Study of ultimate capacity of piles in Bangladesh

Abstract

Soil investigation and foundation design may together account for a significant cost of a building while encountered. with a geologic situation that has low bearing capacity. In this s£tuation, the greatest concem of the geotechnical engineers ought to be design of a foundation from a number of technically acceptable alternatives based on economy of the whole scheme. Recently, different engineering agencies (such as PWD, LGED etc.) of Bangladesh has started using precast and cast-in-situ (bored) piles for their different projects located in different parts of Bangladesh. There are two objectives behind using piles. Firstly, low cost precast RCC Piles of 175 mm by 175 mm with 7m length have been used as a replacement for the timber piles for low-rise structures constructed on soil of low bearing capacities. On the other hand, 300 mm by 300 mm precast square RCC piles with 11 m length and bored piles of 400 mm to 500 mm diameter with 12 m to 18m length are used for medium rise structures. Moreover, for some high rise structures, piles of required dimensions have been used. Ultimate load capacity of at least 1% of all those piles has been estimated by Static Pile Load Test. In this study ultimate pile load capacity of Bangladesh has been studied. The database comprise of pile load test reports at thirty Public Works Department sites in different parts of Bangladesh. Among the pile load test data, twenty one is precast RCC piles and twenty five is RCC cast-in-situ piles. For the estimation of ultimate pile capacity in the static method, the total soil strata has been divided into some reasonable layers with specified soil properties. The precast piles are indexed as PTP-l, 2, 3 etc. and the cast-in-situ piles are indexed as CTp. 1,2,3 etc. . For static analysis CJ. or total stress method, f3 or effective stress method and A. or semi. empirical method have been used. In this study, load settlement curves from pile load test results are used to find out the ultimate capacity of pile by Davisson, Butter & Hoy and British Standard methods. After obtaining the ultimate pile load capacity values, correlation between ultimate pile capacities of tested piles and ultimate capacities obtained from static analysis as well as some correlation between ultimate pile capacities of tested piles and corresponding settlements are drawn. These correlation are made for precast as well as for cast-in-situ piles. The effect of different size and length of piles have also been considered. In all these cases there exist considerable correlation between the static analysis of pile capacity and capacity of pile from pile load test. This study has proved a higher degree of confidence to use the static formulae to find out the ultimate capacity of the piles. The attempt to correlate the settlement corresponding to ultimate capacity estimated from pile load test for the whole data has been found to be futile. Regression Analysis shows that there is little or almost no correlation with the settlement of pile with the ultimate capacity. Correlation has only been established between settlement and ultimate pile capacity for eastin- situ end bearing piles by following Chin's method.