Stability Analysis of the Nonlinear Failure Envelope and its Applications to Shallow Landslides

Abstract

There is a general tendency for many shallow landslides caused by heavy rainfall to occur at less than 1. 5 meters on forest slopes.
Because of this, it is thought that the initial confining stress before the event happens is the range of 0. 2 to 0. 25kgf/cm². The stability analysis based on Coulomb's straight criteria is frequently used to evaluate surface instability such as mentioned above at low confining stresses. However, this usually results in an overestimation of the safety factor because the experimental evidence, for example results of shear tests on sites and in the laboratory, clearly show a nonlinear decrease of the shear strength with a decrease of normal stresses and the failure envelope is curved for almost all types of subsurface soils in forest slopes.
The empirical strength criteria of soil, in the from of a power relationship, has been put forward based on tests of a range of low confining stresses. In this paper, a stability analysis (based on the Fellenius Method of the nonlinear envelope) has been developed and the effect on the safety factors on a model slope of simple geometry with uniform soils and actual surface failures caused by rainfall is discussed.
The value of the safety factor calculated with Coulomb's criteria is about 20 to 30% larger than that of the nonlinear figure using the model slope. Also in actual surface failures, (the difference between the Coulomb's criteria and the nonlinear one ranges from 6 to 8%) the former is larger. However, the safety factors calculated with the two methods in the submerged condition have a smaller variance than in the natural soil-water one.