Boudinage structures causing instability in some rock cuttings along the Southern Expressway, Sri Lanka

Abstract

Abstract The southwestern region of Sri Lanka is underlain mainly by Precambrian metasedimentary rock sequences consisting of charnockites, gneisses and migmatites. Along the Southern Expressway, charnockites occur extensively as boudinage structures of different dimensions (mini- and mega-) enveloped or wrapped by gneissic rock bands. As a result of characteristic tropical intense precipitation and extreme weathering conditions of the area, gneissic rocks decompose to loosely bound lateritic soils or hard lateritic cover. The lowermost parts of rock cuttings along the expressway show the occurrence of charnockite boudins wrapped within less competent gneisses that have weathered into lateritic soils. Weathering and erosion processes weaken the bond between gneisses and the boudins, resulting in some of the more competent boudins becoming detached and falling to the ground, leaving voids in the cut slopes and thereby making the slopes more vulnerable to erosion. However, on weathering, gneissic bands when not associated with boudinage structures were observed to develop hard lateritic covers at the top of cut slopes. In some cuttings, boudins are associated with highly folded, pegmatite-rich migmatitic gneisses that weather more rapidly than banded gneisses. Erosional features such as gullies and cavities cause large-scale instability owing to collapse of weathered rocks and soils, giving rise to slope failures.