Geotechnical Engineering

DHARM

PILE FOUNDATIONS 685

When the piles are resting on a firmer stratum than the overlying soil (end-bearing
piles), the total load is assumed to act at the pile tip itself. If the piles are embedded into the
firmer layer in this case, the load is assumed to be transmitted to a depth equal to two-thirds
of the embedment from the top of the firmer layer. The rest of the settlement analysis proce-
dure is applicable. These are illustrated in Fig. 16.18.

2/3

D

D

Pile cap

Piles

Total load

taken to

act here

Uniform

soil

1

2

1

2

D

Pile cap

Piles

weaker

layer

Total load

taken to

act here

1

2

1

Firmer layer^2

(a) Friction and end-bearing

piles in uniform soil

(b) Pile tips in firm soil
(end-bearing piles)
Fig. 16.18 Conditions assumed for settlement of pile groups in clay
The total pressure may be assumed to be distributed on a slope of 2 vertical to 1 horizon-
tal, for the purpose of computation of increment of stress, in an approximate manner.

*16.8 LATERALLY LOADED PILES

Piles and pile groups may be subjected to vertical loads, lateral loads or a combination of both.
If the lateral loads act at an elevation considerably higher than the base of the foundation,
there will be significant moments acting on it.

Vertical piles may be relied upon to resist large magnitudes of lateral loads. The lateral
load capacity of a vertical pile depends upon the nature of the soil, the size of the pile, and the
conditions at the pile head. If the pile head is fixed rigidly in a pile cap, its lateral load capacity
will be more than when it is free.

Extensive theoretical and experimental studies have been made on laterally loaded piles
by Reese and Matlock (1960), Palmer and Brown (1954), and Murthy (1964). Most of these are
based on the concept of coefficient of sub grade reaction, which is the pressure required to
cause unit deflection.

Winkler’s hypothesis

Most of the theoretical solutions for laterally loaded piles involve the concept of ‘coefficient of
subgrade reaction’, or ‘soil modulus’ as it is sometimes called, based on Winkler’s (1867) hy-
pothesis that a soil medium may be approximated by a series of infinitely closely spaced inde-
pendent elastic springs, which is only an approximation of a beam on an elastic foundation
(Fig. 16.19).