A Blast Analysis Study was Conducted to Simulate Explosions in a Transit Tunnel to Predict Potential Damage to the Tunnel Structure.
Introduction
Terrorist attacks on transit tunnels in Madrid, London, Moscow, and other cities have resulted in enormous cost in loss of life, injuries, property damage, and economic consequences. A blast in a transit tunnel is particularly dangerous because of the confined underground space. According to the Blue Ribbon Panel on Bridge and Tunnel Security1 (2003), there are more than 200 transit tunnels in the
Parsons Brinckerhoff (now part of WSP | Parsons Brinckerhoff) performed extensive in-house research to improve the safety and security of infrastructure facilities2 and developed a systematic approach named TARIF (see Figure 1), which has been applied to the design of several tunnels and underground facilities. This article discusses blast analysis on typical transit tunnel geometry and the effectiveness of protection measures which are widely used in current practice. The information from this study can assist owners and operators of underground infrastructure in making their systems more secure and resilient.
Base Case Study for Blast Analysis
According to the Federal Emergency Management Agency (FEMA), explosive charge weight and stand-off distance are two important parameters used to define a blast threat. The charge weight is usually measured in equivalent pounds of TNT, and the standoff distance is the distance from the charge’s center of gravity to the bearing surface of the structure. For this study, a backpack bomb with a conservative estimate of 100 pounds of TNT and a stand-off distance of 2 feet were selected as a reasonable potential threat.
The generic transit tunnel was modeled with a precast concrete segmental lining that is 11 inches thick and has an internal diameter of 20 feet (see Figure 2). Neighboring segments are connected by radial bolts; adjacent rings are connected by steel dowels.
WSP | Parsons Brinckerhoff used the commercial computer program ANSYS AUTODYN to perform the three-dimensional coupled Euler-Lagrange nonlinear finite element blast analysis to simulate explosions in a generic transit tunnel and to predict the potential damage. The simulation incorporated nonlinear dynamics, large strains and deformations, fluid–structure interactions, and interactions among structures. In the base scenario, the simulation predicted that an area in the lining of about 2.6 square feet would suffer severe damage.
Protection Measures
In order to improve the security, safety, and resilience of transit tunnels, some protection measures are proposed (see Table 1). A series of numerical analysis were performed to quantify the effectiveness of the proposed measures. This study only considered the initial costs for the proposed measures. The durability and life-cycle cost was not included in the following cost-effective analysis. Figure 3 shows damage simulation for the base case and all six protection measures.
Increase Concrete Lining by 1 Inch
In general, a thicker tunnel lining tends to perform better under extreme loading events. In the first scenario, the lining thickness was increased from 11 inches to 12 inches. Compared with the base case, the damage in this case is reduced by 35 percent with a cost increase in the precast lining of 5 to 10 percent.
Additional Rebar
The second measure considered was to double the number of steel reinforcing bars. The damage to the tunnel lining in this scenario was reduced by 45 percent. The cost of the lining was estimated to increase approximately 20 to 40 percent compared to the base scenario.
Use of Steel Fiber Reinforced Concrete (SFRC)
The third protective measure consisted of using SFRC with a dosage of 80 pounds of steel fiber per cubic yard of concrete for the tunnel lining and interior structures. SFRC was modeled by assuming that steel fibers were uniformly distributed throughout the concrete elements. Based on the information from literature, it was assumed that the steel fibers increased the strength and improved the ductility for the concrete members with corresponding steel fiber dosage. This approach reduced the damaged area by 14 percent. Based on data from similar tunnels using SFRC, it was estimated that this approach would cost about 15 to 20 percent less than the base case due to reduced labor and simplified manufacturing process.
Interior Steel Plate
Another measure considered was bonding a 1-inch thick steel plate to the tunnel wall. The simulation showed that the concrete lining suffered more damage compared to the base case. This model demonstrated that the 1-inch plate did not provide substantial impact energy absorption and that the impact of the plate on the lining increased the destructiveness of the blast. The plate could increase the cost of the lining by 30 to 35 percent. Therefore, an interior 1-inch thick steel plate is not effective to mitigate the damage.
Interior Aluminum Foam Panel
Research proved that porous materials such as aluminum foam can effectively delay shock wave propagation and attenuate the amplitude by absorbing the kinetic energy through compaction of the material. This protection measure considers a 4-inch thick interior aluminum foam panel. Additional material cost could be about 2.5 times the cost of the base case (a 250 percent increase) but it nearly fully mitigated the damage to the lining. However, constructabilty may be an issue as interior clearances for installation of the panel may not meet fire life safety requirements.
15 Inch Tunnel Lining + Additional Rebar
This protection measure considers a 15-inch thick concrete lining reinforced by steel bars at reduced spacing. The tunnel lining in this scenario experienced some plastic deformation but the damage on the lining is nearly fully mitigated. Compared to the base case, the cost of tunnel lining, including additional excavation cost, could be doubled.
Conclusion
Simulation reveals that it is possible to reduce the blast impact to the structure of the tunnel to almost nothing, for example by bonding an internal aluminum porous panel, but the cost is high and can be affordable for critical structures only. However, a conventional measure such as an increase in the lining thickness with design optimization using steel rebar can also significantly reduce the damage at a smaller cost increase. Optimization of the design of anchored hooks and transverse reinforcement bars can also be considered to increase resilience and further reduce structural damage.
This study on the effectiveness of proposed protections is valuable to those setting safety and resilience guidelines, especially when applied to problems that are difficult or costly to study experimentally. The objective of this study is to provide owners and operators of underground infrastructure with a guideline to elicit industry discussion on the value of various tunnel protection measures and blast protective design of tunnel linings. As transportation agencies or authorities balance multiple demands, this information can assist in the decision-making process to make their systems more secure and resilient.
Acknowledgement
The authors would like to thank ANSYS, Inc. for providing software tools ANSYS AUTODYN and ANSYS DesignModeler for this study.
Reference
- ANSYS Mechanical User Guide R14.5. ANSYS, 2012.
- Choi, S. “Tunnel Stability under Explosion”, Parsons Brinckerhoff William Barclay Parsons Fellowship Monograph, 2009.
- Choi, S. “Protective Design Guideline of Tunnels”, Fifth International Symposium on Tunnel Safety & Security, New York, NY, 2012.
- Choi, S., Munfakh, G. “Tunnel Design Under Explosion”, Proceedings of Third International Conference on Protection of Structures Against Hazards, Padova, Italy, 2006.
- David, S. “Fiber-Reinforced Concrete for Precast Tunnel Structures”, Parsons Brinckerhoff William Barclay Parsons Fellowship Monograph, 2011.
- FEMA. “Risk Assessment A How-To Guide to Mitigate Potential Terrorist Attacks Against Buildings”, 2005.
- Munfakh, G. “Fixing a TARIF for Security”, World Tunnelling, 2008.
- The Blue Ribbon Panel on Bridge and Security, “Recommendations for Bridge and Tunnel Security”, FHWA, 2003.
- TCRP/NCHRP. “Making Transportation Tunnels Safe and Secure”, Transportation Research Board, Report 525, Volume 12, 2006.
1The Federal Highway Administration (FHWA) and the American Association of State Highway and Transportation Officials (AASHTO) formed a Blue Ribbon Panel on Bridge and Tunnel Security after the terrorist attacks in the United States in 2001.
2Choi, S., Tunnel Stability Under Explosion; Parsons Brinckerhoff William Barclay Parsons Fellowship Monograph, 2009.
3Munfakh, G., Fixing a TARIF for Security, World Tunnelling, 2008.
