Elasto-plastic finite element analysis of twin tunnels – A comparison of excavation in intact and jointed rock mass

Abstract

In the present paper, a comparative study of tunnel excavation has been reported in intact and jointed rock mass. Considering increase in demand of tunneling as a means of transportation and several other purposes in India, a numerical analysis of twin parallel interacting horse shoe tunnels using plain strain elasto-plastic finite element analysis have been carried out. Three pillar width to tunnel diameter ratio (W/D = 1.2, 0.6, 0.3) along with three in-situ stress ratios (Ko = 0.5, 1.0, 1.5) have been considered in the analysis. Hoek-Brown yield criterion has been opted and tunnels have been considered to be excavated simultaneously. Two orthogonal discontinuity sets have been considered in the rock mass to study the effect of joints on tunnel excavation. Variations of principal stresses and deformations along the tunnel boundary and pillar zone have been reported in this presentation. A comparison of results of tunnel excavation in intact and jointed rock mass has been done to observe the extent of behavior of excavation when joints are present. The results obtained in case of jointed rock mass are very much different from that of intact rock mass. Little yielding have been observed only at springing and invert of tunnels in intact rock mass whereas tunnels in jointed rock mass exhibits significant yielding around tunnels and surrounding medium. Largest displacement is observed at springing level on abutment side for Ko = 1.5 for jointed rock mass. The difference between major principal stresses is largest for Ko = 1.5 and Ko = 0.5 at springing level and at invert-crown locations respectively. In general, it has been observed that behavior of interacting tunnels is largely affected by discontinuities in the medium along with pillar width and in-situ stress ratio.