Experimental and numerical investigation on soft soil tunnels under impact loading condition

Background: Tunnels extend the use of underground space for special applications such as transportation, mine development and civil defense. These structures could become susceptible to severe dynamic loads such as traffic loads, pile driving, impact and blast loads. 

Methodology: In this context, the experiment and simulations were conducted on semi-cylindrical tunnels of dimensions 1.2 m length × ∅ 0.5 m center-to-center and 0.05 m lining thickness under impact loading. The drop height was kept at 3.0 m and the burial depths of the soil cushion were varied as 0, 0.05, 0.10 and 0.15 m to study the effect of the cushion layer under repeated impact load. The experimental results were obtained in terms of damage to the tunnel and displacements in the tunnel under repeated impacts. 

Results: It was observed that the impact resistance of tunnels increased significantly when the natural burial depth increased from 0.05 to 0.10 m. It was concluded that the tunnel with 0.15 m burial depth was able to resist up to nine impacts as compared to the tunnel without cushion which offered resistance against two impact only. The numerical investigations were performed for each tunnel under repeated impacts using ABAQUS/Explicit. The numerical results were found to be in good agreement with the experimental results in terms of strain in rebar and mid displacement. The parametric study was performed for the influence of mass and velocity of the impactor in terms of crack pattern, impact force, displacement and energy absorption capacity in the tunnel. On increasing the mass of the impactor from 150 (4.41 kJ) to 400 kg (11.77 kJ), the energy absorption capacity was found to increase from 7.8% to 48.7%; however, on increasing the height of drop from 5 (5.1 kJ) to 20 m (20.40 kJ), the energy absorption capacity of the tunnel was increased from 8.1 to 48.6%. 

Conclusions: It was concluded that under the low-velocity impact, energy absorption in the tunnel is more sensitive to the mass of the impactor as compared to drop height. It was observed that Yang Qixin's algorithm was the best predicting algorithm among others when comparing peak impact force with the numerical result.