Numerical modeling of piled raft foundation system subjected to seismic loading

Abstract

The quick growth of urbanization all over Dhaka city led to a rapid increase in the number and height of high-rise buildings even in unfavorable subsoil conditions. Since the 80's, a new foundation technique called Piled rafts, has been developed and used extensively all over the world in order to reduce the maximum and differential settlements and the associated tilting of the buildings. In recent years, a large number of mega projects are being constructed with piled raft foundation system in Dhaka city. Hence a noticeable attention has been drawn toward better understanding of the performance of piled raft foundation systems under vertical and lateral loading system. This research has been focused to study the effects of engineering factors related to piled raft foundation. However, shore protection work during excavation phase has not been studied here. A parametric study of a selected project with piled raft foundation system in Dhaka city under both vertical and seismic loading condition has been carried out. The selected structure is a 40-storey building with five basements which is located in the seismic zone 2 as per BNBC-2020. The shape of the structure is inverted "L-shape" with an area of about 2716 square meters. The geotechnical parameters have been collected through in-situ and laboratory tests for this research. All the analysis has been carried out by PLAXIS-3D software using Hardening soil method. Earthquake effect has been simulated using prescribed displacement method with appropriate scaling factors. Several previous case studies have also been validated by the PLAXIS-3D software before initiating the current research. Effect of mesh size and raft thickness has been studied for un-piled raft foundation by developing eleven different numerical models. However, for piled raft foundation system, the effects of raft thickness, pile number, pile layout, pile spacing and pile diameter have been observed by performing twenty-seven different models in PLAXIS-3D. In all the parametric study, only one parameter has been varied at a time and constant values have been selected for all other parameters. From this study it has been observed that the effect of mesh size is relatively small. It is also found that regardless of the number and diameter of piles, the raft can carry about 70-80% of the total load if the raft is rested on medium dense to dense Silty sand. This research shows that increase in raft thickness, pile number and pile diameter can reduce a significant amount of raft bending moment, total settlement and differential settlement of the piled-raft foundation. It is also observed from the study that with the increase in thickness of raft the utmost bending moment in un-piled rafts increases, whereas it decreases in piled rafts. Overall, the results of this study show that changes of raft thickness, pile diameter and pile numbers in numerical model helps to optimize the Piled Raft foundation system. This optimization method has high efficiency and stability and can help practicing engineers to optimize the pile numbers and raft thickness to satisfy higher differential settlement requirements.