Axial load capacity of cast-in-situ bored piles in stiff Dhaka clay

Abstract

Cast-in-situ bored piles are the most commonly used deep foundations that are currently being used for different structures in Dhaka city, especially in the reclaimed sites. In most of the cases, the tips of the piles of such type arc embedded in relatively dense sand layers where the end bearing is high. But in areas where such bearing strata are not present, the base of the pile may have to rest on clay layer and the end bearing capacity of the piles decreases considerably which in tum reduces the load carrying capacity of the piles. This study aims at to propose a design method of such piles in more efficient and effective way with the help of proper field investigations and laboratory test results. To fulfill the objective mentioned above axial load capacities of piles having different diameters entirely embedded in stiff Dhaka clay located at Bashundhara, Dhaka, were determined both by standard theoretical methods and from load test results. To determine the load carrying capacity by standard theoretical methods the strength properties of the e1ay along the entire length of pile were determined by collecting undisturbed soil samples by boring and subsequent laboratory tests on these samples. On the other hand the load test results mentioned above that was carried earlier by the project authority was also used to determine the capacities of piles of different diameters. Most of the pile load tests conducted were terminated much earlier before the peak/failure load was reached. As a result, the load-settlement curves could not be used to pin point the failure load by the methods mentioned above. To determine the ultimate load from the load-settlement curves an effort has been made to extrapolate the load-settlement curves and make an estimate on the ultimate load carrying capacity of the piles. The load settlement curves were extrapolated with the assumption that the load settlement curve in stiff Dhaka clay follows the trend of a parabola after the initial straight portion. With the help of these extrapolated curves ultimate carrying capacities of piles of different diameter were derived by methods proposed by Dc Beer. Mazurkiewicz, Van Weele and Davisson. It has been observed that the ultimate load carrying capacity determined from extrapolated load-settlement curves shows close agreement with those determined by theoretical methods, especially with that determined following Indian Standards Institution, while ultimate pile capacity determined in accordance with Tomlinson and API methods tend to be higher than that determined by Indian Standards Institution.