Numerical Study on Optimization of Piled Raft Foundation on Stratified Soil under Static Load

Piled raft foundation (PRF) is a combination of pile and raft that provides adequate bearing capacity under the allowable settlement. This research aims to optimize the PRF design for a 48-story Commercial Bank headquarters building in Addis Ababa, Ethiopia, by conducting parametric study on various pile configuration using FLAC3D (Fast Lagrangian Analysis of Continua in three Dimensions) software. We analyzed 28 piled raft configurations and found that increasing the distance between piles increases the raft load sharing distribution by 11.5% while reducing settlement. Despite variations in pile length and diameter, a close load sharing between the pile and raft is accomplished at a separation of seven times the pile diameter (7D). The largest settlement decrease achieved was 9% for a 2-m pile diameter by doubling the pile length from 10 to 20 m, indicating that increasing pile length is ineffective for settlement reduction. We also proposed two new pile configurations that share load equally among pile and raft, and slightly increase total settlement below 10-cm limit. The results show that the first configuration exceeds differential settlement limit, but the second configuration reduces it by 95% and saves 40% of piles. This study provides a useful reference for designing PRF for high-rise buildings in similar soil conditions.