Plan, Design, Build:
Tunnels

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Explore the latest resources on this topic:
 Tunnels in RTC TrailBlog
 Tunnels in the Library

Tunnels are among the most striking physical features of a trail and often one of the most memorable aspects of a trail experience. However, a tunnel on a trail can also present challenging structural, design and management issues. 

If a corridor includes a tunnel, the first step is to determine its structural soundness. If possible, locate the tunnel's original engineering drawings. These will describe significant structural features of the tunnel that are not apparent from visual inspection.

To make sure your tunnel can be safely and successfully adapted for trail use, obtain a certified report from a structural engineer describing the current condition of the tunnel. The engineer should inspect the structure and provide you with a written report of the tunnel's condition, a list of what needs to be done to make it safe for trail use, and a cost estimate for any repairs.

If the tunnel is found to be structurally deficient and you are not immediately able to make repairs, it is best to close the tunnel temporarily and reroute the trail until repairs can be made. In the event that you need to seal your tunnel, you can do so with thick wood, steel or concrete. Another option is to install strong jail-like bars that prohibit entrance yet allow users to look into the tunnel.

Even if your tunnel is found to be structurally sound, make sure to schedule regular inspections to ensure that it remains safe for trail use.
 
Occasionally, the need arises for a new tunnel. Most new tunnels are constructed with large corrugated metal culverts or pre-cast concrete culverts. The tunnel's vertical clearance should be at least 10 feet, and the width of the tunnel must be at least as wide as your trail, plus a two-foot-wide shoulder on each side.

Water is a potential hazard in tunnels. As with any other segment of trail, proper drainage is critical and can be accomplished either by digging ditches on the sides of the trail or by adding a layer of well-drained ballast in the center of the tunnel to raise the trail above any standing water.

Poor lighting is another potential problem in tunnels. Tunnels should have a source of light for safety, security and to show off the interesting elements of the tunnel itself. Install lights in the tunnel, if possible, or post "flashlight-required signs" if permanent lighting is not an option.
 
Like bridges, tunnels contribute to a memorable trail experience and often act as the signature of their associated trail. Examples of popular tunnels on rail-trails can be found in RTC's "Tunnels on Trails" report, along with additional information about 78 tunnels on rail-trails across the country.

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