Numerical Analysis of Raft Foundation Behavior on Soils with Subsurface Voids Using PLAXIS 3D

Moradi, Gholam; Qasim, Mortatha

doi:10.22034/ijumes.2026.2108969.1424

Numerical Analysis of Raft Foundation Behavior on Soils with Subsurface Voids Using PLAXIS 3D

Document Type : Case Study

Authors

Gholam Moradi ¹

Mortatha Qasim ²

¹ Department of Civil Engineering, University of Tabriz, Tabriz, Iran

² Department of Geotechnical Engineering, University of Tabriz, Tabriz, Iran

10.22034/ijumes.2026.2108969.1424

Abstract

Subsurface voids such as karst cavities, abandoned qanats, and old mine workings interrupt stress transfer beneath shallow foundations and can cause bearing-capacity loss, excessive settlement, and structural rotation. Most previous studies, however, have examined strip or isolated footings, plane-strain conditions, single cavities, or ultimate capacity alone, so the three-dimensional response of finite rafts remains poorly understood. This study aims to quantify how cavity size, depth, horizontal offset, shape, number, and spacing govern the ultimate and serviceability performance of a raft foundation. A 10 m × 15 m reinforced-concrete raft on homogeneous silty sand was modeled in PLAXIS 3D with the Hardening Soil model, and one no-void reference model and 22 voided models were analyzed under identical conditions. Reliability was confirmed through mesh-convergence and boundary-sensitivity analyses, analytical and equilibrium checks, and reconstruction of a published laboratory test. The reference raft reached an ultimate bearing pressure of 2,795 kPa and settled 19.4 mm at 250 kPa. Increasing D/B from 0.10 to 0.40 reduced the bearing-capacity ratio from 0.907 to 0.542 and increased settlement by up to 141%, whereas increasing H/B from 0.25 to 1.50 raised BCR from 0.624 to 0.984, with cavity influence becoming negligible at H/B = 1.00. The critical horizontal offset occurred at X/B = 0.50, which produced the largest differential settlement and tilt, and three closely spaced cavities reduced BCR to 0.565. Because capacity, settlement, and rotation were governed by different configurations, cavity-affected rafts should be assessed against both ultimate and serviceability criteria.

Graphical Abstract

Numerical Analysis of Raft Foundation Behavior on Soils with Subsurface Voids Using PLAXIS 3D

Highlights

· 3D PLAXIS analysis of a raft above 22 subsurface cavity configurations

· Raising D/B from 0.10 to 0.40 cut the bearing-capacity ratio from 0.907 to 0.542

· Cavity influence on capacity and settlement became negligible at H/B = 1.00

· An offset cavity at X/B = 0.50, not a centred one, governed raft tilt

· Capacity, settlement and tilt were each governed by a different configuration

Keywords

Bearing-capacity ratio

cavity interaction

differential settlement

hardening Soil model

Three-dimensional finite-element analysis

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