Seasonal Rainfall Effects on Shear Strength and CBR in Volcanic Ash Soils

This study evaluates the impact of seasonal rainfall on the geotechnical behavior of volcanic ash soils in the Aburrá Valley, Colombia. Samples were collected during low, intermediate, and high rainfall periods and tested for water content , plasticity , unconfined compressive strength , and California Bearing Ratio (CBR) . Results show that moisture content and Atterberg limits increase with rainfall, while CBR decreases exponentially , with values dropping by up to 42% in wet conditions. An empirical correlation (R² = 0.88) between water content and CBR was developed. Unconfined strength also decreased by 43% in dry conditions. Pavement reliability (Zr) declined significantly under saturated conditions, emphasizing the importance of accounting for seasonal variability in design. The findings highlight the need for drainage solutions , conservative designs , and site-specific assessments when building on volcanic ash soils.