DISCONTINUITY GEOMETRY CHARACTERIZATION IN A TUNNEL CLOSE TO THE PROPOSED PERMANENT SHIPLOCK AREA OF THE 3 GORGES DAM SITE IN CHINA

Discontinuity data from a 400 m tunnel located close to the proposed s hiplock area were investigated to characterize the discontinuity geome try of the rock mass around the tunnel. Traces of 39 major discontinui ties (faults and dykes) were found on the tunnel exposures. Over 2000 minor discontinuity (joint) trace data available showed that the rock mass can be separated into about five statistically homogeneous region s. Three to four joint sets were found to exist in each of these regio ns, Available theoretical probability, distributions, were found to be insufficient to represent the statistical distribution of orientation of joint clusters. For about 94% of the joint sets, the gamma distrib ution was found to be the best probability distribution for representi ng a joint size distribution. Exponential and gamma distributions were the best probability distributions for representing joint spacing dis tributions for the joint data studied here, Censored joint trace data showed a significant effect of the trace length biases on the estimate d mean trace length. This indicated that the 2 m wide exposures which were used for collecting the joint trace data may not be sufficient to produce reliable estimates for joint size parameters for the shiplock area. Different estimates were obtained based on, wall data and roof data for the joint size and 3-D intensity parameters. The following ma y have contributed to this result: (a) the effect of different finite sample sizes used; (b) the effect of sampling biases; and (c) the effe ct of modelling uncertainties on the estimated parameters. To obtain b etter estimates, it is suggested to collect joint data for several exp osures which are at least 4-5 m wide and have different orientations. When such data become available for the Three Gorges region, it is sug gested;to perform validation studies to check the applicability of the models, in addition to performing the joint geometry, modelling.