Classification of rock type is one of the most important factors in rock engineering. However, it is always perceived to be the least important factor in the industry. Engineers would like to focus on knowing the strength and the deformation properties of the rock with little attention paid to understanding the mineral constituent, chemical composition, mode of occurrence as well as the depositional environment of a particular rock type, that inherently affect greatly its weathering profile and the rock mass load/deformation behavior. In engineering practice, logging geologists will usually classify rock type solely based on visual inspection of core samples, which might not be adequate in many cases. In Hong Kong, in areas that are underlain by carboniferous sedimentary rocks, there are statutory regulations where extensive ground investigation and special foundation design would be required to overcome potential problem of cavities and sinkholes. Volcanic tuff breccia has been misidentified as carboniferous conglomerate or meta-siltstone with marble clast; the consequence of this had increased the foundation cost and ground investigation cost substantially. A systematic classification system for rock type in the Tuen Mun and Tin Shui Wai areas of Hong Kong would therefore be very crucial for all practitioners to follow in order to minimize any chances of misidentifying a particular rock type. In this paper, the importance of both geological and engineering classification of rock are described and are recommended for adoption in the future for rock found in the Tuen Mun and Tin Shui Wai area of Hong Kong.
Classification of rock type is an important factor in rock engineering. Based on the rock type, it is able to characterize its behavior, which would then help the detailed design of a rock slope, rock foundation and underground excavation. Geologist will usually classify the rock type based on visual inspection of the rock core, which would not be adequate in complicated geological environment. In the cases where detailed field mapping should be supplemented to determine its mode of occurrence and testing to identify its mineral constituents, an inexperienced geologist might skip these crucial factors and try to classify the rock based on their subjective experience in the study area. Misinterpretation of rock type would result in a major implication in the estimation of the rock mass strength as well as the long-term behavior of it. Foundation cost and ground investigation cost would be reduced substantially if the correct rock type can be identified. A systematic rock classification would therefore be very crucial for all practitioners to follow in order to minimize the chances in misidentifying a particular rock type. Both geological and engineering classifications of rock should be adopted because they are complementary to each other and should be adopted altogether for rock classification. This paper discusses some of the problems in rock classification in the Tuen Mun and Tin Shui Wai areas of Hong Kong and how a systematic classification should be adopted in the future.