<p>The explosive growth of cities in developing countries has created a strong demand for new underground development, as has the demand for improved liveability and environmental protection. Underground structures subjected to dynamic loading are used in many civil and mining engineering applications. They must withstand not only static load but also impact and blast loads. An attempt was made to understand the deformation behaviour of unlined parallel twin tunnels under impact loads in the laboratory. The article presents the physical modelling of an unlined parallel twin tunnel in homogeneous and anisotropic rockmass and an experimental study under impact loads in the laboratory. Three types of geo-synthetic materials are selected for the study and named as SM1, SM2, and SM3. The tunnels were studied in homogeneous, horizontal, and vertical layered rockmass. Rockmass dimensions of the model are 40 cm × 30 cm × 30 cm. The tunnel diameter, overburden depth, and pillar width are 5cm for all the model tests. The variable parameters are the anisotropy of the rockmass and impact energy (10 kg, 15 kg, and 20 kg) to understand the deformation behaviour of tunnel crown. A total of 21 tunnel models were impacted with a twin hammer and deformations measured along the tunnel length. It was observed that tunnels in vertically layered rock masses are more critical than tunnels in horizontally layered and homogenous rock masses. The deformation and damage zones were identified at the crown and spring line along the tunnel length. More damage is seen at the spring line and along the tunnel length.</p>