## 3 Physical Modelling of Single and Twin Tunnel  
Physical modelling is a very useful method to simulate field conditions in the laboratory because, in field conditions, it is not possible to conduct all the tests. In this study, physical modelling of a single tunnel and twin tunnel is done under static loading conditions. The purpose of conducting a physical investigation is to determine the effect of cover depth, strength properties of rock, and spacing between the tunnels on the stability behaviour of tunnels. The effect of the presence of liner material on the deformation of the tunnel is also studied. In the case of the single tunnel, the test has been performed on all three types of material, i.e., Plaster of Paris, GM-1, and GM-2 material. All three model materials are characterized in the earlier stage. More than 36 models of the lined and unlined tunnel are prepared in Perspex moulds. The size of the tunnel model is 30 cm \(\times\) 25 cm \(\times\) 23 cm. (L \(\times\) W \(\times\) H). The experimental study is conducted on two different cover depths, i.e., 3 cm and 5 cm, as discussed earlier. Polyvinyl Chloride pipe is used as a liner material. After the preparation of the models, three LVDTs are fixed along the length of the tunnel. For fixing LVDTs, three different locations are selected to take the deformation in the tunnel. After the marking of locations to fix LVDTs in the model, holes are drilled with the help of a drill machine up to the bottom lining of the tunnel. After drilling holes, three LVDTs are fixed at three different locations, i.e., L/3, L/2 and 7L/12,where \"L\" is the length of the tunnel model. In single tunnels, the deformation is taken at points 0, 10 cm, 12 cm, 15 cm,18 cm, 20 cm, and 30 cm along the length of the tunnel.After placing the LVDTs, the tunnel model is kept inside the compression testing unit attached with the 16-Ch LVDT data logger. Then the load is applied with the help of a compression testing machine. After the failure of the sample, it is removed from the platen. LVDTs are then removed gently from the tunnel sample. Figure 3 shows the location of points where the deformation is taken in a single and twin tunnel. In the case of the twin tunnel, the sample is made of 100% Plaster of Paris material at 60% water content. The dimension of the twin tunnel sample is 42.5 cm \(\times\) 37.5 cm \(\times\) 23 cm (L \(\times\) W \(\times\) H). Twin tunnel samples are tested for three different spacings, i.e., 1.5D, 2D, and 2.5D (where 'D' is the diameter of the tunnel). The cover depth of the tunnel is 3 cm and 5 cm from the top of the model surface. Both lined and unlined samples of twin tunnels are prepared in the laboratory. The steps for the preparation of tunnel models are shown in Figures 4 and 5 and show the single and twin tunnel samples prepared in the laboratory. To take the deformation in the tunnel sample, six LVDTs are placed at different locations. The location of LVDTs in the tunnel sample is chosen according to the extent of deformation that occurs in the tunnel when applied to static loading conditions. Three LVDTs are placed in each tunnel at three different locations. LVDTs are placed at a distance L/3, L/2, and 9L/15, where 'L' is the length of the tunnel. In twin tunnels, the deformation is taken at points 0, 14.1 cm, 17 cm, 21.25 cm,25.5 cm, 28.4 cm, and 42.5 cm along the length of the tunnel.To place the LVDTs, the same method as adopted in a single tunnel is followed. For installation, 6 holes of 10 mm diameter are drilled from the bottom of the surface, as shown in Figure 6. The LVDT is fixed with the help of a three-pin clamp to keep it tight so that it does not move from its location. After fixing the LVDTs in the single and twin tunnel samples, the samples are placed in a compression testing machine under the effect of static loading. The load is then applied to the sample, as shown in Figure 7. The loading rate is 1 Kn/s so that the load acts gradually on the tunnel sample.