1. Load Transfer Mechanism
Fig. gives a single
pile of uniform
diameter d
(circular or any
other shape) and
length L driven
into a
homogeneous
mass of soil of
known physical
properties. A
static vertical load
is
applied on the top.
It is required to
determine the
ultimate bearing
capacity Qu of the
pile.
2. Load carrying capacity of piles
A pile foundation should be safe against shear
failure
Settlement should be within the permissible limits
Four different categories to measure the load
carrying capacity of piles
Static methods
Dynamic formulas
In-situ penetration tests
Pile load tests
3. Static Formula
Ultimate load capacity of individual pile or group of piles
(Group pile) (Qu)
Depending upon characteristics of the soil
Qu=Qp+Qs
Qu=Ultimate failure load
Qp=Point resistance of the pile
Qs= Shaft resistance developed by friction between the soil
and pile
Load (Q)
Qp
QsQs
4. Static Analysis
Qf= Qb + Qs
Where Qf = Net ultimate pile load or the
load which can be supported
by a pile top at failure.
Qb = Net load carried by the pile
base or total base resistance.
Qs = The load carried by the pile
shaft or total shaft resistance
Or Qf = fb x Ab + fs x As
8. Meyerhof’s Method
It has been established that the effective vertical
pressure (q) at the pile tip increases with depth
only until a certain depth (Critical Depth Dc)
q
Dc
Depth
q=γxD
9. In Pile Foundation
Critical depth depends upon the angle of internal
friction and width or diameter of the pile
(Charts available)
Dc/B
ɸ
Critical depth can roughly be taken as 10 B for
loose sands and 20 B for dense sands
10.
11. For square foundation
γ
γ
γγ
γ
BNNDq
BNqNq
qcp
qp
4.0
4.0
+=
+=
γγγ BNNDq qcp 3.0+=
For circular foundation
Pile foundations in sand
q=Effective vertical
pressure at the pile tip
γ=Unit weight of the
soil in the zone of pile
tip
Dc=Critical depth
Nq and Nγ= Bearing
capacity factors for
deep foundation
(Charts available)
B= Pile width or
diameter
12. For square foundation
γγγ BNNDq qcp 4.0+=
γγγ BNNDq qcp 3.0+=
For circular foundation
Pile foundations in sand
q=Effective vertical
pressure at the pile tip
γ=Unit weight of the
soil in the zone of pile
tip
Dc=Critical depth
Nq and Nγ= Bearing
capacity factors for
deep foundation
(Charts available)
B= Pile width or
diameter
13. For square foundation
qcp NDq γ=
qcp NDq γ=
For circular foundation
Pile foundations in sand
q=Effective vertical
pressure at the pile tip
γ=Unit weight of the
soil in the zone of pile
tip
Dc=Critical depth
Nq and Nγ= Bearing
capacity factors for
deep foundation
(Charts available)
B= Pile width or
diameter
14.
15. Meyerhof’s method for qp
QP=ApxqxNq ≤ Apxql
ql (kN/m2)=limiting stress=50xNqxtanɸ ≤11000 kPa (110 t/ft2)
qNq≤ ql
Be careful for critical parameters
q=effective vertical pressure at pile tip = γxDc
16. Nq values
Assumed that the soil above the pile tip is similar
to the soil below the pile tip
If pile penetrates the above hard or compact
stratum only slightly and soil above the pile tip is
loose stratum….then…..
It would be more appropriate to use the value
of Nq for a shallow foundation
17. Meyerhof’s method for qp
The point bearing capacity (qp) of pile generally
increases with the depth of embedment (Db) in
the bearing stratum
Load (Q)
Qp
G.L
Loose
stratum
Bearing
stratum Db
18. Meyerhof’s method for qp
Load (Q)
Qp
G.L
Loose
stratum
Bearing
stratum Db
Db reaches to a maximum value at
an embedment ratio of (Db/B)cr
Critical ratio (Db/B)cr depends upon soil friction angle
Determine Nq
20. ql(1)=limiting unit point resistance of loose
sand=50Nq1tanɸ1(kN/m2)
ql(2)=limiting unit point resistance of dense
sand=50Nq2tanɸ2(kN/m2)
Db=Depth of penetration in dense sand
21. ( )
( )
( )depthscouringoranyifsoilweakinportiondeducting
sandwithinembededactuallylengthdepthPileL
esubmergencand
pressureoverburdenforcorrected
basepile
ofvicinitytheinSPTofvalueNAverageN
siltsplasticNonkPaN
SandskPaN
B
L
Nq
p
p
p
,
)(
point)pilethebelowDia4andaboveDia10About(
300
40040
=
=
≤
≤=
22. qp(gross)=cNc + qNq
q =vertical stress=(γxDf)
for ɸ=0, Nq=1
qp(gross)=cNc + q
qp(net)=cNc
Qp=cNcxAp
c=Cohesion
Nc=Bearing capacity factor for deep foundation
Generally 9.0 for pile (but vary in some cases)
For Clay
24. Where qp= fb = unit base resistance
qs= fs = unit skin friction or unit shaft
resistance
Ab = Area of pile base (depends upon
the shape)
As = Surface area of pile shaft.
Pile Capacity in Sand (Effective stress method)
(((( ))))
depthcriticalD
D.pileoflengthEmbeddedEffectiveL
pileforforfactorcapacityBearingN
)pileofbase(pileoftipatpressureoverburdenEffective
)casesallfor(
ft/torkPaNLNf
C
Cp
qp
o
qppqpob
====
≤≤≤≤====
====
====′′′′
≤≤≤≤′′′′====′′′′====
φ
σ
γσ 2
11011000
26. Qs=Shaft resistance developed by friction
between the soil and pile
Qs=fsxAs
fs=Cumulative average unit friction
between the sand and the pile surface
As=Effective surface area of the pile in
contact with the soil (Perimeter X Length)
Load (Q)
QsQs
Floating
27. fs=Average unit friction between the sand and the
pile surface
Ω= tan
'
vs Kf σ
K= Coefficient of earth pressure
σv
’= Effective vertical pressure at the depth under
consideration
=Coefficient of friction between soil and pile material
(critical depth=15 Dia of Pile), after that frictional resistance
remain constant
32. Qs=Shaft resistance developed by friction
between the soil and pile
Qs=fsxAs
fs=αc’
c’=Cumulative average cohesion along the pile shaft
α=adhesion factor (Adhesion factor depends upon cohesion
of soil)
Charts available
α METHOD
33.
34. Qs=Shaft resistance developed by friction
between the soil and pile
Qs=fsxAs
fs=ʎ(σv’+2c) [Vijayvergiya and Fochit (1972)]
c=cohesion of soil
ʎ=friction capacity factor (Friction capacity factor depends
upon embedment length)
Charts available
ʎ METHOD
35.
36. Care while calculating C and C’
Clay gets remolded while pile is driven
Remolding factor must be taken in account
Remolded strength must always be less than the
undisturbed strength
Strength improves with time
Rate of gain of strength depends upon
Consolidation characteristics
Rate of dissipation of excess pore water pressure
38. From CPT (Plastic soils like clay)
(((( ))))
m.Bfor
m.Bfor.safetyoffactoruseloadAllowable
N
C.C.Ofor.
C.C.Nfor
N
q
f
N
N
N
q
Nf
k
o
o
k
avc
os
k
cp
k
c
cpb
503
5052
20
50
1
20
9
>>>>
≤≤≤≤
====
====
========
≅≅≅≅====
====××××====
α
αα
39. Method Based on CPT (for driven
piles) (Non Plastic soils like sand)
siltplasticNon
q
sand
q
f
qf
avc
avc
s
cb
150
200
====
====
====
Allowable load: apply Factor
of safety 2.5-3.0
42. For Bored Piles
( )
4:
2
2
1
2
1
40
3
1
3
1
safetyoffactorApplyLoadAllowable
kPaNpilesDrivenoff
B
L
Npiledrivenofrdf
avs
p
b
==
==
43. The load carrying capacity of bored piles can
be determined using the same procedure as
that of driven piles
The values of soil parameters are different
between driven and bored piles
44. Bored piles in sand
Ultimate load capacity of individual pile or group of piles
(Group pile) (Qu)
Depending upon characteristics of the soil
Qu=Qp+Qs
Qu=Ultimate failure load
Qp=Point resistance of the pile
Qs= Shaft resistance developed by friction between the soil
and pile
Load (Q)
Qp
QsQs
45. Bored piles in sand
( )∑=
Ω+=
n
i
ispqu AvKANqQ
1
''
)(tanσ
Qp Qs
q’ and σv
’= Effective vertical pressure, limited to a
maximum value given by critical depth
K= Lateral earth pressure coefficient for bored foundation
tan =coefficient of friction between soil and pile material
46. K=1-sinɸ Approximate value
K generally varies between 0.3 and 0.75
An average value of 0.5 is usually adopted
K values
tan values
Can be taken equal to tanɸ for bored piles excavated in
dry soil
If slurry is used while excavation value of tan will
reduced
47. In general, for a given value of ɸ, bored piles
have a unit point resistance of 1/2 to 2/3 of
that of corresponding driven piles
Why???
In driven piles there is densification
48. Ultimate load capacity of individual pile or group of piles
(Group pile) (Qu)
Depending upon characteristics of the soil
Qu=Qp+Qs
Qu=Ultimate failure load
Qp=Point resistance of the pile
Qs= Shaft resistance developed by friction between the soil
and pile
Load (Q)
Qp
QsQs
Static method for bored piles in clay
49. Bored piles in clay
''
spcu AcAcNQ α+=
Qp Qs
As’=Area of shaft that is effective in developing skin
friction
50. Value of α
Pile type
Method of drilling
For straight shafts
α=0.5 in dry soils without slurry
α=0.3 in dry soils with slurry
For belled shafts
α=0.3 in dry soils without slurry
α=0.15 in dry soils with slurry
51. Effective area of shaft for skin friction (As’)
Depends upon embedded length of shaft in soil that is
effective in developing skin friction
The lower 1.5 m (or 2B) and top 1.5 m (or 2B) is
neglected
Why?
Because of the disturbance
Generally, for bored piles installed in stiff clay
C=0.75 C (obtained from Triaxial test)
52. Negative Skin Friction
When soil layer
surrounding the
portion of the
pile shaft settles
more than pile
A downward
drag occurs on
the pile
The drag is
known as
negative skin
friction
Extra
downward
load on pile
53. nsfuu QQQ −='
Negative Skin Friction
Qu’= net ultimate load
Qnsf= Drag down fce
Qnsf is computed using the same method as we
discussed for the positive frictional resistance
67. When pile load test shall be conducted
3 days after installation in sandy soils
One month after installation in silts
and clays (soft clays)
How shall be the application of load
Equal increments of about 20% of the
allowable load (Load at which pile will be
tested)
(Test load= Twice the safe load or the
load at which the total settlement reaches
the specified value
68. How to record settlement
At least with three dial gauges
When to increase/change the load
Rate of movement of the pile top is not
more than
0.1 mm/hour for sandy soils
0.02 mm/hour for clayey soils
Two hours maximum
69. Time interval to observe settlement
0.5 min, 1 min, 2 min, 4 min, 8 min, 15
min, 30 min, 1 hour and 2 hours
The last stage of loading will be
continued for 24 hours or even more
depending upon the settlement
70. How to remove the load
With the same increments and 1 hour
interval
Final rebound is recorded for 24 hours
after the entire load has been removed
72. Safe load??
2/3 of the final load at which total
settlement is 12 mm
2/3 of the load corresponding to a net
settlement of 6 mm
1/2 of load corresponding to a total
settlement of (B/10) (7.5% incase of under-
reamed pile)
The least of below
73. The limiting criteria (sometimes
specified)
Under the total load (twice the safe
load)
The net settlement should not be
more than 20 mm
The gross settlement should not be
more than 25 mm
74. To construct a pre-stressed bridge, a 30
cm diameter pile of length 12 m was
subjected to a pile load test and the
following results were obtained.
Determine the allowable load
6.05.85.55.24.64.0Settlement during
unloading (cm)
6.03.82.551.650.850Settlement during
loading (cm)
25002000150010005000Load (kN)
