Evaluation of Rock Pressures in Tunnels Through Squeezing Ground in Lower Himalayas

Abstract

Construction of several hydroelectric tunnels in recent years through the lower Himalayan region has been delayed due to support problems in the weak and squeezing grounds under complex geological and tectonic conditions. Besides the lack of adequate geological details,these problems could be attri buted to the heterogeneous rocks and their behaviour,inadequate knowledge of Rock Mechanics and the indiscriminate use of the existing classification systems for tunnel support design. These practices disregard the influence of high cover pressure and ignore the benefits of allowing tunnel-wall displacements. The real problems are : (i) how to anticipate squeezing condi tions and assess the degree of squeezing and (ii) how to estimate the short-term and long-term rock pressures as well as the allowable tunnel-wall displacements. It is possible to solve these problems if 'tunnel mechanics1 of squeezing ground in particular is better understood. As such, field observations with load cells, borehole extensometers etc. in five hydroelectric tunnels and in a roadway of a deep coal mine have been analysed with a new graphical method. The data analysis of 12 test-sections shows that the classification systems are not a panacea for all tunnel support problems. The Q-system of Barton et al (1975) proved to be the most reliable for predicting the short-term rock pressures in non-squeezing grounds only. For tunnels in squeezing groundsfthe elasto-plastic theory has shown consi derable promise in predicting the short-term rock pressure and also the allowable tunnel-wall displacement. iv A three dimensional numerical solution for the shortterm rock pressure on tunnel supports in squeezing ground conditions has been developed. The corrected ground reaction curves obtained from the above solution compare qualitatively with the observed support reaction curves in two steel suppor ted tunnels. The corrected ground reaction curves indicate that the short-term rock pressure on steel ribs is considerably higher at the face than the stable pressures far behind it because the broken zone expanded gradually and stablized at a face advance about four times the radius of the broken zone. Observations of the tunnel-wall displacements and the borehole extensometers indicate that the radial displacements within the broken zone decrease lineraly with the logarithm of the radial distance. Surprisingly,the failed rock masses tend to get compacted adjacent to the supports. The overall coefficient of volumetric expansion ranges from O.OOl to 0.01 and the observed ratio of the radius of broken zone to the tunnel radius varies from 3 to 5.6. A semi-empirical theory for ultimate creep pressure has been proposed assuming that the 'compacting zone' undergoes visco-elastic creep relaxation. The charts prepared on the basis of this theory indicate that the ultimate rock pressure on tunnel supports in squeezing condition would increase with the cover pressure.Obviously,weak rock masses would mobilize higher ultimate pressures. Finally,the theoretical estimates of the ultimate rock pressure are compared with the capacities of the existing tunnel linings for the nine cases discussed in the thesis and a safety factor of 3 for designing permanent tunnel linings is recommended.