Composites perform water rescue in high-speed rail tunnel
Initial tunnel construction was completed in 2009, but unprecedented water infiltration threatened to thwart rail line installation. Water was seeping in from the surrounding soils at a rate a full order of magnitude greater than is seen in most tunnels — up to 2,200 liters/sec, enough to fill an Olympic-sized swimming pool every 20 minutes. The culprits were cracks in the tunnels’ concrete walls. Conventional approaches to waterproofing — injecting polymeric paste into the cracks, installing a polymeric membrane — and other attempts to seal tunnel walls or retrofit tunnels with an existing abatement solution proved ineffective.
One key consideration that led to the choice of composites over other material options was the need to maintain sufficient tunnel diameter. The raw tunnel cut could have been larger, potentially allowing for a noncomposite water-channeling system. Every additional millimeter that has to be excavated is expensive. Of course, additional excavation at this point in the project was infeasible, because construction of the tunnel walls was already substantially complete. The water channeling system’s thickness, therefore, could not compromise tunnel clearances. That ruled out conventional options. To meet structural requirements, cement panels, for example, would have had to be 10 cm thick, unacceptably diminishing the tunnel’s size. But the thickness of the laminate in the composite design is a mere 4 mm; and the panel profile, which creates water pathways in the form of integrally manufactured longitudinal ribs/channels, decreases tunnel diameter by an acceptable amount.
Another key consideration was that the water-channeling system’s panels would have to provide adequate radial strength once they were fastened in place against the curved tunnel walls. High-speed trains create significant air pressure as they enter a tunnel. But even more significantly, when they exit, they create a vacuum that, along with other forces, will try to pull the panels out of their fixtures. A specially designed nut-and-bolt anchorage system helps keep the panels in place, while the panels themselves are designed to handle the kinds of loads created both by the speeding trains and by the ongoing water seepage.
