Real-time determination of fracture size and shape using trace data on tunnel walls

Abstract

A large amount of geological data can be obtained during tunnelling excavation. An on-going research project, AMADEUS (Adaptive Real-Time Geological Mapping Analysis of Underground Space), is being conducted to develop new methods to measure, process and implement this new geological datain real-time. This paper describes a statistical process developed to determine the shape and size of fractures, using the trace data that is observed along the tunnel wall. From field observations, fractures in sedimentary rocks are commonly rectangular and elongated parallel to their strikes. According to the possible geometric relationships between a fracture and a tunnel, four types of intersections (complete, longedge, short-edge and corner) are defined when fracture width is greater than tunnel diameter. The probabilities of occurrence of these different intersection types are related to tunnel diameter, size, and aspect ratio of fractures. The size and aspect ratio of fractures are then estimated directly from the observed number of occurrences of different types of intersection. Monte Carlo simulations using Excel and custom software are used to generate synthetic fracture populations and a cylindrical tunnel in a three-dimensional sedimentary rock mass. The results of these simulations, which validate the approach, are presented. Use of the estimators is demonstrated by an example. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.