SRC rock mass classification of tunnels under high tectonic stress excavated in weak rocks

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This paper describes the application of the SRC rockmass classification system to tunnels under high horizontal tectonic stress excavated in weak rocks. The analysis was performed on 25 tunnels in Spain and Italy, for which it was found that much heavier supports than those estimated by the RMR index were required. SRC and RMR indices and other relevant geomechanical data were obtained during the site investigation and construction stages. Data corresponding to in situ stress measurements, analysis of tectonic structures and instability problems arising during construction were used to asses the state of stress. The relationship between tunnel section convergence and the SRC and RMR indices was also analysed. Support measurements based on SRC and RMR classification were compared with those actually used during construction. These analyses indicate that for most of the tunnels examined, supports estimated using the SRC were much closer to those actually installed than those predicted by the RMR index. Based on the case histories presented, the factors mainly contributing to deformability and consequently to assessing support measurements were: high horizontal tectonic stress, low strength of rocks, overburden thickness and structural anisotropy related to tunnel axis orientation. According to these factors, the tunnels investigated were classified as three types. Tunnels classed as type I were those of low overburden thickness under high horizontal tectonic stress excavated in low strength rocks. The supports installed for these tunnels were much heavier than those predicted by the RMR index, being more in line with those indicated by the SRC index. The type II tunnels had thick overburdens and showed similar stress and strength conditions to the former. The supports installed were practically those foreseen by the SRC index, appreciably differing with respect to the RMR index. Finally, tunnels included in the type III class were those under low to moderate tectonics stress, irrespective of overburden thickness. These tunnels gave rise to RMR and SRC indices that provided acceptable results.