Cost comparison of pile foundation with reference to other foundation

1. In general the pile require foundation is likely to be more expansive than spread footings or mat.
2. Weak soils with heavy column loads require either rafts or piles. Rafts, in general are more economical than piles. But, when rafts become very large, piles have to be used for restricting both cost and settlement.
3. When piles and rafts are both equal in cost, then piles are preferable to rafts, as the settlement for piles is considerably less than that of rafts. Thus, pile foundations have to be used when raft foundations are not suitable on grounds of economy or settlement considerations.
4. Generally up to 3 to 4 floors (storeys), provision of raft foundation works out to be economical when a medium bearing/moderate bearing soil is available upto a depth of 2 to 3m. However for structure having more than 4 storeys and a hard strata is available only 10m depth below ground level, then adoption of pile foundation is the best in addition to be economical.
5. The key to economy in the pile foundations is to keep the area of pile cap in plan to the minimum, for which high capacity piles should be used. 
6. Capacity of a pile is increased by increasing either the diameter or depth of a pile, pile spacing (equal to three times the diameter) will increase and this will lead to a large pile cap, which will result in increased cost. So, pile depth should be increased to get high capacity piles. Thus, economy requires the use of a few high capacity deep piles under a column rather than a large number of low capacity shallow piles. 

Construction Problems on Soft Compressible Soils

In case of highly compressible saturated soft clay, imposition of load generates excess pore water pressure in soft layer. This excess pore water pressure is initially borne by water molecules existing within the soft layer. Initial excess pore pressure in the water is equal to the applied external pressure on the soil. Slowly and slowly, as water oozes out of pore space depending upon permeability of soil, excess pore water pressure dissipates and the load gets transferred to soil grains. With the passage of time, excess pore water pressure decreases and correspondingly, the effective stresses in the layer increases. After a very long time the excess hydrostatic pressure becomes zero and the entire consolidation pressure becomes an effective stress transmitted from grain to grain of the soil.
As a result, soft soil gets compressed and the consolidation process continues over a long period of time unless measures are taken for quick dissipation of excess pore water pressure. Throughout this process, the foundation of structure would settle until the complete consolidation takes place. By this time, irreparable damage to structure occurs.
Because of low permeability and poor drainage characteristics of soft soil, it becomes essential to drain out pore water before construction begins, otherwise the added weight of new structure will cause water to squeeze out for a long period of time. Moreover, due to high compressibility of these soils, the consolidation settlements are of a very high order and from structural safety point of view, it would be better if major portion of this consolidation settlement takes place before/during construction phase itself.

CONSOLIDATION METHODOLOGY

For constructing any structure, it is necessary to accelerate the process of consolidation. To achieve this, the following methods are generally adopted-
i) Preloading
ii) Stage Construction
iii) Vertical Sand Drains
iv) Prefabricated Vertical Drains
v) Vacuum Assisted Consolidation

The choice of method depends upon the importance of structure, applied loading, site conditions and time period available for construction etc. Therefore, it is important to select the appropriate method for a given situation.

Slope instabilities throughout pan India have been repaired using existing soils or imported soils, with or without modifications in an effort to minimize costs and reconstruction time. Consideration for the use of Geosynthetic materials as partial replacement for fill soils used in slope repair projects appears to provide a significant improvement to long term stability. This is significant for all type of soils and overburden materials.

For such project precise Geotechnical investigation is proposed to know the subsurface conditions of the project area, Petral Geo Engineers has suggested the use of geophysical techniques such as Seismic Refraction Test.

Analysis has been carried out by using both Geotechnical and Geophysical Data.