So the FRP Construction team used a recently-developed adaptation of the StifPipe technology, where the host pipe itself serves as the mold for creating the pipe insert. Because of StifPipe’s lightweight core, the team was able to get the desired strength with only eight layers of material.

This approach also provided a new liner that exactly fit the existing tunnel pipe. “Sometimes pipes may not be truly cylindrical, because they have become more oval over the years. With this approach, you don’t need to worry about taking measurements ahead of time, because you just apply it to the surface of the pipe,” says Ehsani.

After PCi Roads dried out the tunnel areas, FRP Construction brought in the required rolls of composite fabric and buckets of resin through the access shaft, transporting 3,250 feet to the repair area. Then they sealed off the area to get the desired humidity and temperature. After applying a primer, crews installed the StifPipe system, which included both glass fiber and carbon fiber, as well as the 3D core, followed by a chemical-resistant, paint-like top coat. When completed, the structural liner measured 1.47 inches thick.

The entire 20-foot tunnel length was completed in three weeks and should not require additional repairs for a very long time. “Unlike steel or concrete pipes that corrode, these materials are pretty much inert so they will never corrode,” Ehsani says. “There’s really every reason to believe that these should last 70 to 80 years.”

Ehsani believes that the in-place StifPipe application could be a solution that DOT and municipal engineers will welcome. “In terms of cost-effectiveness, this is a much more competitive solution compared to some other available techniques,” he says. “Nothing is a cure-all, but this is definitely another tool in the tool box for engineers that are designing these special challenging projects.”