Predicting Pressure Effects on a Resilient Tunnel Plug Installation in a Transit Tunnel

In a Potential Flooding Event, the RTP Can be Remotely Inflated with Air to Fill a Tunnel Cross Section.

Introduction

New technologies that enhance tunnel safety and resiliency have recently been developed in response to extreme weather conditions. One such technology is a “Resilient Tunnel Plug” (RTP) made of state-of-the-art high strength material. The material can be folded into a compact shape and can be installed in a tunnel with minimal tunnel modification.

Figure 1 - Demonstration of RTP Inflation - Courtesy of ILC Dover

Figure 2 — Resilient Tunnel Plug (with permission from ILC Dover)

In a potential flooding event, the tunnel operator can remotely activate the pressurization system which will inflate the RTP with air or an inert gas to a specified pressure. The pressure will cause the RTP to fill the tunnel cross section (see Figure 1) and remain in place due to the friction between the RTP and the tunnel walls (see Figure 2). The non-uniform shape of the tunnel cross section will allow some flood water to leak past the RTP, but the amount of leakage can be mitigated with some minimal modifications to the tunnel cross section at the RTP location to make the cross section as smooth as possible. Once the flood event has subsided and the water has been pumped out, the RTP can be deflated and repacked into its storage container, ready for the next deployment.

For a majority of the RTP’s life in a transit tunnel, it will sit in a unique and difficult tunnel environment, subjected to high cyclic pressure loads due to passing trains. The RTP container and deployment structure should be constructed to withstand the daily cyclic pressure loads in a transit tunnel environment. The pressure signature of passing trains is predicted using the Subway Environment Simulation (SES) program¹ and that information is passed to the RTP designer to construct a suitable storage and deployment device.

SES Analysis

SES analysis was undertaken to predict the tunnel air pressure signatures that the RTP would experience from normal train traffic. The train speeds along the routes were modeled according to their normal operating speeds. Dwell times of 20 seconds were assumed for each station. Train headways of 3, 4, 5, 6, and 7 minutes were modeled to simulate worst case peak hour conditions. Relative train positions were modeled to determine the maximum pressure due to trains operating in both tunnels. The blockage introduced by the RTP installation was modeled as an airflow resistance assuming the airflow resistance effect of the RTP installation was similar to that of a thick-edged orifice. Figure 3 shows a cross section of the tunnel, the proposed RTP location is crosshatched.

Table 1 - Summary of results for train piston effect analysis Figure 3 — RTP proposed cross section

A total of ten (10) SES simulations were performed to determine the maximum tunnel pressure signature generated at the RTP location by the movement of trains. The summary of the results for the train piston effect analysis are presented in Table 1.

The results of the study indicate that the maximum positive/maximum negative tunnel air pressure that the RTP would experience is 2.51 / 2.07 inch water gauge (this is with an added safety margin of 30 percent). The maximum positive tunnel air pressure occurs when trains are operating on a three-minute headway. The RTP designer should take into account the short duration of the pressure pulse due to the passing trains.

Figure 4 - Typical pressure profile plot at the RTP location

In addition to the maximum and minimum pressures, the pressure signature was plotted against time as the train passed the RTP location. The pressure signature profile gives information on the time duration of the pressure profile. A pressure signature that increases and decreases slowly will affect wayside equipment differently than a pressure signature that sharply increases and sharply changes from positive to negative or vice versa. The shape of the pressure profile is dependent on relative train position and whether trains are accelerating or decelerating. Figure 4 shows a typical pressure profile plot at the RTP location. The left plot shows the shape of the profile when the maximum pressure occurs, and the right plot shows when the minimum pressure occurs. The positive and minimum pressures often occur at long intervals. The plot was split up to show a higher resolution of the data near the maximum and minimum pressure times.

Conclusion

The results of the study indicate that the maximum positive/maximum negative tunnel air pressure that the RTP would experience is 2.51 / 2.07 inch water gauge. SES is a useful tool for predicting the pressure effects of passing trains in a tunnel system. Innovative technologies that improve the safety and resiliency of tunnel systems can be developed more cost effectively without the need for expensive full-scale testing. In addition, SES can be used to evaluate many tunnel environment scenarios that cannot be achieved with full-scale testing, thereby giving the greatest confidence that the design of the RTP or any other tunnel technology would be designed properly to withstand the challenging environment of a transit tunnel system.

¹ Subway Environmental Design Handbook. Volume II. Subway Environment Simulation Computer Program (SES). Part 1. User’s Manual, 1975