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

摘要

A large amount of geological data can be obtained during tunnelling excavation processes. 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 data in real-time. In relation to this research project, a statistical process has been 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, long-edge, short-edge and corner) are denned when fracture width is greater than tunnel diameter. Assuming independence between the tunnel and the fracture population, the probabilities of occurrence of these different intersection types are related to tunnel diameter, size, and aspect ratio of fractures. On the axis-normal plane, the center of the tunnel must be inside a specific region around the projected fracture in order for an intersection to occur (see Figure). Each intersection type, therefore, has a corresponding locus with respect to the projected fracture on the axis-normal plane (see Figure). The size and aspect ratio of fractures are then estimated directly from the observed number of occurrences of different types of intersection.