Effect of fly ash on shear strength of a volcanic soil as embankment material based on direct shear test

Fly ash is a non-hazardous by-product of high-temperature coal combustion and is commonly found in large quantities in coal mining areas. This condition creates an opportunity to reuse fly ash as an alternative material, particularly when blended with volcanic soil for embankment construction on mining haul roads. In haul road design, the shear strength of embankment materials is an essential geotechnical parameter. This study evaluates the influence of fly ash on the shear strength of a fly ash (FA) + volcanic soil (Soil-C) mixture. Shear strength parameters were obtained through laboratory direct shear testing in accordance with ASTM D3080. The fly ash was sourced from a stockpile in the Tanjung Selor coal mining area, North Kalimantan, while the volcanic soil originated from Lagadar, West Java. Two mixture proportions were tested: 35% FA + 65% Soil-C and 50% FA + 50% Soil-C. The results indicate that increasing the fly ash content from 35% to 50% enhances the internal friction angle by approximately 10°, while reducing cohesion by about 6 kPa. These findings demonstrate that higher fly ash content can improve frictional resistance but decreases the cohesive bonding within the FA-volcanic soil embankment material.