Maintaining the Pressure in Minnesota

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.