Parametric study of unconnected piled rafts in clayey soil using finite element method

Piled rafts are widely used to provide the foundations for high-rise buildings with a high bearing capacity and exhibit acceptable settlement characteristics. A high level of axial stress is created in the few piles that are structurally attached to the raft when piles are utilized as settlement reducers in piled raft foundations. Unconnected piled raft foundation (UCPR) is a cost-effective and efficient system in which the piles are separated from the raft by a structural fill sand-gravel cushion. The piles are treated as a method of soil reinforcement rather than structural members. The current study aims at conducting a parametric study on the behavior and performance of UCPR foundations in clayey soil under vertical loading. The three-dimensional finite element method (3D FEM) was used in conjunction with Plaxis 3D to create a model of this UCPR's (24) full scale and to analyze the complex interactions between (raft-piles-cushion-soil). The study examines the effect of the length of piles, using geogrids at various levels within the cushion, and estimates the total and differential settlement of the raft and load-sharing ratio between the raft and piles. The findings demonstrate the effectiveness of the UCPR in lowering settlement, raising the raft stress ratio, and lowering the pile stress ratio. For example, the cushion thickness decreases the settlement by (8.4, 12.5, 16.7, and 20.9%), and decreases the raft stress ratio by (40, 25, 20, and 18%), as the cushion's thickness increases by (0.5, 1, 1.5, and 2 m), respectively. As the pile spacing changes from 2 to 3, 4, and 4.5D, respectively, the settlements are reduced, the raft stresses ratio is increased, and the pile stresses ratio is decreased by 90, 90, 56, and 44%, respectively, where D represents the diameter of the pile. The results also show that the raft stresses ratio increases with the presence of the geogrid layer, and the best location for the geogrid layer is at the pile's head. In addition, the increasing the number of geogrid layers leads to an increase in the raft stresses ratio by 15.8, 27.7, 33.8, and 33.8% as the number of geogrid layers increases from 1 to 2, 3, and 4 layers, respectively.