The Long-Term Performance of Surface Support Liners 

for Ground Control in A Underground Limestone Mine 

Deno M. Pappas, Timothy M. Barton, Eric S. Weiss 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, P.O. Box 18070, 


Cochrans Mill Road, Pittsburgh, PA, USA, 15236
 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, P.O. Box 18070, 


Cochrans Mill Road, Pittsburgh, PA, USA, 15236
 

National Institute for Occupational Safety and Health, Lake Lynn Laboratory, P.O. Box 528, Fairchance, PA, 


USA, 15436
 

Abstract 

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational 

Safety and Health (NIOSH) has been evaluating developments with surface support liners which includes 

shotcrete and thin spray-on liners (also referred to as sealants, coatings and membranes) from a ground 

control perspective. The proper selection and use of a surface support material can significantly enhance 

ground control which may result in a decrease in worker related injuries due to ground falls. The purpose of 

this paper is to evaluate the preliminary findings of a long-term underground study of various types of surface 

support liners, and to examine installation practices that are critical for an effective liner material 

application. The underground study utilizes NIOSH’s Lake Lynn Laboratory Experimental Mine to evaluate 

the long-term performance of several types of shotcrete and thin spray-on liner materials. Liner material 

performance to date was evaluated on a regular basis over a three year period. Although the study is still 

ongoing, critical mining practices were identified that may seriously affect the bond of the liner material to 

the mine roof and rib. 

1 INTRODUCTION 

Most underground mines in the United States are composed of deteriorated rock of some

degree. When this rock is exposed to a cyclical weathering process, it often accelerates the 

deterioration of the mine roof or rib. This presents a stability problem and potential safety 

hazard for underground workers. In underground coal mines for instance, it is estimated

that over 400 injuries occur annually due to skin failure of the roof or rib in permanently 

supported areas of the mine (Molinda, et al., 2002). Critical and long-life areas in the mine, 

such as the shafts, shops, beltways, haulage ways, and overcasts, can often experience

weathering over time which may require remedial support to arrest this process. These 

surface support methods include additional bolting, wire meshing, or sealing the weathered 

rock with materials such as shotcrete. Shotcrete, or pneumatically sprayed concrete, offers

the benefits of low cost, high compressive strength, high tolerance for humid conditions, 

and a good long-term performance record. 

The newest generation of surface support liners are materials known as thin spray-on

liner materials (also referred to sealants, coatings and membranes). Liners can generically 

be classified as multi-component polymeric material and offer these advantages: fast 

application rates, fast setting times, reduced material handling underground, good tensile

strength, excellent elongation properties, and good bond strength. Although shotcrete 

offers excellent rigid support, most thin spray-on liner materials can provide active support 

where the elastic liner supplies resistance as it deforms due to failure of the rock. Thin 
spray-on liner materials are considered an emerging technology and are gaining favor in an 

increasing number of mines. 

More information on recent developments with the use of shotcrete and thin spray-on 

liner materials is discussed further in other publications (Espley, et al., 2001; Rispin, et al., 

1999; Espley, et al., 2001; Baiden, 1999; Spearing and Champa, 2001; Pappas, et al., 2002). 

This paper presents preliminary results of a three year comparison study of several types 

of surface support liners in an underground environment and reviews critical practices in 

effectively using liner materials. 

Figure 1 - Shale bands dividing limestone formation at LLEM. 

2 LAKE LYNN EXPERIMENTAL MINE STUDY 

2.1 Background

The three-year study of surface support liner materials was conducted at the NIOSH, 

Pittsburgh Research Laboratory’s Lake Lynn Laboratory (Triebsch and Sapko, 1990). 

Developed in 1979, the Lake Lynn Laboratory is one of the world’s foremost facilities for

conducting mining safety and health research. Located at the site of a former underground 

limestone mine and quarry, Lake Lynn is a multipurpose research lab designed to provide a 

modern, full-scale, realistic environment for performing research in mining safety and

health technology. The Lake Lynn Experimental Mine (LLEM) is located in the Greenbrier 

limestone formation which has three to five shale bands horizontally bedded in the 

limestone throughout the mine (figure 1). Large seasonal variations of the temperature and

humidity have caused the shale bands to weather and degrade. Frequent explosion tests 

generate overpressures as high as 550 kPa which accelerates the degradation of the mine 

ribs. Vibration effects often dislodge loose rib and roof rock onto the mine floor and

thereby exposing fresh rock to weathering. This continual process presents long-term 

structural degradation issues for the rib lines and pillars as well as a potential safety 

concern. Additionally, each year significant worker hours are committed to facility

maintenance for the removal of the spalled and loose material. To alleviate this problem, a 

long-term maintenance solution of sealing the mine ribs and roof with shotcrete was 

initiated. This provided an ideal opportunity to initiate a research study to evaluate various 
types of surface support liner materials in conditions typically encountered in underground

mine environments. 

The goal of this study is to evaluate the long-term performance of various types of 

shotcrete and thin spray-on liner materials at critical mine locations (predominately on mine 

ribs). The LLEM offers the advantages of an underground mine environment without the 

constraints of a production mine. The uniform presence of shale bands throughout the mine 

provides a good opportunity to evaluate the liner adherence to weathered rock. The mine 

setting also allows the liner materials to be equally exposed to cyclical changes in 

temperature, humidity, vibration, stress conditions, and other factors. These variable 

conditions may help identify those materials that are most effective under different

underground environments. 

2.2 Study Methodology

The study at the LLEM is very basic. Before a site is sealed, the rib is mechanically 

scaled, washed with water at pressures ranging from 690 to 1000 kPa, and photographed. 

The photographs are taken to record any structural defects that may affect the liner

performance after application. A second series of photographs are taken to record the 

appearance of sealed rib after application. This series of photographs provide a baseline of 

the rib’s appearance for future comparisons.

Most of the study sites are clustered in areas of the mine that are more prone to 

weathering, vibration effects, and high stress conditions. The weather effects are most 

severe near the entrance portal (e.g., study site M in figure 2), the vibration effects are more 

intense near where the explosion tests are initiated (e.g., study sites D, Q, R, S, and X in 

figure 2), and the high stress conditions are more pronounced where the mine pillars have 

spalled in the old mine workings near the hydrostatic chamber area of the mine (e.g. study 

sites A and C in figure 2). Shotcrete parameters examined include bedded shale effects and 

debonded shotcrete. These factors were quantified and evaluated with a Schmidt Hammer 

tool to estimate the compressive strength of the applied shotcrete. A few locations of 

shotcrete were cored, examined and photographed to identify problems within the shotcrete 

that may have weakened or debonded the shotcrete from the mine rib. 

Figure 2 - Diagram of LLEM (not to scale). 
 

2.3 Schmidt Rebound Hammer 

According to Beaupre (1994), the rebounding Schmidt Hammer was found to be an 

effective tool to determine shotcrete hardness and correlates well with compressive 

strength. Each shotcrete site has 4-6 test areas upon which each area has 10 Schmidt

Hammer tests conducted on a bimonthly or monthly basis. Each test surface area is first 

smoothed with a grinder to provide a uniform test surface. The Schmidt Hammer contains 

a spring-loaded mass that is released against a plunger when the hammer is pressed against 

the shotcrete. Once the plunger impacts the shotcrete and the mass recoils, the rebound 

value of the mass is measured by a gauge on the hammer’s side. Testing is done according 

to ASTM C 805 (2001). This procedure requires 10 readings at each site, and if any of the

readings exceed the average by 6 units, those readings are removed and a new average is 

determined. The average is then multiplied by an average calibration correction factor 

based on a series of 10 tests using a calibration anvil before and after each mine visit.

Based on a series of calibration curves provided by the Schmidt Hammer manufacturer, the 

estimated compressive strength can be interpolated based on the calibrated average value 

and the angle of the hammer with regards to the horizontal.

Since the Schmidt Hammer provides only an estimate of the shotcrete compressive 

strength, a verification test was conducted to compare the Schmidt Hammer results with 

laboratory unconfined compressive strength test results of shotcrete samples. The coarse 

shotcrete was poured into a test mold, cured for 8 months, and cut into two inch cubes. A 

surface grinding machine was then used to finish the cube surfaces at the required 

tolerances. Unconfined compressive strength tests were run on these shotcrete cube

samples. The Schmidt Hammer results produced an estimated compressive strength of 

48 MPa while the laboratory unconfined compressive strength of the shotcrete was 50 MPa 

(both shotcrete series were cured more than 8 months). From these results, it appears that 

the Schmidt Hammer produces a fairly reasonable estimate of the actual compressive 

strength. 

The non shotcrete materials cannot be tested with the Schmidt Hammer due to the nonrigid behavior of those materials. 

2.4 Other Monitoring Techniques

Weather data loggers are stationed in the vicinity of the study sites to track the 

temperature and humidity every hour. Periodic explosion tests are conducted in A, C, and 

D drifts and at the hydrostatic chamber. These test results are documented including the 

date and the corresponding maximum overpressure generated during the test. Defining the 

stress conditions is a more difficult task 

2.5 Surface Support Liners Studied 

Two types of commonly used shotcrete materials and three types of promising thin 

spray-on liner materials were selected and evaluated in this long-term study. The materials 

will be referred to by their generic names to avoid any appearance of NIOSH endorsement 

of individual products. 

� Coarse shotcrete - contains cement, sand, “BB-sized” gravel aggregate, micro silica

fume and 1.3 cm acid resistant glass fibers. The shotcrete is applied using dry process 

equipment primarily composed of a large rotary barrel gun. As with all dry process 

shotcrete machines, the shotcrete is conveyed dry through the hose and water is added to 

the dry shotcrete at the nozzle. The nozzleman controls the amount of water to obtain 

the proper consistency which can vary depending on the particular application. The 

coarse shotcrete was installed at sites C, D, M, R, S, and X as shown in figure 2. 
 

� Fine shotcrete - contains cement, well graded silica aggregate, micro silica fume and 1.3 

cm acid resistant glass fibers. The same dry process shotcrete machine used to apply the 

coarse shotcrete was used to apply the fine shotcrete. The fine shotcrete was installed at 

site Q as shown in figure 2. 

� Latex/cement thin spray-on liner - contains two separate components, a liquid polymer 

latex and hydraulic cement powder. The two components are mixed in a single bin and 

an air compressor provides a nozzle pressure of 690 kPa to apply the liner. This thin 

spray-on liner material was installed at sites BP1 and A. 

� Methacrylate-based thin spray-on liner - contains two liquid, high viscosity, multicomponent parts. One part is a methacrylate monomer and the second is a peroxide 

initiator. The two parts are placed into separate bins and pumped to the nozzle in 

separate hoses. At the nozzle they are combined using air pressure of 413-690 kPa. The 

air acts as both the mixing and propelling medium for the thin spray-on liner as it is

applied to the substrate. Different experimental versions were tested due to the wide 

range of performance of the material based on the chemistry of the monomer chosen and 

the specific formulation 

� Polyurea thin spray-on liner - contains two separate liquid components, an isocyanate 

material and resin mixture. The two materials are mixed in separate barrels, heated, and 

pumped to the nozzle in separate hoses. At the nozzle they are combined under a

pressure of 17 MPa. Three different layers of polyurea are applied: first layer is a primer 

that provides a tacky surface for a strong initial bond to the rock, second layer forms a 

low strength (polyurea is heated to 35° C) stringy material that can span large cracks and 

fissures, and the third layer forms a high strength (polyurea is heated to 63° C) which 

provides durable and glossy final coating. The third coating contains graphite chips to 

extinguish flames. The polyurea was installed at sites F and K.

Table 1 shows a summary of some of the properties of the materials chosen for the 

study. Most of the liner properties were obtained from an extensive study by Archibald and 

DeGagne at Queen’s University (2001). After a decade of study, Archibald and DeGagne

(2001) found that thin spray-on liners offer significant ground support potential and, from a 

health and safety perspective, the majority of liners were found to be acceptable for 

underground use. 

Table 1. Summary of liner material properties. 

Support liner 

type 

Manufacturer’s 

compressive 

strength1 

(MPa) 

Estimated 

tensile 

strength1 

(MPa) 

Percent 

elongation 

(%) 

Set time 

(Minutes) Flammability Health issues 

Coarse Shotcrete 55 6.9 0 45-602 Nonflamable Dust 

Fine Shotcrete 56 6.5 0 45-602 Nonflamable Dust 

Latex/Cement - >1 (1 hr) 12-30 <3 Self Extinguishing Respiration 

Methacrylate - >2 (1 hr) >100 <3 Self Extinguishing Respiration 

Polyurea - 18.5 125 10 sec Self Extinguishing Respiration 

1

Strength in 28 days.


2

Initial set time.
 

Portions of this table are taken from laboratory studies conducted by Archibald (2001).
 

The two shotcrete materials evaluated at the LLEM have high compressive strengths but 

no ability to elongate. This makes the shotcrete behave as a rigid restraint whereas the 
more elastic spray-on liner materials deform and confine the pillar which increases the 

overall pillar strength. The Material Safety Data Sheets (MSDS) indicate that they are

either nonflammable or self extinguishing. Set (cure) times of the thin spray-on liner 

materials are much quicker than the shotcrete materials. Although the Mine Safety and 

Health Administration (MSHA) does not require certification of ground support liner

materials, MSHA has developed flammability guidelines for polyurethane foams. 

2.6 Preliminary Study Results

Most of the quantitative data compiled for this study is associated with the Schmidt 

Hammer results on the shotcrete which have been evaluated on a regular basis for the past 3 

years for the coarse shotcrete and 1-1/2 years for the fine shotcrete. The study observations

of the thin spray-on liner materials will be discussed in the later section. Table 2 lists some 

of the results for shotcretes and thin spray-on liner materials. 

Table 2 - Summary of liner material study information. 

Support liner 

type 

Sites Thickness 

(mm) 

Date 

installed 

Estimated 

cost (US$/m2

) 

Materi 

al loss2 

(%) 

In situ 

compressive 

strength1 (MPa) 

Coarse Shotcete R,X 

C,D,M,S,R,X 

C,M,R,X 

12.7 

25.4 

50.8 

11/99 

11/99-2/00 

11/99-2/00 

7.10 

13.34 

21.63 

14.9 

15.0 

15.0 

51.1 

41.4 

47.3 

Fine Shotcrete Q 

Q 

Q 

12.7 

25.4 

50.8 

9/01 

9/01 

9/01 

5.16 

7.96 

18.73 

17.6 

15.3 

14.9 

30.0 

38.7 

44.1 

Latex/Cement A 

BP1 

7.6 

3.8-7.6 

10/99 

10/99 

17.54 

10.55 

n/a n/a 

Methacrylate L,E 

E 

E 

2 

3-5 

3.6-5 

1/01-3/01 

9/01 

11/02 

24.5 

n/a 

34.75 

n/a n/a 

Polyurea F,K 5 4/02 43.00 n/a n/a 

1

Estimated (based on recent Schmidt Hammer results).
 

2

Loss due to rebound and overspray combined.
 

n/a = not available
 

2.7 Coarse Shotcrete 

The coarse shotcrete was first installed in November 1999 at sites S, M, X and R 

followed by sites C and D in February and June 2000, respectively. None of the coarse 

shotcrete sites have shown any signs of failure or deterioration, except Site M has shown a

few hairline cracks but no signs of failure. Over the years some of the Schmidt Hammer 

test sites have deteriorated from the repeated hammer blows and had to be re-ground to 

provide a smooth test surface. The hammer test results provide a good quantitative

indicator of the shotcrete’s performance. The most noticeable problem with shotcrete is the 

loss of material due to overspray and rebound during installation. During application, 15% 

of the mixed shotcrete falls on the mine floor (table 2). According to Browning (2002),

shotcrete (with silica fume) applied to the rib using the dry method will experience a 
material loss of 5%, while overhead application to the roof is about 8% loss. The higher 

loss of shotcrete at the LLEM may be due to irregular rib surfaces. Sometimes the 

horizontal ledges make it nearly impossible to apply the shotcrete perpendicular to the 

surface. Other contributing factors could be the age of the equipment and the infrequent 

replacement of the nozzle. 

2.8 Weather Effects 

Figure 3 shows a large increase in the coarse shotcrete average compressive strength 

over the three-year period. Examining the cumulative rate of strength increase from the 

averaged hammer test results for all the test sites, as shown in figure 4, indicates a 60% 

increase during the spring and summer of 2000. However by the fall of 2000, the rate of 

increase had lessened. Shotcrete is normally expected to reach its design strength in 28 

days, but may increase in strength another 5-10% after 56 days (Browning, 2002). The 

coarse shotcrete was installed between November 1999 and early February 2000, so the 

shotcrete should have reached at least its 56 days strength by the time the hammer tests 

were started. Consequently, most of the strength increase may be attributed to other 

factors, perhaps the high humidity levels present during the time in question. Typically, the 

humidity levels at the LLEM during the summer months are 100% which produces 

condensation on the rib and roof of the mine (figures 5-6). Possibly, the continual presence 

of moisture prolonged the curing process of the shotcrete and resulted in the large increase 

in shotcrete strength. This seems to indicate that spraying the shotcrete with water, in dry 

mines or dry winter months, may help cure the shotcrete and allow it to reach its full 

strength. 

50 

40 

30 

20

Average Strength (MPa)

Coarse Shotcrete 

10 

Fine Shotcrete 

0 

0 5 10 15 20 25 30 35 40 

Months After Installation 

Figure 3 - Schmidt Hammer averaged for coarse and fine shotcrete results - time versus 

average of compressive strength. 
Mar-00 

May-00

Jul-00 

Sep-00

Nov-00

Jan-01 

Mar-01

May-01

Jul-01 

Sep-01

Nov-01

Jan-02 

Mar-02

May-02

Jul-02 

Sep-02

Nov-02

Jan-03 

0% 

10% 

20% 

30% 

40% 

50% 

60% 

70% 

80% 

Cumulatative Percent Increase 

Date 

Figure 4 - Schmidt Hammer coarse shotcrete results - time versus cumulative percent 

increase in strength. 

Dec-99 

Jan-00 

Feb-00 

Mar-00 

Apr-00 

May-00 

Jun-00 

Jul-00 

Aug-00 

Sep-00 

Oct-00 

Nov-00 

Dec-00 

Jan-01 

Feb-01 

Mar-01 

Apr-01 

May-01 

Jun-01 

Jul-01 

Aug-01 

Sep-01 

Oct-01 

Nov-01 

0 

20 

40 

60 

80 

100 

Humidity (%) 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Temperature (C o ) 

Humidity (%) 

Temperature C 

Date 

Figure 5 - Weather station data at BP-1 site – time versus temperature and humidity 

versus time. 
Dec-01

Dec-01

Jan-02

Jan-02

Feb-02

Feb-02

Mar-02

Mar-02

Apr-02

Apr-02

May-02

May-02

May-02

Jun-02

Jun-02

Jul-02

Jul-02

Aug-02

Aug-02

Sep-02

Sep-02

Oct-02

Oct-02

Oct-02

Nov-02

Nov-02

Dec-02

Dec-02

Jan-03

Jan-03

Feb-03

Feb-03

Mar-03 

100 

80 

60 

40 

Humidity (%) 

Temperature (C) 

0 0 

35 

30 

25 

20 

15 

10 

Humidity (%) 

20 

Temperature (Co ) 

Date 

Figure 6 -

humidity. 

Weather station data near Pillar Q site – time versus temperature and 

5 

10/31/02 Sites S (227 kPa) 

5/16/2000 Sites X (214 kPa) 

12/20/2000 Sites X (213 kPai) 

3/13/2001 Sites X (241 kPa) 

8/20/2001

sion

Sites X (131 kPa) 

9/25/2001

xplo

Sites X (207 kPa) 

2/27/2001

Date of E

Sites D,R (103 kPa) 

4/4/2001 Sites D,R (117 kPa) 

03/12/02 Sites D,R (117 kPa) 

Before Explosion 

5/31/2000 Site C (538 kPa) 

After Explosion 

6/12/2000 Site C (669 kPa) 

9/7/2000 Site C (221 kPa) Note: In parenthese shows 

size of explosion in kPa 

0 10 20 30 40 50 60 70 80 90 100 

Strength (MPa) 

Figure 7 - Comparison of coarse shotcrete strength before and, after explosion tests ­

estimated compressive strength versus time. 

2.9 Explosion Effects

The effects of the frequent mine explosion tests conducted within the LLEM were 

evaluated by comparing the average hammer strength of the shotcrete before and after each 
explosion test. As can be seen in figure 7, there is no significant decrease in the shotcrete 

strength. In addition, examinations of the sites before and after each mine explosion test

did not reveal any effects on the physical integrity of the shotcrete. It was apparent after 

the explosion tests, that the shotcrete had maintained its integrity and stayed completely 

intact. No fallen roof or rib debris was detected in the areas that were shotcreted. A few 

areas near site D have scrapings in the shotcrete due to debris hitting the face from 

explosions tests in the D drift. 

2.10 Bedded Shale Effects 

Mar-00 

May-00

Jul-00 

Sep-00 

Nov-00 

Jan-01 

Mar-01 

May-01 

Jul-01 

Sep-01 

Nov-01 

Jan-02 

Mar-02 

May-02 

Jul-02 

Sep-02 

Nov-02 

Jan-03 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Averaged Strength (MPa) 

Limestone 

Bedded Shale 

Date 

Figure 8 - Comparison of coarse shotcrete strength in the limestone and bedded shale ­

time versus estimated compressive strength. 

Test sites were also established to determine if the shotcrete strength is different on the 

shale than on limestone surfaces. The preliminary short-term results, as shown in figure 8, 

do not reveal any significant difference in the shotcrete strength, which appears to indicate 

that the shotcrete adheres equally well on the bedded shale as it does with the limestone. If 

the shale binder did not provide a good bond or had detached from the shotcrete, the

hammer results would have been significantly lower. 

2.11 Bonding Effects 

Following the shotcrete cure period (greater than 56 days), the shotcrete was sounded 

with a geologist’s hammer to indicate isolated areas in the shotcrete that produced a hollow

(drummy) sound compared to the solid sound elsewhere. This hollow sound may be the 

result of an air cavity behind the shotcrete. To determine if cavities behind the shotcrete 

affects its strength, a representative sample of hollow and solid sounding areas were

evaluated. Figure 9 shows that the hollow cavity areas have 25% lower strength than the 

solid areas. This trend is fairly consistent through the three year study period. The periodic 
Mar-00 

May-00 

Jul-00 

Sep-00 

Nov-00 

Jan-01 

Mar-01 

May-01 

Jul-01 

Sep-01 

Nov-01 

Jan-02 

Mar-02 

May-02 

Jul-02 

Sep-02 

Nov-02 

Jan-03 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Averaged Strength (MPa)

Solid 

Hollow 

Figure 9 - Comparison of coarse shotcrete strength by bond effects -time versus 

estimated compressive strength. 

“valleys” in figure 9 of the hollow site curve is due to gradual degradation of the hollow 

shotcrete site after several series of hammer tests. The hammer test data usually rebounded

following regrinding of the degraded test site. A few areas were cored to visually examine 

the hollow sounding areas. Figure 10 shows a cored sample of a void in the coarse 

shotcrete from site M. Most likely, these hollow soundings are due to the formation of air

voids or pockets of shotcrete that were loosely cemented during application. Similar types 

of voids were found with the fine shotrete and will be discussed later. 

Figure 10.- Coarse shotcrete coring with large void. 
 

2.12 Fine Shotcrete 

Fine shotcrete was installed in September 2001 at site Q in A drift. As listed in table 2, a 

16% overspray/rebound rate was estimated with the fine shotcrete which was comparable to 

the coarse shotcrete rate. After 16 months of monitoring its strength using the Schmidt

Hammer the estimated compressive strength of the fine shotcrete had increased 45% 

compared to 72% for the coarse shotcrete for the same time period (figure 4 and 11). 

Figure 3 shows that the initial 56-day strength of the fine shotcrete was higher than coarse 

shotcrete (29 MPa versus 22 MPa) however, the maximum strengths appear to be similar 

(42 MPa vs. 43 MPa). According to the manufacturer’s specification sheet, the maximum 

strength is 55 MPa for the coarse shotcrete and 57 MPa for the fine shotcrete. In addition, 

the greatest increase in strength for both the coarse and fine shotcrete seems to coincide 

with the late spring and early summer period, 60% and 35% increase respectively (figure 4 

and 11). Perhaps the full curing of the shotcrete (after 56 days) is not so much a function of

time, but rather a function of greatest time exposure to moisture. To date, the fine shotcrete 

has not shown any signs of deterioration. 

Oct-01 

Dec-01 

Feb-02 

Apr-02 

Jun-02 

Aug-02 

Oct-02 

Dec-02 

0% 

5% 

10% 

15% 

20% 

25% 

30% 

35% 

40% 

45% 

50% 

Cumulative Percent Increase 

Date 

Figure 11- Schmidt Hammer fine shotcrete results - time versus cumulative percent

increase in strength. 

2.13 Sample Cores

Cores were taken of the in situ fine shotcrete from an accessible site. Several of these 

cores were found to have vertical bedded voids, inadequately mixed shotcrete, and 

shotcrete areas that were poorly bonded to the rib rock. Figures 12-13 show core samples 

with large vertical voids parallel to the rib face. Figure 13 shows a void that originates 

from shotcrete that is poorly mixed and cemented together. Perhaps the loosely cemented 
aggregate and glass fibers allows the shotcrete to bridge air gaps resulting in narrow 

vertical voids as each layer is applied. 

Figure 12 - Fine shotcrete coring with large void. 

Figure 13 - Fine shotcrete coring with voids originating from poorly mixed material. 
Also detected in some of the fine shotcrete core samples was a weak bond between the 

shotcrete and the rib rock. Figure 14 shows a fine shotcrete core cross section that is poorly

mixed. Portions of the shotcrete near the rock interface actually crumble loose under the 

pressure of a finger rubbed across the sample. As a result, the poorly mixed fine shotcrete 

has a weak bond to the adjacent rib rock. These conditions seem to occur more often with 

the fine shotcrete than with the coarse shotcrete. Perhaps the larger aggregate size with the 

coarse shotcrete allowed the shotcrete to mix better than the smaller aggregate size with the 

fine shotcrete. Another cause for the loosely cemented shotcrete could be a water 

deficiency in the shotcrete mix. 

Figure 14 – Fine shotcrete coring with improper water content and loosely cemented 

material at shotcrete/rock interface. 

Another bond issue is the presence of dust or air bubbles at or near the shotcrete and rib 

rock interface. Figures 15-16 show the remnants of air bubbles on this interface which 

reduces the bond of the shotcrete to the rock. This problem may be related to an improper 

surface preparation of the test site which will be discussed later in more detail. 

It should be noted that these sample cores may not fairly represent all test site areas.

They may just be an anomaly in the fine shotcrete areas that were sampled and not 

necessarily a general representation of the overall behavior of the fine shotcrete. However, 

these observations provide some insight about the cause of hollow soundings and the lower 

shotcrete strength findings found with both types of shotcrete. 

Future components of this study may include evaluating the performance of a fine 

shotcrete with polypropylene fibers compared to the glass fibers contained in this current

type of shotcrete. 
Figure 15 - Fine shotcrete coring with poor bonding at the rock/shotcrete interface due 

to air pockets and bubbles. 

Figure 16 - Fine shotcrete coring with small air bubbles formed at the shotcrete/rock 

interface. 
 

2.14 Latex/Cement Thin Spray-on Liner 

The latex/cement thin spray-on liner has been the longest standing material in the study.

It was applied in October 1999 and has performed favorably. A double thickness of 8 mm 

of spray-on liner material was applied on pillar A in the hydrostatic test chamber area 

(figure 2) of the mine. In this area, ventilation control structures are frequently evaluated

through explosion testing to study failure mechanisms. These explosion tests generate 

considerable blast vibrations in the area. Over the past 40 years, the pillars in this area have 

spalled considerably due to weathering as described earlier and more recently due to

damage caused by debris propelled at the pillar as the result of the explosion tests. Added 

stress conditions due to the greater overburden at this site may also be influencing the pillar 

spalling in this area. To date it appears that despite the severe conditions, the latex/cement

liner has effectively sealed the pillar from additional weathering mechanisms. No rock 

spalling has occurred since application of the liner. Although the liner material does not 

contribute significant structural support (table 1-2) it may provide some additional

confinement to the pillar which, in turn, may increase its load carrying capacity.

 The latex/cement thin spray-on liner was also applied on a 76 m section of the large 

barrier pillar (BP1) near the entrance portal (figure 2). This area of the mine is subject to

frequent freeze thaw cycles and large changes in temperature and humidity (figure 5). An 8 

mm thick double coating of liner material was applied around a corner area and on the 

unmeshed areas of the adjacent roof. The liner at this site has also performed well, with the

exception of four small patches (approximately 3.7 m2

 total area) that have detached from 

the rib. These patches represent less than 1% of the total area covered. Upon close 

observation, it appeared the cause of failure was more related to the failure of the rock than

failure of the liner material. In three of the cases, the area was located at the bedded shale 

material near the rib/roof corner. Most likely, the fractured shale was partially degraded 

when initially sealed. Close examination of the detached liner material indicates that the 

dominant failure mechanism was the rock failing and not the liner material. However, the 

weight of the loose rib rock eventually exceeded the strength of the liner (which in two of 

the cases was a single thickness of 4 mm), allowing a section to tip and fall from the rib. If 

this site had been properly scaled and the damaged rock removed, in all probability the 

latex/cement liner would not have detached. 

2.15 Methacrylate-Based Thin Spray-on Liner 

Several different experimental versions of the methacrylate-based thin spray-on liner 

were installed on small trial areas at sites L and E (figure 2) in 2001 and 2002. The liner 

trials that did not perform adequately were removed and the methacrylate formulation was 

adjusted for the next trial. Several adjustments were made to the formulation to decrease 

the viscosity and improve its tolerance to moisture. Pump and application problems were

corrected to provide a more uniform thickness of the spray-on liner to the mine ribs. 

The reformulated methacrylate thin spray-on liner was applied to three different types 

of surface conditions at the site L. The first was a test surface that had been previously

coated with an earlier methacrylate-based thin spray-on liner that did not mix and set 

properly. A portion of the un-reacted liner material on this surface was removed from the 

rib however, a considerable amount of material remained. Application of the reformulated

methacrylate material over the un-reacted material did not provide a good base for 

adherence. After a few days, most of the new application debonded from the mine wall. At 

the second L site, a surface that had been scaled and then saturated with water without 

drying the surface was evaluated The reformulated methacrylate material debonded from 

the saturated mine wall, due to the excess water on the rock surface. At the third site, the 
surface that had been scaled and then blown dry was evaluated. In this case, with the 

surface moist but with no excess water on the rock surface, the reformulated methacrylate 

material performed well. In a few minor areas where the reformulated methacrylate liner 

was applied in a very thin coating, some debonding occurred. After four months of 

application, the reformulated methacrylate-based thin spray-on liner continues to

sufficiently seal the mine rib and prevent the rib from deterioration. 

2.16 Polyurea Thin Spray-on Liner

A polyurea liner material was installed in April 2002 at sites K and F. The rib face was 

scaled and washed before application of the three layers of polyurea. Both test sites had 

small trial areas where the face was not washed and areas that were saturated with water. 

After nearly a year of application, it does not appear that these trial areas were adversely 

affected by the dirt or moisture. 

At the site F, three trial areas were established without applying the initial primer layer.

In the first area, only the second and third layer of polyurea was applied. In the second 

area, only the second layer of polyurea was applied. In the third area, only the third layer 

of polyurea was applied to the unprimed rib rock. After nearly a year of application, only

the second layer (web layer) showed some areas of debonding. The other trial areas appear 

to be unaffected with the absence of the primer layer. The remaining areas of the site F 

appear to be securely sealed. 

2.17 MDI Exposure Evaluation 

The polyurea “A” component contains diphenylmethane diisocyanate (MDI), which can

be a powerful irritant to the respiratory system. According to the Coal Mine Health 

Inspections Procedures Manual (MSHA, 2003), workers exposed to isocyanate foams can 

become sensitized, making them subject to severe asthma attacks if they are re-exposed,

even at low concentrations. To address these concerns, the Mine Safety Health 

Administration (MSHA), Physical and Toxic Agents Division were contacted to monitor 

the MDI concentration levels during the application of the polyurea liner material. MSHA 

used five midget impingers with 20 mL of solution (NIOSH Method #5522) with sampling 

pumps to sample the air (both vapor and droplets) at both application sites. Figure 17, 

shows the placement of the impingers relative to the pillar.

According to the MSHA report (Arnold, et al., 2002), the sample analysis showed a 

nearly unquantifiable concentration of airborne MDI present during and following the 

application of the polyurea (figure 17). All of the measured concentrations were well 

below the MSHA ceiling value of 0.2 mL/m3 or 20 ppb. Sixteen of the nineteen samples 

taken showed no measurable MDI. The first set of readings (readings noted on figure 17 

adjacent to the test sites) monitored the application of the polyurea primer and web layers

and the second set of readings monitored the application of the final coating. The samples 

that showed some presence of MDI were mostly located downwind or near the rib during 

the application of the first two layers at site F. These very low but measurable levels are

associated with the polyurea primer and web layers that were applied to site F. Perhaps, the 

primer layer may be responsible for these readings since this material is atomized longer in 

the air than the web layer. At site K, no measurable MDI were detected for any of the

polyurea layers applied. 

Although, the air sampling results showed that all MDI concentrations were well below 

the MSHA ceiling value in the work area and downwind, workers applying the polyurea 

should follow the respirator requirements detailed in the MSDS (1999). A positive pressure 

supplied respirator with full face-piece or hood or self contained apparatus is required for 
MDI concentrations less than 250 ppb. A NIOSH type C respirator, self contained or 

supplied positive, demand or continuous flow is required for MDI concentrations greater 

than 250 ppb. 

Figure 17 - Diagram of sampling locations and MDI concentration levels at the F and K 

sites during the polyurea installation (not to scale). 

3 CRITICAL PRACTICES 

The shotcrete and thin spray-on liner materials are only as strong as their weakest link. 

The physical properties of these materials are severely diminished if they are not properly

installed. The direct experience gained while working with these materials as well, as 

numerous discussions with manufacturers (Spearing and Champa, 2001; Browning, 2002; 

Degville, 2002) and mining personnel (Espley, et al., 2001; Espley, et al., 2001;), indicate 

that the following critical practices should be carefully considered before using any type of 

shotcrete or thin spray-on liner material. 

3.1 Surface Preparation 

Probably the most critical and most overlooked component that may hinder the 

successful application of a shotcrete or thin spray-on liner material is the surface 

preparation prior to application. For the LLEM study, the rib and roof were scaled the day 

before application and washed (689-1,034 kPa water pressure) the day of application. The 

scaling removes most of the loose material and provides a clean surface for the shotcrete to 

adhere. Washing the site with water removes most of the smaller size broken rock and dust 
 

that coats the rib and horizontal ledges. A recent study by Kuchta (2002) found an increase 

in the shotcrete adhesion strength by a factor of four on a concrete wall cleaned with water

at 21 MPa as compared to surface cleaned at 689 kPa. If diesel equipment is operated in 

the area, a high water pressure cleaning may be capable of removing the slick diesel soot or 

any oil that may have accumulated on the rib and roof. The Colorado School of Mines 

included LLEM as a test site for their field study of high pressure water treatment of 

shotcrete. 

3.2 Mine Conditions 

In the selection of an effective liner material, several questions need to be considered to 

fit the needs of the mine. How long must the liner material perform; i.e., what is the

anticipated life of the site? Generally, if it is a temporary site, a lower strength liner 

material can be used with a minimum thickness. If it is a critical site that may play a longterm role in the life of the mine, for example the main beltway, a high strength material

with a greater thickness than is typically used may be appropriate. 

If shotcrete is being installed during the drier winter months, it appears to be beneficial 

to spray water on the shotcrete on a consistent basis after application. According to the

preliminary results from the LLEM study, shotcrete applied during the winter mo nths 

increased in strength by 35-60% (figure 4 and 11) during the following summer possibly 

due to the high humidity that may enhance the curing process and result in the added

shotcrete strength. Humidity levels at the LLEM during the summer months are typically 

over 100% which produce condensation on the rib and roof of the mine. This seems to 

indicate that spraying the shotcrete with water may help to better cure the shotcrete to reach

its maximum strength. 

Not one of the thin spray-on liner or shotcrete materials will perform adequately when 

applied under flowing water conditions. Flowing water will prevent the material from

properly curing and adhering to the rock. The source of the water should be identified, 

diverted, drained, and the substrate relatively dry before any liner material applications. 

3.3 Logistical Issues 

Critical logistical issues that may hinder the effectiveness of the support liner materials 

include the proper conditions for storing and applying the material. Most of the liner 

materials specify a storage temperature range of 4-43° C, however, some polyurethane 

materials require a minimum storage temperature of 20° C. In addition, all of the materials 

specify dry storage conditions, especially the shotcrete. Maintaining dry conditions may be 

difficult to control especially if the materials are stored underground. In mines of limited 

mining extent, such as LLEM, there are large fluctuations in humidity as shown in the 

humidity charts (figures 5-6), which can result in condensation. The liner materials should 

be stored in heated warehouses until required underground and then tarps and shrink wrap 

should be used to protect the material from condensation. 

Another critical factor is tracking the shelf life of the materials. While shotcretes have 

an indefinite shelf life (if kept in a warm and dry location), most thin spray-on liner 

materials have a shelf life of just 3 to 6 months. Materials that exceed the shelf life may 

have a reduced strength.

Finally, the project conditions may affect the curing and bonding of the liner to the rock 

surface. Some specifications for shotcrete require the surface temperature of the rock be 

greater than 9° C. In addition, the shotcrete should not be exposed to temperatures below 

9° C for a minimum of 96 hours after application. During some of the summer months, 

areas in close proximity to intake air in many mines experience nearly 100% humidity 
which may seriously affect the curing and bonding of some thin spray-on liner materials. 

The temperature and humidity levels of the application site need to be considered when 

scheduling the thin spray-on liner and shotcrete work. 

3.4 Human Factors 

Also critical to the performance of the shotcrete is the experience and technique of the 

nozzleman installing the material. The basic technique for the LLEM application involved 

keeping the nozzle perpendicular and maintaining the nozzle approximately 1 m distant

from the rock surface. According to Browning (2002), the art of shotcreting is building up 

the proper thickness of shotcrete on the rock at the optimum rate so it adheres but not so 

fast so that it starts to rebound. Applying shotcrete to the roof is even more difficult due to

overspray and rebound of the shotcrete. Rispin and Brooks (2001) mentions that there are 

industry wide training programs for nozzlemen, as well as other associated positions, that 

offer a blend of practical and theoretical knowledge and the importance of proper

placement of the shotcrete. Usually application of thin spray-on liner material is not quite 

as challenging as shotcrete due to shotcrete’s denser consistency and sensitive application 

process. Critical issues for thin spray-on liner material application are maintaining a proper 

thickness so that the liner will cure properly and obtaining the proper pump mixing ratios of 

the two components. 

Some other factors that may indirectly benefit the nozzleman’s performance include:

•	 providing the optimal airflow to minimize dust; 

•	 using additional lighting to enhance visual attention to ensure proper material adherence 

and thickness; 

•	 using the required personal protective equipment including a fitted respirator (according 

to MSDS specifications); 

•	 providing hearing protection;

•	 providing disposable water/chemical resistance coveralls, gloves, and boots; and 

•	 following the recommended maintenance and replacement schedules for the nozzle and 

other parts of the liner material application equipment. 

4 CONCLUSIONS 

An ongoing study of various types of thin spray-on liner and shotcrete materials at

NIOSH’s LLEM is providing a unique forum for evaluating the long-term behavior of 

liners in an underground environment. Preliminary results indicate that the latex/cement 

performed favorably, with less than 1% of the material surface area debonded from the 

mine rib three years after application. After several trials, a reformulated experimental 

methacrylate provides a competent seal to prevent weathering on pillars that are relatively 

dry. The three layer polyurea system provides a durable bond to the pillar and appears to 

work well even under saturated conditions. In addition, the MDI monitoring of polyurea 

found that these levels are very low or undetectable. 

The coarse and fine shotcrete have performed favorably over the study period. Although

no observations of shotcrete failure occurred to date during the study, several areas 

indicated an incomplete bond with the rock according to the results obtained from the 

Schmidt Hammer tests. The hollow sounding areas in the coarse shotcrete resulted in 25% 

lower compressive strengths. Sample corings indicate these hollow sounding areas may be 

related to voids, poor mixing, water deficiency in the mix, and/or air bubbles in the 

shotcrete. Hammer test results indicate that the shotcrete strength does not seem to be 

affected by the bedded shale or vibrations induced by the experimental explosion tests. 

However, a delay in the full curing process appeared to have occurred for both types of 
 

shotcrete. Full curing was not achieved until the humid summer months. Then a 35-60% 

increase in the shotcrete strength was achieved as documented by the Schmidt Hammer test

results. Humidity levels at the LLEM during the summer months are typically over 100% 

which produces condensation on the rib and roof of the mine which may promote the 

shotcrete reaction and additional strength.

Other critical practices that need to be addressed specifically include proper surface 

preparation of the mine strata prior to application. A clean surface that is devoid of loose 

rocks, dust, or diesel soot ensures a good bond with the thin spray-on liner or shotcrete 

materials. Additional critical practices are related to the mine conditions, logistical issues, 

and human factors. 

With the emerging technology of thin spray-on liner materials, more U.S. mines are

starting to use these materials to rehabilitate critical underground locations afflicted with 

weathering and unraveling ground conditions. The use of these liner materials is expected 

to reduce the occurrence of groundfalls which will provide an additional tool to enhance 

worker safety and extend the longevity of the underground workings. In the not too distant 

future, both shotcrete and thin spray-on liner materials may be used in the underground 

metal mi nes as the primary ground control component during the mining development 

cycle. Perhaps segments of this technology may be applied to underground coal mines and 

provide a partial solution to reducing the high number of roof and rib fall injuries during 

face development. 

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