SOME CAUSES OF SLOPE FAILURE
Slope failures are caused, in general, by natural forces, human misjudgment and activities, and burrowing animals. We will describe below some of the main factors that provoke slope failures.
Water and wind continuously erode natural and man-made slopes. Erosion changes the geometry of the slope (Figure , ultimately resulting in slope failure or, more aptly, a landslide. Rivers and streams continuously scour their banks, undermining their natural or man-made slopes (Figure).
Some causes of slope failure.
Long periods of rainfall saturate, soften, and erode soils. Water enters into existing cracks and may weaken underlying soil layers, leading to failure, for example, mud slides.
Earthquakes induce dynamic forces (Figure ), especially dynamic shear forces that reduce the shear strength and stiffness of the soil. Porewater pressures in saturated coarse-grained soils could rise to a value equal to the total mean stress and cause these soils to behave like viscous fluids—a phenomenon known as dynamic liquefaction. Structures founded on these soils would collapse; structures buried within them would rise. The quickness (a few seconds) with which the dynamic forces are induced prevents even coarse-grained soils from draining the excess porewater pressures. Thus, failure in a seismic event often occurs under undrained condition.
Many failures commonly result from unidentifi ed geological features. A thin seam of silt (a few millimeters thick) under a thick deposit of stiff clay can easily be overlooked in drilling operations, or one may be careless in assessing borehole logs only to fi nd later that the presence of the silt caused a catastrophic failure. Sloping, stratifi ed soils are prone to translational slide along weak layers (Figure ). You must pay particular attention to geological features in assessing slope stability.
Loads placed on the crest of a slope (the top of the slope) add to the gravitational load and may cause slope failure (Figure). A load placed at the toe, called a berm, will increase the stability of the slope. Berms are often used to remediate problem slopes.