This study investigates the impact of varying stress and wetting conditions on the engineering properties of clay and sand mixtures, specifically focusing on how these factors influence soil strength. Two mixtures composed of 70% clay and 30% sand (70C30S), and 70% sand and 30% clay (70S30C), were prepared at their respective optimum water content (OWC) values. The samples were subjected to different durations under sustained vertical loads to simulate stress history, followed by direct shear and unconfined compressive strength tests. The results show that when the water content exceeded the OWC, the unconfined compressive strength of the 70S30C mixture decreased by up to 42%, while that of the 70C30S mixture decreased by c. 58%. The internal friction angle dropped by 6° to 11° under similar conditions. Cohesion increased with water content up to the OWC, but then decreased by as much as 35% beyond that level. Stress history also played a significant role; prolonged loading resulted in strength changes of up to 25% depending on the mixture type. These findings highlight the importance of both the water content and past stress conditions in determining soil behaviour, and offer valuable insights for geotechnical engineering applications, particularly in environments subject to moisture and load variations.

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