SUMMARY:

In Indian subcontinent single horse shoe tunnel section has been generally used in the hydroelectric power projects as well as in railways and highways projects. But with increasing demand multiple horse shoe tunnels are also a feasible option. Therefore the need arises for a detailed interaction analysis between multiple horse shoe tunnels and hence a systematic parametric analysis has been carried out and reported in this paper. The present study depicts the plain strain elastic and elasto-plastic finite element analysis of two parallel interacting horse shoe deep tunnels. For the elasto-plastic analysis Hoek-Brown yield criteria has been adopted. Three pillar width (W) to diameter (D) ratios (W/D) have been introduced for each of the three considered in situ stress ratios (Ko = 0.5, 1.0 and 1.5) in the rock mass. The tunnels have been considered to be excavated simultaneously. Variation of principal stresses in the pillar zone and at the tunnel boundary have been studied in the analysis. The results have also been compared with the results of that excavation of a single horse shoe tunnel. It is concluded that for W/D = 0.3, in the case of elasto-plastic analysis of interacting tunnels most of the rock mass in the pillar zone shows yielding resulting in the development of very low principal stresses. For elasto-plastic analysis of interacting and single tunnels, it is observed that the difference in major principal stresses are largest for Ko = 1.5 and least for Ko = 0.5 at W/D = 0.3. From the study of comparison of principal stresses in interacting tunnels (elasto-plastic and elastic analysis), it is founded that difference in principal stresses are maximum for Ko = 0.5 and least for Ko = 1.5 at the centre of pillar for higher W/D of 1.2. For lower W/D of 0.3, marginal difference is observed between principal stresses for the three in situ stress ratios.

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