ABSTRACT 

 Extremely difficult ground conditions were encountered in the 

main belt entry in a room and pillar mine in West Virginia. Several 

generations of supplemental support, including cable bolts, 

cribbing, and Heintzmann jacks and beams failed to halt roof falls 

that threatened workers safety and the life of the mine. 

Polyurethane (PUR) was injected into 27 intersections in the roof 

and the results were monitored by borehole video camera. The 

mine had averaged 2-3 falls in the beltway per year and, since the 

roof injection, no roof falls have occurred. Pre- and post injection 

monitoring of test holes showed that stability can be achieved 

without filling all of the existing void spaces. Video logging 

showed that the project goal of building a beam from 2-6 ft into the 

roof was successful. Pre-injection monitoring of roof fractures 

uphole can delineate the zones to target for reinforcement, and 

avoid pumping PUR into large voids to little effect. Multipleinjection zones can be identified, and the entire intersection drill 

hole array can be optimized based on knowledge of the fractured 

zones. In addition, pre-monitoring can prevent excessive 

hydrofracturing in solid zones. Polyurethane injection is a proven 

method of rock stabilization in even the weakest, most broken 

ground. Optimization of the injection design by pre-injection video 

diagnostics can greatly contribute to the successful and efficient 

roof stabilization. 

BACKGROUND 

 Polyurethane injection for ground stabilization in coal mines 

was first developed by the German coal mine research organization 

Bergbau-Forschung GmbH in the early 1960’s (Stewart and Hesse, 

1985). It became a standard stabilization method in Germany since 

its commercial introduction in 1971 (Knoblauch, 1994). With the 

introduction of the RokLok binder system in 1977, polyurethane 

stabilization, particularly in longwall recovery, has become 

common in the U.S. (Stewart and Hesse, 1985). 

 Polyurethane injection in coal mines is most commonly used 

in difficult ground conditions including fractured rock in headgates 

and tailgates, and as a stabilization remedy to prevent longwall face 

caving. It may also be used as a replacement for roof meshing in 

shield recovery, and as a sealant to prevent groundwater inflow, but 

often it is applied as a last resort where conventional roof 

reinforcement and support has failed. 

 Polyurethane is typically a two component system that has 

several advantages to conventional support. It has the ability to 

chemically bond to the rock, unlike other supports which rely on 

frictional contact. Because it is injected under pressure, it 

inherently “targets” fractures, which are the paths of least 

resistance. It also has a low viscosity which allows it to penetrate 

cracks as small as 0.04 mm wide (Knoblauch, 1994). It has 

engineered expansion properties (1:1 to 1:12) which also allow for 

penetration (Shaller and Russell, 1986). It is both strong and 

plastic, preserving its’ integrity under load and racking-type 

deformations. Finally, it does not present the obstacle of standing 

support. 

 In designing a polyurethane stabilization the goal is to 

reinforce the fractured rock to the point where it can support its 

own weight and the weight of unconsolidated rock above it. 

Mechanically, the polyurethane forms a beam with rock that has 

been separated along bedding or is broken into key blocks. It is the 

size and strength of this beam which enhances the stability of the 

roof. 

 There are a number of variables which must be considered: 

1. The location of fractures – This information will help 

determine the zone to target for polyurethane injection. 

2. The extent of the fracture zone – An estimation of the total 

void space could be used to calculate the volume of PUR 

needed. In highly fractured roof, more test holes may be 

required. 

3. Character of the fractures – A determination of the nature of 

fractures, whether they are bedding separations or a rubbleized 

zone, will indicate the permeability of the zone. 

 PUR injection arrays can have a number of configurations. A 

typical injection pattern for an intersection will have four injection 

holes angled over the rib on 10 ft centers spanning each crosscut in 

the intersection (Figure 1). PUR may be injected over the rib on 

each side of the intersection to erect a “grout curtain” which will 

act as a barrier and permit infilling of the intersection. These 

injection holes will be packed off to the destabilized zone, and then 

PUR is injected. The holes will be pumped to a predetermined 

volume or pressure or they may be injected to refusal. Then holes 

will be drilled and pumped in the center of the intersection to 

complete the beam. The exact specifications of the design are often 

determined by the experience of the contractor. 

EVALUATION OF POLYURETHANE INJECTION FOR BELTWAY ROOF STABILIZATION 

IN A WEST VIRGINIA COAL MINE 

Gregory M. Molinda, Research Geologist 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania USA 
BELTWAY STABILIZATION 

 A coal mine in West Virginia was experiencing extremely 

difficult roof conditions in its main beltway throughout the life of 

the mine leading up to the autumn of 2002 (figure 2). The 18 ft 

wide belt entry was averaging 2-3 falls per year which threatened 

workers safety and resulted in costly delays due to cleanup and 

rehabilitation. The roof rock is an extremely weak clay shale 

which is highly moisture-sensitive. In August, roof falls occur 

almost 2.5 times more frequently than the annual monthly average. 

In addition, it is suspected that frequent clay veins also react to 

moisture, swell, and apply bulking pressures on the roof sequence. 

The roof begins to unravel between bolts in a relatively short time, 

leading to a progressive upward failure and finally a roof fall. 

Mine-wide, 63% of roof falls occurred in intersections. In the 

beltway from the portal to the first submains, 15 of 43 intersections 

had fallen (figure 2). The project goal was to stabilize all the 

unfallen intersections by polyurethane injection. 

 In the beltway several generations of supplemental support 

including cable bolts, roof screen, pizza pans, posts and beams, and 

cribs were beginning to restrict travel. At this point options 

included adding additional support, moving the beltline, or 

polyurethane injection. Polyurethane injection was selected 

because, based on past experience, it had the greatest likelihood of 

success. 

 Using an injection design pattern with 11 pumped holes per 

intersection, the project began (figure 1). After injection of two test 

intersections on the track, the job was stopped in order to evaluate 

the PUR reinforcement by using video monitoring. It had been 

difficult to build any pump pressure and questions arose as to 

where the polyurethane was going. Large voids were detected at 

10-12 ft up into the roof. (It should be noted that this intersection 

was heavily supported with steel beams and posts). From these 

observations it was decided to concentrate the PUR injection on 

reinforcing the roof beam from 2-6 ft up into the roof. It would be 

impossible to fill the large void spaces in the roof and, in fact, 

would be unnecessary if the lower beam could be reinforced. 

Testing Method 

 Because of its demonstrated value, it was decided to continue 

video monitoring through the project, and to use the information to 

refine the design of the PUR injection. Figure 2 shows the Mains 

and the beltway, and intersections selected for polyurethane 

stabilization. All intersections that had not fallen in the beltway 

were selected for PUR-injection stabilization. A total of 27 

intersections had PUR injected for stabilization. The reinforced 

beam in the Mains was created by pumping PUR in a zone from 2-

6 ft. Two pump zones were isolated. PUR was pumped first from 

4-6 ft, allowed to set, and then was pumped from 2-4 ft. Each 

intersection averaged 12 injection holes and these holes averaged 6-

7 ft long. The average amount of PUR injected per intersection 

was 425 gal. This volume was calculated to allow 2 - 55 gal drums 

of PUR mix to pump 3 holes. Injection pressures ranged from 0-

2,000 psi and averaged about 400-500 psi. 

 The injection pattern was typically 4 angled holes on each side 

of the beltway across the intersection, and 4 holes along the middle 

of the intersection. A total of 17 video logs from 11 intersections 

were used in the analysis. Five intersections had both pre-injection 

and post-injection, and 6 intersections had only post-injection 

videos. Monitoring holes were drilled on the walkway side of the 

belt in the middle of the intersection crosscut, and approximately 

3 ft from the rib. 

PRE AND POST INJECTION MONITORING OF 

FRACTURES IN THE BELTWAY ROOF 

 Video logs, some pre-and post injection, revealed the 

condition of the roof in selected intersections along the Mains 

project area (figures 3a and b). Pre-pumping video logs showed 

significant voids in the roof at two intersections (No. 23 and. 32) 

(figure 3a). At No. 32, highly fractured roof rock was loading 

standing support and falling between supports. Using the design 

plan of reinforcing a roof beam from 2-6 ft into the roof, PUR 

injection continued. Video logging was available at 16 postinjection test holes at 11 intersections. The test holes showed PUR 

successfully injected into numerous void spaces in the target zone 

in each of 11 intersections. Individual cracks ranging from paper 

thin up to 3⁄4 in wide, and rubbleized zones up to 1.5 in were filled 

with PUR (figures 4a and 4b). This information allowed for an 

intersection-by-intersection evaluation of the PUR injection 

performance. 

 In 5 intersections (Nos. 20, 21, 22, 23, 32) both pre and postinjection test holes were video-logged in order to determine which 

pre-existing fractures were filled with PUR (figures 3a and 3b). 

Intersection No. 21 had a rubble zone 3/8 in thick at 2.9 ft into the 

roof prior to PUR injection (figure 3a). Twelve injection holes 

pumped 399 gal of PUR into the intersection in two isolated zones 

(4 ft and 2 ft) (table 1.). In a post-injection test hole (figure 3b), 

there are three prominent PUR “shows” from 2.7-3.5 ft. effectively 

filling the rubble zone (figure 5). 100% of the void space at this 

hole location in the intersection was filled with PUR (figure 3b, No. 

21 post-injection hole). 

Figure 1. Idealized PUR injection array. 
Figure 2. Intersections stabilized with polyurethane. 
 

Figure 3a. Borehole camera logs of monitoring holes prior to PUR injection. 

(These are the only pre-injection logs available.) 

Figure 3b. Borehole camera logs of monitoring holes after PUR injection. (Some 

intersections listed here do not have pre-injection logs available.) 
Table 1. Void Space Filled by PUR in 7 ft high monitoring zone. 

Intersection no. 

Void space 

filled (%) in 

test hole 

(2-7 ft zone) 

Total void 

space (in) 

2-7 ft zone 

No. of 

injection 

holes 

PUR 

pumped 

(gal) 

43 100 0.11 21 699 

42 100 1.5 16 532 

40 100 0.69 17 566 

37 100 0.5 20 666 

36 100 3.0 19 632 

35 100 2.87 21 699 

33 71 1.75 12 466 

32 43 0.87 8 233 

 0 11.75 8 233 

29 1 4.10 No data 600 

28 93 0.27 No data No data 

26 9 2.43 No data 600 

23 54 0.81 12 399 

22 100 0.31 12 380 

21 100 1.06 12 399 

20 100 0.56 14 432 

 In intersection No. 23, two fractures existed at 4.2 ft (3/8 in 

wide) and 6.0 ft (3/8 in wide) prior to injection into the roof 

(figure 3a, test hole No. 23a). One-hundred ten gal of PUR was 

pumped into a zone packed at 4 ft and 289 gal of PUR was pumped 

into a zone packed at 2 ft (table 1). Both zones were filled with 

PUR, with another 3/8 in void left unfilled at 6.5 ft (figure 3b). 

This void was above the height of the injection holes. 

 In intersection No. 20 the pre-injection hole showed solid roof 

and no voids or even separations (figure 3a). Fourteen holes were 

drilled and approximately 432 gal of PUR was pumped (table 1). 

After injection, a video log revealed that PUR was injected into a 

zone at 1.7 ft and from 3.5-3.8 ft into the roof (figure 3b). It is 

assumed that PUR was injected into weak, unseparated bedding 

planes or that the bedding may have been hydrofractured with 

injection pressures up to 1,800 psi. Another possibility is that 

fractures were present that were not revealed by the pre-injection 

test hole. PUR could have been injected into these zones. 

 At intersection No. 22 pre-injection video monitoring again 

showed a solid roof with no voids or separations (figure 3a). 

Twelve injection holes were pumped. At 4 ft of each hole a packer 

was set and 110 gal total was pumped between all holes. The hole 

was repacked at 2 ft and 271 gal was pumped, between 2 and 4 ft. 

The post-injection video monitoring hole showed significant PUR 

injected into 6 zones from 1.2 ft to 6 ft (figure 3b). These zones 

may have been hydrofractured. It is also possible that PUR was 

injected into tight bedding plane separations not visible in the preinjection videos. In this case, PUR may still have some reinforcing 

value, especially when roof movement begins. In tight roof, premonitoring can indicate that lesser volumes of PUR should be 

pumped. 

 The last intersection with both pre and post injection 

monitoring was No. 32. At this intersection PUR injection was less 

successful. Several inches of void space was measured in the preinjection zone (2-7 ft into the roof) (figure 3a and 6). At 8 injection 

holes a total of 233 gal of PUR was pumped (table 1). Two holes 

were video monitored after injection (figure 3b, holes nos. 32, 32a). 

No PUR was observed in one post-injection monitoring hole. But 

in the other test hole, several fractures in a zone from 0-1 ft had 

PUR shows (figure 7). PUR shows below the packed injection 

zone highlight the extreme fracturing in the intersection. The PUR 

found fracture conduits below the packed zone and was seen 

dripping from the roof. The one pre injection hole and two post 

injection holes show large variations in fracture location in the 

intersection. This indicates that additional monitoring holes may 

Figure 4. Borehole camera images showing PUR zones in the 

roof. 

Figure 5. PUR-filled separations at intersection No. 21.
be necessary to delineate the variation in highly fractured 

intersections. 

 Six other intersections (Nos. 43, 40, 37, 36, 35, 33) had only 

post-injection video logs available (figure 3b). Table 1 summarizes 

the PUR injection history of the remaining intersections. It shows 

the amount of void space filled by PUR in the 7 ft monitoring zone 

(the injection zone was from 2-6 ft.). 

 Of the 16 holes that were video logged (15 intersections) 9 had 

100% of the void space in the monitoring zone (2-7 ft) filled. That 

is, there was no remaining void. Six of the holes had voids filled 

ranging from 1-93%, and one had no observed PUR “shows.” In 

some intersections with multiple test holes, large differences in 

void space were seen across the intersection (No. 32 intersection 

and No. 28). In No. 28 intersection 4 test holes in the intersection 

spanning one crosscut showed voids ranging from 0 – 1.5 in. The 

variation in void space over short distances may explain the partial 

filling of void in some test holes. Even though test holes are near 

injection holes, PUR may follow a circuitous route depending on 

the fracture permeability of the intersection. In two intersections 

(Nos. 32 and 29) monitoring holes detected 0 and 1% of the voids 

filled, indicating loss of the pumped PUR into the mine opening or 

away from the intersection monitoring hole. Monitoring holes in 

both of these intersections revealed large void spaces above the 

bolted horizon (3/4 in – 6 in wide) (figure 3b). These intersections 

are currently controlled by heavy standing support. 

 The amount of PUR pumped into each intersection was also 

recorded. The volume ranged from 233 to 699 gal (table 1). 

Several intersections that were injected with large volumes (600 

gal) of PUR (Nos. 29 and 26) were still left with a large percent of 

the void spaces unfilled. The location of the PUR injection up in 

the roof, in regards to building a stable roof beam, appears to be 

just as important as the volume of PUR pumped per hole. If the 

beam is constructed too high in the roof then fractured rock below 

it may fall. Additionally, if PUR is injected into large voids it may 

migrate away from the intersection and be of little value. Void 

spaces open an inch or more may be difficult to completely fill with 

PUR. A better strategy in the beltway was to concentrate PUR 

injection to building a stable beam below these large openings. At 

intersection No. 26, even though large voids exist from 10-13 ft 

into the roof, a stable beam has been created from 2-6 ft in the roof. 

DISCUSSION 

 The design of a PUR stabilization site necessarily requires an 

estimate of the amount of chemical to inject. Current estimates are 

based on calculations of area (ft2

) of roof to be stabilized and the 

contractors’ experience. The polyurethane grout used at the site 

was rated at 70 lbs/ft3

 and not intended to expand, although some 

expansion may occur if water is encountered. By using prepumping video monitoring the condition of the roof can be assessed 

and the zone of beam reinforcement can be determined. In the 

Mains beltway, in most of the intersections, a stable beam was 

created at 2-6 ft into the roof. Eleven of 12 post-gluing monitor 

holes showed PUR in the targeted beam zone, with 10 of 12 

monitoring holes showing no open fractures in the target zone. 

Some of the intersections had large voids above the reinforced zone 

indicating that the reinforced beam was strong enough to hold at 

least 7 ft of fractured rock. 

 The reinforced beam in the Mains was created by pumping 

PUR in a zone from 2-6 ft. Two pump zones were isolated. PUR 

was pumped first from 4-6 ft, allowed to set, and then was pumped 

from 2-4 ft. The roof is highly fractured from 0-2 ft and it would 

be very difficult to saturate this zone with PUR because most of the 

chemical would leak out of the roof. In addition, high pumping 

pressures may dislodge roof blocks. Therefore, PUR is not 

typically designed for skin control but meant to prevent massive 

roof falls. 

 One method to estimate the volume of PUR needed for 

stabilization is to do a volume calculation based on the void space 

detected in a pre injection test hole. Void volume calculations were 

done for all injected intersections. When comparing the amount of 

voids filled with the amount of pumped PUR, the results were 

highly varied. Some intersections showed more PUR pumped than 

required, and some test holes showed less PUR was required to fill 

100% of the void space than calculated. The discrepancy appears 

to lie in the monitoring. In highly fractured rock, fracture 

permeability requires more test holes to correctly delineate the void 

space and permeability of the target beam. 

 A void volume calculation was also done on intersection 

No. 35. In this highly fractured intersection, a total of 2.87 in of 

void space was measured in the target zone (2-6 ft). At 379 gal/in 

of void space, it would take 1,087 gal of PUR to fill the voids space 

of the entire intersection. Six-hundred ninety-nine gallons were 

pumped into 21 holes at that intersection, filling 100% of the void 

space detected at the test hole site. While 100% of the void space 

at the test hole was filled with PUR, more unfilled void space may 

possibly exist throughout the remainder of the intersection. In this 

case stability was achieved with less than complete filling of the 

theoretical void space. More test holes are required to completely 

delineate the extent of PUR infiltration throughout the intersection. 

Figure 6. Large void at 6 ft into the roof at intersection 

No. 32 prior to PUR injection. 

Figure 7. PUR filling fractures at 1 ft into the roof at 

intersection No. 32. 
 The determination of how much PUR to pump into each 

intersection is a function of roof condition and experience. The use 

of pre-injection monitoring can: 

1. Locate the position of the zone requiring reinforcement. 

2. Provide void separation data for a preliminary estimate of the 

necessary PUR volume. 

In highly fractured roof it may be impossible to fill all available 

void space. An estimate of void space in the target zone will 

provide a starting point for a PUR volume calculation. 

 In tight roof (no open voids) less PUR is needed. In 

intersection 16, a total of 3/8 in of total void space was found in the 

target zone in a pre-injection hole, amounting to 142 gal of PUR 

necessary to fill the intersection roof void space (no post-injection 

hole was available for analysis). 765 gal of PUR were injected into 

the intersection. The additional PUR may be injected above the 

target zone, lost to the mine opening, or injected into weak bedding 

planes in the roof. Once again, a third possibility is that PUR was 

injected into existing fractures not detected by the monitoring hole. 

More monitoring holes would more accurately delineate the 

permeability of the target zone and provide a more accurate starting 

point for a PUR injection volume estimate. 

PERMEABILITY 

 The permeability of the fractured rock plays an important role 

in designing the number and location of injection holes. 

Unfortunately, permeability in highly fractured rock can vary 

tremendously even in short distances. Two monitor holes in 

intersection No. 32 separated by 10 feet showed 0.68 in roof void 

and 11 in of roof void, respectively, in the target zone (2-6 ft). 

Schaller and Russell (1986) found that after injecting one roof hole 

in an intersection, PUR was found distributed over 20 m2

. This 

was in roof with voids up to 4 in wide. Rather than pumping PUR 

to refusal, contractors may often pump a predetermined amount of 

PUR around the perimeter of the intersection, allow it to set up (25 

seconds set time in this instance), and fill in the intersection with 

holes along the center line. Pumping to refusal happened rarely in 

the Mains beltway indicating that it was not possible (or necessary) 

to fill all the existing voids to achieve stabilization. The path of 

PUR migration in the highly fractured rock is difficult to predict. 

At intersection No. 32, 3 injection holes were within 1-5 ft of a 

monitor hole (32a) and no PUR was detected. For this reason more 

injection holes, more closely spaced, would enhance PUR 

distribution. 

 In the more highly fractured intersections more difficulty was 

encountered in getting the PUR into the zone intended for beam 

reinforcement. This problem may be addressed when designing the 

grout curtain to act as a barrier to subsequent PUR injections. It 

may be necessary to install several layers of grout curtain to ensure 

an effective barrier to PUR loss. More injection holes in 2 or more 

concentric layers around the perimeter of the intersection may be 

necessary before infilling of the interior of the intersection. Along 

the same lines, shorter packed zones (1 ft) within the beam zone 

would allow the beam to be reinforced in several lifts. Instead of 

two injection zones from 4-6 and 2-4 ft, four zones (2-3, 3-4, 4-5 

and 5-6 ft) may provide more effective infill. 

 Fundamental questions remain about the optimal volume and 

location of PUR needed to stabilize an intersection. Numerical 

modeling studies may shed light on the mechanics of PUR 

stabilization and provide answers to these questions. 

SUMMARY 

 Polyurethane injection in the Mains beltway appears to have 

successfully stabilized the remaining unfallen intersections. This is 

demonstrated by the fact that no roof falls have occurred through 

April, 2004 when 2-3 falls per year previously occurred. Video 

monitoring of PUR injection results have shown that polyurethane 

was contained in the target zone in 11 of 12 cases. This 

containment depends on the extent of fracturing and permeability 

of the roof strata. In highly fractured roof more injection holes may 

be needed. Additional monitoring holes may also be needed to 

verify the location of the polyurethane. 

 The PUR injection strategy of building a roof beam from 2-6 

ft in the roof appears to have successfully supported up to 7 ft + of 

fractured rock load height above the injection zone. In 9 of 16 

video monitoring holes (15 intersections, some logs not presented), 

PUR filled 100% of void spaces, 5 holes showed partial filling of 

voids (ranging from 1-93% void filling), and 2 monitoring holes in 

intersections showed little or no PUR. From these data and the 

success in stopping roof falls, it was determined that it is not 

necessary to fill all of the void spaces in the roof strata. It is 

sufficient to concentrate on stabilizing a target roof beam which 

can then carry dead load from unconsolidated strata above. 

 Pre-injection monitoring holes showed where voids occurred 

in the roof and can be used to guide the target zones for PUR 

injection, thereby avoiding losing PUR into vast voids and costly 

over-pumping into tight roof. Post-injection video monitoring can 

also be used to assess the success of the injection by calculating the 

amount and location of void space filled. Video monitoring of roof 

holes also showed rapid changes in permeability in short distances 

can occur in intersections, thus affecting the zone of influence of 

the injection holes. The accuracy of estimates of the injection 

volumes needed to stabilize the intersection is dependent on the 

number of test holes available for void calculations. More test 

holes will allow more accurate estimates of void space and 

permeability in the target zone. 

 It is concluded that pre and post-monitoring of roof void space 

can help to optimize the design of PUR injection for roof 

stabilization.

REFERENCES 

Knoblauch, K. (1994). Bolting, Sealing, Consolidation, Cavity 

Filling, Repairing Economic and Reliable Formulas by CarboTech. 

Proceedings, 10th International Conference on Coal Research, 

Coal: Energy for the Future, Vol. 1, Oct. 9-12, Brisbane, Australia, 

pp. 417-435. 

Stewart, J.G. and Hesse, M. (1985) Roof Control with 

Polyurethane for Recovery of Kit Energy’s 1,000-foot Longwall. 

Proceedings, 4th International Conference on Ground Control in 

Mining, July 22-24, pp. 78-82. 

Schaller, S. and Russell, P. (1986) Trial Application of 

Polyurethane Resin in Australian Coal Mines. In Ground 

Movement and Control Related to Coal Mining, Symposium on 

Ground Movement and Control Related to Coal Mining, 

Wollongong, Australia, Aug. 26, pp. 134-142.