Effect of normal stress and relative compaction on secant friction angle of sands

In most geotechnical projects the linear Mohr{Coulomb envelope is used. This envelope is curved for coarse- grained soils and is affected by some factors such as con ning pressure, relative density, mineralogy, particle crushing, ne content, and gradation of sand particles. The curved strength envelope could be used to study the behavior of deep foundations, earth dams, soil slopes, and other earth structures in which failure occurs under considerable normal stresses. A deep failure surface may be more critical than a shallow surface when the phenomenon of curved strength envelopes is considered, because the secant friction angle decreases with increasing depth of soil layers and con ning pressure. The main purpose of this research is to express the secant friction angle of sands as a function of normal stress and relative compaction. In this study, direct shear tests are performed on air-dried and saturated sand samples at different normal stresses to evaluate the variation of secant friction angle with these factors.