Numerical Study Of Piled Raft Foundation Under Vertical Static And Seismic Loads

Abstract

Piled raft foundation (PRF) is a combination of pile and raft working together to provide adequate bearing capacity under the allowable settlement. A uniform pile positioning has been used in PRF, however, there is a wide room for optimization through parametric study to provide a lesser number of piles within the required design criteria under vertical load. Addis Ababa is found in seismic zone 3 with a peak ground acceleration (PGA) above the threshold of damage that makes investigating the performance of PRF under seismic load considering the dynamic kinematic soil-structure interaction (SSI) vital. The study area is located in Addis Ababa around Mexico (commercial bank) and Kirkos (Nib, Zemen and United Bank) in which input parameters (pile length, pile diameter, pile spacing, raft area, raft thickness and load) are taken. A finite difference based numerical software, FLAC3D V6 is used for the analysis. The 1940 El Centro earthquake is selected, scaled to the required PGA and deconvolution analysis was done. The static analysis showed that increasing spacing results rise of load sharing of the raft by 11.5% while settlement has reduced non uniformly. A close load sharing between pile and raft was achieved at a spacing of 7D with different pile lengths and diameters. The maximum settlement reduction achieved is 9% for a pile of 2 m diameter by increasing length from 10 m to 20 m, which show pile length is not effective in reducing settlement. The installation of piles results increases of negative bending moment of the raft compared with unpiled raft. Hence, the optimized design depends on pile spacing and the raft edge length from the outermost pile, while pile length and diameter are not significant parameters. An optimized piled raft configuration proposed has reduced pile number by 40% and differential settlement by 95%. The dynamic analysis shows the acceleration plot at the top of the friction piled raft has PGA of 1.5 𝑚/𝑠𝑒𝑐2 and 3 𝑚/𝑠𝑒𝑐2 under El Centro earthquake respectively due to amplification of seismic waves. There was no observed residual pile head displacement and resonance by the El Centro earthquake. Compared with friction PRF, an end bearing PRF has lower amplification, pile head displacement and vertical settlement under the 1940 El Centro earthquake with different peak ground acceleration, hence, PRF perform very well under seismic load.