Mt. Washington Tunnel

MICON Polyurethane Grouting returns structural integrity through void filling at Mt. Washington Tunnel-Port Authority of Allegheny County, PA

During routine inspection of the 76 year old Mt. Washington tunnel, deterioration of the brick liner was observed. Further studies showed that the original timber sets had rotted out causing incompetence of the immediate rock strata and uneven load distribution on the brickwork. This caused the occurrence of void spaces throughout the entire 1000 meter length of tunnel. The void spaces varied between a few millimeters to several meters in height

Grouting was permitted only between 1 and 5 AM. Liquidated damages of $500 per minute were to be levied for each minute that the contractor delayed tunnel traffic after 5 AM.

Using technology MICON developed in the coal mining industry to support incompetent rock, MICON injected a low density high strength polyurethane foam into the void areas, not only filling the voids but also consolidating the surrounding strata. During the pumping process, the brickwork was moni­tored for the slightest movement; no movement was detected at any time during the project.

The result was a removal of the problem of point loading and uneven load distribution on the brickwork. This allowed the liner to function as designed and maintain its structural integrity. The polyurethane grout provided a semi-rigid liner behind the rigid brickwork, with capability of absorbing uneven or isolated loading conditions without transmitting these loads onto the liner itself. Subsequent core samples showed that all void spaces were filled by this method.

The project was completed on schedule in five months operating within the tight working periods. The 174,000 gallons of Hquid polyure­thane makes this project one of the largest applications of rigid polyurethane foam grout.