Effect of Matric Suction on Monotonic and Cyclic Behaviour of a Low-Plastic Silty Soil from Sakarya Region

Shallow soil layers subjected to seasonal wetting and drying cycles are typically located above the groundwater table and experience negative pore water pressures. Many geotechnical problems are associated with the behaviour of these soils. Therefore, instead of relying solely on saturated soil mechanics, incorporating unsaturated soil mechanics principles provides more realistic results. In unsaturated soils, compressible air within voids leads to matric suction and net stress, two critical stress variables governing strength and deformation characteristics. The complex behaviour of unsaturated soils under static and dynamic loading continues to be a research focus. This study investigates the effect of matric suction on the monotonic and cyclic behaviour of low-plasticity silty soil from the Sakarya Basin. Laboratory tests employed a sophisticated unsaturated triaxial testing system that applies matric suction using the axis translation technique, enabling controlled monotonic and cyclic loading. Cylindrical specimens, reconstituted by using the moist tamping method to replicate in situ relative density, were consolidated under a specific net stress at three different matric suction levels before fully undrained axial loading. Throughout the tests, pore water and air pressure responses, as well as axial and volumetric deformations, were monitored. Results demonstrated that matric suction significantly influences soil behaviour. In both monotonic and cyclic tests, increasing matric suction enhanced strength and reduced pore water pressure increments. In particular, in cyclic tests, conducted at a single cyclic stress ratio, specimens with lower suction experienced large deformations and cyclic failure, whereas specimens with higher matric suction did not fail. These findings contribute to understanding the mechanical behaviour of unsaturated silty soils and provide a foundation for future research.