DISPLACEMENT-DEPENDENT VARIATION IN GROUND DEFORMATION IN LANDSLIDES IN NORTHERN UTAH

Observations at recently active landslides in northern Utah indicate the intensity and distribution of ground deformation varies with the amount of landslide movement. Landslides can be characterized into three groups where displacements: (1) are less than 50 centimeters (2 ft), (2) range from 50 centimeters (2 ft) to 3 meters (10 ft), and (3) exceed 3 meters (10 ft). However, transitional cases are common because displacement along the width of a landslide is typically not uniform.

Landslides in group one lack significant ground deformation features. Where displacements are only a few centimeters, ground deformation features consist of rare transverse ground cracks and lateral shears generally developed in brittle construction materials such as concrete and asphalt. Landslides where displacements approach the upper bound value for the group typically have a continuous main scarp, but lack flanking shears or a well-defined toe. Downslope propagation of the surface of rupture in these landslides is incomplete and the landslide movement is confined.

Landslides in group two generally develop localized ground deformation in the head and near the toe of the landslide. Minor scarps and transverse ground cracks are generally restricted to the head. Ground cracking in the toe and along the flanks of the landslide also occurs, but continuous or stepping lateral shears on the flanks are generally in an incipient stage.

Landslides in group three generally develop complex toe thrust systems and flanking and internal lateral shears. Toe thrust systems develop to accommodate the total displacement of the upslope landslide mass in response to dewatering-induced abandonment of individual toe thrusts and downslope loading caused by soil overthrusting. Displacement compatibility requires complex internal deformation of the landslide mass and/or transition to flow rather than sliding.

Careful mapping of the intensity and distribution of ground deformation features in landslides allows general estimation of the amount of displacement, average movement rate, and residual shear strength in the surface of rupture zone.