#041 Soil Condition and selection of Pile foundation according to it

Pile foundation selection depends on the soil data received from soil exploration bore holes at different depths.

Selection of pile foundation types and length depends on following conditions:

1. Soil conditions
2. Loads from structures
3. Nature of loads
4. Number of piles to be used
5. Cost of construction

There are three types of pile foundation based on load transfer mechanism:

1. End bearing piles
2. Friction bearing piles
3. Combination of end bearing and friction bearing piles.

Selection of End Bearing Pile Foundation:

End bearing piles also called as point-bearing piles are selected when the depth of hard soil strata or bedrock at site is within reasonable depth. The length of pile to be used can be easily computed based on bedrock depth obtained from soil exploration borehole records.

In this case, the loads from structures are directly transferred the hard soil through bearing action of pile bottom tip and it does not require the use of skin friction to resist loads. The cost of construction of piles in such cases is optimum.

The ultimate capacity of pile or pile group depends on the bearing capacity of bedrock or hard strata. Number of piles to be used in this case depends on the loads from structure and individual capacity of piles.

In this case,

QU = QP

Where QU is the ultimate load capacity of pile foundation

QP is the load carried by the end bearing pile or pile group.

In case when the hard bedrock is not available at reasonable depth and fairly compacted hard strata of soil exists, then piles should be extended a few meters into the hard soil strata.

Selection of Friction Pile Foundation:

Friction piles resist the loads from structures due its skin friction with soil. This type of pile foundation is selected when a hard stratum is available at large depth and construction of end bearing pile becomes uneconomical. Then number of piles in a group is selected to resist the load from structure through its skin friction. This type of pile foundation also resists loads due to end bearing but its value is small, thus it is neglected in calculation.

The length of friction pile to be selected in this case depends on the shear strength of soil, loads from structures and size of piles. The capacity of individual pile is calculated based on skin friction resistance provided by selected length of pile. Optimum length of this pile should be used considering economy. The number of piles required in a group can be calculated from individual pile capacity.

In this case,

QU = QS

Where QU is the ultimate load capacity of pile foundation

QS is the load carried by the friction pile or pile group.

The load is transferred to the soil through friction in case of sandy soil and adhesion in case of clayey soil. Loose sand and soft clays may not provide sufficient skin friction or adhesion resistance to heavy loads from structures.

Combined End Bearing and friction Pile Foundation:

This type of pile foundation is mostly used in construction. The advantage of using this pile is that it can resist loads from structures through both end bearing and friction resistance. This pile has high pile capacity and is economical.

This pile is used when the soil exploration results shows hard bedrock or fairly compacted soils at reasonable depth and soil above bed rock supports skin friction resistance.

In this case,

QU = QS + QP

Where QU is the ultimate load capacity of pile foundation

QS is the load carried by the friction pile or pile group

QP is the load carried by the end bearing pile or pile group.

The load is transferred to the soil through friction in case of sandy soil and adhesion in case of clayey soil.