The Aspen Grove landslide, Ephraim Canyon, central Utah

The Aspen Grove landslide, central Utah, occurred in older landslide debris. The debris is about 6-15 meters thick, and consists of medium- to high-plasticity clays and silty clays. Persistent landslide structures, including toes, hollows, and flank ridges, outline dimly preserved landslide masses in the older debris.;Tension cracks defined the head of the landslide, and segmented strike-slip faults formed the flanks. The terminus of the slide was poorly defined. Cracks, domes, flank ridges, scarps, and other structures formed at the surface of the landslide as movement progressed.;The landslide began moving each spring when infiltrating snow-meltwater raised the water table to 0.5-1.0 meters below the ground surface. The landslide accelerated as the water table rose to the ground surface, and reached a maximum velocity of 20 centimeters per day. Subsequently, the slide slowed as the water table fell during the early summer, and the slide stopped in July or August. Annual displacements of 0.1-4.5 meters accumulated in the upper part, 4.5-8.8 meters accumulated near midlength, and 0.1-2.5 meters accumulated in the distal part of the landslide during 1984, 1985, and 1986.;A theory for sliding on uneven surfaces might explain the observed slow movement of the landslide. The flanks and basal slip surface of the Aspen Grove landslide are uneven. Sliding is more difficult on an uneven slip surface than on a smooth one because the debris must deform in order to slide over the uneven surface. The deformation causes redistribution of pore water, and resistance to this deformation is proportional to the velocity of sliding and the permeability of the landslide debris. Slow movement of the slide is consistent with low permeability of the clayey landslide debris.;The landslide was 600 meters long and 25-50 meters wide when it started moving about May 31, 1983. It enlarged to 1000 meters long and 25-100 meters wide in 1984 and 1985. The landslide is wide at its head and toe, and narrow at mid length.