A Study on Piled Raft System in Collapsible Clayey Gypseous Soil

• Plaxis 3D foundation program version 2013 is used by taking full scale models that consist of different types of piled-raft foundation system • Gypseous soil in saturated and unsaturated states. • The effect of number of piles and spacing on the pile-raft behavior in saturated and unsaturated states. • The collapse strain and consolidation stage in gypseous soil are taken into consideration. • The behavior of load-deformation relation is not affected by changing the spacing between the piles. • The behavior of piled raft foundation system in gypseous soil at saturated state can be represented by time-loading relation. This paper presents a numerical investigation of piled raft foundation behavior in a specific collapsible gypseous soil from Al-Qayiara, Iraq, using Plaxis 3D with experimentally derived soil parameters. The study examines load-deformation response, settlement components, and load sharing for 4-pile and 9-pile raft configurations under both saturated and unsaturated conditions, with particular attention to the effects of soil collapse potential (Cp=14%). The analysis demonstrates that for this specific soil, saturation reduces ultimate capacity by approximately 500% for 4-pile rafts and 220% for 9-pile rafts, while collapse settlement contributes 66-75% of total displacement—far exceeding consolidation effects (3.8-10.7%). The upper portions for load-settlement curves for pile spacing 3d and 5d are merged then they drop and became diverged at lower parts of load-settlement curve. This contradicts the general conclusion that spacing has no effect. It suggests that spacing affects post-yield behavior. Comparison with classical bearing capacity methods reveals discrepancies of 220-560% in unsaturated conditions, confirming the inapplicability of conventional analytical approaches to gypseous soils. While the findings are site-specific and the modeling approach involves simplifications (notably in collapse simulation and unsaturated representation), the study provides quantitative benchmarks for this problematic soil type and demonstrates a framework for incorporating collapse as a time-dependent process in piled raft analysis. In unsaturated soil condition, the pile load sharing ratio in piled-raft foundation with small number of piles (4 piles) increases with spacing, then tends to be constant. While in piled-raft foundation with large number of piles (9 piles). The maximum load sharing is recorded at spacing 7d.