Reinforcing Coal Mine Roof with Polyurethane Injection:
 

4 Case Studies 

Gregory Molinda 

G. Molinda 

Pittsburgh Research Laboratory, National Institute for 

Occupational Safety and Health (NIOSH), Cochrans Mill 

Road, P. O. Box 18070, Pittsburgh, PA, USA 

e-mail: Gdm4@cdc.gov 

Abstract NIOSH has recently completed a study of 

the interaction between polyurethane (PUR) and coal 

mine roof in order to determine the mechanism of 

reinforcement, in both highly fractured rock and 

unfractured rock. Four case studies of PUR reinforcement are presented. At a West Virginia site, a 

borehole camera revealed the location of roof voids 

and guided the PUR injection. By injecting polyurethane into a zone from 0.6 to 1.8 m (2–6 ft.) high in 

the roof, a roof beam was created and ongoing 

intersection falls were halted. In another highly 

fractured roof in a western Pennsylvania mine, a 

total of 5.8 cm (2.3 in.) of rubbleized rock was found 

in a zone up to 3 m (10 ft.) into the roof. Untargeted 

PUR injection filled approximately 1/2 of the fractures. These two case studies showed that it is not 

necessary to fill up 100% of the void space to create 

stability. In the other two field sites, polyurethane was 

injected into weak, but unfractured roof. Post-injection video monitoring showed that weak bedding 

planes were hydraulically wedged open and polyurethane injected along bedding. The reinforcement 

value of this injection method is limited because of 

the wafer-thin layer of PUR introduced along 

bedding, and the lack of a PUR ‘‘webbing’’ which 

would serve as a structural framework to provide 

strength. It was determined that video inspection 

prior to PUR injection can aid in identifying the 

fracture zones to target, and minimize ‘‘blind’’ 

pumping and loss of PUR. 

1 Background 

Coal mine roof strata has been successfully reinforced with polyurethane (PUR) for over 40 years. 

Applications have included headgate stabilizations, 

rock consolidation over coal panels in advance of 

mining, and roof reinforcement for shield recovery. 

Polyurethane injection for ground stabilization in 

coal mines was first developed by the German coal 

mine research organization Bergbau-Forschung 

GmbH in the early 1960s (Jankowski 1972). It 

became a standard stabilization method in Germany 

after 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 US (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 over 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 0.05 mm (0.002 in.) 

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 (Micon 2003). Finally, it 

does not obstruct roadways like standing support. 

Fig. 1 Idealized PUR injection design 

2 Current PUR Injection Design Process 

Injection designs currently have a ‘‘one size fits all’’ 

approach, with drilling patterns and chemical grout 

volumes decided in advance, often for the sake of 

convenience rather than engineering design. A design 

process is needed to determine the optimum location 

for injecting polyurethane grout in order to maximize 

the reinforcement benefit and prevent wasting large 

volumes of chemical. 

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 rubbleized zones, will indicate the 

permeability of the zone (Molinda 2004). Uphole 

mapping of fractures will help define permeability. 

4.	 Injection pressures—Often the injection proceeds 

until a pre-determined injection pressure is 

achieved, indicating that the fractures are filled. 

If no back pressure is ever achieved the indication is that the fracture zone is infinitely large. 

Conversely, if a high back pressure is reached 

immediately or very quickly, then the roof is 

considered to be unfractured, and further pumping may hydrofracture the roof, which may 

loosen roof rocks. 

5.	 Injection arrays—These pumping patterns can 

have a number of configurations. A typical 

injection pattern for an intersection will have 

injection holes angled over the rib on 3 m (10 ft.) 

centers spanning each crosscut in the intersection 

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

each side of the intersection. These injection 

holes will be packed off to the destabilized zone, 

and then PUR is injected to erect a ‘‘grout 

curtain’’ which will act as a barrier and permit 

infilling of the intersection. The holes are either 

pumped to a predetermined volume or pressure 

or injected to refusal. Then holes will be drilled 

and pumped in the center of the intersection to 

complete infilling of the pattern. The exact 

specifications of the design are often determined 

by the experience of the contractor. 

3 Case Histories 

Four case histories of polyurethane injection for coal 

mine roof stabilization are presented. Two of the 

histories were in highly fractured rock and two were 

in unfractured rock. 
3.1 West Virginia Coal Mine—Fractured Roof 

A coal mine in north-central 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 (Fig. 2). The 5.5 m (18 ft.) 

wide belt entry was averaging 2–3 roof falls per year 

which resulted in costly delays due to cleanup and 

rehabilitation. The roof rock was extremely weak and 

highly moisture-sensitive clay shale. August was the 

worst month, with roof falls occurring almost 2.5 

times more frequently than the annual monthly 

average. In addition, it was suspected that frequent 

clay veins reacted to moisture, swelled, and applied 

bulking pressures on the roof sequence. The roof 

began to unravel between bolts soon after mining, 

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 (Fig. 2). 

Fig. 2 Roof falls and PUR injection sites in West Virginia 

mine 

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, building a false roof, moving the beltline, or 

polyurethane injection. Polyurethane injection was 

selected to stabilize all the unfallen intersections in the 

main beltway because, based on past experience, it had 

the greatest likelihood of success. 

Beltway PUR pumping began using an injection 

pattern with 11 pump holes per intersection. It was 

difficult to build any pump pressure and questions 

immediately arose as to where the polyurethane was 

going. (It should be noted that this intersection was 

heavily supported with steel beams and posts). Cold 

air was blowing down the test hole indicating 

communication over the crosscut to the intake entry. 

During injection of two test intersections on the track, 

the job was stopped in order to evaluate the PUR 

reinforcement by using video monitoring. 

3.1.1 Video Diagnostics 

A total of 16 video logs from 15 intersections were 

used in the analysis. Monitoring holes were drilled on 

the walkway side of the belt in the middle of the 

intersection crosscut and approximately 0.9 m (3 ft.) 

from the rib. 

Video monitoring of the first PUR injection test 

intersection on the track (intersection No. 26) 

detected large voids at 2.9–3.7 m (9.5–12 ft.) up into 

the roof (Fig. 3). A large void (27.9 cm (11 in.)) was 

detected in two test holes in the intersection at 3.4 m 

(11 ft.) above the roof line. A total of 48 cm (19 in.) 

of void space was observed in the roof. From these 

observations, and the lack of pump pressure, it 

appeared that large volumes of PUR were being lost 

into the voids. Video logs also revealed the condition 

of the roof in selected intersections along the Mains 

project area. Pre-pumping video logs showed significant voids in the roof at two intersections (No. 23 

and No. 32) (Fig. 4). At No. 32, highly fractured roof 

rock was loading standing support and falling 

between supports (Fig. 5). Three-3.7 m (10–12 in.) 

of deflection on the steel beam in this intersection 

indicates the sum of separate fracture voids up in the 

roof and can be used as a de facto roof extensometer. 

3.1.2 PUR Injection into the Beltway Roof 

Because of the large separations detected in the roof, it 

would be impossible to fill all the voids with the full 

strength non-foaming PUR. After considering cavityfilling foam, a decision was made to target zones for 

reinforcement with non-foaming PUR. The concept 

was that if the lower beam could be reinforced, it 

would be unnecessary to fill all the voids. It was 

decided to concentrate the PUR injection on reinforcing the roof beam from 0.6–1.8 m (2–6 ft.) up into the 

roof. An injection procedure was designed which 

would target two isolated zones for PUR injection, 

creating a reinforced beam. The reinforced beam in 

A-Mains was created by pumping PUR in an isolated 
zone from 1.2–1.8 m (4–6 ft.). The chemical was 

allowed to harden (30 s set time). A packer was then 

set and PUR was pumped from 0.6–1.2 m (2–4 ft.). 

Each intersection averaged 12 injection holes and 

these holes averaged 1.8–2.1 m (6–7 ft.) long. The 

average amount of PUR injected per intersection was 

1,608 l (425 gal). This volume was calculated to 

allow 2.2–207.9 l (55 gal) drums of PUR mix to pump 

three holes. Injection pressures ranged from 0– 

13.8 MPa (0–2,000 psi) and averaged about 2.8– 

3.5 MPa (400–500 psi). The injection pattern was 

typically four angled holes on each side of the beltway 

in the intersection, and four holes along the middle of 

the intersection. 

Fig. 3 Large voids detected by videoscope above bolt 

anchorage 

All intersections that had not fallen in the beltway 

were treated with PUR injection stabilization (Fig. 2). 

A total of 27 intersections had PUR injected. 

3.1.3 Location of PUR After Injection into the 

Beltway Roof 

Video logging was available at 16 post-injection test 

holes at 15 intersections. The test holes showed PUR 

successfully injected into numerous void spaces in 

the target zone in each of 15 intersections. Individual 

cracks ranging from paper thin up to 1.9 cm 

(0.75 in.) wide, and rubbleized zones up to 1.5 in. 

(3.8 cm) were filled with PUR (Fig. 6). This information allowed for an intersection-by-intersection 

evaluation of the PUR injection performance. 

In five intersections (Nos. 20, 21, 22, 23, 32) both 

pre and post-injection test holes were video-logged in 

order to determine which pre-existing fractures were 

filled with PUR (Figs. 4, 6). In intersections No. 21 

and 23, all of the pre-existing fractures, in the zone of 

reinforcement, were filled with PUR. 

In intersection No. 20 and 22 pre-injection holes 

showed solid roof and no voids or even separations 

(Fig. 4). After injection, a video log revealed that 

PUR was injected into a zone at 0.5 m (1.7 ft.) and 

from 1.1–1.2 m (3.5–3.8 ft.) into the roof in hole 

No. 20 (Fig. 6). It seems that in these holes PUR 

was injected either into weak, unseparated bedding 

planes or that it hydrofractured the bedding planes 

with injection pressures up to 12.4 MPa (1,800 

psi.) Hole No. 22 showed similar evidence of 

hydrofracturing. 

At intersection No. 32 PUR injection was less 

successful. PUR injection was stopped because no 

back pressures could be built up indicating flow out 

of the intersection. Several centimeters of void space 

was measured in the pre-injection pump zone .6– 

2.1 m (2–7 ft.) into the roof) (Fig. 4). No PUR was 

observed in one post-injection monitoring hole 

(Fig. 6, No. 32a), indicating loss of PUR into voids. 

The other post injection test hole in the intersection 

showed much less severe fracturing in the target zone 

at 0.6–2.1 m (2–7 ft.), with some PUR showing at 

1.2 m (4 ft.). Several fractures in a zone from 0– 

0.2 m (0–1 ft.) had PUR shows. PUR shows in this 

zone, below the packed injection zone, indicated the 

extreme fracturing in this intersection. The PUR 

found fracture conduits below the packed zone and 

was seen dripping from the roof. In intersection No. 

32 the one pre-injection hole and two post injection 

holes showed large variations in fracture location in 

the intersection. This indicates that additional 
monitoring holes may be necessary to delineate the 

variation in highly fractured intersections. 

Fig. 4 Video logs of roof 

holes in selected 

intersections before PUR 

injection 

Fig. 5 Heavily loaded standing support and roof damage in 

intersection 

Table 1 summarizes the PUR injection history of 

the remaining intersections. It shows the amount of 

void space filled by PUR in monitoring zone (the 

injection zone was from 0.6–1.8 m (2–6 ft.)) and the 

amount of PUR pumped. 

Of the 16 holes that were video logged in 15 

intersections, 9 had 100% of the void space in the 

monitoring zone 0.6–2.1 m (2–7 ft.) filled with PUR. 

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 four test holes in the intersection showed voids 

ranging from 0–3.8 cm (0–1.5 in.) wide. The variation in void space over short distances may explain 

the partial filling of voids 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 three intersections 

(Nos. 32, 29, 26) monitoring holes detected 0, 1 and 

9% of the voids filled, indicating loss of the pumped 

PUR into the mine opening or away from the 

intersection monitoring hole. Monitoring holes in 

each of these intersections revealed large void spaces 

above the bolted horizon 1.9–15.2 cm (0.75–6 in. 

wide voids). PUR injection was unsuccessful in these 

instances. The intersections are currently controlled 

by heavy standing support. 

The amount of PUR pumped into each intersection 

was also recorded. The volume ranged from 880– 

2,642 l (233–699 gal) (Table 1). 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. If PUR is injected too low, roof 
Fig. 6 Fractures filled with PUR in selected intersections after PUR injection 

Table 1 Void space filled 

by PUR in the monitoring 

zone (0.6-2.1 m (2–7 ft.)) 

Intersection no/hole PUR 

pumped l (gal) 

Total void space 

cm (in.) 0.6–2.1 m 

(2–7 ft.) zone 

No. of 

injection 

holes 

Void space filled 

(%) in test hole 

0.6–2.1 m 

(2–7 ft. zone) 

43 2,642 (699) 0.28 (0.11) 21 100 

42 2,010 (532) 3.8 (1.50) 16 100 

40 2,139 (566) 1.8 (0.69) 17 100 

37 2,517 (666) 1.3 (0.50) 20 100 

36 2,388 (632) 7.7 (3.0) 19 100 

35 2,642 (699) 7.4 (2.9) 21 100 

33 1,761 (466) 4.6 (1.8) 12 71 

32 880 (233) 2.2 (0.87) 8 43 

32a 880 (233) 6.3 (11.8) 8 0 

29 2,268 (600) 10.5 (4.1) No data 1 

28 No data 0.69 (0.27) No data 93 

26 2,268 (600) 6.23 (2.43) No data 9 

23 1,508 (399) 2.0 (0.81) 12 54 

22 1,436 (380) 0.79 (0.31) 12 100 

21 1,508 (399) 2.7 (1.06) 12 100 

20 1,632 (432) 1.4 (0.56) 14 100 
blocks may be dislodged. Additionally, if PUR is 

injected into large voids it may migrate away from the 

intersection and be of little value. Void spaces open 

2.54 cm (1 in.) 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 3.0–4.0 m (10– 

13 ft.) into the roof, a stable beam has been created 

from 0.6–1.8 m (2–6 ft.) in the roof. In 2.5 years of 

monitoring since the injection project, 26 of the 27 

reinforced intersections were stable. 

3.2 Western Pennsylvania Coal Mine—Fractured 

Roof 

A longwall mine in southwestern Pennsylvania was 

experiencing heavy roof conditions in surrounding 

rooms after a roof fall in the headgate of a future 

longwall panel. It was decided to inject PUR into the 

roof in order to stabilize several hundred feet of 

headgate entry in preparation of the upcoming 

longwall. An opportunity was presented to observe 

the fracture condition of the roof before injection and 

then after PUR injection. Additionally, heavy roof 

conditions were observed in the center track entry 

and this entry was chosen as a test site for PUR 

injection. Figure 7 shows the intersection where PUR 

injection and monitoring took place. Five pre-injection monitoring holes revealed the lithology and 

fractured condition of the roof (Fig. 8). The roof 

consisted of approximately 0.6 m (2 ft.) of roof coal, 

followed by gray shale from 0.6–2.1 m (2–7 ft.), 

followed by a coarse sandstone with coal streaks. 

Fig. 7 Intersection holes monitored before and after PUR 

injection 

Hole No. 2 showed a total of 2.8 cm (1.1 in.) of 

open void or rubbleized zones of fractured rock up in 

the monitoring holes which were drilled to 3.0 m 

(10 ft.). The individual cracks ranged from 0.15– 

1.28 cm (0.06–0.5 in.), in bedding separations and 

zones of rubble. In holes 1, 2, 3 most fractures 

occurred above 1.5 m (5 ft.) and in holes 4, 5 most 

fractures occurred below 1.2 m (4 ft.), indicating 

some variability across the intersection. Polyurethane 

was injected into five vertical holes adjacent (30– 

60 cm (1–2 ft.) away) to the observation holes in the 

intersection. The holes were packed at approximately 

30 cm (1 ft.) above the roof line. 

Figure 9 shows the logs of test holes drilled after 

PUR injection. Approximately 50% of the total 

fractures were filled with PUR, while the other 50% 

remained open. Figure 10 shows one of the gluefilled fractures after PUR injection. Since the holes 

were packed at the bottom, the PUR was free to find 

the path of least resistance up hole. Due to the 

tortuous nature of the fracture permeability the PUR 

found pathways which filled some fracture zones and 

bypassed others, as observed in the post-PUR monitoring holes. From experience at the mine in West 

Virginia this amount of void space filling should be 

enough to stabilize the intersection. The mine was 

closed shortly after the project and no assessment of 

the success of the PUR stabilization project was 

possible. By using multiple packed zones, as was the 

case in the West Virginia project, the chances of 

targeting particular zones of significant fracturing can 

be greatly increased. 

3.3 Bruceton Safety Research Coal Mine— 

Unfractured Roof 

The previous two case studies illustrate the reinforcement mechanism of PUR in highly fractured 
rock. The remaining two cases show the behavior of 

PUR when injected into unfractured rock. A test of 

PUR injection was undertaken at the NIOSH Bruceton Safety Research Coal Mine in southwestern Pa. 

The mine is located in the Pittsburgh coal bed with an 

immediate roof that consists of a sequence of rider 

coals and shale (Fig. 11). 

Fig. 8 Pre-injection 

monitoring holes in the 

intersection 

Fig. 9 Post-injection 

monitoring holes in the 

intersection 

Six 2.4 m (8 ft.) long 

injection holes were drilled in one intersection 

(Fig. 12) into which a total of 477 kg (1,050 lbs) 

(397 l (105 gal)) of polyurethane was injected. The 

holes were isolated with packers located at 0.9 m 

(3 ft.) up in the hole, and PUR was pumped into open 
hole from 0.9–2.4 m (3–8 ft.). Injection pressures 

went as high as 8.3 MPa (1,200 psi) and averaged 

about 3.5 MPa (500) psi. 

Two coreholes were drilled after injection to 

determine the location of the polyurethane 

(Fig. 11). PUR was found injected along bedding 

at 1.2 m (3.9 ft.) in corehole ACH-2 and at 1.3 m 

(4.2 ft.) and 1.6 m (5.2 ft.) in ACH-3. This is 

essentially the same horizon due to small variables 

in the roof line. The PUR had hydrofractured the 

weak bedding in the second rider coalbed and left a 

1.9 cm (0.75 in.) thick plug along bedding 

(Fig. 13). PUR was pumped into 1.5 m (5 ft.) of 

open hole at six locations around the intersection 

and the path of least resistance was the weak 

bedding found in the second rider coalbed 

(Fig. 11). 

Fig. 10 Glue-filled fractures after PUR injection into hole 2-1 

Fig. 11 Core holes drilled after PUR injection show glue 

injected at &1.2 m 

Fig. 12 Injection and monitoring holes at the Safety Research 

Coal Mine 

The results from this site indicate that, in unfractured ground, multiple horizons may not be 

hydrofractured and reinforced, but that only one 

reinforced zone can be expected. The support value 

of this zone containing a 1.9 cm (0.75 in.) layer of 

PUR is questionable. If multiple zones of reinforcement are desired, it will be necessary to isolate each 

such zone with a packer and pump the zone until 

failure and injection. This may be done from several 

injection holes packed at different heights or from the 

same hole with multiple packers. 
Fig. 13 Polyurethane injected into roof core Ach-2 at 1.2 m 

Bruceton Research Safety Coal Mine 

Fig. 14 PUR injected into solid roof over longwall panel 

migrated back to the recovery room through a cutter 

3.4 Western Pennsylvania Coal Mine— 

Unfractured Roof 

A western PA longwall mine was using a pre-driven 

entry for longwall recovery. The entry was heavily 

supported including 2.4 m (8 ft.) combination bolts, 

3.7 m (12 ft.) cable bolts, double channels every row 

of bolts, screen, and pumpable cribs. This heavy 

support is necessary to resist the front abutment load 

which will come on the room as the shearer 

approaches and cuts into the room. In addition, from 

the recovery room, the mine injected PUR over the 

panel and also over the opposite rib side of the entry. 

This was an attempt to reinforce the rock mass above 

the final panel cutout prior to the longwall pass. This 

rock mass would be subjected to front abutment 

loading when the shearer cut into the recovery room. 

Often, in weak ground, emergency PUR stabilization 

is needed in the final cut-through before longwall 

recovery. It was hoped that pre-grouting the roof 

would head off the need for an emergency PUR 

injection. 

Ten feet angled ‘‘forepole’’ holes were drilled on 

3.0 m (10 ft.) centers and PUR was injected in a 

zone from 0.6–3.0 m (2–10 ft.) into the roof rock at 

45� over the panel. High pressures were built up and 

it was extremely difficult to force PUR into the 

tight, unfractured rock. PUR migrated back towards 

the recovery room and was observed leaking into the 

room via a cutter developed on the panel side of the 

room during development (Fig. 14). No observation 

was possible over the panel, but it is clear that PUR 

could not migrate over the solid panel as originally 

planned, but hydrofractured a weak bedding plane and 

followed the path of least resistance back into the 

entry. Two adjacent recovery chutes off of the 

recovery room were also selected for a test of the 

injection of PUR into unfractured, but undermined, 

roof rock. Chutes C and D were injected with PUR and 

the results monitored via videoscope (Figs. 15, 16). 

Figure 17 shows the lithology and final location of the 

PUR in the immediate roof after the injection in room 

C. No open separations occurred in the immediate 

3.0 m (10 ft.) of roof rock prior to the injection. In 

room C one hole (PH-1) was drilled to 2.4 m (8 ft.), 

packed at 1.5 m (5 ft.), and injected with 4.0 l (15 gal) 

of PUR (Fig. 15). Then the hole was re-packed at 

0.6 m (2 ft.) and injected with another 4.0 l (15 gal) of 

PUR. A number of cable and combination bolt holes 

experienced leaks as the PUR migrated through the 

entry roof. Five monitor holes were videoscoped 

after the PUR injection (Fig. 17). The first PUR 

injection zone was isolated at 1.5–2.1 m (5–7 ft.). This 

was the contact between the sandstone and the 

underlying shale. This was done to see if the PUR 

could hydrofracture the coarse sandstone with coal 

spars. No PUR was observed in the sandstone in any of 

the monitoring holes (Fig. 17). It appears that the 

bedding in the sandstone was too strong to be 

hydrofractured and the PUR must have found some 

other conduit for relief. The other injection zone was 

isolated at 0.6 m (2 ft.) (PH-1). PUR was injected 

from 0.6–1.5 m (2–5 ft.). A thin 0.08–0.64 cm (0.03– 

0.25 in.) PUR wafer was observed at 42.7–57.9 cm 
(1.4–1.9 ft.) up in the roof in two of the video holes 

(C-3, 5). These shows were below the packer at 0.6 m 

(2 ft.). This indicates that PUR migrated below 

the packer and found weak bedding planes at 

42.7–57.9 cm (1.4 and 1.9 ft.). Similar to the Bruceton 

unfractured roof case, only one zone was hydrofractured and reinforced. 

Fig. 15 First recovery 

chute with PUR injection 

and monitoring holes 

Fig. 16 Second recovery 

chute with PUR injection 

and monitoring holes 

The adjacent chute (D) roof was also injected with 

PUR and videomonitored to locate the PUR (Fig. 16). 

Injection hole PH-1 was packed at 30 cm (1 ft.) and 

pumped with 62.7 l (16.6 gal) of PUR Injection hole 

PH-2 was packed at 90 cm (3 ft.) and pumped with 

62.7 l (16.6 gal) of PUR. Video monitor holes D-1 

thru 6 recorded the results (Fig. 18). The sandstone/ 
shale contact occurred at 1.1 m (3.5 ft.). All six 

monitor holes showed PUR layers ranging from 0.08– 

0.64 cm (0.03–0.25 in.) thick right near the contact of 

the sandstone and shale. Clearly the weakest bedding 

contact was this shale/sandstone contact. The PUR 

that was injected into the roof of both test rooms 

found the bedding horizon that was the weakest and 

wedged it open. In only one monitor hole (D-1) was 

there evidence of multiple injection zones over a 

0.45 m (1.5 ft.) zone. 

Fig. 17 Location of PUR after injection 

Fig. 18 Location of PUR 

after injection 

4 Discussion 

The design and performance of a roof stabilization 

using PUR injection depends greatly on the condition 

of the rock mass. Typically, highly fractured rock 

masses benefit the most from the chemical bonding 

and inherent strength of PUR. PUR injection is more 

suitable for reinforcing highly fractured rock masses 

where fractures propagate across bedding resulting in 

isolated key blocks. In the two fractured roof cases 

described above, large voids and rubbleized zones 

allowed easy access for PUR, permitting a webbing 

structure of PUR-supported key blocks to form a 

beam in the roof. If the beam is significant enough 

to support the overlying dead load of detached 

rock, then this detached zone does not have to be 

reinforced. 

The two case histories in unfractured ground 

indicate that significant reinforcement is unlikely. 

More likely PUR will hydrofracture the bedding and 

remain only on that one bedding plane. Monitoring 

data of the two test sites in unfractured ground 

indicate that injecting PUR ‘‘on the solid’’ will not 

reinforce the ground in any significant way because 

open fractures are not available. While the pressures 

realized (13.8 MPa (2,000 psi)) when injecting PUR 

are certainly enough to hydrofracture weak bedding 
planes, the resulting thin layers of PUR do not form a 

significant reinforcing web. 

The 1.9 cm (0.75 in.) thick layer of polyurethane 

injected into the Pittsburgh roof bedding at the 

Bruceton site cannot be considered a consistent and 

continuous layer, considering the variability of bedding strength. The most likely occurrence, seen at 

both test sites, is that the PUR will be injected on 

only one horizon. Similarly, in the second site at the 

longwall recovery chute, only one horizon was 

hydrofractured. This single layer cannot be expected 

to provide substantial resistance to thick, overlying, 

detached roof blocks. These results are consistent 

with results obtained by using the hydrofracture 

method to measure in situ stress (Enever et al. 1990). 

In this procedure, only one bedding fracture is 

obtained in weak rock, indicating the path of least 

resistance for the fluid. Once the fracture has been 

created and the PUR is being injected, large volumes 

may be pumped into the single bedding plane 

fracture. This additional PUR will provide little 

additional reinforcement as it is usually a thin wafer 

confined to only one horizon. If substantial reinforcement is desired in unfractured rock, that goal 

must be accomplished by specific design. Multiple 

injection zones must be isolated with packers and the 

reinforcement will be obtained by the sum of the 

strength of several layers of PUR injected into weak 

bedding planes. 

In reinforcing intersections, current designs utilize 

holes drilled in the corner of an entry angled up over 

the ribline. The idea is to create a ‘‘grout curtain’’ 

which could act to contain the polyurethane which 

then sets up and forms a barrier over the entry 

shoulder. The experience in the unfractured rock 

injection in the recovery room shows that this ‘‘overthe-rib grout curtain’’ is unlikely to be successful 

because PUR will migrate towards the undermined 

entry and not over the solid rib. Additionally, the 

polyurethane is thought to resist shearing of roof 

layers along the ribline. From the experience in 

unfractured rock, a single layer of PUR injected along 

bedding is unlikely to provide much resistance to 

shearing. 

In extremely fractured rock, difficulty was encountered in getting the PUR into the zone targeted for 

beam reinforcement. This problem may be addressed 

when designing the ‘‘grout curtain’’ to prevent 

unwanted PUR loss into voids. This is a barrier 

established by injecting PUR around the perimeter of 

the intersection, and allowing it to set up before any 

subsequent round of PUR injection. Injection holes, 

pumping a set volume of PUR, may be drilled in 

concentric circles around an intersection, working 

towards the center of the intersection. With a 30 s set 

time, the PUR will have enough time to form a 

barrier before the next injection hole is started. This 

method will help to avoid pumping large volumes 

into large void spaces. 

The study also demonstrated the value of using 

video monitoring of fractures prior to PUR injection. 

In the West Virginia case, the presence of large open 

voids, some as large as 28.2 cm (11 in.) wide, 

became the path of least resistance for PUR. Large 

volumes of PUR were being pumped into big voids 

resulting in wasted resin and little reinforcement. 

Video data showed that a reinforced beam from 0.6– 

1.8 m (2–6 ft.) could be created which would support 

the overlying broken rock. In many intersections 

video logs revealed that the roof was extremely 

broken up from 0–0.6 m (0–2 ft.) into the roof. 

Without this information attempts to inject PUR 

under pressure into this zone could result in hazards 

from dislodged roof blocks. 

5 Conclusions 

When using polyurethane injection to stabilize a rock 

mass, an understanding of the fracture condition of 

the rock mass in advance can help in the design of the 

injection. By knowing the location and extent of the 

fracture permeability, design parameters; including 

volume and expansion properties of chemical grout, 

target horizon, and density/geometry of injection 

holes, the injection of polyurethane can be optimized. 

Pre and post video-monitoring can provide valuable 

fracture information for both designing the injection 

parameters and evaluating the success of the PUR 

stabilization. 

In designing a polyurethane stabilization, the goal 

should not necessarily be to fill all the fractures in the 

roof. Complete void-filling may not be achievable, 

except with expanding foam. In extremely fractured 

roof, an alternative is a beam-building design where 

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. The concept is similar 
to beam building with roof bolts. If the rock beam can 

be maintained intact it can transfer the load of its own 

weight to the pillars and act to support the weight of a 

limited amount of fractured rock above. Mechanically, the polyurethane forms a beam out of rock that 

has been separated along bedding or is broken into 

key blocks. It is the size and strength of this beam 

which determines the stability of the roof. 

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