<p>Undrained shear strength (S_u) is a critical parameter for evaluating the short-term mechanical behavior of fine-grained soils under saturated conditions. This study experimentally investigates the effects of temperature and plasticity characteristics on the S_u of kaolinite clay. Kaolinite was selected as the base soil, and silica powder (SiO₂) was added at 10%, 30%, and 50% by dry weight to produce samples with varying plasticity indices while maintaining similar mineralogy, chemical composition, and particle size distribution. The resulting liquid limits (LL) and plastic limits (PL) ranged from 31 to 47% and 15– 27%, respectively. The specimens were saturated and tested under undrained conditions at 20, 30, 50, and 70&#xa0;°C. The results revealed that as temperature (T) increased, pore water pressure (PWP) increased significantly, leading to a corresponding decrease in S_u. The increase in PWP is attributed to the higher kinetic energy of water molecules and the softening of the soil structure due to reduced effective stress. With temperature rising from 20&#xa0;°C to 70&#xa0;°C, PWP increased by a factor of 3.7–5.2, and S_u decreased by 54–60%. Samples with higher LL and PL exhibited greater resistance to S_u reduction, indicating that higher plasticity enhances the soil’s ability to maintain structural integrity and resist thermal softening. The Su–T curves were similar for all samples, but plastic soils consistently maintained higher S_u values. The PWP–T relationship showed three stages: increase, stabilization, and decrease. At certain temperatures, PWP decreased with increasing LL and PL, likely due to reduced permeability in more plastic soils, which restricts water movement and delays pore pressure buildup. A decrease in clay mineral content led to lower LL and PL, higher PWP, and ultimately lower S_u. These findings provide important insights into the thermal behavior of clays in geotechnical and energy-related applications.</p>