This report provides a review of polyurethane resin (PUR) injection grouting as a technique for stabilization of rock masses.
PUR is a subset of the polyurethane chemical grout category. PUR is implemented by injection under pressure with the goal of consolidating or “gluing” a highly fractured mass. PUR injection grouting may be used as the sole reinforcement method or in conjunction with other rock stabilization methods. PUR has been used extensively as a coal mine roof and longwall stabilization technique, and has since been expanded to rock slope stabilization applications.
This report is based on a review of the available literature on PUR pertaining to stabilization of rock masses. It provides a brief overview of polyurethane chemical grout materials, and narrows its focus to that of PUR, and its applications to stability of rock masses. It goes on to discuss implementation of PUR injection grouting for stabilization and design considerations when utilizing PUR grout. Finally, two case studies of PUR injection grouting for stabilization are reviewed: Roof stabilization in a West Virginia coal mine, and a highway tunnel face stabilization in Colorado.
The broad category of polyurethane chemical grout has a number of applications to rock mechanics and civil engineering as a whole due to the wide range of strengths and chemical properties that are available. Concerning rock, applications range from water cutoff under dams and sealing water inflow in tunnels and mines, to structural support of rock for underground applications and rock slope stability. Because of the wide range of polyurethanes available, it is clear from the literature that there is some ambiguity in naming convention of these different types of PUR products, which generally causes confusion. Most research recognizes and separates polyurethane grouts based on their chemical and physical properties, however the names each category are given become interchanged amongst various sources. In describing the various types of polyurethane products for this report, as it will be necessary to separate my focus on PUR from the other types of polyurethane grout materials, I will describe and follow the conventions detailed below, from Arndt et al. (2008).
Single-stage polyurethane (PU) is the first category of polyurethane grout. PU is composed of only one component, and reacts with water to expand by the production of CO2 foam, and harden within rock fractures. PU products are typically described as hydrophilic, meaning they react with water both during and after the curing process, or hydrophobic, meaning they react with water only during the curing process (Bruce et al. 1999). Because PU rapidly expands to fill voids in the rock, it is predominantly used for sealing or water cutoff applications. Because its density and strength decrease significantly during its reaction with water, PU is not typically used for increasing the strength of rock masses (Arndt et al. 2008). The expansion properties of PU type materials is qualitatively demonstrated below in Figure 1.
Figure 1. PU Reaction with Water (Joyce 1992)
Polyurethane resin (PUR) is the second category of polyurethane grout, and the focus of this paper. PUR is composed of two stages of mixing, where the components react with each other to form the final product. PUR does not require water in the reaction process, but may still foam in the presence of water to a lesser extent than PU products. This typically helps infiltration of the PUR into rock fractures, however density and strength are reduced, although to a lesser extent than PU products. PUR products can attain much higher strengths than PU products, and are therefore used primarily for consolidating and increasing the strength of highly fractured rock masses (Arndt et al. 2008).
Epoxy is the third category of polyurethane grout. Epoxy, like PUR, is composed of two stages of mixing, however it is truly hydrophobic and will not react in the presence of water. Epoxy is typically used in structural foundations, where only a small quantity of product is needed. It is not typically used in rock mechanics applications (Arndt et al. 2008).
Table 1 describes the properties of the three types of polyurethane grout products.
Table 1. Comparison of Polyurethane Products (Arndt et al. 2008)
This report will focus on the applications of PUR as described above. Note that in the literature, various forms of the abbreviations PU and PUR may be used interchangeably, however observing the number of mixing components, reactivity with water, and application setting (i.e. strength v. water cutoff) can resolve the true nature of the polyurethane product if there is any confusion.
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.
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.
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.
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.
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.
PUR has a number of advantages over other types of grouting materials. It has a low viscosity, allowing it to penetrate small fractures. It also has varied expansion properties, and set times that can be on the order of seconds, as well as its strong ability to bond to the surface of the rock (Schaller and Russell 1986). In addition, the material itself can reach strengths that are 3 to 4 times those reached by cementitious grouts (Arndt et al. 2008). Furthermore, PUR injection systems are easily mobile, making PUR injection grouting an ideal candidate for areas that are hard to access, such as underground mines. Also important is the fact that the final PUR product is environmentally inert, which makes PUR advantageous in environmentally sensitive areas.
Perhaps the biggest advantage of PUR over other types of grouting materials is the control over the properties of the material. Through chemistry of the PUR mixture alone, viscosity, set time, strength, and expansion properties can be controlled at a much higher resolution than other types of grouting materials.
Finally, using PUR injection grouting for support of a rock mass does not result in any blockage of pathways that are typical of structural supports, which is important in underground mining applications. It also does not require any surface hardware, which preserves the natural aesthetics of the rock mass and is important in above-ground applications, such as rock slope stability along highways.
PUR products are typically harder to pump than less viscous cementitious grouts (Arndt et al. 2008). Additionally, with water, PUR products may foam and expel through open fractures on a rock face, requiring quick cleanup of the overrun material, before it hardens and removal becomes a more difficult task. Also, PUR is typically higher in cost than other types of cementitious grouts. For this reason, PUR is sometimes used in conjunction with other types of stabilization to lower project costs. PUR can be cost effective for a project provided a good characterization of the rock mass is performed and material is not wasted on poor field implementation. Furthermore, flow of the PUR within the rock mass is unknown until after pumping has been performed and boreholes are drilled to verify infiltration. Finally, hydrofracturing and rock falls may be an issue if pressure is not adequately controlled during pumping operations.
PUR has been used in underground coal mine support applications since the 1960’s, and was adopted by Germany as the standard method for stabilization in coal mines in the early 1970’s (Molinda 2004). As the process has been refined and studied, PUR has more recently seen effective application in rock slope stabilization along transportation routes. The following case studies detail some of the procedures used to successfully implement PUR in both the underground coal mining applications that have widely implemented PUR over the past 40 years, as well as the more recent rock slope stabilization applications.
The following case history, described in both Molinda (2004) and Molinda (2008), is used to demonstrate more specifically the design, implementation, and monitoring procedures typically used for PUR injection grouting in underground mining applications.
This project involved stabilization of a room and pillar coal mine roof in West Virginia, which had experienced multiple roof falls each year due to a weak clay shale roof, which constantly threatened the safety of the mine workers. The shale was extremely moisture sensitive, and the researchers also suspected that there were additional clay-filled veins, which would swell and apply additional pressure on the roof. Previous structural roof supports were restricting passage through the mine, such as one shown in Figure 4, therefore PUR injection grouting was selected as the technique to reach the goal of stabilizing all remaining intersections along the beltway in the mine.
Figure 4. Standing Support at Intersection (Molinda 2008)
The map in Figure 5 shows the unfallen sections, as well as the PUR treated sections.
Figure 5. Plan View of Roof Falls and PUR Injection Sites (Molinda 2008)
The injection array and sequence of the PUR followed a typical pattern seen in coal mine roof support applications, which involves injecting PUR at the perimeter of the intersections at a 45 degree angle, creating a grout curtain, and then injecting PUR into vertical holes along the centerline of the intersection, drilled offset those along the perimeter. This particular intersection geometry called for 11 boreholes in total, spaced at 10 ft. along center. A plan view and cross-section is shown below in Figures 6 and 7 to illustrate this design.
Figure 6. Plan View of Injection Array Figure 7. Cross Section of Injection Array
(Molinda 2004) (Molinda 2008)
For this project, two design options were proposed. The first involved constructing a beam within a targeted zone in the roof (see Figure 7) with non-expanding (hydrophobic) PUR, and the second involved cavity filling above the intersections with a hydrophilic PUR. The hydrophobic PUR beam approach was the selected design choice. It was determined for this project that utilizing a higher strength, non-expanding PUR would be satisfactory, even if the void spaces were not filled completely, however the chosen design method required a target zone for improvement, as opposed to the cavity filling approach. This zone was selected to be from 2 ft to 6 ft above the roof. This selection is critical, as an improved zone that is too high does not protect from shallower roof failures, and a zone that is too low may cause shallow roof failures during injection, due to pressure. The target zone was sequentially constructed by injecting PUR first within the 4 ft to 6 ft zone, allowing the PUR to harden, and then filling in the 2 ft to 4 ft zone.
For testing purposes, 16 boreholes were drilled, one in each of 15 treated intersections with one additional borehole. Fracture voids were monitored by borehole camera before and after PUR injection.
Nine out of the 16 boreholes monitored by camera showed complete filling of voids by the injected PUR material. Three monitored intersections showed 0%, 1%, and 9% filling of voids, indicating loss of PUR into large cavities or away from the monitoring boreholes by a different fracture path. The PUR injection in these 3 intersections was deemed unsuccessful, therefore heavy standing support was erected to support these intersections. The final 4 boreholes monitored showed partial filling of voids, between 43% and 93%. These measurements were taken from the borehole camera logs, illustrated in Figure 8 below.
Figure 8. Illustration of Selected Monitored Intersection Boreholes
Notice in Figure 8 that voids are shown in black, and PUR filled fractures are shown in grey. Color photographs of select fractures within the monitored boreholes are shown in Figure 2.
Most importantly, after 2.5 years of monitoring, 26 of the 27 intersections treated with PUR injection were stable (Molinda 2008).
This case study provides a good example of how some of the considerations discussed in the earlier sections of this report are implemented in underground coal mining. In this study, intersection zones for stabilization were targeted, and monitoring holes were drilled to estimate the location of cavities and void spaces. A design sequence was implemented involving construction of a grout curtain to minimize loss of PUR to unknown pathways away from the target zone. Additionally, the target zone was sequentially filled by injecting the upper two feet first, followed by the lower two feet. The monitoring boreholes were extremely important in determining the extent of PUR infiltration, which drove the decision to install standing supports at the intersections where PUR injection was deemed unsuccessful. Throughout the process, injection pressures and volumes pumped were monitored to ensure that they fell within acceptable estimations for the project.
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.
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.
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.
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).
Polyurethanes are a versatile and widely applicable material for civil engineering applications. Regarding stabilization in rock, PUR has been advantageous to the coal mining industry for decades, due to the ease of mobilization, fast set time, and high strength of the PUR material. Only recently has it been extended to above ground applications for rock slope stabilization, where cost considerations and other stabilization methods are more competitive.
For underground mining applications, proper characterization of the rock mass is vital to both estimating the amount of product required for injection as well as targeting injection zones. Additionally, having an expected volume of material allows the contractor to compare to the amount of material injected during the grouting process, which could point to multiple issues such as loss of PUR to the target zone, or unknown void spaces. Targeting the zone for improvement is performed by constructing a grout curtain, after which the targeted zone is filled by injecting PUR in a typical grid pattern. Spacing of the injection boreholes is the design choice of the grouting contractor, as it is sensitive to the fracture patterns and geometry of the project. It has been shown that 100% filling of void space is not required to stabilize mine roofs, however verification of void space filling is important to ensure that the roof is stabilized and other stabilization methods are not required.
For rock slope stability, more recent studies by the CFLHD have focused on stabilization of rock slopes along transportation routes. These studies have concluded that PUR grouting is an effective stabilization technique that can reduce the number of rock anchors or dowels required to stabilize a rock slope against rock falls. They recommend using PUR in fractured rock with fracture apertures greater than 2 mm. The injection sequence should be performed from the bottom up using a staged pumping approach, with borehole spacing approximately 8 ft to 16 ft apart. Pumping should be performed at low pressures (less than 250 psi) to avoid fracturing the rock mass further and causing rock falls. Finally, PUR may not be cost effective in above ground applications if the void space of the fractured rock mass is excessive, requiring an excessive amount of PUR material to fill.
In general, PUR grouting is a proven method for roof and longwall stabilization in the coal mining industry, and is becoming much more prevalent in above ground rock slope stability applications. The ease of mobilization, as well as the chemical control of set time, strength, and viscosity of the PUR material make PUR grouting an attractive option for rock stabilization, however material costs, as well as the difficulty of flow control after injection and the difficulty of verification may make it less attractive than other stabilization methods.