An unsaturated Mohr-Coulomb model with tuneable near-dry suction-driven strength and attenuation: implications for bearing capacity

We extend the Mohr-Coulomb framework to unsaturated soils by employing an enriched effective stress formulation that captures the drying-induced strength loss often observed in granular and compacted soils. The model also incorporates a hysteretic soil–water retention curve that is coupled with the mechanical response. This coupled system is integrated approximately using explicit, adaptive substepping with local error control for both stress and degree of saturation, along with yield surface consistency checks. Numerical examples show that in near-surface settings with low overburden and limited body-force effects, suction stress provides a dominant share of the available strength. We demonstrate that the enrichment produces a tuneable peak strength followed by attenuation in the suction-related strength contribution, consistent with capillary-bridge breakdown at low saturation. Sensitivity analyses indicate that a moderate integration tolerance offers a favourable accuracy-cost trade-off in numerical analyses of unsaturated soil behaviour, and evaluation of the retention law and its scanning curves significantly influence computer runtime. Validation against experimental plate load test data indicates that the prediction accuracy is generally sensitive to retention-related uncertainty. This issue arises when path-independent retention curves are used to estimate missing suction data without accounting for incomplete knowledge of the wetting/drying history. Overall, the approach furnishes a Mohr-Coulomb model with a shear-based effective stress that can be calibrated from standard laboratory tests (e.g., drained triaxial and direct-shear tests) for practical assessment of bearing capacity under variable moisture conditions.