Enhancements to the Coupled Hydro-Mechanical Hypoplastic Model for Unsaturated Soils

The conventional design of geotechnical structures adheres to the principles of saturated soil mechanics. However, these conventional practices often overlook the impact of matric suction associated with alternating wetting and drying scenarios within the unsaturated zone. Recognizing the importance of this factor, developing a constitutive model becomes crucial for designing geotechnical structures in unsaturated soils. This paper introduces an enhanced version of the coupled hydro-mechanical hypoplastic model proposed by Tafili and Machaček (2023). The reference model can simulate volumetric collapse upon wetting and incorporates suction effects on stiffness and strength. Its mechanical constitutive equation links Bishop’s effective stress with strain and suction. This paper introduces two advancements to the reference model: (i) improvement in the bulk modulus (K) to account for the influence of the degree of saturation and (ii) accurate projection of the overconsolidation ratio (OCR) dependency of the unsaturated soil. The model's performance is analyzed through the back-calculation of three experimental studies: (i) variation of bulk modulus during the wetting-drying process for Bonny silt, (ii) shearing at different suction and mean net stress for Loess and (iii) wetting at different mean net stress for Lower Cromer Till. The results confirm that the proposed model has significantly improved the predictive capability of the experimentally observed hydro-mechanical behaviour of unsaturated soils.