Pile leng th 20m Check sh aft capacity with Vesic and Meyerho f Metho ds By co m parsion, check capaci ty of 550mm d iam eter
scr ewed pile
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Lateral load o n sin g le p i le
Calcu lat ion of u l t im ate lateral resis tance(refer webs ite/han do uts fo r detai ls)
Lateral pi le def lect io n (us e us e su bg radereact ion m etho d, p-y analysis)
Roc k s oc keted p ile (us e ro ck et , Carter etal . 1992 m etho d )