DEVELOPMENTS IN SEALANT SUPPORT SYSTEMS FOR GROUND CONTROL

Deno M. Pappas, Civil Engineer

Timothy M. Barton, Mining Engineer

Eric S. Weiss, Supervisory Mining Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

During the past few years, the Pittsburgh Research Laboratory of

the National Institute for Occupational Safety and Health (NIOSH)

has been evaluating developments in sealant support systems from a

ground control perspective. The proper selection and use of a sealant

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 threefold: to briefly review the current stateof-the-art technology in shotcrete and membrane developments, to

evaluate the preliminary findings of a long-term underground study

of various types of sealant materials, and to examine installation

practices that are critical for an effective sealant material. The

underground study utilizes NIOSH’s Lake Lynn Laboratory

Experimental Mine to evaluate the long-term performance of several

types of shotcrete and membrane materials. Sealant performance to

date have been evaluated on a regular basis over a two year period.

Although the study is still ongoing, critical mining practices were

identified that may seriously effect the bond of the sealant materials

to the mine roof and rib; most notably, the importance of scaling and

thorough cleaning of the rib prior to application. Also, results from

an extensive series of Schmidt Hammer tests found that the shotcrete

increased in strength by 70% during the humid summer months.

INTRODUCTION

Most underground mines in the United States are composed of

deteriorated rock of some degree. When this rock is exposed to the

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 (1). 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 support methods include additional bolting, wire

meshing, or sealing the weathered rock with sealant material such as

shotcrete. Shotcrete, or pneumatically sprayed concrete, offers the

benefits of being low cost, exhibits high compressive strength,

performs well under wet conditions, and has a good long-term

performance record.

The newest generation of sealants are materials known as

membrane or sprayable liner materials. Membranes 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 membrane materials can provide active

support where the elastic membrane supplies resistance as it deforms

due to failure of the rock. Membrane materials are considered an

emerging technology and are gaining favor in an increasing number

of mines.

The paper presents a brief summary of shotcrete and membrane

developments, preliminary results of a 2-year comparison study of

several types of sealants in a underground environment, and reviews

critical practices in effectively using sealant materials.

SHOTCRETE DEVELOPMENTS

Shotcrete was first developed in the early 1900's and the

technology was eventually extended to underground mines.

Currently, it is estimated that 7 million cubic feet of shotcrete is used

annually in North American mining operations (2). Thicker

applications of shotcrete (greater than 3 in) is typically used to

provide structural support to a critical underground location whereas,

thinner applications (less than 3 in) are used to seal and protect the

rock from weathering and unraveling. Currently, shotcrete is used at

the face and as part of the mining development cycle at several

underground metal mines in Canada (3). Often shotcrete is used in

conjunction with bolts to eliminate wire mesh and to permit wider

bolt spacing (4). Use of synthetic and steel fiber reinforced shotcrete

can increase the tensile strength of the shotcrete and provide the

necessary ground support for a wide range of ground conditions.

Current trends indicate that increasing numbers of metal mines are

relying on shotcrete as an integral ground support component during

the mining development cycle (5).

According to Rispin et al. (2, 5), a mine has recently

experimented with automated shotcrete equipment, using a computer

controlled, laser driven system for mapping headings to be sprayed

and the computer regulating part or all of the shotcrete spraying

process. Some of the benefits of this state-of-the-art technology may

provide a reduction in material rebound, a more precise control over
Figure 1. Shale bands dividing the limestone formation at LLEM.

application thickness, and the capability to map the applied concrete.

Inco’s North Mine (nickel and copper) in Copper Cliffs, Ontario (5)

is integrating this technology with tele-operation. Tele-operation is

an innovative process where the face development is operated from a

remote surface location. Shotcrete is used as a ground control

component in their plan to tele-operate the mining development cycle.

This remote mining process may eventually remove underground

workers from dangerous ground fall conditions that occur during face

advancement.

MEMBRANE DEVELOPMENTS

According to the Rand Institute’s publication New Workforce at

Work in Mining (4), spray-on coatings have been identified as one of

the top three emerging ground support technologies for underground

mining. Membrane materials have been found to be an effective tool

to seal the rib from weathering, to prevent the rib from unraveling,

and will gradually deform with any rock movements. According to

Archibald and DeGagne (6), membranes have demonstrated

exceptionally high installation rates, quick set times (within minutes

to hours of application), easy handling procedures, and improved rock

reinforcement than traditional support methods.

Although membranes do not provide as much structural support

as shotcrete, in situations where large rock deformations occur, the

more flexible membrane materials may provide better support over the

full range of deformations (7). Large deformation may not be a

problem so long as the confined rock deforms the membrane in a

uniform manner.

Most application of membranes in the U.S. are used for repairing

critical underground locations that have severely weathered; however,

in Canada some progress has been made in using membrane materials

during face development and other applications. Significant

membrane research studies have been conducted at Inco’s Research

Mine in Copper Cliff, Ontario where these ground support roles for

membranes were identified by Espley et al. (8):

• membranes may be used during mine development, instead of

wire meshing, as a secondary means of support along with

bolting

• membrane materials may be sprayed over mesh and bolts which

prohibits rusting and also provides significant improvements in

the support capabilities of the overall system

• membrane materials may be sprayed over shotcrete to form a

tough composite super liner that is able to withstand severe

ground bursting conditions

• high-strength membrane materials may be used for stand alone

support with delayed and wide-spaced bolting, this is a longterm goal. 

Based on Inco’s experience at their research copper mine (8), a

membrane material for replacing bolts and screens (stand alone

support) should have a tensile strength greater than 700 psi, as well

as a quick set time. Development of high tensile strength membrane

material may not be too far off in the future. According to Lacerda

and Rispin (9), most membrane materials under development may

generate tensile strengths over 2,200 psi in 15 minutes and should

become a reality within the next few years. This would be quite a

breakthrough considering most membrane materials (excluding

polyurethanes) have tensile strengths of 100-700 psi. In addition, the

application of high strength membranes have the potential of being

automated and integrated with tele-remote mining techniques from a

surface control room, so that exposure of mine personnel to hazardous

ground conditions during the mining cycle may be significantly

reduced. 

LAKE LYNN EXPERIMENTAL MINE STUDY

Background

The sealant study was conducted at the NIOSH, Pittsburgh

Research Laboratory’s Lake Lynn Laboratory (10). 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 generating over pressures as high

as 100 psi have accelerated the degradation process by dislodging the

loose rib and roof rock onto the mine floor and thereby exposing fresh

rock to weathering. This continual degradation process presents longterm 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 longterm 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 sealant 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 membrane 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

sealant adherence to weathered rock. The mine setting also allows the

sealant 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.
Figure 2. Diagram of LLEM.

Study Methodology

The comparison evaluation conducted at the LLEM is a very basic

observational study. 

Before a study site is sealed, the rib is mechanically scaled,

washed with water at pressures ranging from 100 to 150 psi, and

photographed. The photographs are taken in sequential order to

record any structural defects that may affect the sealant’s performance

after application. A second sequential 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.

In an attempt to examine various mine factors more prevalent in

certain areas of the mine, 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 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 being examined

include: shotcrete thicknesses, 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.

Schmidt Rebound Hammer. According to Beaupre (11), 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 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. A wide range of solid and non-solid areas are

selected to equitably sample the estimated compressive strength of the

shotcrete. 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 (12),

which requires 10 readings at each site. If any of the readings exceed

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

is determined. This 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 shotcrete

was poured into a test mold , cured for 8 months, cut into two inch

cubes, a surface grinding machine was used to finish the cube surfaces

at the required tolerances and unconfined compressive strength tests

were run on the shotcrete cube samples. The Schmidt Hammer results

produced an estimated compressive strength of 6,900 psi while the

laboratory unconfined compressive strength of the shotcrete was

7,275 psi (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 non-rigid behavior of those materials.

Other Monitoring Techniques. Weather data loggers are stationed in

the vicinity of the study sites to track the temperature and humidity

every hour. When explosion tests are conducted in C and D drifts and

at the hydrostatic chamber, they are documented including the date

and the corresponding maximum over pressure generated. Defining

the stress conditions is a more difficult task. Future plans include the

use of the boundary element model LAMODEL to estimate the stress

conditions in the hydrostatic chamber area of the mine.

Sealant Materials Studied

Two types of commonly used shotcrete materials and three types

of promising membrane materials were selected to be evaluated in this

long-term study. The materials will be referred to by their generic

names to avoid any appearance of NIOSH endorsement of one

product over another.

• Coarse shotcrete - contains cement, sand, “BB-sized” gravel

aggregate, micro silica fume and 0.5-in 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. Due to the LLEM entry size (6.5 ft high

by 18-20 ft wide), the compressed air valve on the pneumatic

gun is set at a lower adjustment level to minimize rebound.

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 0.5-in 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 membrane - 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 100 psi to apply the membrane.

This membrane was installed at sites BP1 and A.

• Methacrylate membrane - contains two separate liquid

components, a methacrylate resin and an initiator (epoxy resin).

The two components are mixed in separate bins and pumped to

the nozzle in separate hoses. At the nozzle they are combined

under a pressure of 60-100 psi. This membrane was installed

at sites E and L. The liquid methacrylate component contains

graphite chips to extinguish flames.

Table 1 shows a summary of some of the properties of the

materials chosen for the study. Most of the membrane properties were

obtained from an extensive study by Archibald at Queen’s University

(13). After a decade of study, Archibald and DeGagne (6) found that

membranes offer significant ground support potential and, from a

health and safety perspective, the majority of membranes were found

t be acceptable for underground use.

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

membrane materials deform and confine the pillar which probably

increases the overall pillar strength. The Material Safety Data Sheets

(MSDS) of the materials indicate that they are either nonflammable

or self extinguishing. Set (cure) times for the membrane materials are

much shorter than for the shotcrete materials. Although the Mine

Safety and Health Administration (MSHA) does not require

certification of ground support membrane materials, MSHA has

developed flammability guidelines for polyurethane foams.

Preliminary Study Results

Most of the quantitative data compiled for this study is associated

with the Schmidt Hammer results on the coarse shotcrete which have

been evaluated on a regular basis for the past 2 years. The study

observations of the other membrane and shotcrete materials will be

discussed in the later section. Table 2 lists some of the study results

of the shotcretes and membrane materials. 

Table 1. Summary of sealant material properties.

Sealant type

Manufacturer’s

compressive

strength1

 (psi)

Estimated tensile

strength1

 (psi)

Percent elongation

(%)

Set time

(Minutes) Flammability Health issues

Coarse Shotcrete 8,000 1,000 0 45-602 Nonflamable Dust

Fine Shotcrete 8,200 950 0 45-602 Nonflamable Dust

Latex/Cement - >145 12-30 <3 Self Extinguishing Respiration

Methacrylate - >290 >100 <3 Self Extinguishing Respiration

1

Strength in 28 days.

2

Initial set time.

Portions of this table are taken from laboratory studies conducted by Archibald (13).

Tabl111e 2 - Summary of sealant material study information.

Sealant type Sites Thickness

(inch) Date installed Estimated

cost1

 ($/ft2)

Material

loss2 (%)

In situ compressive

strength3 (psi)

Coarse Shotcrete R,X

C,D,M,S,R,X

C,M,R,X

0.5

1.0

2.0

11/99

11/99-6/00

11/99-6/00

0.66

1.24

2.01

14.9

15.0

15.0

6,425

5,911

6,618

Fine Shotcrete Q

Q

Q

0.5

1.0

2.0

9/01

9/01

9/01

0.48

0.74

1.74

17.6

15.3

14.9

2,938

3,972

4,526

Latex/Cement A

BP1

0.3

0.15-0.30

10/99

10/99

1.63

0.98 n/a n/a

Methacrylate L,E

E

0.08

0.12-0.20

1/01-3/01

9/01

2.28

n/a n/a n/a

1

Material cost.

2

Loss due to rebound and overspray combined.

3

 Estimated (based on Schmidt Hammer results) and most recent data (January 2002).

n/a = not available
0

1000

2000

3000

4000

5000

6000

7000

Mar-00May-00

Jul-00

Sep-00

Nov-00Jan-01

Mar-01May-01

Jul-01

Sep-01

Nov-01Jan-02

Date

Average Strength (psi)

Figure 3. Schmidt Hammer averaged coarse shotcrete results -

time versus estimated compressive strength.

0%

10%

20%

30%

40%

50%

60%

70%

80%

Mar-00May-00Jul-00Sep-00Nov-00Jan-01Mar-01May-01Jul-01Sep-01Nov-01Jan-02

Date

Cumulatative Percent Increase

Figure 4. Schmidt Hammer coarse shotcrete results - time

versus cumulative percent increase in strength.

0

20

40

60

80

100

Dec-99

Jan-00

Feb-00Apr-00May-00

Jul-00Aug-00

Sep-00

Nov-00

Jan-01

Mar-01Apr-01May-01

Jul-01

Aug-01

Date

Temp (F) / Humidity (%)

Temp (f)

Humidity (%)

Figure 6. Weather station data at BP-1 site -temperature and

humidity versus time.

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

May-00Jul-00Sep-00Nov-00Jan-01Mar-01May-01Jul-01Sep-01Nov-01Jan-02

Date

Cumulative Percent Increase

Site C

Site M

Site R

Site X

Site S Site D

Figure 5. Schmidt Hammer coarse shotcrete results - time versus

cumulative percent increase in strength by test site.

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. 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 were required 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 fell on to the mine floor (table 2).

According to Browning (14), 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 attributed to the age of the equipment and the

infrequent replacement of the nozzle.

Figure 3 shows a 70% increase in the coarse shotcrete average

compressive strength over the two year period. Technically

shotcrete reaches its design strength in 28 days but may increase in

strength another 5-10% after 56 days (14). 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.

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.

Weather Effects. There appears to be a considerable lag time in

reaching the maximum strength of the in situ coarse shotcrete.

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 higher rate of strength increase during the

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

of increase had lessened. Further analyses of the data by test sites

(figure 5) shows that study sites C, M, and R had the greatest

cumulative increase in strength, especially during the spring and

summer of 2000. Weather data from these sites (figures 6-8),

shows that during the late spring and early summer the humidity

levels rapidly increase and reach a 100% saturation level by mid

summer. Perhaps the added moisture at these sites may have

assisted in fully curing the shotcrete on the mine ribs. Site M is

located near the entrance portal where there are frequent changes in

temperature and humidity which mirror the exterior daily weather

cycles. However, site C is located 1,000 ft from the portal where

the temperature and humidity cycles are significantly less than

Site M, although humidity is still high. Site R is located in the D

drift near the ventilation fan. Frequently the fan is operating during

the night, in the blowing mode, to ventilate the mine after fire or

explosion experiments. During this mode, the D drift experiences

frequent temperature and humidity cycles due to exterior climate

changes. 
0

20

40

60

80

100

Dec-99Jan-00Feb-00Apr-00May-00Jul-00Aug-00Sep-00

Nov-00

Dec-00Jan-01Mar-01Apr-01Jun-01Jul-01Aug-01

Date

Temp (F) / Humidity (%)

Temp (f)

Humidity (%)

Figure 7. Weather station data near Pillar C site - temperature

and humidity versus time.

0 2000 4000 6000 8000 10000

3/29/2000

5/16/2000

12/20/2000

3/13/2001

8/14/2001

8/20/2001

9/4/2001

9/25/2001

2/27/2001

3/1/2001

3/20/2001

3/28/2001

4/4/2001

5/31/2000

6/12/2000

9/14/2000

9/7/2000

Date of Explosion

 Averaged Strength (psi)

Before Explosion

After Explosion

Sites X,S (20 psi)

Sites X,S (31 psi)

Sites X,S (31 psi)

Sites X,S (35 psi)

Sites X,S (24 psi)

Sites X,S (19 psi)

Sites X,S (29 psi)

Sites X,S (30 psi)

Sites D,R (15 psi)

Sites D,R (16 psi)

Sites D,R (28 psi)

Sites D,R (16 psi)

Sites D,R (17 psi)

Site C (78 psi)

Site C (97 psi)

Site C (32 psi)

Site C (32 psi)

Note: In parenthese shows size of 

explosion in psi.

Figure 9. Comparison of coarse shotcrete strength before and after explosion tests - estimated compressive strength versus time.

0

20

40

60

80

100

Dec-99Jan-00Feb-00Mar-00Apr-00May-00Jun-00Jul-00Aug-00Sep-00Oct-00

Date

Temp (F) / Humidity (%)

Temp (f)

Humidity (%)

Figure 8. Weather station data near R site - temperature and

humidity versus time.

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 test.

As can be seen in figure 9, 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. Increases in the shotcrete strength

after the mine tests is probably due to the long-term curing 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.

Bedded Shale Effects. The limestone strata within the LLEM is

divided by several bedded shale layers and these shale layers were

significantly susceptible to the original weathering problem. To

determine if there is a change in shotcrete strengths between the

shale and limestone sites, a representative sample of hammer sites

that had shotcrete applied were located in the shale and limestone

areas. The preliminary short-term results, as shown in figure 10, do

not reveal any significant difference in the shotcrete strength, which

appears to indicate that the shotcrete adheres equally well to the

bedded shale as it does to 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. 

Thickness Effects. Although it is difficult to maintain a consistent

thickness with shotcrete, three thickness variations were evaluated.

Protruding nails were used as a guide by the nozzleman to measure

the desired shotcrete thickness during the application process. Since
0

1000

2000

3000

4000

5000

6000

7000

8000

Mar-00May-00Jul-00Sep-00Nov-00Jan-01

Mar-01May-01Jul-01Sep-01Nov-01Jan-02

Date

Averaged Strength (psi)

1/2 inch

1 inch

2 inch

Figure 11. Comparison of coarse shotcrete strength by

thickness - time versus estimated compressive strength.

0

1000

2000

3000

4000

5000

6000

7000

8000

Mar-00May-00

Jul-00

Sep-00

Nov-00Jan-01

Mar-01May-01

Jul-01

Sep-01

Nov-01Jan-02

Date

Averaged Strength (psi)

Limestone

Bedded Shale

Figure 10. Comparison of coarse shotcrete strength in the

limestone and bedded shale - time versus estimated

compressive strength.

0

1000

2000

3000

4000

5000

6000

7000

8000

Mar-00May-00

Jul-00

Sep-00Nov-00Jan-01

Mar-01May-01

Jul-01

Sep-01Nov-01Jan-02

Date

Averaged Strength (psi)

Hollow

Solid

Figure 12. Comparison of coarse shotcrete strength by bond

effects - time versus estimated compressive strength.

one inch is a common thickness for non-structural shotcrete, half the

common thickness (1⁄2 in) and twice the common thickness (2 in)

were selected along with the 1 in thickness for this study. A series

of hammer tests were conducted at each of the sites with the three

different shotcrete thicknesses to evaluate the strength of the

shotcrete as a function of thickness. The data shown in figure 11

indicates that there are some differences in the shotcrete

compressive strength based on its thickness, however, these

differences are random and not consistent. Technically the thickness

of the shotcrete should not produce different strengths. However,

the differences may have been related to test variability and effects

of the hammer blows penetrating the thin shotcrete layers into the

rock. 

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 sound

compared to the solid sound elsewhere. This hollow sound may be

the result of an air cavity behind the shotcrete, probably due to the

shotcrete not properly bonding to the underlying rock. To determine

if cavities behind the shotcrete effects its strength, a representative

sample of hollow and solid sounding areas were evaluated. Figure

12 shows that the hollow cavity areas have 25% lower strength than

the solid areas. This trend is fairly consistent through the two year

study period.

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 five months of monitoring its strength

using the Schmidt Hammer results, the compressive strengths of the

fine shotcrete had increased 19% compared to 52% for the coarse

shotcrete for the same time period. It is conceivable that seasonal

factors affected the rate of curing, as the coarse shotcrete was first

cured in the winter/spring and the fine shotcrete initial curing was

in the fall/winter. In the fall and winter, the humidity levels are

decreasing and in spring the humidity levels are increasing. To date,

fine shotcrete has not shown any signs of deterioration. Due to its

recent application minimal data is available for analysis. Future

components of this study may include evaluating the performance of

a fine shotcrete with polyproylene fibers compared to the glass fibers

contained in this current type of shotcrete.

L

atex/Cement Membrane

The latex/cement membrane has been the longest standing

material in the study. It was applied in October 1999 and has

performed favorably. A double thickness of 0.31 in (8 mm) sealant

was applied on pillar A in the hydrostatic test chamber area

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

such as stoppings, seals, and bulkheads 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 have also spalled considerably due

to weathering as described earlier and more recently due to damage

caused by block and other debris propelled at the pillar as the result

of the seal explosion evaluations. 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 the latex/cement

membrane has effectively sealed the pillar from additional

weathering mechanisms. No rock spalling has occurred since

application of the membrane. Although the membrane does not

contribute significant structural support (tables 1-2) it may provide

some additional confinement to the pillar which, in turn, may

increase its load carrying capacity.

The latex/cement membrane was also applied on a 250 ft 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 6). A double

coating of sealant 0.31 in (8 mm) thick was applied around a corner

area and on the unmeshed areas of the adjacent roof. The membrane

at this site has also performed well, with the exception of three small

patches (approximately 36 ft2

 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 membrane. In

two of the cases, the area was located at the bedded shale material

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

degraded when initially sealed. Close examination of the detached

membrane indicates that the dominate failure mechanism was the

rock failing and not the membrane. However, the weight of the

loose rib eventually exceeded the strength of the membrane (which

in both cases was a single thickness of 0.15 in (4 mm)), allowing a

section to detach and fall from the rib. Probably, if this site was

properly scaled and the damaged rock was removed, the

latex/cement membrane would not have detached.

Methacrylate Membrane

The methacrylate membrane was installed at sites L and E

(figure 2) in January, March and September 2001. Pump problems

produced incorrect mixing ratios and viscosity which resulted in

inadequate application thickness and material composition.

Possibly, operator inexperience may have also contributed to the

improper application. The inadequate thickness and the improper

mixing resulted in not generating sufficient exotherm to cure the

membrane. Consequently, the material did not set properly or bond

to the limestone, and after several months it began to split and peel

off the rib and roof. However, the sealant appeared to bond

adequately in the areas where both the pump was providing the

correct mixing ratios and the operator applied the sealant at the

required coating thickness. Unfortunately, during this particular

application at the LLEM the combination of correct mixing ratios

and proper sealant thickness was only achieved over a very limited

area. 

The previous applications were removed in September 2001 and

a reformulated methacrylate was applied with a thickness of about

0.15 in (4 mm) on Pillar E using a different type of pump.

Observations over several months appear to indicate that the

reformulated sealant is bonding better than the previous material.

However, during this period, about 5% of the sealed area has

separated from the rib. Particular bonding problems have been

observed along the horizontal rock ledges. Although the pillars

were washed (100-150 psi water pressure) prior to sealing, perhaps

moisture collected on these ledges resulting in a poor bond.

Discussions are ongoing with the manufacturer to resolve these

application issues, as well as to evaluate other innovative membrane

materials that were recently developed. 

CRITICAL PRACTICES

The membrane and shotcrete materials are only as strong as their

weakest link. The physical properties of these materials are severely

diminished if they are not properly installed. After direct experience

with working with these materials as well as numerous discussions

with manufacturers (14, 15, 16) and mining personnel (3, 8), the

following critical practices should be carefully considered before

using any type of shotcrete or membrane sealant material.

Surface Preparation

Probably the most critical and most overlooked component that

may hinder the successful application of a shotcrete or membrane

material is the surface preparation prior to application. For the

LLEM study, the rib and roof were scaled the day before application

and washed (100-150 psi 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 (17) found an

increase in the shotcrete adhesion strength by a factor of four on a

concrete wall cleaned with water at 3,000 psi as compared to surface

cleaned at 100 psi. 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.

In the upcoming year, the Colorado School of Mines will include

LLEM in their field study of high pressure water treatment of

shotcrete.

Mine Conditions

In the selection of an effective sealant material, several questions

need to be considered to fit the needs of the mine. How long must

the sealant material perform; i.e., what is the anticipated life of the

site? Generally, if it is a temporary site, a lower strength membrane

material can be used with a minimum thickness. If it is a critical site

that may play a long-term 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

months increased in strength by 70% during the following summer

possibly due to the high humidity that may prolong the curing

process and result in the added shotcrete strength. Humidity levels

at the LLEM during the summer months are typically over 100%

which produces condensation on the rib and roof of the mine. This

seems to indicate that spraying the shotcrete with water may help

cure the shotcrete to reach its full strength.

None of the membrane 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 dried before any membrane or shotcrete applications.

Logistical Issues

Critical logistical issues that may hinder the effectiveness of the

sealant material include the proper conditions for storing and

applying the material. Most of the membrane materials specify a

storage temperature range of 40-110o F, but some polyurethane

materials require a minimum storage temperature of 68o F. In

addition, all of the materials specify dry storage conditions,

especially the shotcrete. Maintaining dry conditions may be

difficult, especially if the materials are stored underground. In mines

of limited mining extent, there may be large fluctuations in humidity

(as shown in the humidity charts, figures 6-8), which can result in

condensation. The material 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.

Most membrane materials have a shelf life of 3 to 6 months, while

shotcretes have an indefinite shelf life if kept in a warm and dry

location. Materials that exceed the shelf life may have a reduced

strength. 
Finally, the project conditions may affect its curing and bonding

to the rock surface. Some specifications for shotcrete require the

surface temperature of the rock be greater than 48o

 F. In addition,

the shotcrete should not be exposed to temperatures below 48o

 F 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 membrane materials. The temperature

and humidity levels of the application site need to be considered

when scheduling the membrane and shotcrete work.

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 3 ft

distance from the rock surface. According to Browning (14), the art

of shotcreting is building up the proper thickness of shotcrete on the

rock fast enough that it adheres but not too 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 (2) mentions that

there are industry wide training programs for nozzleman, 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 membrane material is not quite

as challenging as shotcrete, due to shotcrete’s denser consistency

and sensitive application process. Critical issues for membrane

application are maintaining a proper thickness so that the membrane

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 sealant

equipment.

CONCLUSIONS

An ongoing study of various types of membrane and shotcrete

materials at NIOSH’s LLEM is providing a unique forum for

evaluating the long-term behavior of the sealants 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 two years after application. The coarse

shotcrete also has performed favorably over the two year 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 resulted in 25% lower

compressive strengths. 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 the

shotcrete applied during the drier winter months; full curing was not

achieved until the humid summer months. During the summer

months, a 70% 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 helps ensure a good bond with the membrane or

shotcrete. Additional critical practices are related to the mine

conditions, logistical issues, and human factors.

With the emerging technology of mine membrane 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 sealant 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 pillars. Several Canadian metal mines are using

shotcrete at the face during face development. They are also starting

to experiment with membranes during face development as a

substitute for wire meshing used for secondary support along with

bolting. In the not too distant future, both shotcrete and membrane

materials may be used in the underground metal mines as a primary

ground control component in the remote mining development cycle.

Perhaps segments of this technology may be applied to underground

coal mines and provide a partial solution to reducing roof and rib fall

injuries during face development.

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Exploration, Inc. (SME), Annual Meeting and Exhibit, Phoenix,

AZ, February 25-27, 2002.

2. Rispin, M. and Brooks, J. Shotcrete in North American

Underground Mines: Yesterday, Today and Tomorrow. CIM

Bulletin, Vol. 94, No. 1052, 2001, pp. 76-79.

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