Effect of Beach Sand and Curing on CBR and Compressive Strength of Expansive Soil

Abstract

Expansive soil is a fine-grained clay that undergoes large volume changes as its moisture content fluctuates, which often produces differential settlement, cracking, and structural damage to the infrastructure built upon it. Improving such soil before construction is therefore essential. This study evaluates beach sand, a locally abundant granular material in Tarakan City, as a stabilizing agent for expansive soil, with particular attention to how curing time affects bearing capacity. Expansive clay sampled from Gunung Lingkas, Tarakan, was classified as high-plasticity clay. It was blended with beach sand at 20%, 40%, and 60% by weight and cured for 10, 20, and 30 days. Physical tests (sieve analysis, moisture content, unit weight, specific gravity, and Atterberg limits) and mechanical tests (compaction, California Bearing Ratio, and unconfined compressive strength) were carried out in the laboratory. Adding beach sand lowered plasticity to the point that several mixtures became non-plastic, while it markedly improved mechanical performance: the maximum dry density rose from 1.393 to 1.679 g/cm³, the bearing ratio increased from 9.59% to 25.47%, and the unconfined compressive strength grew from 0.0502 to 0.115 kg/cm². The most effective combination was 60% beach sand cured for 30 days. The results show that local beach sand is a promising granular stabilizer for improving the bearing capacity of expansive subgrade, although its low unconfined compressive strength and unverified mechanism call for further evaluation.