Influence of a Weathered Zone on the Susceptibility of a Slope to Rainfall Induced Instability

Numerous research studies are being carried out to examine the behaviour of unsaturated slopes subjected to rainfall. In many cases the research is a consequence of transportation infrastructure that has shown instabilities triggered by specific rainfall events. This paper presents results of generic modeling to assess the impact of the thickness of a weathered crust on the susceptibility of a typical highway embankment slope subjected to rainfall induced instability. Engineered embankment fills and underpass cut slopes will undergo stress relaxation after construction. This relaxation near the surface and exposure to the environment can result in development of a secondary structure with decreased strength and increased permeability. The extent of this weathered zone depends on the geometry of the engineered slope and the nature of the soil. The model developed in this work presents generic modeling results that examine the importance of the weathered zone by examining the sensitivity of some generic slopes with varying weathered zone thickness under specified rainfall events. The results provide some insights as to the importance of characterizing the characteristics of the weathered zone for this class of problem. Specifically the results show that the hydraulic properties are more important in terms of assessing the susceptibility of slopes to this mode of failure as opposed to the strengths. Results also show that in this generic modeling the dissipation of suction was more important that any mounding of the groundwater level.