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Section 5: Excavations and slopes |
1 Engineering Geology and Geophysics Group, British Geological Survey, Keyworth, Nottingham NG 12 5GG, UK
2 Earth Science Systems Ltd, 1 Barnfield Court, Harpenden, Herts AL5 5TL, UK
Rock excavation conditions at engineering construction sites such as roads and pipeline routes can be assessed from a measurement of the seismic P-wave velocity. From a knowledge of the seismic velocity it is possible to establish where blasting may be required along the route rather than conventional excavation procedures. The popular term `ripping' is applied to excavation without blasting and manufacturers of excavation equipment have produced charts relating the rippability of a rockmass to its measured in situ seismic velocity. These charts are a basic guide to the assessment of rippability and cover a limited range of rock types; in addition, they do not always include the effects of weathering or the degree of stress relief in the rock mass resulting from removal of the overburden.
The effectiveness of the use of seismic velocity measurements for predicting rippability is examined. It is shown that, provided each lithological type is considered, in conjunction with an assessment of the degree of fracturing and weathering in the rock mass and an appraisal of the effects of stress relief, then seismic velocity is a valuable parameter for predicting rippability. This can lead to a consequent reduction in costs due to the decreased use of blasting.
