Field-scale assessment of effective dilation angle and peak shear displacement for a footwall slab failure surface

When designing high steep footwall slopes, an accurate assessment of the dilational shear strength component is necessary. Bedding surface profiles taken from a detailed digital terrain model of a slip surface exposed by a large slab-type footwall failure were used to characterize the geometric roughness and dilation angles in the sliding direction. The dilation angles predicted from best-fit dilatancy curves are sensitive to the base distance or shear displacement. The dilation angle for a given shear displacement (or base distance) is not sensitive to the profile length if it exceeds the typical in situ block size. For the case history, a 2D finite element analysis was performed to back-calculate the effective dilation angle assuming no cohesion was present and the basic friction angle was 25°. This analysis suggested that a dilation angle of 12° was required to create a condition of incipient slab failure. Using dilatancy curve equations, it is possible to convert dilation angles back-calculated from finite element stability analyses into shear displacements required to mobilize the peak strength. The shear displacement required to mobilize a dilation angle of 12° is estimated to be 0.02–0.15 m for the slab failure that was studied. Analysis of profile geometries to estimate the shear displacement needed to reach peak shear strength will improve the interpretation of slope displacement monitoring data.