Modelling discontinuity control on the development of Hell’s Mouth landslide

This paper focuses on numerical modelling and back analysis of the Hell’s Mouth landslide to provide improved understanding of the evolution of a section of the north coast of Cornwall, UK. Discontinuity control is highlighted through the formation of a ‘zawn’ or inlet, the occurrence of two successive landslides and evidence of ongoing instability through opening of tension cracks behind the cliff top. Several integrated remote sensing (RS) techniques have been utilised for data acquisition to characterise the slope geometry, landslide features and tension crack extent and development. In view of the structural control on the rock slope failures, a 3D distinct element method (DEM) code incorporating a discrete fracture network and rigid blocks has been adopted for the stability analysis. The onset and opening of tension cracks behind the modelled slope failure zones has also been studied by analysing the displacements of two adjoining landslide blocks, between which, a joint-related tension crack developed. In addition, a sensitivity analysis has been undertaken to provide further insight into the influence of key discontinuity parameters (i.e. dip, dip direction, persistence and friction angle) on the stability of this section of the coastline. Numerical modelling and field observations indicate that block removal and preferential erosion along a fault resulted in the formation of the inlet. The development of the inlet provides daylighting conditions for discontinuities exposed on the inlet slope wall, triggering the initial landslide which occurred on 23rd September 2011. Numerical modelling, and evidence from a video of the initial landslide, suggests that the cliff instability is characterised by a combination of planar sliding, wedge sliding and toppling modes of failure controlled by the discrete fracture network geometry.

Psuedo static stability analysis of rock slope using patton’s shear criterion

A deterministic model for the factor of safety of an idealized rock mass for planar mode of failure is developed adopting Limit Equilibrium Method (LEM) using Patton’s shear strength criterion and considering practically occurring conditions such as the effect of tension crack, water filled up in tension crack, horizontal and vertical seismic acceleration, rock bolt stabilizing force and surcharge. In the Pseudo-static analysis horizontal seismic acceleration is taken outward from the slope and vertical seismic acceleration is considered in both the direction i.e. towards the direction of gravity (downward) and opposite to the direction of gravity (upward). An expression of normal stresses as limiting criterion has been derived in order to compare the field normal stresses along the failure surface. A detailed parametric study has been presented to investigate the influence of vertical seismic coefficient for both the direction on the stability of rock slope using developed expression. For high normal stress along the failure plane, it is observed that the factor of safety decreases with increase in the value of vertical seismic coefficient towards the direction of gravity and increases linearly with increase in the value of vertical seismic coefficient against the direction of gravity and the opposite trend has been found for lower normal stress. The vertical seismic coefficient against the direction of gravity has predominant effect on factor of safety of rock slope as the rate of increase/decrease of factor of safety with vertical seismic coefficient is more against the direction of gravity. Hence in determining the critical factor of safety, effect of vertical seismic coefficient against the direction of gravity should be considered.

Geological and Geotechnical State of the Nisane Khola Landslide, Dharan, Sunsari, Nepal: a case study

Geological and landslide mapping was carried out in order to delineate the geological and geotechnical state of the Nisane landslide, situated in Dharan Sub-Metropolitan City, Sunsari, Nepal, which has been obstructing the Koshi Highway time and again. The affected area bears metasedimentary and sedimentary rock sequences of the Lesser Himalaya and the Siwalik, respectively. The Dharapani Thrust separates the Chiuribas Formation and the Tamrang Formation of the Lesser Himalaya whereas the Main Boundary Thrust separates the Tamrang Formation and the Siwalik. The Nisane landslide is an old landslide situated in the structurally weak zone having variable activation period. Recently, the landslide reactivated as intense slide after the Gorkha Earthquake, 2015 and is occasionally obstructing the Koshi Highway. The subsidence in highway indicates the landmass is creeping along the tension crack. Kinematic analysis, Rock Mass Rating (RMR) and Slope Mass Rating (SMR), based on the field observation, measurement and laboratory testing, was carried out in eight different slopes of the slide. The results suggested majority of slopes suffers the wedge and plain failures along the major joint sets. The rock masses are of fair class and the slopes are unstable to partially stable. The slopes of major slide are vulnerable and seeks for immediate sustainable treatment.