The International Information Center for Geotechnical Engineers

Review of Polyurethane Resin Grouting for Rock Mass Stabilization



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.

Standing west virginia mine roof support

Figure 4. Standing Support at Intersection (Molinda 2008)

The map in Figure 5 shows the unfallen sections, as well as the PUR treated sections.

West Virginia Roof Falls and PUR stabilization points

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. 

Plan View of PUR Injection Array West Virginia Mine Cross Section of Injection Array West Virginia Mine

                          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.

Borehole Logs 

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.


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