Influence of varying suction conditions on the shear strength of a clayey silt soil

Abstract

Unsaturated soil mechanics is a crucial area of geotechnical engineering that has gained increasing importance in recent years. However, despite its relevance, no standard guidelines have yet been implemented to study unsaturated behaviour in laboratories. This gap limits professionals’ ability to address practical challenges in arid and tropical regions, particularly in problematic soils under shallow foundations, as well as in collapsible and expansive soils. In addition, critical geotechnical structures such as earth dams, slopes, and pavements require expertise in this field. In this context, the main objective of this study was to analyse the influence of varying suction conditions on the shear strength of a problematic clayey silt soil, which exhibits a macrostructure and microstructure with highly compressible characteristics and sensitivity to moisture variations. The research was conducted following ASTM standards for physical and compressibility characterisation. Specific methods included the consolidated drained triaxial compression test with controlled suction (CID-CA-CS) and the unconsolidated undrained (UU) triaxial test for unsaturated mechanical characterisation. The PRISMA model was also applied to define the strength parameters of saturated soil and estimate the linear failure envelope of unsaturated soil. The key findings include an increase in shear strength to 102.8, 128.8, and 157.4 kPa for matric suction values of 81.3, 160.2, and 246.9 kPa, respectively. This research concludes that suction plays a fundamental role in soil strength and stiffness. Additionally, when suction is lost, the soil undergoes significant deformations under loading. Finally, the reconstituted state of the material and unconsolidated undrained conditions are not representative to evaluate its unsaturated soil behaviour.