A3 Slope materials, landslide causes and landslide mechanisms

Abstract

Soils in hilly or mountainous areas are normally divided into two types: in situ weathered soils and transported soils. Table A3.1 provides a simplified classification and description of the common soils encountered in the humid tropics and subtropics and their engineering behaviour, based mainly on Fookes (1997). View this table: Table A3.1. Common soil types found in the humid tropics and subtropics ### A3.1.1 In situ weathered soils Deep in situ weathered soils are often developed under humid tropical and subtropical climates (e.g. Ruxton & Berry 1957) and are the result of intense chemical decomposition and, to a lesser extent, mechanical disintegration of the parent rock. The classification of these soils is based on the degree of weathering that has occurred to the parent rock (Moye 1955; Ruxton & Berry 1957; Little 1969; Anon 1977, 1995; IAEG 1981; BSI 1999). Further discussion on weathering classifications is provided in Hencher (2008) and Norbury (2010). Figure A3.1 shows the classification used to describe weathered rock and soil sequences in the humid tropics and subtropics by Fookes (1997). Text box A3.1 outlines some of the engineering properties associated with each of these weathering grades. Weathering grades IV, V and VI are usually classified as soil because, from an engineering perspective, they tend to behave more as soil than as rock. They are termed highly weathered rock, completely weathered rock and residual soil, respectively (BSI 1999), but because all three effectively behave as soil, they are also frequently referred to collectively as tropical residual soil (Fookes 1997). Due to processes of erosion and mass movement, it is usual in hilly and mountainous areas for deep residual soils to be limited in extent and confined to areas of gentle slope such as on geologically-controlled benches and broad ridge crests. Frequently, the weathering profiles found on most mountain …