## 5 Conclusions  
In the present analysis, an attempt is made to understand the deformation action of single and twin tunnels. The experimental research is performed in a laboratory for this purpose. Three different materials are used in the case of a single tunnel to adjust the strength parameters of the rock. The effect of overburden pressure and liner material on the stability of single tunnels is investigated. Similarly, the effect of a change in cover  
Figure 14: Deformation Profile of Lined and Unlined Tunnel for 2.5D Spacing. Figure 15: Comparison of deformation profiles for Twin Tunnels of different spacing. depth and distance between the tunnels is studied in the case of twin tunnels. The following findings can be made from the experimental research conducted on both single and twin tunnels:  
1. In the case of a single tunnel, from the experiment results, it has been observed that the stability of the tunnel depends upon various factors such as the strength of rock, overburden pressure, and liner material. 2. For the unlined samples of Paster of Paris material, the value of deformation observed at L/2 distance in 3 cm cover depth samples is 21.05% more than the deformation value observed at L/2 distance in 5 cm cover depth samples. Similarly, for the unlined samples of GM-1 material, the value of deformation observed at L/2 distance in 3 cm cover depth samples is 24% more than the deformation value observed at L/2 distance in 5 cm cover depth samples of GM-1 material, whereas for the unlined samples of GM-2 material, the value of deformation observed at L/2 distance in 3 cm cover depth samples is 27.58% more than the deformation value observed at L/2 distance in 5 cm cover depth samples of GM-2 material. 3. For the lined samples of Paster of Paris material, the value of deformation observed at L/2 distance in 3 cm cover depth samples is 11% more than the deformation value observed at L/2 distance in 5 cm cover depth samples. Similarly, for the lined samples of GM-1 material, the value of deformation observed at L/2 distance in 3 cm cover depth samples is 16% more than the deformation value observed at L/2 distance in 5 cm cover depth samples of GM-1 material, whereas for the lined samples of GM-2 material, the value of deformation observed at L/2 distance in 3 cm cover depth samples is 21.42% more than the deformation value observed at L/2 distance in 5 cm cover depth samples of GM-2 material. 4. It has been observed that in all the cases, the maximum deformation is noted at the centre of the tunnel, i.e., L/2 distance, and the minimum deformation is recorded at distance L/3. From the results, it has been concluded that as the distance from the centre increases, the deformation value will decrease. The deformation value is noted as zero at the face of the tunnel in all the cases. 5. In the case of the twin tunnel samples, from the experimental results, it has been concluded that the value of the deformation observed in 3 cm unlined tunnels of 1.5D c/c spacing distance is 20% more than the deformation observed in 5 cm unlined tunnels of 1.5D c/c spacing distance. At the same time, the value of the deformation observed in 3 cm unlined tunnels of 2D c/c spacing distance is 18.18% more than the deformation observed in 5 cm unlined tunnels of 2D c/c spacing distance. Similarly, the value of the deformation observed in 3 cm unlined tunnels of 2.5D c/c spacing distance is 15.78% more than the deformation observed in 5 cm unlined tunnels of 2D c/c spacing distance. P.K. performed and processed the experimental work; P.K., writing--original draft; A.K.S., supervision; P.K., investigation and methodology; A.K.S., review--writing and finalized the final draft for submission and publication. All authors have read and agreed to the published version of the manuscript. This research work is funded and supported by Delhi Technological University, Delhi. The authors would like to thank Delhi Technological University for its financial support. Not applicable. Data sharing is not applicable to this article.