Slope failure processes in saprolitic soils of a tropical rain forest, Puerto Rico

¹U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA, 98661, USA

²U.S. Geological Survey GSA Center 651 Federal Drive Guaynabo, Puerto Rico 00965-5703, USA

Abstract

Rotational failures, with associated downslope earthflow component, appear to be the primary mechanism of hillslope denudation in the rainforest of eastern Puerto Rico. Annual rainfall in excess of 3,000 mm, and thick sequences (up to 20 m) of residual soils (saprolite) combine to produce specific failure mechanisms. Shear strength testing and observatons of tension cracks indicate that slope failure initiates by tensile failure of saprolitic material. The load on a potential failure plane in the sub surface is thereby increased by the additional weight of the detached block and by saturation of the overlying soil. During or shortly after intense or prolonged precipitation, shear failure then occurs along zones of reduced permeability within the saprolite.

Fifty-two 'naturally occurring' landslides were mapped using aerial photography and field observation. Although forest elevations exceed 1000 m, the majority of these failures were found between 600 to 800 m in elevation, in areas where there there is significant concentration of throughflow. Slope failures show no specific trend with respect to slope aspect. More than 100 'road-related' failures were also mapped and can be attributed to such factors as toe-removal, oversteepening, and increased hydraulic gradients. Tensile stresses in the unsaturated, upper saprolite zone range from 3 to 10 kPa, and have mean seismic refraction velocities of 476 +/- 1 27 m/s. Denser, clay-rich, less permeable saprolitic zones with mean seismic velocities of 1420 +/- 157 m/s indicate probable failure planes where exess pore-water pressure decreases shear strength, leading to shear failure. These zones of translocated clay show marked density increases as shown by an order of magnitude increase in blow counts.

Slickenside surfaces in the saprolite are found along relict fractures and jo ints derived from the parent rock- quartz diorite and marine-deposited volcaniclastics. This evidence of differential movement suggest that failures, once initiated occur along specific planes of weakness in the saprolite. However, the largest landslides (areas greater than 25,000 m²), fail along saprolite-weathered-bedrock boundary. It is therefore apparent that failure planes occur along zones of contrasting density and permeability within the weathered profile.

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