Uses of Piles.
Piles are used to:
(i) carry vertical compressive loads,
(ii) carry uplift or tensile forces,
and (iii) resist horizontal or inclined loads.
Bearing piles are used to support vertical loads from the foundations of buildings and bridges.
Point-bearing and friction piles fall in this category.
Tension piles are used to resist upward forces such as uplift in buildings with basements below ground water level, aprons of dams, etc.
Laterally loaded piles support horizontal or inclined forces such as the foundations of retaining walls, bridges, and dams.
Large lateral loads are better resisted by batter piles.
These are illustrated in the above figure.
Bearing Capacity of Pile Foundation.
The ultimate bearing capacity of a pile foundation is the maximum load it can carry without any shear failure or excessive settlement.
The allowable load on a pile is the load which can be imposed on it with an adequate margin of safety; it can be the ultimate load divided by a suitable factor of safety.
Or the load at which the settlement reaches the allowable value, whichever is smaller.
The bearing capacity of a pile foundation depends primarily on the type of soil through which it passes and/or on which it rests and on the method of installation.
It also depends upon the cross-section and length of the pile.
The pile shaft is a structural column that is fixed at the point (bottom) and is usually restrained at the top.
The elastic stability of piles, or their resistance against buckling, has been investigated both theoretically and by load tests (Bjerrum, 1957).
Both theory and experience demonstrate that buckling rarely occurs because of the lateral support of the soil.
It may occur only in slender piles in soft clays or in piles that extend through open air or water.
Hence, an ordinary pile in sand or clay can be designed as if it were a short column.
A pile transfers the load into the soil in two ways.
Firstly through the tip in compression, called point-bearing or end-bearing, and secondly, by shear resistance along the surface, called skin friction.
If the strata through which the pile is driven are weak, the tip, resting on a hard stratum, transfers a large part of the load; the pile is then said to be an 'end-bearing pile'.
Piles in homogeneous soils transfer the greater part of the load by skin friction, and are called 'friction piles'.
However, nearly all piles develop both end-bearing and skin friction resistance.
The methods available for determining pile bearing capacity are;
(i) static analysis,
(ii) dynamic analysis,
(iii) load tests on the pile,
and (iv) penetration tests.
The first two tests are analytical, and the other two tests are the field or practical-oriented methods.
Design aspects of Pile Foundation.
The design of a pile foundation consists of assuming the dimensions of the pile, depth of driving, and other relevant details.
Then checking the proposed design for safety, and revising it, if necessary, until it is found to be satisfactory.
Economy and speed of construction are the criteria in choosing any one of the available alternatives.
The following are the important design aspects.
1. Length of Piles.
The selection of the 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 excessive settlement.
Friction piles must be long enough to distribute the stresses through the soil mass so as to minimize the settlement and obtain adequate safety.
2. Type and Material of Piles.
The following points should be considered before selecting the type and material of piles:
(i) the loads,
(ii) time available for completion of the job,
(iii) the characteristics of the soil strata involved,
(iv) the ground water conditions,
(v) the availability of equipment,
and (vi) the statutory requirements of building codes.
If the structure is a bridge abutment or a waterfront structure, the characteristics of the flow of the water and scour must be considered.
3. Pile Bearing Capacity.
This design aspect should be determined for an individual pile and for the group acting as a unit.
An appropriate factor of safety shall be applied to determine the allowable load.
4. Pile Spacing.
The piles are placed so that the capacity of the piles acting as a unit is equal to the sum of the capacities of individual piles.
A tolerance of shift at the top of 5 cm to 15 cm and out of plumb of 1 to 2% of the length may be permitted in the planned position of piles.
5. Inspection and Records.
Competent engineering inspection and keeping complete records of driving of every pile is an essential part of any important job.
All details such as those relating to the hammer, pile, number of blows, penetration, length driven, heaving and shrinkage of adjacent ground, and details of pile cap shall be recorded.