STABILITY ANALYSIS OF WEATHERED VOLCANIC ROCK SLOPES IN THE CARIBBEAN

Recent slope failures were investigated on the islands of Dominica in 2017 and Saint Lucia in 2018. The islands are situated in the Lesser Antilles island arc and are experiencing rapid uplift as they are on the leading edge of an active subduction zone. The areas selected for investigation are typical of many sites on the islands where roads have cut into slopes at the angle of repose. Landslides damage and bury the roads under debris that may take several months to repair. They occur during large rainfall events associated with hurricanes that have been moving farther south over time and inundating the islands for longer periods.

Shallow core samples were obtained by hand from previously failed slopes. During sample collection, the permeability was measured in-situ using a mini-infiltrometer. Two different size cores were collected: a three-inch diameter core for shear analysis and a two-inch diameter core for permeability testing. Drained direct shear strength tests were conducted to determine the effective friction angle and cohesion at normal loads of 500, 1000 and 2000 psi.

Direct shear testing estimated that the overall critical angle of the landslide on St. Lucia was around 33 degrees while the results of several landslides on Dominica ranged from 26-47 degrees. XRD analysis of the Dominica soils detected a halloysite rich matrix which is indicative of kandoid soils. These soils are found in older volcanic areas with high rainfall where leaching is moderate but continuous.

Slope stability analysis was conducted with a limit equilibrium approach using the infinite slope method. This method allowed back calculations on previously failed slopes to determine the friction angle needed for the current slope to stay stable under different conditions and the height above or below the failure surface that the water table reached when the failure occurred.