International Journal of Impact Engineering 178 (2023) 104604 Available online 11 April 2023 0734-743X/© 2023 Elsevier Ltd. All rights reserved. Experimental and numerical investigation of transient dynamic response on reinforced concrete tunnels against repeated impact loads K. Senthil a, * , L. Pelecanos b , S. Rupali a , R. Sharma a , K. Saini a , M.A. Iqbal c , N.K. Gupta d a Department of Civil Engineering, National Institute Technology Jalandhar, Jalandhar, Punjab, 144011, India b Department of Architecture and Civil Engineering, University of Bath, Bath, United Kingdom c Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, 247667, India d Department of Applied Mechanics, Indian Institute of Technology Delhi, New Delhi, 110016, India A R T I C L E INFO Keywords: Semi-cylindrical tunnels Repeated-hit impact load Varying drop height Experiment Simulations ABSTRACT 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/m 3 . 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. 1. Introduction The use of underground tunnels for transportation, water pipelines and other utility lines is gaining continuous popularity. As a result, the dynamic loads inclusive of impact and blast loads on the underground tunnels are very important. Circumferential faults around tunnels could develop during blast excavation or drop impacts; these fractures can generate, expand, and coalesce, degrading the tunnel structure and contributing to a type of geotechnical risk, such as a rock-faults [1-5]. As a result, cracks may have a significant impact on tunnel stability and a further investigation of failure characteristics under varying impact loading was also necessary. Abdul et al. [6] developed a logical impact resistant modelling methodology for arch-type shelters utilizing the 3-D elastoplastic Finite Element Model (FEM) to calculate the maximum input energy for achieving the end state and also created a Three-Layer Absorbent System (TLAS). It was made from a sand sheet on top, a thick RC slab in the middle and a deep Expanded Poly-Sterol (EPS) block on the bottom. When TLAS was used as an absorption system, the trans- mitted impact force was reduced by half and the displacement at the pithead section was reduced by 75% at the crown, indicating that the RC * Corresponding author. E-mail address: kasilingams@nitj.ac.in (K. Senthil). Contents lists available at ScienceDirect International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng https://doi.org/10.1016/j.ijimpeng.2023.104604 Received 3 February 2022; Received in revised form 19 February 2023; Accepted 8 April 2023