Many cities around the world are choosing the Metro rail network as an answer to their transport problems including namma bengaluru. Since most cities are old and densely populated, it is not always feasible to have metro lines over the ground. Especially in the business centres of the city, in Bengaluru, 8.2 km out of 42 km of Metro railway line is being built underground in its busiest areas.
With terrorist activities on the rise, it has become necessary to take into account the effects of a blast on an underground tunnel. An underground blast has several induced repercussions - fire, gas leaks, and the possibility of flooding by groundwater, not to mention its effect on nearby buildings on the surface. The study of stability and resilience of underground tunnel systems, and incorporating the learning into design becomes essential.
A team of four researchers from IISc Bangalore, and Sastra University, Thanjavur have studied the effects of a blast on the twin-tunnel system of the Underground Metro in Bangalore. They have published a paper based on their results. This attempt is a part of a preliminary study for a proposed project in an Indo-German partnership with the IGCST under the Civil Security programme. The paper intends to study blast effects on metro tunnels, and evacuation in the event of a blast.
The tunnels section chosen for the hypothetical study is a twin tunnel section located on the East-West corridor of the Bangalore metro, in the City Railway Station-Majestic-Central College-Raj Bhavan stretch. These tunnels have a diameter of 5.6 m, with a concrete lining that is 28 cm thick. The tunnels are about 15 to 18 m below the ground, and the centres of the two tunnels are about 15 m apart.
The researchers have developed a Finite Element Model of the tunnel, to study its strength and response to various loads and stresses. Finite Element Analysis (FEA) is used to find out the effect of forces on a solid object. A complex solid is broken down into simpler parts, or "finite elements." The response of these finite elements to the forces applied is used to predict the behaviour of the original large structure. This is a preliminary study of the FEM analyses.
Modelling is only a theoretical simulation study. It is a complex process that takes into account several parameters, like the weight of the tunnel, the gravitational force acting on them, and the pressure of the water in the soil around the tunnels. The data necessary for the model was available from previous studies on properties of different soil layers in Bangalore.
With the help of their model, they have determined that the “hydrostatic” pressure -- pressure of the water that is in the soil around the tunnels -- is quite substantial, and has to be taken into consideration while designing the system.
Next, they have studied the effects of a TNT blast by simulating a pressure wave on the model, and then observing the effects of the blast at every few milliseconds after the moment of explosion.
The study has found that a 30 kg TNT blast in one tunnel results in blast waves travelling along the tunnel and affecting the twin tunnel as well. A 75 kg TNT blast will cause the tunnel with concrete lining to fail. In places where the tunnel is shallow, the blast will be absorbed by the soil and the surface of the ground will be affected. Nearby buildings might also face the consequences of the blast.
Prof TG Sitharam from the Department of Civil Engineering, IISc., one of the authors of this paper, says, "This is a very initial study in preparation for the Indo-German project. This is a very preliminary numerical modelling work. Modelling is a complex process in ground and it is not possible to include the whole tunnel in the model."
He says that in further studies (if the project is approved), they intend to work on evacuation, crowd behaviour and safety drills. They also intend to involve the authorities of Bangalore Metro Rail Corporation and Delhi metro corporation in the studies, and work with them in future to incorporate the results in the field.
About the authors
Lead author T G Sitharam is a Professor at the Department of Civil Engineering, IISc.
The paper “Finite Element Analysis of Underground Metro Tunnels” was published in the International Journal of Civil Engineering and Technology (IJCIET) in Volume 6, Issue 2, February (2015), pp.06-15. Link to PDF.