The International Information Center for Geotechnical Engineers

Review of Polyurethane Resin Grouting for Rock Mass Stabilization

 

3.0 PUR APPLIED TO ROCK STABILITY

The following sections give a general overview of the effects of PUR injection on the structure of rock, as well as the implementation of PUR injection grouting for stability and the main considerations required to successfully use the technique.

3.1 EFFECTS ON FRACTURED ROCK STRUCTURE

PUR is most useful when applied to a fractured rock mass. PUR chemically binds to the rock itself, and has a low viscosity, meaning that it can penetrate very small fractures and fissures, as small as 0.5 mm in aperture (Molinda 2008). Provided the PUR has penetrated and filled all void spaces, the overall strength of the rock mass is much greater than before, as the PUR typically has compressive and tensile strengths that are much greater than the fractured rock mass. Additionally, the rock mass is no longer permeable through the treated area, providing water cutoff if necessary. The filling of fractures by PUR is demonstrated in Figure 2 below.

PUR in Rock Color 1    PUR in Rock Color 2

Figure 2. PUR Penetration of Rock Fractures (Molinda 2004)

Figure 2 shows two color photos, taken with a borehole camera in a West Virginia coal mine, of rock fractures filled in with PUR after injection. The light colored material is the PUR, and the dark colored material is the surrounding rock. Picture A shows the PUR penetrating fractures that are less than 1/16 inch in aperture. Picture B shows a larger fractured zone to demonstrate the infiltration of the PUR through a wider fracture pattern. These borehole images clearly show that the PUR is able to penetrate extremely small fractures (Molinda 2004). In essence, PUR works to fill the void spaces of the rock, and chemically bind the fractured zones to one another forming one competent mass. Because the PUR itself has strengths comparable to the intact rock, the strength of the PUR treated rock mass will be comparable to that of the intact rock.           

3.2 IMPLEMENTION AND DESIGN CONSIDERATIONS

To implement PUR grouting in a fractured rock mass, a borehole is drilled, into which the grout tube assembly is inserted. The grouting assembly contains a number of parts, which serve the purpose of transferring pressure through the grout tube, stopping backflow of the grout through the tube, and targeting specific zones to grout along the borehole. Specifically, grout packers are used to seal the borehole around the grouting tube, and also to target specific fracture zones along the borehole for injection (“Grouting Packers”). The mixing components of the PUR are poured into the grout pump, either in one chamber for one-component mixing, which is typical of PU foam products, or in two separate chambers for two-component mixing, which is typical for PUR grouting applications. The separate grouting materials are pumped into a mixing chamber where they are combined, and subsequently pumped into the grouting tube, where the grout enters the rock mass, under pressure, through fractures along zones in the borehole specified by placement of the grouting packers. It is important that mixing occurs as close as possible to the borehole, because set times of polyurethane grouts are typically around 1 to 2 minutes, and may be less (Bodi et al. 2012).

Before grouting a rock mass, there are a number of important considerations that will help maximize the efficiency and effectiveness of PUR grouting.

Firstly, rock mass characterization is important. From the characterization of the rock mass, one can estimate the void space within the target zone, which is extremely important when applying PUR grouting for a number of reasons. An estimation of void space can drive the estimated amount of PUR needed (Arndt et al. 2008). From this void space estimation, comparing the amount of PUR being pumped into the rock during injection to the estimated void space may point to a number of issues. If the true amount is much greater than the estimated amount, PUR may be flowing away from the targeted zone through other persistent fractures, or there may be a much larger void space than initially estimated. If the true amount is much less than the estimated amount, there may be a lack of fracture persistence, non-intersecting joint sets along the targeted zone, or a problem with the design placement of the grouting boreholes. As part of the rock mass characterization, moisture content within fractures is also extremely important to determine, because PUR may interact with the water present in fractures, resulting in volume expansion and density and strength reduction. It is very important to perform an accurate and thorough investigation of the rock mass, because these considerations will subsequently drive the design decisions for the project, including the selection of the most appropriate PUR product.

Secondly, injection design and sequencing must be determined. Amongst the design and sequencing considerations are borehole spacing, borehole orientation, and location of grout curtains to provide guidance for the grout into the targeted zone. Additionally, it is important to consider when injection boreholes are drilled. If too many are drilled before injection, or the spacing is too close, grout may flow between adjacent boreholes and result in wasted PUR product and drilling time. The rock mass characterization and the experience of the contractor performing the grouting drives many of these considerations.

Thirdly, injection pressure must be considered and closely monitored. If injection pressure is too high, there is a risk of scaling the face of the rock and causing rock falls, or hydrofracturing competent rock. Additionally, grouting procedures are typically ceased when the measured back pressures significantly increase, indicating complete filling of the void spaces surrounding the grout borehole (Molinda 2008). If back pressures never increase, it may indicate a much larger fracture area, or grout that is flowing away from the targeted area. These insights are similar to those gathered when monitoring the total amount of PUR injected against the estimated void space.

Fourthly, it is important to be aware of the effects of temperature on the viscosity and set time of PUR. In general, PUR should be injected at ambient temperatures between 55°F and 90°F. If too cold, the viscosity of the PUR may increase and it may not be able to penetrate smaller fractures. If too hot, the PUR may set too quickly, causing a number of issues (Arndt et al. 2008). Figure 3 below shows a graph of viscosity versus temperature for two different types of PUR.

viscosity v temperature of PUR

Figure 3. Viscosity v. Temperature for Two Types of PUR Product (Arndt et al. 2008)

Finally, it is important to consider the hazards of using the separate components that result in a cured PUR product. Polymeric isocyanate, one possible PUR component, is a skin, eye, and mucous membrane irritant, and polyol resin may be a slight skin irritant. It is important to handle both with care. Also important to note, when the components of PUR are mixed and cured, they are chemically and environmentally inert (Arndt et al. 2008).     

Depending on the application of PUR grouting, the design methods are much different. PUR for consolidation and support of rock masses has most commonly been used in underground coal mine roof and longwall stabilization. More recently, PUR has been investigated and implemented in transportation projects regarding the stability of rock cuts along highways. These two very different applications have different design considerations and goals, therefore case histories have been included in Section 6.0 of this report to provide more detailed information on the implementation of PUR grouting in each field.

 

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