Abstract
In extensively used empirical rock-mass classification systems, the rock-mass rating (RMR) and tunneling quality index (Q) system, rock-mass quality, and tunnel span are used for the selection of rock bolt length and spacing and shotcrete thickness. In both systems, the rock bolt spacing and shotcrete thickness selection are based on the same principle, which is used for the back-calculation of the rock-mass quality. For back-calculation, there is no criterion for the selection of rock-bolt-spacing-based rock-mass quality weightage and shotcrete thickness along with tunnel-span-based rock-mass quality weightage. To determine this weightage effect during the back-calculation, five weightage cases are selected, explained through example, and applied using published data. In the RMR system, the weightage effect is expressed in terms of the difference between the calculated and back-calculated rock-mass quality in the two versions of RMR. In the Q system, the weightage effect is presented in plots of stress reduction factor versus relative block size. The results show that the weightage effect during back-calculation not only depends on the difference in rock-bolt-spacing-based rock-mass quality and shotcrete along with tunnel-span-based rock-mass quality, but also on their corresponding values.
Highlights
The depth of underground excavations is increasing day by day for different civil and mining engineering projects posing more challenges for safe and economic construction [1,2,3]
During the back-calculation of the rock-mass quality (RMR* and Q*), the difference between the rock-mass quality obtained from the rock bolt spacing (RMR1 * and Q1 *) and fiber-reinforced shotcrete thickness along with the tunnel span (RMR2 * and Q2 *) are responsible for the weightage effect
The back-calculation approach is used for the extension of empirical classification systems to highly stress-jointed rock-mass environment and due to low difference between the back-calculated rock-mass quality through different support media, the weightage effect is small
Summary
The depth of underground excavations is increasing day by day for different civil and mining engineering projects posing more challenges for safe and economic construction [1,2,3]. The difficulties for modelling such underground excavations due to the complex nature of rock mass are decreasing due to advancement in the research and technology, materials and systems in the conventional excavations methods, and in mechanized one [4,5,6,7]. The ability to forecast the rock-mass quality and support pattern during the design of underground excavation is a challenging and risky task [10].
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