Shallow soil slips, earth and debris slides appear to be a primary mechanism of hillslope denudation in the rainforest of eastern Puerto Rico. Annual rainfall in excess of 4,000 mm, and thick sequences (up to 20 m) of residual soils (saprolite) combine to produce these landslides. Shear strength testing and observatons of tension cracks indicate that landslides may start as tensile failure of saprolitic material. The tension cracks provide avenues for rapid infiltration of rainwater and saturation of the underlying soil. During or shortly after intense or prolonged precipitation, shear failure then occurs as a result of pore-pressure buildup along zones of low permeability within the saprolite.
Tensile stresses in the unsaturated, low-density upper saprolite zone range from 3 to 10 kPa, and mean seismic refraction velocities are 476 +/- 127 m/s. Denser, clay-rich, less permeable saprolitic zones having mean seismic velocities of 1420 +/- 157 m/s represent zones that are commonly associated with perched ground water and are zones where exess pore-water pressure is most likely to trigger movement. These zones of translocated clay show marked density increases.
Slickensides are present in the saprolite along relict fractures and joints derived from the parent rock; they are common in quartz-diorite bedrock, and less so in marine-deposited volcaniclastic bedrock. The failure planes of many landslides have exposed these relict fractures and joints as slickensides, and landslides appear to move on these pre-existing planes of weakness in the saprolite. The larges landslides (areas greater than 20,000 m2, however, are those that fail along saprolite-bedrock boundaries, which are zones of contrasting density and permeability within or at the base of the weathered profile.
