FOAM GROUT SAVES TUNNEL

Pittsburgh's 81 year old Mt. Washington tunnel has been made safe for many more decades of light-rail transit use, with the use of foamed chemical grout. The job is the first major U.S. civil engineering application where foamed polyurethane grout was specified from the start. Over much of its length the tunnel was built without grout or backfill in the space between the rock and the brick lining. This made it less than optimally safe over the long term, since a brick lining is not strong in bending. Normally cement grout is used, but before setting it might easily overload the lining. The lower the grout's weight, its other properties being acceptable, the better. This led engineers to foamed polyurethane grout. In the formulation chosen it is very light, 5 pcf, yet it has nearly 3 times the required 35 psi compressive strength.