The shear and compressive strength characteristics of the foundation materials are required for assessment of foundation stability and its bearing capacity. Where foundation conditions are complex, or foundation soils are fine grained, soft or susceptible to consolidation, pore pressure generation or other adverse effects, more detailed field and laboratory testing would be required. The number and type of tests to be conducted, and conditions of testing, depend on the complexity of site conditions, the nature of the soil to be tested and the loading conditions to which it will be subjected.
The effective shear strength of dump materials depends on a wide variety of inter-related parameters including intact particle strength and strength anisotropy, particle angularity, gradation, basic surface roughness and frictional properties, lithologic composition, mineralogy, degree of saturation, and others. Shear strength may also change with time due to consolidation, degradation due to freeze-thaw, swelling or slaking, oxidation, leaching or other chemical changes or strains induced by foundation or internal adjustments.
The common practice in assessing the shear strength of dump materials for analysis and design is to assume a linear Mohr-Coulomb type failure criteria, with no cohesion and a friction angle represented by the natural repose angle of the materials. Repose angle of mine dumps typically range from 35° to 40°. This relatively simplistic approach for evaluating shear strength is considered reasonable for relatively small to moderate size dumps where internal stresses are low in comparison to the intact rock strength and dump materials contain only limited amounts of fines (i.e. <10% passing No. 200 mesh), and dump materials are not subject to degradation.