Nowadays, the wave of construction of high-rise buildings, which usually use the concept of piled group foundation in their design, has increased in Ho Chi Minh City as well as other cities in Viet Nam. In this study, settlement, the load shared by the raft, and the behavior of the piled raft were considered via monitoring settlement and the Poulos-Davis-Randolph (PDR) method where the settlement of a varying number of piles, the pile length, and raft embedment were determined. The results of settlement monitoring of a high-rise building in Ho Chi Minh City showed that the foundation design of this building was conservative, with a ratio of allowable-to-actual settlement of 9.3. In the simplified method, the proportion of load share by raft was 2.8% (which was ignored in the piled foundation concept), with the settlement results being in good agreement with the measurement results. The parametric analysis indicated that the piled spacing/piled diameter was 5 -7 times the recommended optimum value. Furthermore, increasing the pile length decreased settlement. The pile length was equal to 30 times the pile diameter, which was effective for the settlement ratio. In addition, the raft load share reached 30% of the applied load when the raft was put in the second layer of stiff-to-very-stiff clay. The study indicated the simplified method was effective for evaluating the preliminary conditions of the foundation, settlement, and that a piled raft was feasible for Ho Chi Minh City's subsoil geology.
Load-displacement behavior of a pile is one of the key parameters in the foundation design of a high-rise building. This research studied the relationship between the load and displacement of a pile in sand using a physical model test and the finite element method (FEM). Five types of sandy soil, including Bangkok sandy soil, were tested using the physical model test with a pile 1 cm in diameter and 30 cm in penetrated length. The results showed that the relationship between the load and actual pile displacements in all soil types was nonlinear with an elastoplastic-strain hardening pattern. This relationship was expressed using a polynomial function for a displacement in the range 0-20 percent of the pile diameter. For easy use, the relationship can be considered a linear function only in the first stage of displacement (0-10 percent of pile diameter), confirming that linear pile spring stiffness can be reasonably used for the design of the foundation. In addition, the finite element method was used to investigate the effect of various factors on pile spring stiffness. The results showed that pile spring stiffness depended on diameter, length, and modulus of the pile and on the soil modulus. In natural soil, the modulus of soil was related to the soil strength and the pile spring stiffness also depended on the soil strength.
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