Field and Laboratory Investigation of the Effect of Water Saturation Variation on the Progressive Instability of Soil Slopes Along the Al-Naqaza Mountain Road, Libya

Abstract

This study aims to analyze the instability mechanism of cut earth slopes along the Al-Naqaza mountain road west of the city of Al-Khums, Libya, which consists of silty clay soil, by linking field observations with laboratory tests to assess the effect of moisture content on shear strength and slope stability. The study relied on direct field observations, in addition to sieve analysis and direct shear tests under different moisture conditions. The field results showed the presence of cracks and near-vertical joints, exposed plant roots, localized disintegration of surface layers, and the bending of some tree trunks, indicating the occurrence of soil creep. Sieve analysis also showed that the percentage of fine materials passing through a 0.075 mm sieve was approximately 60%, indicating a predominance of silt and clay particles and a poor grain size distribution of the soil. Direct shear tests showed that increasing the degree of saturation from partially saturated conditions to full saturation Sr=100% resulted in a reduction in the angle of internal friction from 32° to 10° and a decrease in cohesion from 8 t/m² to nearly 0 t/m², while the bulk density increased from 1.4 g/cm³ to 1.98 g/cm³, leading to a significant decline in shear strength and slope safety factor. The results indicate that the slope is undergoing progressive deterioration, manifested as surface collapses and shallow slides associated with increased saturation, soil weakness, and loss of natural support due to cutting operations.