Aim of the study:
This study investigates the mechanisms controlling recurrent road subgrade failures in Aceh Besar, Indonesia, by integrating geoelectrical imaging and granulometric analysis to identify the dominant hydro-mechanical factors affecting pavement stability.

Material and methods:
Electrical Resistivity Tomography (ERT) surveys using the Wenner–Schlumberger configuration were conducted to delineate subsurface weak zones. Six soil samples were analyzed through specific gravity and grain-size distribution tests to evaluate the physical characteristics of the subgrade material. Parameters including soil classification and coefficient of uniformity (Cu) were used to assess granulometric stability.

Results and conclusions:
The ERT results revealed extensive low-resistivity anomalies (<20 Ωm) at depths of 2–10 m, indicating saturated and mechanically weak subsurface zones. Laboratory analyses showed relatively uniform specific gravity values (Gs ≈ 2.67), suggesting that mineralogical composition is not the primary cause of failure. Granulometric analysis classified the soil as Poorly Graded Sand (SP) with low coefficients of uniformity (Cu ≤ 2.67), indicating weak particle interlocking and high void ratios. The integrated interpretation demonstrates that pavement failure is controlled by a coupled hydro-mechanical mechanism in which saturated poorly graded sands experience increased pore-water pressure and reduced effective stress under repeated traffic loading. The study concludes that subgrade stabilization requires both soil densification and groundwater control. Compaction grouting and deep French drainage systems are recommended to improve long-term pavement performance in alluvial environments.