Question

Should laboratory-tested core sample shear strength be factored up, down, or unchanged to develop an allowable rock mass shear strength in design of the foundation system?

Answer 1

Rock discontinuity surfaces exhibit a diversity of features consisting of a range of micro to macro wavelengths. These features may be very small asperities to large scale undulations. The valleys and peaks formed by shorter wavelengths may be termed as roughness and the longer wavelengths as waviness. Usually the roughness is superimposed on waviness. In complex undulating joints, the actual shear strength may be function of both the roughness and waviness components. When a non-planar joint system is subjected to direct shear under constant normal stress, the shear stress against displacement curve typically shows a curved pre-maximum, distinct

maximum and post minimum shear strength response. This can be explained as follows. Before applying the shear force, the two sliding surfaces are in perfectly matched (mated)position. At small shear displacement, the shear stress increases rapidly (i.e. high shear stiffness. Yielding or breakage of asperities (diminishing roughness) is reflected by a significantly reduced shear stiffness in the pre - peak region. The point of maximum shear stress coincide with the totally mismatched position of the two sliding surfaces. The downward movement along the asperities is charcterised by a quasi-linear reduction in shear strength. The shear strength attains a minimum value after continued shearing. This measures peak and residual direct shear strength as a function of stress normal to the sheared plane.