The International Information Center for Geotechnical Engineers

Review of Polyurethane Resin Grouting for Rock Mass Stabilization



The next case history details the use of PUR for rock slope stabilization against rock falls along transportation routes. The use of PUR for rock slope stabilization against rock falls has been recently studied by the Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) in 2006 and 2007. One such study was conducted on the Poudre Canyon Tunnel, and is discussed below.

The Poudre Canyon Tunnel project was performed by the Colorado Department of Transportation (CDOT) in 2006 as a demonstration project for the CFLHD study of PUR injection for rock slope stabilization. The Poudre Canyon Tunnel is a 75-foot long tunnel in vertically foliated gneiss. It was constructed to allow passage of a two-lane stretch of highway SH-14 west of Fort Collins, CO. The site had issues with rockfalls along the western portal, and non-tensioned rock dowels had previously been installed, as shown in Figure 9.


Previous Rock Dowel and rock block Poudre Canyon

Figure 9. Rock Block and Rock Dowels along Poudre Canyon Tunnel (Arndt et al. 2008)

The goal of the project was to successfully inject PUR into fractures to support the previous rock dowel installation and prevent further rock falls. To achieve this goal, a mild hydrophilic (indicating some, but not extensive, foaming due to water) PUR product was selected and injected per the sequence shown in Figure 10. The sequence included sixteen injection holes of 1.5 in diameter, drilled 10 ft to 12 ft deep, and then packed upon completion. 

Poudre Canyon Tunnel Injection Sequence

Figure 10. Poudre Canyon Tunnel Injection Hole Location and Sequence

Notice that the injection sequence was completed from the bottom to the top of the tunnel face. In each borehole, an initial injection was performed to allow gravity to carry the resin downward through the fractures. After this initial injection had set, which took approximately one minute, additional injection pressure was used force the resin radially outward and upward. This sequential injection maximized filling of the fractures and minimized blockage of resin flow by hardened PUR product from past injections. Pumping was performed at low pressures, not exceeding 50 psi, to avoid fracturing the rock and causing rock falls. Pumping was performed until PUR was seen extruding out of fractures at the face above the current injection borehole, which indicates that additional pumping will not provide further infiltration of the PUR product into the fractures. PUR migration distances, which refer to the distance at which PUR was seen extruding out of the rock face measured radially from the injection borehole, averaged 4 to 8 ft., however it was noted that persistent fractures could carry PUR as far as 10 to 15 ft. before initial PUR set.

This CFLHD study did not perform validation measures by drilling boreholes to verify the void space filled by PUR after completion of the project, however volume estimations of treated rock were performed.                

The project treated approximately 850 ft2 of rock face area, and estimated a treatment depth into the rock of 10 ft, making the total treated rock volume approximately 8500 ft3. This volume is not in reference to void space measurements. Due to the lack of initial rock void space estimates and the dependence of the chosen PUR product on water and expansion once it was pumped into the rock, the total filled void space within the rock was unknown, however it was expected, based on resin set time testing on rock samples at the site and visual observation of PUR extruding from adjacent fractures during pumping, that a significant amount of voids were filled due to the PUR injection.

In addition to stability, the aesthetics of the rock face were also an important aspect of the project. For this project, it was important to remove extruded PUR immediately, which was performed by hand. After initial set, removal is typically much more difficult, and requires chipping the hardened PUR off the rock face (Arndt et al. 2008).

While the underground stabilization methods are typically driven by the ease of mobilization, in which PUR grouting often wins out, above ground applications such as rock slope stabilization are significantly driven by cost of material. In a very highly fractured rock mass with a large void space, the cost of PUR product may be too high when compared to other rock slope stabilization methods, however with these more recent investigations by the CFLHD, the use of PUR grouting for rock slope stabilization is becoming increasingly more popular. In the following subsection, the recommendations made by the CFLHD for rock slope stabilization are discussed.


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