Estimation of support pressure during tunnelling through squeezing grounds

High in situ stresses and poor quality of rock mass are primarily responsible for the squeezing behaviour of rock masses. This phenomenon is prevalent especially in the Himalayan region and hence rock engineers and engineering geologists have frequently encountered problems of stability during construction in this region. High in-situ stresses, poor rock mass quality, large overburden depth and large radius or span width of a tunnel or cavern in weak rocks are the factors which are responsible for the occurrence of squeezing ground condition. The present study involves development of a dimensionally correct empirical correlation for assessment of support pressure in tunnels which are excavated in squeezing ground condition. The correlation uses the concept of ‘joint factor’ as a measure of rock mass quality, allowable closure, depth and radius of opening as the governing parameters. Data from 52 different tunnel sections and one set of data from a mine gallery have been considered for analysis. The predicted results have been compared with the results obtained via existing approaches, based on rock mass quality (Q) and rock mass number (N). It was observed that the proposed correlation holds better with a correlation coefficient of 0.92 and estimated values of support pressure from the approach show better accordance with the observed values of support pressure as compared to other existing correlations based on Q and N values. The proposed correlation makes use of parameters which can be easily obtained at project sites. Therefore, it can become a handy tool for site engineers to predict the support pressure in squeezing ground conditions and take appropriate measures for the stability of underground excavations.