FINITE ELEMENT ANALYSIS OF UNDERGROUND STRUCTURE SUBJECTED TO BLAST LOADING

Abstract

In the present study, three dimensional numerical simulations have been carried out to ascertain the behavior of the underground concrete structure under the combined effect of missile penetration and blast load. Bomb attacks, currently much in evidence in different parts of the world, could also be possible due to sabotage and terrorism activities. Explosions and bomb attacks exert impulsive loading (shock) of high intensity and short duration on structure in the vicinity of the blast. Hence, a few of the large and complex structures may need to be designed for overall safety against such effects. The material behavior of concrete and steel is highly complex to be predicted under impact and blast. The modeling of material behavior has been discussed in detail with the help of experiments as well as numerical simulations. Concrete damage plasticity model was used for simulating the behavior of the concrete and Druker-Pager Plasticity model for sand and rock whereas Johnson Cook Damage Plasticity model for ductile material was used for the reinforcing steel. A 3D continuum element with reduced integration was used for concrete, sand and rock.3D truss element was used for the steel reinforcement. The study carried out with structures of different configuration consist of rock layer, protective reinforced buster slab and sand layer of varied thickness, which was used as a protective cover over the underground tunnel structure as mentioned below: (i) Configuration I- 2m thick rock cover, 1.2m thick buster slab, 2m thick sand layer and 1.2m thick tunnel (ii) Configuration II- 1.2m thick buster slab, 2m thick sand Iayer and 1.2m thick tunnel. (iii) Configuration III- 6.5m thick rock cover, 2m thick sand layer and 1.2m thick tunnel. The impact velocity of the rigid missile was considered as 200 m/s to study the depth of penetration into the target structure. Blast loads have been found to have significant influence on the overall structure response. Finite Element solver Abaqus/Explicit 6.9 code was used for the simulations. Analysis results are ultimately given in terms of compression as well as tension damage, Maximum Principal Stresses and Von-Mises Stresses accounting a wide variety of input conditions. Maximum penetration of missile in complete tunnel assembly is 4.8m in rock cover and the maximum blast load generated due to this penetration mechanism is 5.54 Mpa. However, it has been observed that the tunnel successfully withstood these stresses without any significant damage and the stresses developed in tunnel were found under permissible limits. Hence, tunnel was safe in different configurations as mentioned above.