Discussion on the Relationship Between Bearing Capacity Safety and Settlement of Piled Rafts

Aiming at the problems existing in the calculation of piled raft foundations, including inconsistent theories for ultimate bearing capacity calculation, lack of correlation between bearing capacity safety and settlement, and unclear collaborative settlement reduction effect of pile length and pile number, a three-dimensional nonlinear finite element method was adopted to establish a calculation model for piled raft foundations on soft soil ground. This study investigated the coupling effect of pile length, pile spacing, and raft size on the bearing capacity safety and settlement. By comparing the limit equilibrium theory with the load-settlement curve extrapolation method, the reasons for the differences between the two methods in evaluating the ultimate bearing capacity of piled raft foundations were revealed. The research results show that: The collaborative effect of pile length and pile number is significant. When the pile spacing is less than 4d (d is the pile diameter), the improvement effect of increasing the number of piles on the foundation stiffness decreases sharply by more than 50%. When the pile length is relatively short (<25 m) or the pile spacing is relatively large (>6d), it is difficult to effectively control the foundation settlement by simply increasing the number of piles or extending the pile length. The ultimate bearing capacity obtained by the hyperbolic extrapolation method based on a relative settlement of 10%B_r (B_r is the width of the raft) is on the safe side, and it has a good correlation with the calculation results of theoretical formulas, which can provide a complementary verification basis for engineering design. There is a non-monotonic relationship between the total length of group piles and settlement, and there exists an optimal pile volume. Excessively increasing the pile volume will lead to settlement rebound. When the foundation settlement is within the allowable deformation range, the overall safety factor is positively correlated with the foundation width, while the degree of raft bearing capacity utilization is generally low. The research results provide theoretical support for the optimization of the "strength-settlement" dual-control design of piled raft foundations in deep soft soil strata.