Tunnels in Dams

State dam safety program responses to the following inquiry:

We have a proposal for a new, 34-ft tall earthen dam, with a concrete tunnel in the dam. The tunnel is designed to carry 1 lane of traffic through the dam (the other lane will be on the dam crest). The roads are access for buses and cars to a large resort/ retreat complex.

To accommodate the tunnel, the designer has modified the dam’s cross-section so that the crest is wider, and the downstream face is steeper (2:1 slope), but a template for a typical dam (ex: 3:1 slopes, 12-ft crest) does not fit upstream of the tunnel. He has also added a core wall under the upstream edge of the crest. Reportedly bedrock is near the foundation of the dam, and the core wall will sit on rock, but the tunnel will sit on compacted earthen embankment material, because its bottom will be at a higher elevation. It will be downstream of, and not attached to the core wall.

Questions:
1. Have you ever, or would you ever, approve such a concept?
2. Based on the original design, the dam would have been classified as a Low Hazard dam. Should the presence of a road/ tunnel within the embankment raise the dam’s hazard class?
3. If the concept is approvable, what concerns/ details would you want to see addressed in the design?

The ASDSO surveys archive is available to members through the members only section of DamSafety.org