• They are used for large structures, and in situations where the soil under is not suitable to prevent excessive settlement.
• Piles are required for setting out of towers for high power transmission lines and in some cases are also used for supporting the multistory buildings.
Factors Affecting Selection of Piles
Among the factors that will influence the selection of type of piles for a given project are the following:
1. Type, size, and weight of the structure to be supported.
2. Physical properties of the soil at the site.
3. Depth to a stratum capable of supporting the piles.
4. Possibility of variations in the depth to a supporting stratum.
5. Availability of materials for piles.
6. Number of piles required.
7. Facilities for driving piles.
8. Comparative costs in place.
9. Durability required.
10. Types of structures adjacent to the project.
11. Depth and kind of water, if any, above the ground into which the piles will be driven.
Situations Which Demand Pile Foundations
Sub-soil water table is so high that it can easily affect the other foundations.
Load coming from the structure is heavy and non uniform.
Where grillage or raft foundations are either very costly or their adoption impossible due to local difficulties.
When it is not possible to maintain foundation trenches in dry condition by pumping, due to very heavy inflow of seepage or capillary water.
When it is not possible to timber the excavation trenches in the case of deep strip foundation. (strip foundation-spread footing under wall ).
When overlay soil is compressible, and water-logged and firm hard bearing strata is located at quite a large depth.
When structures are located on river-bed or sea-shore and foundations are likely to be scoured due to action of water.
Large fluctuations in sub-soil water level.
Canal or deep drainage lines exist near the foundations.