Optimized Use of Settlement Reducing Piles for Raft Foundation on Clay Soil

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Addis Ababa University


In this study, a concept of using piles for settlement control has been examined through a Finite Element modeling technique. A raft alone may be adequate in terms of bearing capacity but predicted settlement may exceed tolerable values. Hence, piles may be provided under the raft foundation to make the system sound. The study begins with an analysis of an unpiled raft foundation to decide the requirement of piles followed by an extensive parametric study to optimize the pile usage through three-dimensional analysis. The results indicated that the di erential settlement requirement can be meet with an increase in raft thickness but the vertical settlement increases in the same order. Vertical and di erential settlement decreases with an increase in length and number of piles. For the considered loading and subsoil condition a central pile group area ratio of 26% reduced the di erential settlement signi cantly and the corresponding load shared by the piles reach more than 40%. The pile spacing is an important parameter and the study shows a vertical settlement decreases with an increase in pile spacing owing to a reduction of interaction among piles while di erential settlement increases with increasing pile spacing due to the sti ness reduction of the central portion of the piled raft in which maximum settlement is expected . The load sharing ratio increases with an increase in pile spacing. Strategic use of non-uniform pile length con gurations, shorter piles at the center, and longer at edges, enhances the settlement performance of piles raft foundations for the same total pile length due to relatively longer piles at edges supports the cantilevered raft portion. Pile diameter variation shows no signi cant change in settlement values as the settlement increase associated with increase in pile diameter is compensated by the frictional resistance increase due to pile circumference frictional area increase. The e ect of consolidation in the load sharing ratio reaches up to 14% along with its e ect in increasing the settlement and hence the usual practice of pre-proportioning the super- structure load to the foundation units may not be the best solution as the long term behavior of these units will be altered due to consolidation.



Piled raft foundation, Finite Element modeling, Clay soil