2.4 Geotechnical Properties of Material

The important geotechnical properties affecting stability of a slope are shear strength of material, particle size distribution, density, permeability, moisture content, plasticity and angle of repose. The strength of rockmass is a very important factor that affects the stability of slopes. It is a function of strain rate, drainage condition during shear, effective stresses acting on the soil prior to shear, the stress history of the soil, stress path, and any changes in water content and density that may occur over time. It consists of cohesion and friction angle of material. Friction is a resisting force between two surfaces. Cohesion results from a bonding between the surfaces of particles. It is dependent upon many factors, including material properties, magnitude and direction of the applied force and the rate of application, drainage conditions in the mass, and the magnitude of the confining pressure.

The relationship between the peak shear strength  and the normal stress σ can be represented by the Mohr-Coulomb equation (figure 7):

where c is the cohesive strength and is the angle of friction.

Figure 7: Shear testing of discontinuities or between two plane

The shear strength of Patton's saw-tooth specimens (figure 8) can be represented by:

where is the basic friction angle of the surface and is the angle of the saw-tooth face.

 

Figure 8: Patton’s experiment on the shear strength of saw-tooth specimens.

 

Materials that are coarse or have a rough texture have greater opposing frictional forces or shear strength to resist the movement. However, unconsolidated materials such as sediment and soil that have no strong cementing material or interlocking crystal structure is far less stable than hard rock. Rate of loading, degree of compaction and moisture content of the rockmass also affect its slope stability.

Density is also important factor in slope stability. However, its effect is more in mine waste dumps where it is a function of the manner of deposition, gradation, and loading history. A relatively small increase in density can increase the shear strength of waste dump, but it also increases the stresses due to gravity loading.

Permeability of the soil or waste material affects seepage pattern and water levels in the slope. This, in turn, can affects shear resistance of the material depending on the size and shapes of the particles, degree of compaction and the gradation of soil and its density (Campbell, 1975 and Aubeny and Lytton, 2004).

Angle of repose of loose material is influenced by the size and shape of its particles. Smooth, rounded particles have a lower angle of repose than rough, angular particles. Coarse fragments can maintain a greater slope than fine fragments.