Challenges and Solutions for Carrier Pipe Design and Installation in Tunnels

Water line in primary tunnel liner or a similar note is much easier said than done. Ignoring the fact that the tunnel needs to be installed, that’s a topic for a much different paper, installing carrier pipe in said tunnel is no easy feat. The primary challenges are constructability, corrosion protection, and accessibility for repairs after construction, if needed. Freese and Nichols, Inc., has seen its fair share of pipe installed in tunnels and has developed innovative solutions for large diameter pipelines to mitigate challenges during construction and over the life of the tunnel. Polyurethane coated steel pipe with a mortar overcoat and mortar bands is one of the best solutions to install steel pipe in a tunnel since it can be pushed/pulled into place without damaging the pipe and does not require casing spacers that may get damaged or stuck. The steel pipe also allows the exterior heat shrink sleeve joints to be easily made up outside of the tunnel and tack welded before they are pushed into the tunnel and fully welded. This method also allows for minimizing the tunnel diameter. However, pipe axial loading may limit the length of pipe installed with this method. Corrosion protection and electrical isolation of the tunnel liner and carrier pipe is a challenge, and grouting the annular space is a great solution, since the liner can become sacrificial and the tunnel structure will still be secure. Grouting is recommended for large diameter steel pipe as it can be repaired from the inside but not for rigid concrete pipe, which is more likely to be replaced than repaired. Metallic pipe and tunnel liners may need cathodic protection to avoid corrosive effects from foreign metallic pipelines. They also may need to be isolated from each other to avoid corrosion of the carrier pipe. Fiberglass casing pipe is resistant to corrosion and can be great for isolating the carrier pipe from foreign pipelines outside of the tunnel. Installing the carrier pipe inside fiberglass casing can be done by lubricating the casing surface and pushing the pipe through while taking care to ensure the friction is low enough to not damage the casing. This can be a great option for jacked casing since it is one of the more cost-effective options. Improvements and innovations like this help better face the challenges of installing pipe in tunnels and deliver successful projects.