BEHAVIOUR OF SKIRTED GRANULAR PILE FOUNDATION

Abstract

Designing safe and economical foundations in weak sub-soil stratum is adifficult engineering problem. The problem becomes more compli cated when the design loads are high and corresponding settlements are restricted. The common practice is to improve the capacity of ground using drains with a combination of preload. Other means of ground improvements are use of lime piles, woven fabric reinforce ment, compaction by explosives and falling heavy weights etc. Stone columns (granular piles) installed through vibrofloatation process or vibro-replacement methods also provide a suitable solution. However, the process necessitates the import of vibroflot and need trained personnel. Despite wide application of granular piles in soft cohesive soils the design methods are semi-empirical and con servative. Also their use in reinforcing loose to medium dense cohesionless soils is not so common. In the present investigation, arational analytical design approach using the anology of expansion of cylinderical cavity in an infinite soil mass expanded by uniformly distributed pressure has been modi fied to arrive at the ultimate load carrying capacity of single plain granular piles. The analysis is also extended to groups of piles individually or collectively skirted. Utilizing the modulus of compressibility of composit soil mass con sisting of granular piles and the virgin soil, method to compute the total settlement has also been developed. Further, analytical solutions have been developed to predict the ultimate uplift capacity of single granular pile when the pull load is applied at the pile toe. This has also been extended for pile groups. A simple, and speedy construction technique of granular pile based on indigeneous knowhow, of different diameters and depths had also been developed. Using this technique, granular piles have been constructed at five different sites. These have then been load tested. Full scale in-situ load tests have been carried out at 5 sites, on plain and skirted granular piles (individually and collectively), single and in groups of 2, 3and 4with varying diameters of 25cm, 35cm having 3.5m and 8m lengths in loose-medium dense cohesionless soil deposits. On other two sites having clayey silt and soft clay deposits, piles of 60 and 55cm with 9m and 14m lengths have been constructed. Also, collectively skirted 4-pile groups with central unit piles have also been load tested. Tests with skirts made of different materials ranging from r.c.c. to timber piles have been carried out with a view to study their application in actual practice. Full scale pull out load tests on plain granular piles 25cm diameter, 3.5m deep, single and in groups of 2, 3and 4granular pile group with a unit pile cast at the centre have also been carried out. Further horizontal load tests on full scale skirted 4-granular pile group with unit pile cast in the centre has also been carried out. Based on Analytical and Experimental investigations the following conclusions have been drawn: 0) The ultimate load carrying capacity of weak sub soil stratum reinforced with plain granular pile under compressive loads, increases considerably and a signif icant reduction in settlement is noted. The increase in ultimate load and reduction in settlement is further enhanced with the pile group are skirted collectively or individually. TV (1i) Also ultimate up-lift capacity of plain granular pile groups are noted to be high with much smaller pull movements. (iii) Though reinforced cement concrete skirting is found to be best however, other type of skirts using different materials have also shown promise provided, these are rigid enough to confine the reinforced soil plug under applied load. (iv) The observed ultimate loads and deformations have shown a reasonably good agreement with the predicted values based on proposed analytical approaches. (v) A 3000 cu.m. capacity molasses tank founded on skirted