Shear strength and slope stability at sites in the Caribbean National Forest and Coamo area, Puerto Rico

¹U.S. Geological Survey GSA Center 651 Federal Drive Guaynabo, Puerto Rico 00965-5703, USA

²U.S. Geological Survey, Nashville, Tennessee, USA

Abstract

In-situ shear strength tests, used in conjunction with slope-stability analyses are used to determine factors of safety, and failure mechanisms in landslide-prone areas in Puerto Rico. Slope failures in the Caribbean National Forest appear to occur by initial tensile failure of saprolite materials at the crown, followed by shear failure along the slope. Many failures are related to road cuts and occur during periods of heavy, prolonged rainfall. This progressive-type failure does not generally occur in the Coamo area, however intense, prolonged rainfall events (often hurricane related) have led to debris flows and debris avalanches. Plastic clays that overly the saprolite in the National Forest show limited cohesive strengths (mean = 10.7 kPa), because of almost continuous water saturation. In contrast, saprolite above and near head scarps is considerably drier, and maintains tensile stresses of 3.0 - 7.0 kPa. This finding supports the assertion that initial failure occurs in the saprolite by loading with saturated clays and excess pore-water pressure, resulting in tensile failure. Sites in the Coamo area receive less rainfall than the National Forest and contain much less saprolite. Plastic clays are not continuously saturated and therefore maintain moderate cohesive strengths (21 - 35 kPa). This combination of factors usually provides more stable slopes in the Coamo area than in the National Forest.

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