Publication: Weathering and discontinuity evaluation of tunnel face using image analysis technique
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Date
2022-09-01
Authors
Yusoff, Intan Norsheira
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Abstract
Face collapse, block failure controlled by rock mass discontinuities, and high water intrusion are common risks associated with tunnel excavation in weathered regions of mountainous terrain. Two main parameters used to evaluate the tunnel face for tunnel support determination is generally weathering degree and discontinuity, which generally controls rock mass deformability. Human bias in the weathering and discontinuity evaluation of a tunnel face is one of the key concerns that must be reduced in many tunnelling projects. It is proposed here to quantify tunnel face weathering using the CIEL*a*b colour space and image analysis. The digital RGB images of the tunnel face were transformed to L*a*b values using JudGeo, a built image processing software. The Japanese Highway (JH) Rock Mass Classification System was used to classify the rock mass tunnel face. Seventy-one data were collected at the study area to determine the relationship between the CIEL*a*b color space and the different weathering grades of the granitic rock samples. Four samples of different rock classes from the JH Rock Mass Classification (Class B; CII; DII, and E) were used to verify the effectiveness of the quantitative evaluation system compared with that of the manual tunnel face mapping performed by competent geologist personnel at the site. Finding showed good correlations for Class B and CI. However, for Class DII and E showed more conservative results than the other rock mass classes, with one classification above the rock mass grade obtained using the manual observation of the tunnel face mapping. This technique can effectively identify the degree of weathering grades of the tunnel face, especially for fresh to moderatelyweathered tunnel faces. This study also proposes a new measurement system based on SfM photogrammetry for evaluating tunnel face. The aim is to determine the discontinuity pattern and orientation of the tunnel face as input for the tunnel face stability evaluation. A set of overlapping images obtained from the tunnel face replica using 3D polystyrene using SfM photogrammetry approach is used to generate a 3D point cloud model. The discontinuities pattern and orientation are determined using facet extraction of KD-tree plugin in CloudCompare. The discontinuities obtained from the high-quality setting of the point cloud to the manual measurement of the orientation using manual compass measurement to highlight the similarity of the
concentration of both discontinuities set for the replica tunnel face. Two sets of discontinuity are derived. The means of dip and dip direction from the cluster analysis are 82°/164° and 84°/307°, respectively. The orientations are verified through manual compass measurement. The orientations for manual compass measurement for Set 1 and Set 2 are 86°/151° and 80°/303°, respectively. This objective is to help geologists and tunnel engineers determine the appropriate rating of weathering degree as an input for the rock mass classification system.