An Empirical Approach for Tunnel Support Design through <i>Q</i> and <i>RMi</i> Systems in Fractured Rock Mass

Abstract

Empirical systems for the classification of rock mass are used primarily for preliminary support design in tunneling. When applying the existing acceptable international systems for tunnel preliminary supports in high-stress environments, the tunneling quality index (Q) and the rock mass index (RMi) systems that are preferred over geomechanical classification due to the stress characterization parameters that are incorporated into the two systems. However, these two systems are not appropriate when applied in a location where the rock is jointed and experiencing high stresses. This paper empirically extends the application of the two systems to tunnel support design in excavations in such locations. Here, the rock mass characterizations and installed support data of six tunnel projects are used. The back-calculation approach is used to determine the Q value using the Q-system support chart, and these values are then used to develop the equations and charts to characterize the stress reduction factor (SRF), which is also numerically evaluated. These equations and charts reveal that the SRF is a function of relative block size, strength⁻stress ratio, and intact rock compressive strength. Furthermore, the RMi-suggested supports were heavier than the actual installed ones. If the approximate inverse relation between stress level (SL) and SRF is used, the difference between the actual and the recommended supports increases when using the RMi-recommended rock support chart for blocky ground. An alternate system is made for support recommendation using a Q-system support chart. In this system, the ground condition factor is modified from the available parameters, and a correlation is developed with a modified Q system.

Keywords

• high in situ stresses
• fractured rock mass
• <i>SRF</i>
• stress level