Emergency Tunnel Stabilization

Emergency Tunnel Stabilization, Washington, DC


Contractor digging box tunnel for a utility relocation project ran into collapsing soil, causing extensive voids to form between top of box tunnel and bottom of overhead roadway. The tunnel was approximately 18 feet below an arterial roadway, and the voids had to be filled quickly in order to prevent collapse of the roadway.


High density polyurethane grouting to fill the voids which had formed above the tunnel and stabilize the area to prevent collapse.


The mobilization had to be fast, and the material had to be able to be sprayed into a large void with minimal underlying support. In order to stabilize the overhead roadway, the material also had to have sufficient strength for supporting up to 13 feet of overburden soil, the roadway, and its traffic.

All work had to be completed from inside the tunnel.  Lightweight material was required due to inadequate lateral support of the box tunnel walls due to collapse of the material on the sides.


Concrete Jack confined space trained polyurethane grouting crews placed nearly 40 cubic yards of NCFI high density polyurethane foam to fill the voids above and around the tunnel on two different occasions. Two different foams were used to facilitate filling both large, bulk voids, and smaller, more diffuse voids in the most economical way.