This paper presents an example of 3DEC modelling of a rock tunnel using geological mapping obtained from 3D images of terrestrial laser scanner and the discrete fracture network approach. Three main discontinuity sets were identified in an 8-m-long section of a tunnel constructed in gneiss. The amount of discontinuity data obtained by 3D Terrestrial laser scanner mapping allowed the application of several sophisticated discontinuity analysis (orientation, size and frequency), and probability density functions were defined. These functions were used as input to discrete fracture network modeling, leading to a more representative 3DEC block model. Thus, if the mechanical characterization of the rock mass is also properly made, this approach can be used for stability analysis in rock mechanics. Specifically, it could be very useful for analyzing sections of uncoated tunnels in which the rock mass is strongly fractured.

1. Introduction

Several old tunnels in Brazilian roads and railways were constructed in rock masses without any support system. Recently, some of these tunnels have been presenting localized rockfall problems related mainly with rock discontinuities distributions and some punctual weathering processes. The Vitoria-Minas Railway (VMR), owned by VALE SA, is one of the oldest railways in Brazil and has about twenty tunnels in this condition, requiring geological and geotechnical investigations to provide parameters and evaluations of the stability state. Considering this problem, a partnership between the Engineering School of the University of São Paulo and VALE SA was initiated to propose a methodology for studying these tunnels. The lengths of these tunnels vary from hundreds of meters to a few kilometers, which turns conventional field discontinuity mapping into a strenuous and time-consuming job when considerable detail is required. In addition, with the conventional geological mapping in tunnels, it is difficult to obtain the actual position and geometrical parameters (generally described as orientation, frequency and trace length) of all discontinuities, mainly in the higher regions of the tunnel section.

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