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A limit equilibrium method for progressive failure analysis of slopes is proposed based on a technique of non-vertical slices . The local factors of safety are calculated by defining variable factor of safety instead of conventional single value factor of safety along a shear surface. Reasonable simplifying assumptions about the inter-slice forces and the line of thrust are made to render the problem statically determinate. Strain softening, though in an approximate manner, is considered in the solution procedure. The analysis of a field case history indicates that the proposed method realistically represent the behaviour of progressive failure of an actual slope. Another case study revealed that in certain conditions a reasonable solution can be obtained using non -vertical slice approach, but the vertical slice analysis does not work well in obtaining reliable results . The proposed method, which can also handle vertical slice division, enables the user to choose more appropriate results either from vertical or from non-vertical slice analyses.

The effectiveness of plant root reinforcement in protecting shallow landslide was analyzed using a method of slope stability analysis. The study has investigated the effective contribution of roots to the local safety factors as well as over-all safety factor. The roots of natural plants increased the local safety factor by providing additional shear strength locally to the failure plane. During propagation of failure, the strength of soil at certain location along the shear plane drastically reduced to a residual level and the failure was imminent locally, it was found that the roots significantly contributed additional strength locally and increased the safety factor at the place. The shear strengths of roots were determined with root diameters, number of roots and tensile strength of roots. The shear strengths of plants were incorporated as reinforcements into the slope stability analysis. The analysis calculated the local as well as over-all safety factor with such reinforcement of roots. Existing published data regarding the roots were used in the analyses. An example case of slope stability analysis was done with different root reinforcement situation on the slope. Effects of different cases considering the positions of root reinforcements on the safety factor and the shear strength were also examined.

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The occurrences of shallow landslides in residual soils of Penang hilly areas are common in rainy days. The failure mechanisms of a shallow landslide occurred at km 3.9 road in Tun-Sardon area of Penang Island have been simulated using two different methods of slope stability analysis. The results indicate that the failure was initiated locally inside the slope and then propagated further to induce total failure. The failure propagation was started from initial local failure zone and was driven by mobilized shear strength along the shear plane. The slope was marginally stable with an overall factor of safety of 1.32 before it failed to a rainfall event on September 6, 2008. It is found from back calculation that the rain infiltration raised the temporary water level and reduced the shearing strength of soil to a minimum level with increased pore water pressure to trigger the failure. This paper suggests further research on shallow landslide of Penang Island considering the direct rainfall infiltration effect in terms of groundwater pressure-head distribution inside the slope.