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• Rc;d – design resistance as the capacity parameters (1) The results of static load tests, which have been
determined from designing standards, considered demonstrated by means of calculations or other-
in the present article, wise, to be consistent with other relevant experi-
• Rtest – in-situ static test result on top pile, ence,
• Qlim – limit resistance defined as rapid settlement (2) Empirical or analytical calculation methods whose
occurs under sustained or slight increase of the ap- validity has been demonstrated by static load tests
plied load – the pile plunges. in comparable situations,
Ration of Qlim/Rc;d = γt presents the total safety (3) The results of dynamic load tests whose validity
factor. has been demonstrated by static load tests in com-
parable situations,
(4) The observed performance of a comparable piles
foundation, provided that this approach is sup-
ported by the results of site investigation and
ground testing.

Equilibrium equation
The equilibrium equation to be satisfied in the ul-
timate limit state design of axially loaded piles in
compression is
Fc; d ≤ Rc; d , (1)

where Fc;d is the design axial compression load and

Rc;d is the pile compressive design resistance.
Fig. 2a. Capacity parameters:
Rc;d – design resistance, Rtest – static test result, Design axial load
Qlim – limit resistance, s(Q) – load-settlement curve
from top pile measurement sk – characteristic settlement The design axial compressive load Fc;d is obtained
by multiplying the representative permanent and vari-
Figure 2b shows typical load/settlement curves for able loads, G and Q by the corresponding partial ac-
compressive load of the shaft Qs and the base Qb load tion factors γG and γQ
capacity and the total load capacity Qt characteristic
Fc; d = γ G Grep + γ Q Qrep . (2)
depending on soil layers: (a) for friction pile and
(b) for end-bearing pile.
The two sets of recommended partial factors on
actions and the effects of actions are provided in Ta-
ble A3 of Annex A of EN 1997-1.

Characteristic pile resistance

Eurocode 7 describes three procedures for obtain-
ing the characteristic compressive resistance Rc,k of
a pile:
(a) Directly from static pile load tests with coefficient
ξ1 and ξ2 for n pile load tests, given in Table A.9
Fig. 2b. Typical load/settlement curves for compressive load tests: of EN 1997-1 Annex A,
(a) friction pile; (b) end-bearing pile (b) By calculation from profiles of ground test results or
by calculation from ground parameters with coeffi-
cient given in Table A.10 of EN 1997-1 Annex A,
2. APPROACHES OF PILE DESIGN (c) Directly from dynamic pile load tests with coeffi-
ACC. TO EN-1997 cientgiven in Table A.11 of EN 1997-1 Annex A.
In the case of procedures (a) and (b) Eurocode 7
provides correlation factors to convert the measured
EN 1997-1 §7.4(1)P states that the design of piles pile resistances or pile resistances calculated from
shall be based on one of the following approaches: profiles of test results into characteristic resistances.

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 Pile load capacity – calculation methods 85

Case (c) is referred to as the alternative procedure in or by separating it into base and shaft components Rb;d
the Note to EN 1997-1 §7.6.2.3(8), even though it is and Rs;k, using the relevant partial factors, γb and γs
the most common method in some countries.
Rb; k Rs ; k
Rc; d = + . (6)
Characteristic pile resistance from profiles γb γs
of ground test results The combinations of sets of partial factor values
Part 2 of EN 1997 includes the following Annexes that should be used for Design Approach 2 are as fol-
with methods to calculate the compressive resistance, lows
Rc,cal of a single pile from profiles of ground test re- DA2.C1: A1 “+” M1 “+” R2
sults:
(a) D.6 Example to determine Rc;cal based on cone where R2 for base, shaft and total: γt = γb = γs = 1.1 in
penetration resistance: relating the pile’s unit base case of compression, and γt = 1.15 in case of shaft in
resistances qb at different normalised pile settle- tension.
ments, s/D, and the shaft resistance qs to average
cone penetration resistance qc values. The values
in Tables D.3 and D.4 are used to calculate the pile 3. DRAINED AND UNDRAINED
base and shaft resistances in the pile. LOADING CONDITIONS
(b) D.7 Example to determine Rc;cal base on maximum
base resistance and shaft resistance from the qc val-
ues obtained from an electrical CPT. Drained loading occurs when soils are loaded
(c) E.3 Example to determine Rc;cal based on results of slowly, resulting in slightexcess pore pressures that
an MPM test. dissipate due to permeability.On the other hand,
Characteristic total pile compressive resistance Rc;k undrained loading occurs when fine-grained soils are
or the base and shaft resistances Rb;k and Rs;k may be loaded at a high rate, they generate excess pore pres-
determined directly by applying correlation factors ξ3 sures because these soils have very low permeabil-
and ξ4 to the set of pile resistances calculated from the ities.
test profiles. This procedure is referred to as the Model The drained (or long-term) strength parameters of
Pile procedure by Frank et al. (2004) to determine Rc;cal. a soil, c′ and φ ′ must be used in drained (long-term)
analysis of piles.
The undrained (or short-term) strength parameter
Characteristic pile resistance
of a soil, cu, must be used in undrained (short-term)
from the ground parameters
analysis of piles.
The characteristic base and shaft resistances may
also be determined directly from the ground parame-
ters using the following equations given in EN 1997-1 4. ESTIMATING LOAD CAPACITY
§7.6.2.3(8) OF PILES
Rb; k = Ab ⋅ qb; k (3)
Pile load carrying capacity depends on various
Rs ; k = ∑A s;i ⋅ qs ; i ; k (4) factors, including: (1) pile characteristics such as pile
length, cross section, and shape; (2) soil configuration
where and short and long-term soil properties; and (3) pile
qb;k – characteristics of unit base resistance, installation method. Two widely used methods for pile
qs;i;k – characteristics of unit shaft resistance in the design will be described:
i-th layer. • α – method used to calculate the short-term load
capacity (total stress) of piles in cohesive soils,
Design compressive pile resistance • β – method used to calculate the long-term load
The design compressive resistance of a pile Rc;d capacity (effective stress) of piles in both cohesive
may be obtained either by treating the pile resistance and cohesionless soils.
as a total resistance Piles resist applied loads through side friction
(shaft or skin friction) and end bearing as indicated
Rc; k in Fig. 3. Friction piles resist a significant portion of
Rc; d = (5)
γt their loads by the interface friction developed be-

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86 B. WRANA

tween their surface and the surrounding soils. On the stress and undrained shear strength but decreases for
other hand, end-bearing piles rely on the bearing long piles.
capacity of the soil underlying their bases. Usually, Niazi and Mayne [24] presented 25 methods of
end-bearing piles are used to transfer most of their estimating pile unit shaft resistance within α-method
loads to a stronger stratum that exists at a reasonable and compared them. They showed main differences
depth. with respect to parameters: length effect, stress his-
Design bearing capacity (resistance) can be de- tory, Ip, su, σ v′ , progressive failure, plugging effect.
fined as Belowthe main methods estimating skin friction in
claysare shown:
Rc , d = Qb + Qs = Ab ⋅ qb + ∑A s ,i ⋅ qs ; i ; d . (7) (a) American Petroleum Institute (API, 1984, 1987)
The equation by API (1984, 1987) suggests values
for α as a function of cu as follows
⎧ cu − 25
⎪1 − 90 for 25 kPa < cu < 70 kPa,
⎪
α = ⎨1.0 for cu ≤ 25 kPa, (9)
⎪0.5 for cu ≥ 70 kPa.
⎪
⎩
(b) NAVFAC DM 7.2 (1984). Proposition for α coef-
ficient depends on type of pile (Table 1)

Table 1. α vs. undrained shear strength (NAVFAC DM 7.2)

Soil Undrained shear

Pile type α
consistency strength su [kPa]
Fig. 3. Pile’s side friction (shaftor skin friction)
and end bearing Very soft 0–12 1.00
Soft 12–24 1.00–0.96
Timber and
Medium stiff 24–48 0.96–0.75
concrete piles
5. α-METHOD, SHORT-TERM LOAD Stiff 48–96 0.75–0.48
CAPACITY FOR COHESIVE SOIL Very stiff 96–192 0.48–0.33
Very soft 0–12 1.00
Soft 12–24 1.00–0.92
5.1. UNIT SKIN FRICTION qs(z) Steel piles Medium stiff 24–48 0.92–0.70
Stiff 48–96 0.70–0.36
Very stiff 96–192 0.36–0.19
The method is based on the undrained shear
strength of cohesive soils; thus, it is well suited for
As in the API method, effective stress effects are
short-term pile load capacity calculations. In this
neglected in the DM 7.2 method.
method, the skin friction is assumed to be propor-
(c) Equation based on undrained shear strength and
tional to the undrained shear strength su, of the cohe-
effective vertical stress, Kolk and Van der Velde
sive soil as follows and the interface shear stress qs
method [18]. Coefficient α is based on the ratio
between the pile surface and the surrounding soil is
of undrained shear strength and effective stress.
determined as
A large database of pile skin friction results was
qs ( z ) = α ( z ) su ( z ) (8) analyzed and correlated to obtain α value (Table 2).
(d) Simple rules to obtain coefficient α based on
where
su/ σ v′ proposed standard DNV-OS-J101-2007
su – undrained shear strength,
α – adhesion coefficient depending on pile mate- ⎧ 1
rial and clay type. ⎪2 or su / σ v′ ≤ 1,
⎪ su / σ v′
It is usually assumed that ultimate skin friction is α =⎨ (10)
1
independent of the effective stress and depth. In real- ⎪ 4 or su / σ v′ > 1.
ity, the skin friction is dependent on the effective ⎪⎩ 2 su / σ v′

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 Pile load capacity – calculation methods 87

Table 2. Skin friction factor dependent on su/ σ v′

su/ σ v′ 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
α 0.95 0.77 0.70 0.65 0.62 0.60 0.56 0.55 0.53 0.52 0.50 0.49 0.48
su/ σ v′ 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 3.0 4.0
α 0.47 0.42 0.41 0.41 0.42 0.41 0.41 0.40 0.40 0.40 0.40 0.39 0.39

(e) Mechanism controlling friction fatigue, Randolph Figure 4 presents mobilized values of α versus
[26] sud/ σ v′ 0 for all piles discussed in this paper. Studies
Randolph [26] suggested that progressive failure, have shown that the plasticity index has a largeimpact
which occurs in strain softening soil, was a possible on the mobilized ultimate shaft friction and corre-
mechanism controlling friction fatigue. The progres- sponding α-values.
sive failure from the peak (τpeak) to the residual (τres)
shaft resistance is shown in Fig. 4. Randolph [26]
proposed a reduction factor (Rf) which depends on the 5.2. UNIT BASE RESISTANCE qB
degree of softening ξ and the pile compressibility K
2
⎛ 1 ⎞ For cohesive soils it can be shown, using Ter-
R f = 1 − (1 − ξ )⎜1 − ⎟ (11) zaghi’s bearing capacity equation, that the unit base
⎝ 2 K⎠
resistance of the pile is
where
qb = ( su )b N c (13)
τ peak
π DL2 where (su)b is the undrained shear strength of the co-
τ ( EA) pile
hesive soil under the base of the pile, and Nc is the
ξ = res , K = , (12)
τ peak Δwres bearing capacity coefficient that can be assumed equal
to 9.0 (Skempton [29]).
EA – axial stiffnes of pile,
Δwres – post-peak displacement required to mobi-
lize the residual shaft resistance.
6. β-METHOD, LONG-TERM
(f) Norwegian Geotechnical Institute, NGI-05 LOAD CAPACITY FOR COHESIVE
Karlsrud et al. [15], proposed modification of the
AND COHESIONLESS SOILS
NGI method by introducing correlation of sud/ σ v′ 0 and
Ip with α – coefficient presented by the trend lines
shown in Fig. 4. New data are included herein, all
6.1. UNIT SKIN RESISTANCE qs(z)
previous data have been re-interpreted.

The method is based on effective stress analysis

and is suited for long-term (drained) analyses of pile
load capacity. The unit skin resistance qs, between
the pile and the surrounding soil is calculated by
multiplying the friction factor, μ, between the pile
and soil by σ h′

qs ( z ) = μσ h′ = μ ( z ) K ( z )σ v′ ( z ) = β ( z )σ v′ ( z ) (14)

where at rest pressure coefficient depends on the

installation mode, usually K = K0, with K0 =
(1 – sinφ′)(OCR)0.5 ≤ 3,
OCR – overconsolidation ratio,
σ v′ – vertical effective stress.
Niazi and Mayne [24] presented 15 methods of
Fig. 4. Measured values of α in relation to normalized strength, estimating pile unit shaft resistance within β-method
for all piles (Karlsrud et al. [15]) and compared them. They showed main differences

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88 B. WRANA

between them with respect to parameters: σ r′ , δ, φ ′,

OCR, K, σ v′ , L, d, su, ID, Ip. The main methods esti-
mating skin frictionare shown below:
(a) according to NAVFAC DM 7.2(1984), β =
μ (z)K(z) = tan δ (z)K(z), Tables 3 and 4.

Table 3. Pile skin friction angle (δ)

Pile type Pile-soil interface friction angle (δ)

Steel piles 20o
Timber piles 3/4 φ′
Concrete piles 3/4 φ′

Table 4. Lateral earth pressure coefficient (K)

Fig. 5. Chart for determination of β-values
K (piles under K (piles
Pile type dependent on OCR and Ip, Karlsrud [16]
compression) under tension)
Driven H-piles 0.5–1.0 0.3–0.5
Driven displacement piles 6.2. UNIT BASE RESISTANCE qb
1.0–1.5 0.6–1.0
(round and square)
Driven displacement tapered Using Terzaghi’s bearing capacity equation, the
1.5–2.0 1.0–1.3
piles
unit base resistance at the base of the pile can be cal-
Driven jetted piles 0.4–0.9 0.3–0.6
culated
Bored piles (less than 60 cm
0.7 0.4
in diameter) qb = (σ v′ )b N q + cb′ N c (15)

where
(b) proposition value of β = μ (z)K(z) can be estimated (σ v′ ) b – vertical effective stress at the base of the
according to the following propositions:
pile,
Author Proposition of β value cb′ – cohesion of the soil under the base of the pile,
McClelland [21] β = 0.15 to 0.35 for compression, Nc = (Nq – 1)cotφ ′.
for driven piles β = 0.10 to 0.25 for tension (for uplift piles)
β = 0.15, 0.75, 1.2 for φ ′ = 28°, 35°, 37°, Values of bearing capacity factor Nq
for driven piles
Meyerhof [22] (a) Janbu [13] presented equations to estimate capacity
β = 0.1, 0.2, 0.35 for φ ′ = 33°, 35°, 37°,
for bored piles coefficients Nq and Nc for various soils
β = Ctan(φ′ – 5)
Kraft and Lyons
[19] C = 0.7 for compression, C = 0.5 for tension
(uplift piles)

(c) Average K method

Earth pressure coefficient K can be averaged from
Ka, Kp and K0: K = (K0 + Ka Kp)/3 where: K0 = (1 –
sinφ ′), Ka = tan2(45 – φ ′/2), Kp = tan 2(45 +φ ′/2)
(d) Karlsrud [16]
Karlsrud [16] proposed to take into account the
plasticity index Ip in β-method. Figure 5 shows dia-
gram of β-values from as low as 0.045 for low-
plastic NC clays to about 2.0 to very stiff clays with
OCR of 40, which is the upper range of available pile Fig. 6. Shear surface around the base of a pile:
data. definition of the angle η (Janbu [13])

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 Pile load capacity – calculation methods 89

N q (tan φ ′ + 1 + tan 2 φ ′ ) 2 exp(2η tan φ ′) friction angle decreases with depth. Hence Nq, which
is a function of the friction angle, also would reduce
where η is an angle defining the shape of the shear with depth. Variation of other parameters with depth
has not been researched thoroughly. The end bearing
surface around the tip of a pile as shown in Fig. 6. The
capacity does not increase at the same rate as the in-
angle η ranges from π/3for soft clays to 0.58π for
creasing depth. Figure 7 attempts to formulate the end
dense sands.
bearing capacity of a pile with regard to relative den-
(b) Values of bearing capacity factor Nq according to sity (ID) and vertical effective stress σ v′ (Randolph et
NAVFAC DM 7.2(1984), see Table 5. al. [17]).

Table 5. Friction angle φ ′ vs. Nq

φ ′ [°] 26 28 30 31 32 33 34 35 36 37 38 39 40
Nq for driven piles 10 15 21 24 29 35 42 50 62 77 86 120 145
Nq for bored piles 5 8 10 12 14 17 21 25 30 38 43 60 72

If water jetting is used, φ′ should be limited 6.4. CRITICAL DEPTH FOR SKIN FRICTION
to 28°. This is because water jets tend to loosen the (SANDY SOILS)
soil. Hence, higher friction angle values are not war-
ranted. Skin friction should increase with depth and it be-
comes a constant at a certain depth. This depth was
named a critical depth. The typical experimental
6.3. PARAMETERS THAT AFFECT
variation of skin friction with depth in a pile as evi-
THE END BEARING CAPACITY
dence for critical depth is shown in Fig. 8.

The following parameters affect the end bearing

capacity:
(c) Effective stress at pile tip,
(d) Friction angle at pile tip and below (φ ′),
(e) The dilation angle of soil (ψ),
(f) Shear modulus (G),
(g) Poisson’s ratio (v).

Fig. 8. Variation of skin friction (Randolph et al. [27])

Remarks:
• As one can see, experimental data do not support
the old theory with a constant skin friction below
the critical depth.
• Skin friction tends to increase with depth and just
Fig. 7. End bearing capacity of a pile above the tip of the pile to attain its maximum
with regard to relative density (ID) and effective stress value. Skin friction would drop rapidly after that.
(Randolph et al. [27]) • Skin friction does not increase linearly with depth
as was once believed.
Most of these parameters have been bundled into • No satisfactory theory exists at present to explain
the bearing capacity factor Nq. It is known that the the field data.

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• Due to lack of a better theory, engineers are still 6.5. CRITICAL DEPTH FOR END BEARING
using critical depth theory of the past. CAPACITY (SANDY SOILS)

Reasons for limiting skin friction

Pile end bearing capacity in sandy soils is related
The following reasons have been offered to explain to effective stress. Experimental data indicate that end
why skin friction does not increase with depth indefi- bearing capacity does not increase with depth indefi-
nitely, as suggested by the skin friction equation: nitely. Due to lack of a valid theory, engineers use the
1. K value is a function of the soil friction angle (φ ′). same critical depth concept adopted for skin friction
Friction angle tends to decrease with depth. Hence, as forthe end bearing capacity. As shown in Fig. 11,
K value decreases with depth (Kulhawy [20]). the end bearing capacity was assumed to increase till
2. Skin friction equation does not hold true at high the critical depth. It is clear that there is a connection
stress levels due to readjustment of sand particles. between end bearing capacity and skin friction since
3. Reduction of local shaft friction with increasing the same soil properties act in both cases, such as ef-
pile depth, see Fig. 9 (Rajapakse [28]). fective stress, friction angle, and relative density. On
the other hand, two processes are vastly different in
nature.

Fig. 9. Example of unit skin friction distribution

(Rajapakse [28])
Fig. 11. Unit base bearing capacity and critical depth
Let us assume that a pile was driven to a depth of
3 m and unit skin friction was measured at a depth of The following approximations were assumed for
1.5 m. Then let us assume that the pile was driven fur- the critical depth within the bearing zone:
ther to a depth of 4.5 m and unit skin friction was meas- • Critical depth for loose sand = 10 D (D is the pile
ured at the same depth of 1.5 m. It has been reported that diameter or the width),
unit skin friction at 1.5 m is less in the second case. • Critical depth for medium dense sand = 15 D,
NAVFAC DM 7.2 gives maximum value of skin • Critical depth for dense sand = 20 D.
friction and end bearing capacity is achieved after 20 The critical depth concept is a gross approximation
diameters within the bearing zone. The following that cannot be supported by experimental evidence.
approximations were assumed for the critical depth:
• Critical depth for loose sand = 10 D (D is the pile
diameter or the width), 7. ESTIMATING PILE LOAD CAPACITY
• Critical depth for medium dense sand = 15 D, BASED ON CPT RESULTS
• Critical depth for dense sand = 20 D.

7.1. INTRODUCTION

Owing to the difficulties and the uncertainties in

assessing the pile capacity on the basis of the soil
strength-deformation characteristics, the most fre-
quently followed design practice is to refer to the for-
mulae correlating directly the pile capacity compo-
nents of qb and qs to the results of the prevalent in situ
tests. Within the domain of these in situ methods, the
Fig. 10. Critical depth: dc – critical depth, fcd – unit skin friction cone penetration test (CPT) is one of the most fre-
at critical depth, fcd = K σ c′ tanδ, quently used investigation tools for pile load capacity
σ c′ – effective stress at critical depth evaluations. Ever since the first use of CPT in geo-

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 Pile load capacity – calculation methods 91

technical investigations, research efforts have ad- • Semi-empirical methods – with the purely CPT pa-
vanced the very elementary idea of considering it as rameters, the additional estimated parameters are
mini-pile foundation. This has resulted in plethora of taken into account (σr, δ, φ ′, K, σ v′ 0 , L, d, su, ID).
correlative relationships being developed between the • Indirect CPT methods – employ soil parameters,
CPT readings cone resistance (qc) or more proper such as friction angle and undrained shear strength
corrected cone resistance (qt), sleeve friction ( fs), and obtained from cone data to estimate bearing ca-
shoulder pore water pressure (u2) and the pile capacity pacity. The indirect methods apply strip-footing
components of qb and qs. bearing capacity theories, and neglect soil com-
As commonly reported (e.g., Ardalan et al. [2]; pressibility and strain softening. These methods
Cai et al. [5], [6]), there are two main approaches to are rarely used in engineering practice.
accomplish axial pile capacity analysis from CPT
data: (a) rational (or indirect) methods and (b) direct
methods. 7.2. DIRECT CPT METHODS

In one viewpoint, the cone penetrometer can be

considered as a mini-pile foundation as noted by Ar-
dalan et al. [2] and Eslami and Fellenius (1997). The
mean effective stress, compressibility and rigidity of
the surrounding soil medium affect the pile and the
cone work in a similar manner.This concepthas led to
the development of many direct CPT methods.

Fig. 12. CPT based evaluations of pile capacity

(Niaziand Mayne [24])

• Direct CPT methods – used the similarity of the

cone resistance with the pile unit resistances. Some
methods may use the cone sleeve friction in deter-
mining unit shaft resistance. Several methods mod-
ify the resistance values to consider the difference
in diameter between the pile and the cone. The in-
fluence of mean effective stress, soil compressibil-
ity, and rigidity affect the pile and the cone in equal
measure, which eliminates the need to supplement
the field data with laboratory testing and to calcu-
late intermediate values, such as K, and Nq.
• Pure empirical methods – initial formulations
were based solely on cone resistance (qc) derived
from mechanical cone penetrometers. Subse-
quently, with the introduction of the electrical
cone penetrometer, the additional channels meas- Fig. 13. Upper and lower empirical values
uring sleeve friction ( fs), and porewater pressures of different piles in coarse grained soils for:
(u1 and u2) were considered. (a) qs(z); (b) qb (after Kempfert and Becker [17])

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92 B. WRANA

Unit skin resistance qs(z) and unit base resistance qb mon practice is by means of a static loading test. The
capacity is the total ultimate soil resistance of the
Based on the load test database up to 1000 load
pile determined from the measured load-settlement
tests on precast concrete, cast-in-place concrete, steel
behavior. It can be defined as the load for which
pipe, screw cast-in-place, and micro piles etc.,
rapid settlement occurs under sustained or slight
Kempfert and Becker [17] developed correlations for
increase of the applied load – the pile plunges. This
pile qs(z) and qb from CPT qc and su. Their results,
definition is inadequate, however, because large set-
presented in the form of empirically derived charts
tlement is required for a pile to plunge and is not
with upper and lower bound estimates of qs(z) and qb
obtained in the test. Therefore, the pile capacity or
(Fig. 13), have been integrated into the national Ger-
ultimate load must be determined by some definition
man recommendations for piles.
based on the load-settlement data recorded in the
Partial embedment reduction factor test.
Load-displacement curves obtained from axial
White and Bolton [31] studied, the causes of low load tests on pile foundations exhibit differing shapes
values of qb/qc in sand in contrast with qb = qc for and resulting conclusions. There is only a single value
steady deep penetration (e.g., cavity expansion solu- of load termed “capacity” that is selected from the
tions and strain path method). They examined a data- entire curve for design purposes. Yet, there are at least
base of 29 load tests on a variety of CE piles (steel 45 different criteria available for defining the “axial
pipe piles, Franki piles with enlarged base, and precast capacity” (Hirany and Kulhawy [8]). An example of
square, cylindrical and octagonal concrete piles) and a load test conducted on a 0.76 m diameter, 16.9 m
CPT qc data. The low value of qb/qc, which forms long drilled shaft installed at Georgia Institute of
basis of the apparent scale effect on the diameter, can Technology is shown in Fig. 15.
be attributed topartial embedment in the underlying
hard layer (Fig. 14), whereas, partial mobilization was
explained by defining failure according to a plunging
criterion. They concluded that any reduction of qc
when estimating qb of CE piles in sand should be
linked to the above factors rather than pile diameter.

Fig. 15. Comparison of capacity interpretation criteria

from axial pile load tests (Hirany and Kulhawy [8])

When interpreting loading tests, the failure con-

dition can be interpreted in several different ways.
Tomlinson [30] lists some of the recognized criteria
and list disadvantages and advantages of pile tests in
general. The main interpreted failure loads corre-
Fig. 14. Partial embedment reduction factor on qb spond to settlements equal to 0.1d, where d is the
(after White and Bolton [31]) equivalent pile diameter referring to an equivalent
circle diameter for square and hexagonal piles. Such
definition does not consider the elastic shortening of
8. PILE LOAD CAPACITY DETERMINED the pile, which can be substantial for long piles,
FROM STATIC LOAD TESTING while it is negligible for short piles. In reality, set-
tlement relates to a movement of superstructure (pile
with soil), and it does not relate to the capacity as
The pile bearing capacity is necessary to verify a soil response to the loads applied to the pile in
with the assumption of the design. The most com- a static loading test.

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 Pile load capacity – calculation methods 93

REFERENCES [18] KOLK H.J., VAN DER VELDE A., A Reliable Method to
Determine Friction Capacity of Piles Driven into Clays,
Proc. Offshore Technological Conference, 1996, Vol. 2,
[1] American Petroleum Institute, API Recommended Practice Houston, TX.
for Planning, Designing and Constructing Fixed Off-shore [19] KRAFT L.M., LYONS C.G., State of the Art: Ultimate Axial
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Unauthenticated
Download Date | 3/8/16 7:28 PM
 PILE FOUNDATION DESIGN AS PER IS 2911--2010
The pile code consists of four parts. They are Part 1:Concrete piles Part 2: Timber
Piles Part 3: Under reamed Piles Part 4: Load test on Piles. Out of these Part 1
consists of four section. Among these, Section 1: Driven cast in situ concrete piles.
Section 2: Bored cast in situ concrete Piles and Section 3: Driven precast concrete
Piles are revised in 2010. Some of the important changes in the code are highlighted
as follows:
1. Definitions of various terms have been modified as per the prevailing engineering
practice.
2. Minimum grade of concrete to be used in pile foundations has been revised to
M 25.
3. Design parameters with respect to adhesion factor, earth pressure coefficient,
modulus of subgrade reaction, etc, have been revised to make them consistence
with the outcome of modern research and construction practices.
4. Minimum dia of bored cat in situ pile have been changed as 450mm based on
following parameters:
Clear cover over reinforcement Cl.6.11.4 50 x2 = 100mm
Dia of vertical bar minimum Cl. 6.11.4 12 x 2 = 24 mm
Helical reinforcement minimum dia cl. 6.11.4 8 x2 = 16 mm
Clearance for operation of Tremie Cl.6.11.4 4 x20 = 80 mm
Dia of Tremie pipe minimum Cl.8.4 (c) = 200 mm
Total dia of pile required = 420 mm say
450mm
5. Provisions for special use of large diameter bored cast in-situ reinforced cement
concrete piles in marine structures have been added.
6. Procedures for calculation of bearing capacity, structural capacity, factor of
safety, lateral load capacity, overloading, etc, have also been modified to bring
them at par with the present practices.
7. Minimum time of curing before handling of precast piles has been modified.
8. Provision has been made for use of any established dynamic pile driving
formulae, instead of recommending any specific formula, to control the pile
driving at site, giving due consideration to limitations of various formulae.

Selection of pile foundation:

1. For Poor bearing soils such as Soft clay, Medium Clay or any clay etc. under

reamed pile foundation with pile cap and grade beam or Bored cast insitu

Pile foundation with pile cap may be adopted.

2. For clay soil with G+2F, G+3F, silt+3floors loads, under reamed piles with

pile caps connected with grade beams is best suited.

 If the hard strata available is at 10m depth, End bearing/ Bored cast insitu

piles with pile caps may be adopted. This type of foundation is also best

suited for multi storeyed building also.

3. If the load on foundation is 500KN (say G+2Floor load), Under reamed pile

foundation for a depth of 3.5m to 4.50m with pile caps shall be adopted.

If the load on foundation is in the range of 2000KN to 3000KN, Bored cast

in ‘situ concrete piles with 6.0m to 20.0m depth with pile caps shall be

adopted. This type is adopted in case of multi storeyed building.

Design aspects on Pile:

1. A minimum of three piles are required under a column in order to resist all the
column loads and moments acting on it. If one or two piles are provided under a
column, the grade beams have to be provided in one or more directions which have
to be designed to resist fully column basements since piles will resist the vertical
loads only.
2. Further, column shear will also be resisted by piles, which are assumed to have
.a horizontal load carrying capacity equal to 5% of its vertical load capacity. It may
be noted that horizontal load on a pile is the result of earthquake or wind loads only,
for which 25% excess pile capacity is allowed by codes.
3. The bearing capacity of a single pile is governed by the structural strength of the
pile and the supporting strength of the pile and the supporting strength of soil
stratum and the lower one is used for the design.
4. The selection of the type, length and capacity of pile is usually made from
estimation based on the soil conditions and the magnitude of load. In large cities,
where the soil conditions are well known and where a large number of pile
foundations have been constructed, the experience gained in the past is extremely
useful.
5. Generally the foundation design is made on the preliminary estimated values.
Before the actual construction begins, pile load tests must be made to verify the
design values, the foundation design must be revised accordingly to the test results.
6. Piles in sands are driven to the maximum possible depth because pile load is
proportional to square of the length while its cost increases at a smaller rate. In
clayey soils, increased length does not increase the capacity as rapidly as in sandy
soils. In clays the length is determined by block failure criterion. Provision of a
spread or bulb at the bottom increases the pile load considerably, at a very small
increase in the cost.
Cost comparison of pile foundation with reference to other foundation:.
1. In general the pile require foundation is likely to be more expansive than spread
footings or mat.
2. Weak soils with heavy column loads require either rafts or piles. Rafts, in
general are more economical than piles. But, when rafts become very large, piles
have to be used for restricting both cost and settlement.
3.When piles and rafts are both equal in cost, then piles are preferable to rafts, as
the settlement for piles is considerably less than that of rafts. Thus, pile foundations
have to be used when raft foundations are not suitable on grounds of economy or
settlement considerations.
4. Generally up to 3 to 4 floors (storeys), provision of raft foundation works out to
be economical when a medium bearing/moderate bearing soil is available upto a
depth of 2 to 3m. However for structure having more than 4 storeys and a hard
strata is available only 10m depth below ground level, then adoption of pile
foundation is the best in addition to be economical.
5.The key to economy in the pile foundations is to keep the area of pile cap in plan
to the minimum, for which high capacity piles should be used.
6. Capacity of a pile is increased by increasing either the diameter or depth of a
pile, pile spacing (equal to three times the diameter) will increase and this will lead
to a large pile cap, which will result in increased cost. So, pile depth should be
increased to get high capacity piles. Thus, economy requires the use of a few high
capacity deep piles under a column rather than a large number of low capacity
shallow piles.
The technical details of various types of piles are as detailed below:
1. Driven cast in situ piles: IS 2911 (Part 1/sect.1)-2010
This type of Pile is suitable for situations where accurate determination of pile
length cannot be made on the basis of sub soil investigation and in cases where
lengths of different piles may be different.
Driving of piles in sandy soils results in volume changes. In loose soils
compaction takes place. This has a beneficial effect on the soil strength. Driving
piles are advantageous in sandy soils, when compared with bored cast in situ Piles.
in Sandy soils, the point resistance is large compared to the skin resistance and
compaction at the tip greatly helps to improve the pile load.
For driven piles through sandy soil and resting on a firm cohesionless material, ‘N’
value shall be between 35 and 45.
 Driving piles into clays may alter the structure of the soil and reduce rather than
increase the strength of the soil medium. Driven piles are generally not desirable in
clayey soil since the set value resistance.es are false due to pore pressure
resistance. Driven piles are not suitable in predominantly boundary strata. Design
aspects:
Spacing of piles
End bearing : Piles (hard strata) =2.5 D
Piles resting on rock =2.0 D
Friction Piles =3.0 D where D is dia of piles.
Reinforcement:
Longitudinal reinforcement:
Main rod: Minimum 6Nos. bars shall be used. Minimum dia. of bar is 12mm.
Clear horizontal distance between bars: 4 times max. size of aggregate.
Minimum =0.4% of c.s. (of any type or grade)
Links: Minimum dia. =8 mm
Spacing not less than 150mm
Settlement of driven cast insitu piles:
Not more than 10mm for 20 blows with a 3 tonnes drop hammer over a height of
1.0m as a general guide.
Estimate for driven cat in situ pile:
The cost of pile foundation is roughly one floor cost for the building upto 8 storeys
10% extra for each additional floor may be added.
2. Bored cast insitu Piles: IS 2911 (Part 1/sect. 2)-2010
These piles are ideal as end bearing piles particularly when they are to be socketed
into rock. These piles are suitable where vibrations to the existing adjoining buildings
produces noisy and disturb the structures in the immediate vicinity is not desirable. For
high capacities (150 tonnes to 300 tonnes) these piles are preferable.
For bored piles through sandy soil resting on a firm cohesionless material, ‘N’ value
shall be between 40 and 50. Bored cast in situ piles in clay, the capacity must be
ascertained from shear strength of the supporting soil. The ultimate bearing capacity of
a bored pile in a granular soil is generally taken as being 1/3 to ½ that of the ultimate
bearing resistance of a driven pile within the same soil.
Design aspects:
Spacing of piles:
The minimum centre-to-centre spacing of piles is considered from three aspects,
namely,
a) practical aspects of installing the piles,
b) diameter of the pile, and
c) nature of the load transfer to the soil and possible reduction in the load capacity
of piles group.
Minimum spacing: End bearing piles=2.5D ; Piles resting on rock=2 D;
Friction piles=3.0 D where D is dia of piles.
Reinforcement:
Longitudinal Reinforcement: Mini. Ast =0.4% of sectional area of pile.
Main rod: Minimum 6Nos. bars shall be used. Minimum dia. of bar is 12mm.
Lateral ties: Mini. Dia of links or spiral shall be 8 mm and spacing of links or spirals
not less than 150mm.
Concrete Grade: Minimum M25. The minimum cement content shall be 400 kg/m3 .
However, with proper mix design and use of proper admixture the cement content
may be reduced but in no case the cement content shall be less than 350 kg/m3.
Settlement of Bored cast insitu piles:
Bore may be terminated at a depth where the penetration with 1 tonne chisel
dropped from a height of 1.20m should not exceed 150mm for blows.
3. Driven Precast Piles: IS 2911(Part 1/sec.3)-2010
Precast piles are suitable for places where wooden piles are likely to be weaken by
the white ants or other insects and termites. These piles are preferred for Bridge
foundation. Generally these piles are square or octagonal in shape owing to ease in
casting them. Single length precast pile cannot be designed for a length exceeding
18 to 20m, because of undue handling stresses. It derives its capacity from friction
and bearing. Maximum loads on precast piles are generally limit to 100 tonnes. The
optimum loads generally are 50 to 60 tonnes.
Design aspects:
Spacing of piles:
Minimum spacing
 Hard Stratum: End bearing piles =2.5D
Piles resting on rock=2.0 D
Friction piles =3 D where d is dia. of pile
Reinforcement:
Main rod:
(i) Pile length <30 times least width =1.25% of c.s.area
(ii) Pile length 30 to 40 times least width =1.5% of c.s. area
(iii) Pile length > 40 times least width =2.0% of c.s. area.
Minimum : 6 nos. of rod. Minimum dia not less than 12 mm.
Ties: Mini. Dia of links or spirals =8 mm.
Spacing shall not be less than 150mm.
Spacing of ties: closer near ends. Max. spacing=3 times least width of pile.
Stiffener rings:
Stiffner rings preferably of 16 mm diameter at every 1.5 m centre-to-centre to be
provided along the length of the cage for providing rigidity to reinforcement cage.

4. UNDER –REAMED PILES : IS 2911 (PART III ) – 1980

(1) UNDER-REAMED PILE :

If a short bored pile is provided with one, two or three bulbs, to provide anchorage

and / or to increase the bearing load, it is known as the under-reamed pile. Due to

their enlarged based they provide anchorage to structures founded on expansive soils,

carry higher loads in comparison to uniform diameter bored piles and offer high uplift

resistance. This type of pile is particularly useful in swelling soils like black cotton soils.

(2) SUITABILITY :

The choice of the pile is governed by site conditions, economics and time
considerations. Under-reamed Piles are generally adopted in locations, where
soft / loose soils occur at top and relatively compact layers are available at
lower elevations. Best use is made of the existence of such compact soil
layers by resisting the bulb ( at the bottom of the Pile Stem ) so that an
increased bearing capacity ( Point Bearing ) of the pile is obtained. Under-
 reamed Piles are also used in locations where the top soils are of high
swelling and high shrinking type in which case the bulbs are rested at a
depth well below the zone of variation of moisture content. In case of
expansive soils (e.g) black cotton soils or filled up soils, under- reamed piles
with bulbs provide a good anchorage. It is found that provision of bulbs in
the under-reamed piles increases the lateral load capacity of piles.

Under-reamed Compaction Piles:

For loose to medium sandy and silty soils, bored compaction piles should
be used since in such piles, the compaction process increases the load
bearing capacity of piles.

Under-reamed compaction piles are basically cast-in-situ concrete piles

having one or more bulbs. These combine the advantage of both the bored
and driven piles.

(3) CONSTRUCTION ASPECTS:

General equipments required (a) Auger (b) Under- reamed (c) Boring
Guide (d) General tools like cutting tools, extension rods and general

T & P.

Boring is usually done by manual earth auger, handled by tripod hoist.

Three men can easily advance 3.5 m of a hole of dia. up to 30 cm. in about

6 hours, in normal conditions. For Piles larger than 3.5 m and / or of the

dia. larger than 37.5 cm. stem diameter, a tripod is required.

After reaching the desired depth, the bore is enlarged with a special

under- reaming tool. It consists of two collapsible cross- blades with a

bucket at the bottom to collect the scrapped soil. The under-reamer is

lowered in the bore hole and rotated with progressively increasing

diameter. The scraped soil collected in the bucket is lifted up and disposed
 off from time to time. In sandy soils, the bore hole is to be kept filled with

drilling mud if lowering is to be done under water. After the under-reamer

is formed, boring is further advanced. Bottom spreads are made with tools.

Piles should be concreted soon after boring, under the supervision of a

qualified person. Pre- fabricated reinforcement cage is inserted in the hole

and concrete of suitable workability (M20), slump 70 to 150 mm is poured

down through a funnel. For under water construction, concrete of higher

slump should be placed by displacement method using Tremie Pipe (not

less than 150 mm diameter).

(4) DESIGN ASPECTS:

(1) Minimum length of Pile below ground level =3.0 m.

(2) Minimum diameter of Stem (D) in mud =250 mm.

(3) For strata consisting of harmful constituents such as sulphate

diameter =300 mm.

(4) Bulb diameter (Du) =2 to 3 times diameter

Preferable = 2.5 times diameter

(5) Thickness of bulb =¼D

(6) Maximum spacing of bulbs:

Piles up to 300 mm dia. =1.5 Du

Piles greater than 300 mm dia =1.25 Du

(7) Minimum depth of top most bulb =2 Du or 1750 mm for

expansive soils. The minimum clearance below the underside of

pile cap embedded in the ground and the bulb should be a minimum
 1.5 times the bulb diameter.

(8) Location of bottom most bulb from toe = Bucket length +0.55D

Bucket length for 200 to 250 mm dia. = (40 ‡ 5 ) cm.

300 mm dia. = (45 ‡ 5 ) cm.

375 mm dia. = (50 ‡ 5 ) cm.

400 mm dia. = (55 ‡ 5 ) cm.

450 mm dia. = (65 ‡ 5 ) cm.

500 mm dia. = (70 ‡ 5 ) cm.

(9) Number of bulbs in a Pile = Maximum 2 (Restricted)

In case the site is such that the depth of fill is more and water
table is high, as far as possible choose single under- reamed Pile.

If, on the other hand, the water table at the site is low and the
depth of fill is less, choose single or double under-reamed
depending upon the load capacity required. In case of double
under-reamed pile, the first under- ream should be 2 times the
bulb dia. or 1.75 m below G.L.

(10) Location of Piles :

Piles are provided on the corners of the buildings and wall

junctions. For intermediate piles, they should be arranged in such a
way that the doors and windows openings lie centrally as far as
possible. For the structures with columns (e.g) multistoreyed
complex etc., the piles should be first laid for columns and then for
walls. The maximum spacing between two piles in a beam and pile
construction should not normally exceed 3 m.
(11) Mix: Minimum M20 concrete and minimum cement content shall

be 400 kg/m3 in all conditions. M15 concrete with minimum

cement content 350 kg/m3 shall be used for without provision for

under water concreting and non-aggressive sub soil conditions.

(12) Reinforcement:

Longitudinal reinforcement:

Ast = Tension / σst

Minimum steel = 0.4% of c.s. area of stem for M.S. rod

=0.3% of c.s. area of stem for HYSD bars.

Minimum main rod: 3 Nos. of 8 mm dia RTS. For piles of lengths

exceeding 5 m and or 37.5 cm diameter, a minimum number of six 12-

mm diameter bars of mild or high strength steel shall be provided. For

piles exceeding 40 cm dia, a minimum number of six 12-mm diameter

mild or high strength steel bars shall be provided.

Transverse Reinforcement :

(i) 6 mm dia. M.S. circular stirrups at a spacing not more than

stem dia or 300 mm whichever is less.

(ii) For piles of lengths exceeding 5 m and dia. exceeding 375

mm use 8 Φ stirrups.

(13) Clear cover :

Minimum clear cover to reinforcement

Sides =40 mm (normal); 75 mm in case of sulphate presence.

 Bottom =75 to 100 mm.

(5) ULTIMATE CARRYING CAPACITY OF PILES:

a. From soil properties

b. From load test (as per I.S.2911 (Part IV)-1985).

c. From Table 1 of I.S. 2911 (Part III)-1980.

(6) APPLICATION OF UNDER-REAMED PILES:

Under- reamed piles are useful for both in expansive clays as well as in
loose to medium no-expansive soils. It has been established that under-
reamed piles combine safety, economy and speed in construction, especially in
expansive soils. Under-reamed piles are used for following structures:
(i) Residential buildings.
(ii) Industrial sheds, workshops, godowns.
(iii) Machine foundations.
(iv) Retaining walls, boundary walls, fences
(v) Anchors, reaction frames.
(vi) Transmission line towers and pole footings
(vii) Water tanks, silos and bins, cooling towers
(viii) Bench marks, international boundary posts.

(7) LIMITATIONS OF UNDER-REAMED PILES:

Under-reamed Pile foundation can be used in expansive soil strata and in

other soils where the bore and bulb can be supporting or by mud fluids. In

certain cases it is found that the upper most layer consists of medium to

stiff consistency clay up to 1.50m to 2.0 m and beneath it very very soft

consistency clay (bitter like nature) to considerable depth. In such cases

the under-reamed pile foundation cannot be used.

Under-reamed piles in expansive soils is one of the widely used

solutions, but it cannot be considered the best. Failures of structures

supported on under-reamed piles have been reported. As such, it cannot be

treated as a ‘ fool-proof’ method.

 Important Note:- For determining the ‘N’ value the average may be taken

up to a depth equal to the bulb diameter below the pile toe.

(8) DRAW BACK:

(i) Under-reamed piles are suitable generally only for structures contributing
low loads (20 t to 40 t).
(ii) Because of the presence of unreinforced portion of concrete in the under-
ream bulb, there is a chance of failure of concrete by shear in the bulb
portion. Hence the design engineer must carefully select the sizes of the
pile while proposing under-reamed piles. Further concreting of under-reamed
piles shall be done by tremie only since GW displacement during concreting
must not be allowed.
SKETCH:

Section of single under Section of double under

reamed pile reamed pile
 TABLE 1

SAFE LOAD FOR VERTICAL BORED CAST IN SITU UNDER-REAMED PILES IN

SANDY AND CLAYEY SOILS INCLUDING BLACK COTTON SOILS
Dia.of piles Length in ‘m’ Longitu Rings Safe Loads (KN)
dinal
Compression Uplift Lateral Thrust

Load

D Du= Single Dou No.- 6 mm Single Double Single Dou Single Double
UR bleU DIa Φ@ UR UR ble
2.5D UR UR UR
R
UR

3.5 3.5 3 #10 180 80 120 40 60 10 12

200 500

250 3.5 3.5 4 #10 220 120 180 60 90 15 18

625

300 3.5 3.5 4#12 250 160 240 80 120 20 24

750

375
940 3.5 3.75 5#12 300 240 360 120 180 30 40

1000 3.5 4.0 6#12 300 280 420 140 210 34 40

400

450
1125 3.5 4.5 7 #12 300 350 625 175 256 40 48

500
1250 3.5 5.0 9 #12 300 420 630 210 315 45 54
NOTES TO TABLE 1 OF I.S.2911 (Part III) - 1980 :-

1. This Table apply to both medium compact sandy soils (10 < N <30) and clayey
soils of medium (4 < N < 8) consistency including expansive soils.

2. For dense sandy soil (N ≥ 30) and stiff clayey soils (N ≥ 8) , safe loads in
compression and uplift may be increased by 25%.

3. For loose sandy (4 < N < 10) and soft clayey soils (2 < N 4) safe loads should
be taken 0.75 times Table value.

4. For very loose sandy soil (N ≤ 4) and very soft clayey soils (N ≤ 2 ) the Table
value should be reduced by 50%.

5. For piles with bulb of twice stem dia, the table value should be reduced by 15%.

6. If the pile is full of sub soil water, safe load shall be reduced by 25% as per
clause B 1.6 given in Appendix B.

7. If the spacing between piles is reduced by 1.5 times instead 2 times the under
seamed bulb dia. as per normal requirements safe load shall be reduced by 10%

8. If the under reamed bulb is 2 times pile dia. instead 2.5 times pile dia, safe load
shall be reduced by 10% as per clause 5.2.3.3.