Determination of translational landslide slip surface depth using balancedcross sections

An apparently little-recognized but powerful technique for determining slip surface depth of translational landslides is the use of balanced cross-sec tions. Conceptually, the method is simple and the necessary parameters can be determined without subsurface exploration. To determine the depth of the slip surface for a given landslide, a longitudinal cross-section through t he approximate center of the slide is drawn, From the cross section, the ar ea of the depression (zone of depletion) formed at the head of the slide ab ove the listric part of the slip surface is determined. The displacement al ong the planar (translational) part of the slide is also determined, either by direct field measurement or by some indirect method. Finally, the depth of the slip surface is determined by dividing the area of the depression b y the displacement. Determining the slip surface displacement is likely to be the most uncertain aspect of the method, In some cases, the displacement can be determined directly by measurement of offset features such as roads , fences, or surface drains. However, in many, if not most cases, the displ acement must be determined by indirect methods such as using displacements found along the scarp(s) of the landslide. In using scarps, the style of in ternal deformation of the slide mass is important. Styles of deformation in clude penetrative shear strain and/or the slippage along discrete shear pla nes. Based on the displacement determined from the scarp(s), a reasonable c alculation of the slip surface depth can be made.