In order to describe "the long term stability of clay slopes", this paper will look at factors that make slopes become unstable overtime; the measurement of the instability; impact of long-term stability; and the prevention of instability of clay slopes.
To begin with, long term stability of clay slopes is influenced by several causes which as are embedded in two major factors. These main factors, according to Price (1984) and Ritter (1986, p 121) are shear stress and shear strength. Shear stress refers to the forces that cause movement of materials down slope while shear strength is the force that resists materials from moving down the slope. These forces oppose each other in the sense that, while shear stress wants movement of material to commence, shears strength refuses. This means that movement will only occur when shear stress exceeds shear strength. Otherwise, like Sparks (1964, p 56) puts it, in uniform material, shear stress and shear strength may remain comparatively uniform.
It is worth noting that shear stress has several intricate components that make it provoke movement of material down slope. Ritter (1986, p 121) outlines material internal friction characteristics; material normal stress and material normal cohesion as factors that determine stability of shear stress. Any drop or increase in these factors may alter shear stress. Internal friction is further broken down into plane friction (produced when one grain of soil slides past another) and interlocking friction (which originates when particles are required to move upwards and over one another.
Secondly, Ritter (1986, p.122) further states that effective normal stress has the capacity to hold the material together, thereby increasing internal resistance to shear. It acts perpendicular to a shear surface and is absorbed by the underlying slab at the point of contact between grains. It should also be noted that some of the shear surface is usually occupied by openings which are filled with air or water. And since pore pressure exists in these interstitial spaces, it tends to support part of the normal stress.
Thirdly, Ritter (1986, p.123) further states that clay soils have cohesion, which comes as a result of ions and water by clay minerals, thereby creating a binding structure among particles. Unfortunately though, cohesion decreases with increased water acquisition in the soil material. Clay cohesive strength very much depends on attractive forces between the particles and the lubricating action of the interstitial liquid. The more the clay acquires water, the more the slope becomes unstable. However, it should be noted that the rate at which the slope gains more water and the water drains away determines the time the slope should become unstable. For example, fast gaining and slow draining of water on a particular slope will make the slope fail very fast. On the other hand, slow gaining and fast draining of water will make the slope remain stable for some time. Where clay soils remains in an undisturbed normal cohesive strength, long-term slope stability will be evident. Sparks (1964, p 57) agrees with the above statement and adds that cohesive strength increases with depth thereby exceeding shear stress, hence surface mantle (slope material) is the one to be more unstable. As more water is added , cohesion decreases and when all pores are filled, any further input of water results in complete