Thermally induced transition from expansive clay to a frictional granular state: An experimental investigation

Expansive soils pose persistent challenges to geotechnical design due to their high swelling potential and sensitivity to moisture variations. Thermal stabilization has recently emerged as a promising alternative to conventional chemical treatment, yet its implications for soil strength mechanisms remain insufficiently quantified. This study evaluates the effectiveness of extreme thermal stabilization on a high-plasticity, kaolinite-rich expansive clay, specifically focusing on the evolution of its mechanical strength and shear parameters. Soil specimens were subjected to controlled thermal treatment at 200°C, 400°C, and 600°C for durations of 15, 30, and 120 minutes, followed by systematic evaluation through unconfined compressive strength (UCS) and direct shear tests (DST). Complementary analyses of grain size distribution, consistency limits, and compaction characteristics were conducted to interpret the observed mechanical responses. The results demonstrate that increasing temperature induces a substantial transformation of the soil fabric, marked by aggregation of clay particles into stable, sand-sized clusters and a reduction in plasticity index (PI) from 27.0 to 2.94 at 600°C. While extreme heating led to a substantial reduction in UCS; reflecting the loss of cohesive clay bonds; it simultaneously produced a marked increase in shear resistance under confinement, with the internal friction angle rising from 23.15° to 50.19°. Free swell potential was progressively suppressed and effectively eliminated at 600°C, confirming the permanent mitigation of expansive behavior. The findings highlight a fundamental shift in strength mechanisms from cohesion-dominated to friction-controlled behavior, demonstrating that thermally treated expansive clay can function as a granular-like material with high shear resistance under confinement. These results provide critical insight into the rational use of thermal stabilization in geotechnical applications involving expansive soils.