CORRELATION BETWEEN OBSERVED SUPPORT PRESSURE AND ROCK MASS QUALITY

The correlation between rock mass quality and support pressure proposed by Barton et al. (1974) has proven useful, except in cases of squeezing ground conditions. Field data collected systematically from 20 tunnel sections indicate a clear need for correction factors to account for height of overburden and tunnel closure, which do not seem to be adequately accounted for by the stress reduction factor. As expected, the support pressure decreases rapidly with tunnel closure and then increases beyond a limiting closure. The fact that the observed wall support pressures were always close to zero except in squeezing ground conditions has been taken care of by slightly modifying wall factors for Q-wall. A criterion derived from the field data shows that squeezing ground conditions would be encountered where the height of the overburden is greater than 350 Q1/3. The data reported herein confirm the earlier findings of Barton et al. (1974) that the support pressure is independent of the tunnel size.