Hanging Lake Tunnel (Colorado) repair nearly complete

The tunnel goes between two mountains with the tunnel control center being located in between. That portion of the tunnel appears to have been constructed using cut and cover methods. The tunnel and the control center were partially backfilled to help them blend in to the scenery.

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(Photo by CDOT and Colorado Public Works Journal)

The problems appear to have been related to rockfall that landed on this portion of the tunnel, increasing the loads. At one point there was believed to be up to 500 cubic yards of additional material on top of the structure, increasing the total load on the ceiling to 25,000 kips!

The fix involved stripping the inside of the tunnel and bracing it with structural steel, excavating down to the structure from the top, coring holes in the tunnel ceiling, grouting #7 rebar into the holes (extending both top and bottom), grouting up the crack, and installing a new reinforced slab both above and below the existing ceiling slab for a total thickness of 6-ft.

Once the structural repair was complete, they installed 3-ft of structural backfill, a 10-ft thick layer of geofoam to decrease loading from the overburden, and a 5-ft thick layer of geogrid-reinforced structural backfill. As I understand it, this was to help distribute the impact and loading in the event of any future rockfall events.