The International Information Center for Geotechnical Engineers

Review of Polyurethane Resin Grouting for Rock Mass Stabilization


6.2.1 CFLHD RECOMMENDATIONS         

Based on demonstration projects and studies, of which the Poudre Canyon Tunnel project was just one study performed, the CFLHD developed the “Context Sensitive Rock Slope Design Solutions” manual in 2011. Within the manual, recommendations for “Rock Mass Bonding” using PUR are given.                                   

The manual states that the PUR grouting technique may be applied to rock masses with fracture apertures greater than 2 mm. The manual places an emphasis on keeping pumping pressures to a minimum to avoid the risk of causing rock falls by injection pressure itself, which may be one reason why this minimum aperture is larger than the 0.5 mm minimum aperture reported by Molinda (2008). Additionally, viscosities of the PUR vary greatly, which also controls the minimum fracture aperture penetration.

Considering implementation, the manual states that boreholes should be spaced approximately 8 ft to 16 ft apart, and should intersect the major discontinuities at as close to a 90 degree angle as possible. It also states that boreholes may be drilled ahead of time, but cautions that PUR product may extrude out of adjacent boreholes, leading to overruns. Furthermore, injection should work from bottom to top in a staged process, where initial pumping allows PUR to flow downward through discontinuities and set, and then further pumping forces PUR outward and upward through the rock mass, until PUR overrun is observed above the current borehole. This overrun can be peeled off the rock face immediately, or will need to be chipped off after setting. Pressures should be kept to less than 250 psi at all times to ensure the rock is not fractured further by the PUR injection process.

Finally, for cured PUR density and strength, the manual suggests performing the grouting during dry conditions, as hydrophilic PUR products will foam and expand in reaction to water, leading to lower density and strength.  

The CFLHD suggests that PUR grouting be used only to reduce the number of rock bolts required to stabilize a slope, not as the sole stabilization method, due to the need of additional research (Andrew et al. 2011).

 

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