When tunnels are constructed near existing structures or on shallow soft ground there is possibility of very large deformations of the ground. These deformations may in turn cause failure of the tunnel as well as adjoining structures. Also another primary design issue of tunnels is the determination of stress levels associated with the yield of wall due to excavation. Hence proper steps should be taken to ensure the safety of the tunnel by providing suitable supports and reinforcement. Since the ground behaviour is not properly understood, it is necessary to estimate the appropriate support and reinforcement and their response through numerical modeling. In this study, several simulations have been done to understand the effect of excavation for tunnels on a soft ground through numerical modeling by discrete element method (DEM). The numerical simulations adopting discrete element method predicts the mechanical behaviour of soil/rock masses through grain scale modeling. Here, a three dimensional ground surface is modeled in a weak rock like sandstone and a tunnel is excavated inside the modeled ground surface. The response of the system after excavating the tunnel is studied for various cases viz. tunnel without any support or lining, tunnel with lining alone and tunnel with both lining and reinforcement. The studies clearly indicate that the tunnel without any lining on this considered weak rock fails immediately and the stability considerably improves with lining and reinforcement.


In the modern world due to lack of space, tunnels which are underground passages play a very significant role. The stability of tunnel is one of the most important subjects in the tunnel constructions. Based on site conditions, tunnels are excavated on soft to hard soils/rocks. The stability of tunnels depends on the surrounding material. In order to understand the stability of tunnels constructed in a soft ground, simulations are done on an unlined tunnel, tunnel with lining and tunnel with lining and dowels. Discrete element method is adopted in this study to understand the various aspects of the stress distribution during tunneling. In this method the system is modeled as a conglomeration of particles which are discrete in nature and interact only through the contacts. One of the major advantages of this modeling is that the discontinuous nature of the medium or the presence of joints/ fissures can be easily accounted for. In order to account for the strength of the cementing material which binds together the particles in the case of weak rocks, contact bonds are employed. This idea is based on the bonded particle model suggested by Potion & Kendall [1] wherein the rock mass is represented as a dense packing of circular or spherical particles which are bonded together at their contact points. These contact bonds are breakable in nature and the macro behaviour of the rock mass as a whole depends on the strength of these bonds. Since in this study weak rocks are simulated the contact bond strength used is very less.

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