The roof of a rails to trails tunnel started to collapse. In the worst area, a 150 cubic yard cavity opened up at the tunnel crown. Previous rock bolting slowed, but did not stop the collapse. Due to the unsafe conditions, the tunnel was closed. Trail users had to use a 1.5 mile bypass to go around the tunnel.
GeoBuild & Gannett Fleming teamed up for the design build repair of the tunnel. After choosing a structural steel plate lining system for the repair, they turned to CJGeo to provide a back grouting solution. The tunnel is approximately 850 feet long, and 30′ in diameter, with an annular volume of approximately 2300 cubic yards.
It’s believed that the original tunnel was unlined, with bare rock surfaces. Over time, various lining materials were placed, starting with timbers. Some time after the timber lining was placed, concrete lining was installed. The concrete lining was up to 2′ thick in places. A contributing factor to the deterioration of the concrete liner was water accumulating in the timber lining, freezing and expanding behind the concrete lining.
The daylighting of an adjacent live railroad tunnel also posed challenges for the safety of the pedestrian tunnel. The haul road for disposal of the daylighting spoils passed over the top of the pedestrian tunnel. Approximately 1.5 million cubic yards of spoil were hauled over the roof of the pedestrian tunnel.
The biggest limit on this tunnel back grouting job was access. The tunnel was located 2 miles from the nearest paved road, down a single lane bike path. Once at the site, a 20 ton weight limit bridge stood between the staging area and tunnel. This effectively ruled out using low slump grout as the grouting material. Altneratively, low slump grout would have to be pumped 300 feet over the weight limited bridge to the portal and then another 850 feet through the tunnel. This is an exceptionally long distance to pump very low slump grout safely. Low slump grout is the typical grout recommended by the SSP liner manufacturer, Armtec. Minimizing cost was also important; the customer was a non-profit trail maintenance organization.
Cellular concrete was also considered as an option. Cellular concrete is primarily comprised of atmospheric air by volume. This would cut down by 80% the number of trucks needed to deliver grout to the site. However, cellular concrete is also exceptionally flowable. This posed problems in that the SSP plates are not designed to have water-tight joints. Given the tunnel size, there were nearly 20,000 linear feet of joint which would have to be water tight in order to use cellular concrete. Another limit of cellular concrete on this project was the very high potential for fouling the drainage system.
While contemplating using polyurethane grouting to seal the joints in the SSP liner to facilitate cellular concrete grouting, we thought “why not grout the whole thing with polyurethane.” For this project, polyurethane grouting offered a number of key benefits:
- Logistics; the entire 2300 cubic yard annulus would only require three truck loads of material delivered to the site. This alleviated concerns about bridge capacity & truck traffic on the bike path.
- Speed; CJGeo’s in-house polyurethane grout placement capacity exceeds 100 cubic yards per hour. This allowed the job to be completed in a timely fashion.
- Fast cure; the selected polyurethane grout completed expansion & cured within three minutes. This alleviated concerns about leaks through the joints in the SSP. It also allowed for deep, successive lifts.
- Strength; the selected material had a bearing capacity of 3700PSF, despite only weighing 2PCF.
- Cost; while the raw material cost was higher for polyurethane grout than cellular concrete, the savings from speed, insensitivity to joint leaks & logistics made polyurethane grouting the economical choice.
CJGeo placed 120,000 pounds of polyurethane grout in two weeks to complete the tunnel back grouting. In addition to the annulus, approximately 250 cubic yards were placed in the roof collapse cavities. Polyurethane grout was also used for bulk heading the ends of the annulus. CJGeo’s use of polyurethane grout was an unquestionable success.
Because the chosen polyurethane grout only weighs 2 pounds per cubic foot, it wasn’t necessary to perfectly balance injection across sides. This facilitated continued traffic through the tunnel for other activities associated with the relining.
Infrared inspection was used to confirm complete fill, in conjunction with visual inspection of joints & extrusion of polyurethane grout through vent holes.