Mt. Washington Tunnel

Routine inspection of the 76-year-old Mt. Washington tunnel found a deteriorating brick liner. The original timber sets had rotted out, weakening the immediate rock strata and loading the brickwork unevenly, creating voids from a few millimeters to several meters high along the entire 1,000-meter tunnel. Grouting was allowed only between 1 and 5 AM, with liquidated damages of $500 per minute for any traffic delay after 5 AM.

Micon polyurethane grouting restored structural integrity through void filling at the Mt. Washington Tunnel for the Port Authority of Allegheny County, PA.

Problem

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 created void spaces throughout the entire 1,000-meter length of the tunnel, ranging from 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 the contractor delayed tunnel traffic after 5 AM.

Solution

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, filling the voids and consolidating the surrounding strata. During pumping, the brickwork was monitored for the slightest movement; no movement was detected at any time.

Results

The work removed point loading and uneven load distribution on the brickwork, allowing the liner to function as designed and maintain its structural integrity. The polyurethane grout provides a semi-rigid liner behind the rigid brickwork that absorbs uneven or isolated loading without transmitting it to the liner. Subsequent core samples showed that all void spaces were filled.

The project was completed on schedule in five months within the tight working windows. At 174,000 gallons of liquid polyurethane, it is one of the largest applications of rigid polyurethane foam grout.