Experimental and numerical investigation of transient dynamic response on reinforced concrete tunnels against repeated impact loads

An attempt has been made to study the transient dynamic response of small scale reinforced concrete tunnels against repeated impact loads. A total of four semi-cylindrical tunnels of M25 grade concrete having length, base width and thickness as 1.2, 0.5 and 0.05 m were prepared and tested against the mass of the impactor of 104 kg. The reinforcement mesh of ∅6 mm with 60 mm c/c spacing was used. The specimens were kept at the earth's surface which defines the boundary conditions proximity to the actual tunnels. The yield stress of the steel bars was 415 MPa and the resulting percentage reinforcement was 0.4%. The RCC tunnel rests on a soil bed that has a specific weight of 1850 kg/m³. The drop height against samples 1 and 2 was 1.5 and 2.3 m, respectively, whereas samples 3 and 4 were studied against a 3 m drop height under repeated loading. The experimental results were recorded in terms of impact force, residual displacement and damage pattern. It was observed that sample 1 offered resistance up to five hits whereas samples 2 and 3 were found to offer resistance up to four and two hits, respectively. The numerical investigations were performed using the ABAQUS/EXPLICIT software and the concrete, reinforcement steel and the soil element were modelled by using the Concrete Damaged Plasticity model, Johnson-cook model, and Drucker-Prager model, respectively. The results obtained from the simulations were found in good agreement with the experimental results of samples 2 and 3. However, the numerical results of sample 1 were able to complete up to the fourth hit and simulation on the fifth hit was not performed due to the complete loss of concrete material stiffness at the fourth hit. Further, the simulations were conducted considering parameters such as varying length and base width of the tunnel in order to predict the damage intensity of the tunnel. The Mises stresses were found reduced with an increase of the tunnel length up to 3 m, thereafter the same was found to increase with the increase of tunnel length, however, the deformation in the tunnel was found to decrease with an increase in the length of the tunnel. It was also concluded that the tunnel base width is an important parameter that affects the Mises stress resulting in global stability to the structure and further restricting the stress concentration to a localized failure.