EFFECTS OF WATER ON SLOPE STABILITY

Abstract

A brief state-of-the-art review of the effects of water on slope stability and the techniques for analysis is presented. The effective stress principle and basic considerations of slope stability, including design factors of safety, are discussed briefly. The derivations and effects of seepage forces and rapid drawdown on effective stress are also presented. Various conditions of external loading produce changes in effective stress. These changes are discussed and limiting conditions which should be analyzed are mentioned. Limitations of total stress analyses are discussed in detail. It appears that, for soils having a liquidity index of 0.36 or greater (normally consolidated), the undrained shear strength gives factors of safety close to the actual factor of safety. For soils with a liquidity index less than 0.36 (overconsolidated), the undrained shear strength gives factors of safety that are too high; but the strength parameters can be corrected by the empirical relationship presented herein. Data also show that the difference between vane and calculated shear strength increased as the plasticity index and (or) the liquid limit increased. An empirical relationship for correcting vane shear strength is presented. A discussion of effective stress analysis, including idfferences between peak and residual phi angles for normally consolidated and overconsolidated soils, is presented. The residual psi angle decreases logarithmically with increasing clay fraction. The state of a clay is also defined. Shear strength parameters of a clay tested in that state correspond to the theoretical strength of an overconsolidated clay which has undergone a process of softening. To test a clay in the critical state, it is suggested herein the soil should be remolded to a moisture content equal to 0.36 times the plastic index plus the plastic limit. Water may cause unstable conditions in earth slopes due to changes in geometry. Erosion of the toe or the slope can induce damaging stress. Piping through heaving or erosion of subsurface layers can cause damage. Construction of side-hill embankments can cause damming, resulting in a rise in the water table. Methods of water detection are also summarized. These include tracers, electrical resistivity, and water table observations. The latter method apparently is the most successful. A discussion of ways to monitor water pressures, including the types and operations of piezometers, is given. Finally, suggested guidelines for the design of earth slopes are included.