Effectiveness of fly ash in stabilizing sandy soils for liquefaction mitigation under saline coastal conditions

Indonesia faces a high risk of liquefaction in coastal areas due to its seismic activity. This study investigates the effectiveness of varying Class C fly ash content in stabilizing saturated sandy soils under different salinity conditions, with Unconfined Compressive Strength (UCS) used as a measure for liquefaction mitigation. Experiments were conducted using Bangka sand, with fly ash added at 5%, 10%, and 15% by weight, and sodium chloride (NaCl) solution concentrations ranging from 0 to 30 ppt. Samples were cured for 7 and 14 days (with a relative density of 50% and saturation ratio of 100%). The tested soil is classified as poorly graded, with a specific gravity of 2.53, a uniformity coefficient of 4.42, a gradation coefficient of 1.01, and a void ratio of 0.64. The results indicate that the combined increase of fly ash content and salinity enhances UCS for liquefaction mitigation, reaching a maximum of 268.32 kPa at 15% fly ash and 30 ppt salinity. The most significant improvement was observed at 10% fly ash, where UCS increased up to 3.8 times the initial value, and at 10 ppt salinity, UCS improved by 2.01 times. Prolonged curing also enhanced UCS, with optimal gains at 5% fly ash being 2.1 times the initial value after 14 days. Practically, the addition of 10% fly ash can be applied in coastal engineering projects to reduce liquefaction risk in high-salinity areas. • Fly ash stabilization is effective for liquefaction mitigation in sandy soils. • Class C fly ash increases the UCS of saturated sand in saline conditions. • Salinity does not negatively affect the strength of fly ash–stabilized sand. • A 10% fly ash dosage effectively increases UCS to support liquefaction resistance. • Longer curing time significantly increases UCS up to 14 days.