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Deep Foundations 2

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Load Capacity of a Single Pile

� All calculations of pile capacity are approximate

because it is almost impossible to account for the

variability of soil types and the differences in the quality

of construction practice.

� The ultimate pile capacity Qult consists of two

components: skin/shaft friction or side shear Qf and

end bearing at the pile tip or base Qb.

Qult = Qf + Qb

� The allowable pile capacity is expressed as:

Qall = Qult/factor of safety

A minimum factor of safety of 2.5 is typically maintained.

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Load Capacity of a Single Pile

Cairo University

Load Capacity of a Single Pile

� Some pile movement is needed to mobilize skin friction

and end bearing.

� Pile load tests on driven piles have shown that a

vertical pile movement of 2.5 mm to 10 mm is needed

to fully mobilize skin friction.

� For driven piles, end bearing resistance is fully

mobilized when the vertical pile displacement is about

8-10% of the pile tip diameter (for diameters 40-120

cm in diameter, movements between 32 mm to 120

mm).

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Load Capacity of a Single Pile

� Similar response is

characteristic of bored piles.

� Generally, full mobilization

of skin friction and end

bearing does not occur at

the same displacement.

Skin friction is mobilized at

about 10% of the

displacement required to

mobilize end bearing.

Concept of Mobilized Resistances

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Methods for evaluating capacity of deep foundations

1. Full-scale load tests on prototype

foundations.

2. Analytical methods based on soil

properties obtained from laboratory

and/or insitu tests.

3. Dynamic methods based on pile driving

data or wave propagation.

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Analytical Methods

�

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Analytical Methods

� For a cylindrical pile of uniform diameter (D),

penetrating a homogenous soil, Qf is given by:

Qf = �.su.�.D.L

Where L is the pile length embedded in soil.

� There is a wide range of variability in the values of �

reported in literature.

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Analytical Methods

In this course the following values of � will be adopted:

Bored piles: � = 0.3 to 0.4

Driven piles:

� = 1 for very soft clay (su = 0-12.5 kPa)

� = 1 for soft clay (su = 12.5-25 kPa)

� = 0.8 for medium stiff clay (su = 25-50 kPa)

� = 0.6 for stiff clay (su = 50-100 kPa)

� = 0.4 for very stiff clay (su = 100-200 kPa)

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Analytical Methods

� The end bearing capacity is found by analogy with

shallow foundations as expressed by:

Qb

= qb.A

b= N

c(s

u)b

Ab

Where qb

is the pile base (tip) resistance; Nc

is a bearing

capacity factor typically taken equal to 9; (su)b

is the soil

undrained shear strength at the pile base; Ab

is the pile

cross sectional area at the pile base. The undrained

shear strength should be obtained within 2 pile diameters

below the pile tip.

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Analytical Methods

� The � method is based on an effective stress analysis

and is used to determine the short term and long term

pile load capacities of coarse grained soils and the

long term response of fine grained soils. Friction along

the pile shaft is computed using Coulomb’s friction

law, where the frictional stress is given by

fs

= �.�x\ = tan(�i

\�.�x\

Where � is the coefficient of friction between the pile and

soil; �x\ is the lateral effective stress; �i

\ is the interface

effective angle of friction.

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Analytical Methods

Cairo University

Analytical Methods

The end bearing capacity is calculated by analogy with

the bearing capacity of shallow footings as expressed

below:

Qb = qb.Ab = Nq (�z’)b Ab

Where Nq (�z’)b is the base resistance; Nq is the bearing

capacity coefficient as a function of � \ ; (�z’)b is the

effective vertical stress at the pile base; Ab is the pile

cross sectional area at the pile base.

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Pile Groups� The most practical situations, piles are used in groups.

They are arranged in geometric patterns at a spacing “s”.

� The piles are connected at their heads by a concrete pile

cap.

� The spacing between piles in a pile group should be kept

as large as possible to avoid driving problems and

overlap of stresses which may cause a reduction in pile

capacity or excessive settlement.

� large spacings are impractical since they will require

massive and heavy pile caps.

� The optimum spacing between piles should be between

2.5 and 3 times the diameter of pile.

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Pile Groups� To prevent any side deformation of piles under load, tie

beams in two directions should by used to connect a

single pile-cap to adjacent pile caps.

� Theoretical and experimental investigations have

shown that friction pile groups may fail as a unit before

the load per pile reaches the maximum capacity of a

single pile.

� This means that the load-carrying capacity of a group of

friction piles will be less than the sum of the individual

pile capacities.

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Pile Groups� Each individual pile is supported by the surrounding and

underlying soil. The pile imposes a region of stress

influence on the soil, which is greatest immediately

adjacent to the pile, and decreases with increasing

distance from the pile.

� If multiple piles are used in a group, the regions of stress

influence overlap, and the capacity of the soil to support

the piles may be reduced.

� In addition to the effect of stress overlap, the capacity of a

pile may be influenced by the installation of neighbouring

piles.

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Efficiency of Pile GroupsIt is common to not allow for any increase in capacity due to

densification effects. However, pile group capacity losses

are an effect which engineers must be careful to account for.

Pile group capacity loss is by convention calculated using a

pile group efficiency factor, �.

� = Qult.group/(n.Qult)

Where n is the number of piles in the group, Qult.group is the

total load capacity of the group of piles and Qult is the load

capacity of a single pile.

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Efficiency of Pile Groups� A check of pile-group efficiency (group action) can be

made by considering the group to act as one large pile

whose length equals to the length of individual piles and

its cross-sectional area equals the area enclosed within

the outer perimeter of the group.

� The capacity of this large pile is computed in the same

way as the friction pile capacity is computed. If this

capacity (P') of the large pile is equal to or greater than

the sum of individual pile capacities (np) the group is

100% efficient.

� If not the group efficiency is less than 100% and this

condition can be treated by increasing the pile length

and/or increasing the pile spacing.

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Considerations for Pile Groups� When piles are resting on a strong layer underlain by a

weaker one, it is important to check that the bearing

capacity of the underlying layer is not exceeded so that

the group does not punch through the bearing layer as

illustrated below. Shallow footing analogy may be

applied.

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Negative Skin Friction� Negative skin friction is a force developed between the

soil and the pile in the downward direction due to soil

compressibility. This force may be large enough so that,

in conjunction with the applied load from the

superstructure, the piles will settle excessively and

foundation failure may occur.

� Negative skin friction occurs in response to relative

downward deformation of the surrounding soil to that of

the shaft, and will not develop if downward movement of

the drilled shaft in response to axial compression forces

exceeds the vertical deformation of the soil.

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Negative Skin Friction

� The potential for negative skin friction is greatest when

the soils in the upper zones of the subsurface profile can

settle and where the lower portion of the shaft is founded

in a relatively rigid material such as hard/dense soil or

rock.

� Examples of soils that will undergo settlement after pile

construction include loose sand, soft to medium stiff clay,

recently-placed fill, and soils subjected to earthquake–

induced liquefaction.

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Negative Skin Friction

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Negative Skin Friction

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Negative Skin Friction

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Negative Skin Friction

The negative skin friction can be estimated emperically

from data given in figure where the weight of soil enclosed

in the shaded area is to be added to the structural loading.

Negative Skin Friction

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Displacement of Pile Groups

Friction Piles

After checking the group action of the friction pile group, the

load support.ed by the pile foundation is assumed to be

transferred at the lower third point of the pile length on an

area equal to the area enclosed by the piles. The settlement

will be considered as that due to the consolidation of the

thickness (H) and not of the whole thickness of the

clay layer,

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Displacement of Friction Piles

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Displacement of Pile Groups

End Bearing Piles

The settlement of a pile group is larRer than the settlement

of the single pile and depends on the size of the pile group,

The larger the pile group; the deeper the stress bulb

penetrates the bearing stratum, and consequently the

settlement of the group will be much larger than that of a test

pile although each pile of the group is carrying the same

allowable load determined from test pile.

Displacements of Bearing Piles

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Pile Load Test� In spite of the most thorough efforts to correlate drilled

shaft performance to geomaterial properties, the

behavior of drilled shafts is highly dependent upon the

local geology and details of construction procedures.

� This makes it difficult to accurately predict strength and

serviceability limits from standardized design methods

such as those given in this manual.

� Site-specific field loading tests performed under realistic

conditions help in improving the accuracy of the

predictions of performance and reliability of the

constructed foundations.

Conventional Pile Load Test Setup

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FHWA-NHI-10-016

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Pile Load Test-ECP

According to the Egyptian Code of practice:

� It is advisable to drive test piles and to carry out a loading

test for each 100-200 piles.

� There are two methods for performing the pile load test:

� Maintained Load test الحمل على مراحل

� Constant Rate of Penetration معدل الھبوط الثابت:

0.4 mm/minute for piles resting in clay

2 mm/minute for piles resting in sands.

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Maintained Load Test-ECP

Applied load (% of design load) Duration

25 1 hour

50 1 hour

75 1 hour

100 3 hours

125 3 hours

150 12 hours

125 15 minutes

100 15 minutes

75 15 minutes

50 15 minutes

25 15 minutes

0 4 hours

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Interpretation of Pile Load

� Conceptually, bearing capacity “failure” is defined when a

constant stress is reached.

� However, foundation load tests do not always reach a

well-defined peak stress because of practical limitations

on field equipment and test setups, or because a

progressive failure allows repositioning of soil particles

beneath the foundation, thereby the highest stress is not

fully achieved.

� This creates ambiguity in defining the “true” bearing

capacity,

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Interpretation of Pile LoadModified Chin’s Method - Hyperbolic Asymptote

� One of the simplest forms to represent non-linear curves

is the hyperbola as only two constants are required.

� Fitting a simple hyperbola to load-displacement test data

has been used for evaluating the bearing capacity of

piles (Chin, 1971).

� The simple hyperbolic relationship between stress q and

pseudo-strain es

(s/B) is:

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Interpretation of Pile LoadModified Chin’s Method - Hyperbolic Asymptote

where ki = initial stiffness at zero displacement and qult =

ultimate load (asymptote of the hyperbola). The parameters

ki and qult are determined objectively by plotting the

transformed axes: es/q versus es, which is represented by a

straight line given by:

where 1/ki = y-intercept for zero displacement, and 1/qult is

the slope of the straight line. Thus, the hyperbola requires

two constants (ki and qult) that are determined and have

physical significance: the initial stiffness ki = q/s at s = 0,

and the asymptote qult at infinite displacements ) ( ∞ →s) .

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Interpretation of Pile LoadModified Chin’s Method - Hyperbolic Asymptote

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Interpretation of Pile LoadModified Chin’s Method - Hyperbolic Asymptote

� According to ECP, the ultimate axial pile capacity is:

Qult = 1/1.2 s

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Design of Pile Design Load - ECP

� The ultimate pile is not less than 2 times the design load

considering dead and live load, 1.75 times the design

load considering also wind loads and 1.5 times the

design load taking earthquake loading into consideration.

� Additionally, a pile load test is considered satisfactory if

the measured pile settlement after 12 hours under 1.5

times the design load does not the following equation:

s = 0.02 d + 0.5QL/AE

d=pile diameter, Q =1.5 design pile load, L = pile length, A =

pile cross sectional area, E = pile material modulus of

elasticity.

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Eccentric Loading

� The load per pile (i) Pican be determined from the

following formula:

The above equation is valid provided that the pile cap is rigid and the

pile settlement into the soil is proportional to the load it carries.