In tropical areas, weathering is common down to several tens of metres, and can occur to depths of over a hundred metres. Tunnels and excavations in such areas are therefore very likely to encounter weathered rock. Weathering involves progressive weakening of the rock, ultimately to a soil-like condition. Despite this soil-like nature, the weathered rock retains many structural features of the original rock. This means that the behaviour may be significantly different from an apparently similar deposited soil. Both the pattern of weathering development and this retained structure are important in understanding the behaviour of weathered rocks. Some of the issues involved in tunnelling and underground excavation in tropically weathered rocks are illustrated with examples of observed performance in four selected rocks found in Hong Kong and Singapore. The four selected rocks consist of two granitic rock formations and two sedimentary rock formations. The development of weathering in these rocks is contrasted, and related to the nature of the fresh rock. The observed behaviour of the weaker and more soil-like weathering grades of the rocks is discussed in terms of the ‘Tunnelman's classification’. Particular issues discussed include: mixed face conditions, abrasion, assessment of strength, swelling, relic joints, erosion, hardpans and collapse.
Rocks affected by tropical weathering are prevalent over a significant proportion of the earth's surface. Although such rocks are widespread in tropical and sub-tropical areas, they are also found in more temperate areas where conditions favour weathering. The behaviour of the soil-like materials that are the final result of tropical weathering can also differ in important ways from that of deposited soils. The nature and behaviour of the weathered rocks are affected not just by the nature of the parent rock but also by factors such as climatic conditions and topography.