Study of Arch Formation in Basalt Rock—A Case Study

The arching effect is the most universal phenomenon which exist in an underground engineering due to excavation, encountered by Terzaghi in 1943. The arch theory was widely used in the underground tunnel. A numerical investigations were performed using 3D finite element method (MIDAS GTS NX) to study the formation of arches above the tunnel roof in rock mass. In this case study, a 5-km-long-circular twin tunnel between Range hill and Swargate is being excavated by tunnel boring machine (TBM) to improve traffic system of Pune city, Maharashtra, India is consider. Rock types encountered during tunnelling are amygdaloidal basalt, vesicular basalt and compact basalt. In this study, numerical analysis is performed for prediction of arching action. Arch action is studied with the help of transfer of stresses around tunnel. Various model tests are carried out to analyse the differences of the arch action above tunnel face between dry rock mass and saturated rock mass with varying overburden depth (2D, 2.5D, 3D, 3.5D, 4D, 4.5D, 5D) “D” is diameter of the tunnel. 14 numerical models are studied in which 7 are for dry rock mass and 7 for saturated rock mass. Analysis shows that forming process of arch. Furthermore, factors affecting formation of pressure arch, i.e. overburden depth and water table effect on arch formation, are defined. Overburden depth factor has less influence on the arching action. Test results show that the arching coefficient can be decreased 10–15% by groundwater. The results obtained from this study will increase the ability to deal with similar ground conditions during tunnelling using TBM.