What is the primary factor that causes an infinite slope in saturated granular soil to fail when seepage occurs parallel to the slope surface?

The primary factor that causes an infinite slope in saturated granular soil to fail when seepage occurs parallel to the slope surface is the significant reduction in the effective stress within the soil. Effective stress is the stress carried by the soil's solid particles or skeleton, responsible for holding the soil grains together. It is fundamentally defined as the total stress acting on the soil minus the pore water pressure. Total stress is the overall force per unit area exerted on the soil, primarily due to the weight of the overlying soil in a slope. Pore water pressure is the pressure exerted by the water occupying the void spaces between the soil particles. When granular soil, which consists of individual particles with negligible cohesion, becomes saturated, all its voids are filled with water. If seepage then occurs parallel to the slope surface, this means water is flowing downslope through these saturated voids. This flow establishes a hydraulic gradient within the soil. The presence of this hydraulic gradient and the moving water significantly increases the pore water pressure acting within the soil mass, particularly along potential failure planes, compared to static or dry conditions. As the pore water pressure increases while the total stress from the soil's weight remains essentially constant, the effective stress is directly and proportionally reduced (Effective Stress = Total Stress - Pore Water Pressure). For granular soils, the shear strength, which is the soil's resistance to sliding or deformation, is almost entirely dependent on the effective normal stress and the soil's angle of internal friction. According to the Mohr-Coulomb failure criterion, with negligible cohesion for granular soil, shear strength is directly proportional to the effective normal stress. Therefore, a substantial reduction in effective stress leads to a corresponding, significant reduction in the soil's shear strength. When this reduced shear strength becomes insufficient to resist the shear stress imposed by the downslope component of the soil's weight, the slope fails by sliding along the potential failure plane.