3D numerical modeling of the behavior of piled raft foundations in the case of clay soils

FERCHAT, Abdelkrim (2021) 3D numerical modeling of the behavior of piled raft foundations in the case of clay soils. Doctoral thesis, Université Mohamed Khider – Biskra.

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Abstract

Piled raft foundations are increasingly recommended as an economical and efficient foundation system for high-rise buildings. However, there is a reluctance to use them in the case of soft clays due to concerns about excessive settlement and insufficient bearing capacity. Despite these reasons, the use of piled raft foundations in the case of soft clays is increasing swiftly. The behavior of the piled raft foundations is affected by various parameters such as the raft thickness, number, length, and spacing of the piles. Therefore, economical and efficient design requires integrating all these parameters into the design approach. However, the current analysis methods for piled raft foundation on soft clay are not sufficient, especially for the prediction of the overall bearing capacity. In this context, this research work is interested, on the one hand, in a literature review on the results of field auscultation and model tests, and on the other hand, in the numerical modeling of the behavior of piled raft foundations with a thorough parametric study of the various influencing factors. The finite-difference code FLAC3D was used in this thesis to investigate the behavior of piled raft foundation in soft clay conditions. The overall objective of the present study focuses on the load response and the effects of multiple interactions of the piled raft foundation subjected to vertical loading. For this purpose, several types of foundations were considered, including piled raft, pile group, unpiled raft, and single pile. For optimization purposes, the interaction behavior of the piled raft foundation was also investigated by varying some parameters as piles number and pile spacing. The obtained numerical results are validated by comparing them to those of similar subgrade structures and in the comparable geological conditions provided within the literature. The results prove that even for soft clays, the piled raft components interact with each other producing a fully different behavior from that of the unpiled raft and pile group. Therefore, neglecting these interaction effects could lead to an overestimation of the ultimate bearing capacity of piled raft foundations, essentially for cases of large piles number. The concluded observations from the parametric study provide further insight into the mechanical response of piled raft and aim at helping the engineers in taking a logical path in an iterative design process for a piled raft foundation. A predicted model for the efficiency factor of the pile group is also proposed which is validated with the obtained numerical results to better goodness of fit.

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