Stress reduction factor characterization for highly stressed jointed rock based on tunneling data from Pakistan

Abstract

Stress Reduction Factor (SRF) which is the most difficult parameter to characterize in Q (Tunneling Quality Index) system is targeted in this study for highly stressed jointed rock in tunneling. As there are no criteria for this purpose in empirical tunnel design, an attempt has been made in this regard using 542 NATM tunnel sections mapping of four tunneling projects from Pakistan. These already supported sections are used for the back calculation of SRF. SRF values measured from the already available equations did not match with the calculated values from back analysis based on mining cases and without considering the magnitude of rock fracturing. Empirical equations proposed here are based on the data of these tunnel sections and match well with the calculated values. Two types of calculated SRF (SRFQ and SRFQC) from back analysis are dependent on the intact rock strength. In proposed equations, SRFQ (SRF calculated from original Q-system equation) is dependent on the relative block size and the ratio of intact rock strength to major principal stress. The effect of intact rock strength on SRF is also determined from the available data by plotting SRFQC (SRF calculated from normalized Q-system equation) against relative block size for different ranges of UCS and ratio of intact rock strength to major principal stress. The proposed equations are applied to calculate rock quality (Q or Qc) for highly stressed cases of the jointed rock mass of head race and diversion tunnels of another four hydropower tunnels from Pakistan with various cross sections. The empirical support design of these tunnels based on Q or Qc are numerically analyzed and verified in term of total displacement and plastic zones before and after support installation and the performance of liner was also validated based on capacity diagrams.