LECTURE - Foundations (5)

8/19/2019 LECTURE - Foundations (5)

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Geotechnics I Foundations

Civil Engineering Programme Area, ITB 45

4.

Deep foundations

Pile types4.1

If sufficiently competent ground does not exist at shallow depths

then a deep foundation can be used to transfer the structural loads

to deeper, stronger ground. Piles are the main type of deep

foundation.

Fig. 24: Bored and driven pile installation

There are two general types of pile, according to how they are

installed: bored piles and driven piles.



Bored piles

In bored cast-in-place piles, a borehole is excavated and

then filled with concrete, which hardens to form the pile.

These piles cause little disturbance to the surrounding

ground, so there is no risk of ground heave. Bearing

resistances up to 10,000kN, diameters up to 1m, and

lengths up to 50m are not unusual.

Driven piles

Prefabricated piles in steel, concrete or timber are driven

into the ground. Alternatively, a hollow metal shell can

be driven into the soil and then filled with concrete. Pile

driving causes significant disturbance of the soil around

the pile, so these displacement piles usually cause heave

of the neighboring ground surface, which can affectnearby structures. Bearing resistances up to 4,000kN,

diameters up to 0·5m, and lengths up to 30m are not

unusual, depending on the type of pile used.

Ultimate bearing resistance of piles4.2

When a pile is loaded, it tries to move down vertically further into

the ground. Resistance develops along the pile’s sides as a shear

stress q s , which may vary with depth, and as a bearing pressure qb

on the pile’s base.





Fig. 25: General pattern of stresses on a pile

The base resistance is conventionally calculated using the bearing

resistance formulae developed for shallow foundations with

modified values for the bearing capacity factors. This therefore

treats the base as a footing. For undrained failure:

Rb,d

Ab

qb,d cu,d N c* 9cu ,d

where Ab is the base area, while for drained failure:



Rb,d

Ab

qb,d 0 N q*

where 0’ = effective overburden pressure at base of pile. N q* is

given by an empirical relationship.

d (°) N q

*

28 12

30 17

32 25

34 40

36 58

38 89

40 137

Table 3: Bearing capacity factor N q* for drained failure of a pile

(after Craig, 1997)

The value of 0’ does not increase indefinitely but has a limiting

value at a critical depth 20 pile diameter/width. An example of

the change of 0’ for a single layer soil with deep water table is as

below:





Fig. 26: Example of maximum value of ’v used for calculation

of qb in pile (single layered soil with deep water table)

The shaft resistance is calculated assuming shear failure:

R s,d q s,d A s

where s is incremental surface area of the shaft and the shear

stress is summed over the length of the pile. In an undrained

analysis or total stress analysis, appropriate to rapid loading, the

shaft resistance can be calculated using the soil’s design shear

strength reduced by an adhesion factor, , to take account ofremoulding of the soil around the pile during construction:

q s,d cu,d

where 1 for soft clay, and is between 0.3 and 0.6 for

overconsolidated clay. For a drained analysis or effective stress



analysis, suitable for long term loading, the shaft resistance is

usually estimated assuming an earth pressure coefficient and

frictional shear failure, with:

q s,d

K s

vtan

d

where K s is the horizontal earth pressure coefficient, v the

average vertical effective stress along the embedded pile length,

and d is the design angle of friction between the shaft surface and

the soil. The typical values for and K s (Broms, 1966) are given

below:

Pile material

K s

Relative density of soil

Loose DenseSteel 20 0.5 1.0

Concrete 0.75’ 1.0 2.0

Timber 0.67’ 1.5 4.0

Table 4: Typical values for and K s suggested by Broms (1966)

The individual contributions of the shaft resistance and the base

resistance are added to find the bearing resistance of the pile, Rd :

Rd

Rb,d

R s,d

A pile where R s,d Rb,d is known as a friction pile or a shaft

bearing pile, while if Rb,d R s,d this pile is known as an end-

bearing pile. The design force on the soil is the design structural

force on the foundation, P d , plus the design weight of the pile, W d .





However, in practice W d is small in comparison with

Rd for

typical piles, so that the design requirement is:

R

d

F d

R

d

P d W

d

R

d

P d

1

Negative skin friction4.3

A pile derives its resistance to load because of the relative

movement between the pile and the surrounding soil. In soils with

soft layers, as the soft soil compresses or dries out after pile

installation the soil may move down around a pile and therefore

tend to impose an additional load on the pile instead of providing

support to the pile. This is most commonly encountered in soft

clays. It is known as negative skin friction.



Worked example of pile resistance4.4

Consider a square, end bearing pile, 600mm by 600mm in section,

driven through a 5m thick medium clay and penetrating 6m into a

medium sand layer. The water table is 1m below the surface, as

shown in the figure below. Rapid loading only will be considered,

i.e. undrained failure in the clay, drained failure in the sand.

Fig 4.17 Worked example pile

For the clay, the saturated unit weight is

sat 18kN m3, the

undrained shear strength is

cu 50kPa and the friction angle is

26. For the sand the saturated unit weight is

sat 20kN m3

,and the friction angle is

38. In the clay, take

0 8. In the

sand, take

d 0 75

d 24 and

K s 1 5.

Using Eurocode 7 partial factors EQU, the design values of the

parameters are:





Clay: cu,d =

50

1 4 35kPa

Sand: d = tan1 tan38

125

32

Firstly, the medium clay is sufficiently strong to provide some

support to the pile, the contribution to shaft resistance due to the

clay is:

R s,d ,clay = cu,d s

= 0 8 35 0 6 4 5 336kN

At the centre of the layer of sand around the pile, the vertical

effective stress is:

v 518 3 20 7 10 80kPa

so the contribution to shaft resistance due to the sand is:

R s,d , sand = K s v s tan d

= 15 80 0 6 4 6 tan24

≈ 769kN



At the base of the pile, the overburden effective stress is:

0 518 6 20 1010 110kPa

so the base resistance of the pile is:

Rb,d = 0 N q* Ab

= 110 25 0 6 0 6

≈ 990kN

Hence the total design resistance is:

Rd = 336 769 990 2095kN

the maximum design structural load is:

P d = Rd 2095kN

and the maximum allowable structural load is:

P = Rd

2095

11 1905 1900kN





Worked example of pile resistance with negative skin4.5

friction

If, however, on one part of the site the clay is subsequently found

to be a soft clay with an undrained shear strength is

cu 15kPa

and a friction angle is

22, the negative skin friction must be

accounted for as a load on the pile. The soft clay will compress

and move down past the pile, so that the skin friction from the clay

becomes a part of the design load instead of a part of the design

resistance. Because of this, the undrained shear strength is not

factored down, and the design undrained shear strength in the clay

is:

cu,d =

cu 15kPa

In this case the down drag on the pile is:

Rdowndrag =

cu,d A

s

=

1015 0 6 4 5 180kN

where it is assumed that

1 in order to obtain a conservative

estimate of the load due to negative skin friction. The shaft and

base resistance in the sand are unchanged:

R s,d , sand =

769kN

Rb,d

=

990kN



Hence the total design resistance is:

Rd = 769990 1759kN

the maximum design structural load is:

P d Rd Rdowndrag 1759180 1579kN

and the maximum allowable structural load is:

P = Rd 1579

11 1435 1400kN

In this example, assuming negative skin friction in the clay has

reduced the design structural load by approximately 25%compared with the case where the clay is believed sufficiently stiff

to provide support.

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