DHARM688 GEOTECHNICAL ENGINEERINGsands. ‘Pile constants’ and ‘foundation constants’ have been assumed by them in their analy-
ses. If these constants could be evaluated to a reasonable degree of accuracy, the theories may
be expected to yield satisfactory results. Murthy (1964) tried to evaluate some of the constants
for certain angles of batter, both positive and negative.16 10 Design of P ile Foundations
The design of a pile foundation consists of assuming a design, then checking the proposed
design for safety and revising it until it is satisfactory. The final design is selected on the basis
of cost and time available for construction.
Sometimes piles may be valueless in some locations and may be even harmful under
certain circumstances. For example, a layer of reasonably firm soil over a deep stratum of soft
soil might act as a natural mat to distribute the load of a shallow footing. The driving of piles
into the firm layer might break it up or remould it. The result could be a concentration of
stress in the soft stratum, leading to excessive settlement.16.10.1 Selection of Length of Piles
Selection of the approximate length of the pile is made from a study of the soil profile and the
strength and compressibility of the soil strata. End-bearing piles must reach a stratum that is
capable of supporting the entire foundation load without failure or undue settlement and,
friction piles must be long enough to distribute the stresses through the soil mass so as to
minimise settlement and obtain adequate safety for the piles.16.10.2 Selection of Type of Pile and Material of Pile
The points to be considered in the selection of type of a pile and material of pile are: (i) the
loads, (ii) time available for complection of the job, (iii) the characteristics of the soil strata
involved, (iv) the ground water conditions, (v) the availability of equipment, and (vi) the statu-
tory requirements of building codes.
If the structure is a bridge abutment or a water-front structure, the characteristics of
flow of water and scour must be considered.
16.10.3 Pile Capacity
The pile capacity both for an individual pile and for groups of piles shall be determined in
accordance with the procedures outlined earlier. An appropriate factor of safety shall be applied
to determine the allowable load.16.10.4 Pile Spacing
The piles are placed so that the capacity of the pile group acting as a unit is equal to the sum
of the capacities of the individual piles.
It is impossible to construct piles in exactly the required location or angle because they
tend to drift out of line when hard or soft spots are encountered. The tolerance at the top could
be from 5 cm to 15 cm. A pile may be permitted to be out of plumb by 1 to 2% of the length.