Analysis of Slope Stability for Hill-Based Construction in NE India: A Case Study in Guwahati

Guwahati, the gateway to Northeast India, is surrounded by hilly terrain from three sides, which also constitutes the bulk of the city region. The hill soil consisting of stratified layers formed out of different degrees of weathering of the parent rock. Primarily, two distinct types of soil formations are observed. A cohesive reddish clayey soil layer forming the top layer of the slope formations mostly underlain by a less stable yellowish sandy soil layer building the core and bulk of the slope. The shear behaviour of this yellowish soil primarily governs the overall stability of a slope during any triggering event such as rainfall infiltration. The study analyzes numerically the overall strength and shear behaviour of the hill slopes with varying slope inclination and its effects on slope geometry. Applying the Morgenstern–Price method of stability analysis, the stability comparison between slope geometries of undisturbed and remoulded soil has also been studied. The parametric study of undisturbed soil slopes signifies about 40% loss of stability with respect to 100% increase in slope angle at low moisture content. As the slope becomes more and steeper, the factor of safety decreases and also the variation of factor of safety with respect to slope geometry gradually diminishes. At slope angle more than 70°, the slope becomes unstable owing to its height and shape only; hence, saturation due to infiltration need not play a crucial role in instigating instability. For remoulded soil slopes, the values of factor of safety and shear strength for all slope geometries and moisture content are significantly lower than their counterpart values of undisturbed ones with approximately 15% deficiency. It manifests for the presence of strong cementing properties in the original skeleton of the soil, which, once re-constituted, cannot be fabricated again. It implies the supremacy of models utilizing the application of such properties of undisturbed soil which may be considerably significant for slope-based construction in the region.