A new technique for in-place application of a carbon fiber pipe reinforcement has restored the structural integrity of a stormwater tunnel system under Interstate 35W and Interstate 94 in Minneapolis.

The tunnels, which run through the sandstone soil at depths of 50 to 130 feet, often flood during heavy storms. The volume of water is so large that it pushes up through the tunnels’ access shafts, exerting enough force to blow off the shaft lids. These cycles of pressurization when the tunnel floods and depressurization when the waters recede have caused leaks in the liner of the tunnel’s pre-cast concrete pipe.

Concerned about the impact on the tunnels’ structure, the Minnesota Department of Transportation launched a project to repair them. The general contractor was PCi Roads, and the engineering firm was Brierley. For most tunnel sections, workers would poke holes in the tunnel and inject a chemical grout that would seek out water and expand to create a moisture barrier on the outside of the pipe. That would stop the leaks.

In one 20-foot-long section of the tunnel, however, the pipe lining had eroded, and engineers feared it would not be able to withstand the internal pressures of the water and the external pressure of soil. QuakeWrap, which specializes in FRP products for infrastructure repair and renewal, and its construction arm, FRP Construction, provided the solution. “The objective was to bring the strength of that short tunnel segment up to the strength of the rest of the tunnel,” says Mo Ehsani, president of QuakeWrap.

One of QuakeWrap’s products, StifPipe®, is made from lightweight 3D core fabric with layers of carbon or glass fiber reinforcements. When wound on a mandrel, the material can be used to create freestanding pipe liners that can withstand the kind of internal and external pressures found in the Minnesota tunnels. But StifPipe would not work for this application; it simply wasn’t possible to get 12-foot-diameter liner pipes through 3 to 5-foot-wide access holes.

Lining the interior of the pipe with CFRP fabric in situ wasn’t an option. While three to four layers of CFRP could probably handle the internal pressure, they would not provide enough reinforcement for the external loads of the sandstone soil and the highway traffic above. Ehsani estimates it would take 20 to 25 layers of CFRP fabric to get the desired strength, which would be both time-consuming and expensive.

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.”


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