1 INRODUCTION 

In mining, stoppings are used to direct and separate 

ventilation air courses and are designed primarily to 

withstand air pressure differentials generated by the 

mine fan that exert transverse loading against the 

high-pressure side or face of the stopping. These 

pressures can range from as low as 0.25 psi in the 

working sections of the mine to over 1 psi near the 

area of a bleeder fan. Air blasts from roof falls, gas 

ignitions, and other potential sources can generate 

localized areas of significantly higher pressure that 

can destroy stoppings. As a result, it is critical that 

the ultimate transverse loading capability of 

stoppings be given consideration in their design and 

evaluation. 

There are no full-scale tests required for stoppings 

to determine their load capacity. The current Code of 

Federal Regulations (CFR) requirement is to test 

4x8-ft sections of freestanding walls in accordance 

with ASTM E-72 specifications (30 CFR, Part 

75.333, July 2005). This test inadequately determines 

the transverse load capacity of actual in-mine 

stopping constructions when the mine the roof and 

floor, and ribs of un-mined areas restrain the 

stoppings. This restraint creates significantly greater 

transverse loading capability by taking advantage of 

the compressive forces that are generated as the wall 

arches 

between the mine roof and floor. As a result, the true 

transverse load capacities of mine ventilation 

stoppings are not known using the current evaluation 

testing methodology. 

Recently, a new generation of low strength, 

lightweight blocks has been developed for mine 

ventilation stopping constructions. While stoppings 

utilizing these blocks have all passed the current CFR 

criteria, it is believed that their true transverse load 

capacity varies considerably. This is because the 

material strength of the block types varies by as 

much of an order of magnitude, and the material 

strength of the different blocks correlates to the 

arching capability of the restrained wall in the mine 

during transverse loading. Without such knowledge, 

the design of mine ventilation systems using these 

lighter-weight, but lower-capacity alternative 

constructions can be misleading, potentially exposing 

the mine to inadequate ventilation control under 

some circumstances. 

The objective of this paper is to describe a new 

protocol to determine the transverse load capacity of 

block stopping constructions. Using the unique 

biaxial loading capabilities of the National Institute 

for Occupational Safety and Health’s (NIOSH) Mine 

ABSTRACT: The Code of Federal Regulations (CFR) requires that the transverse load capacity of stoppings be 

equal to or greater than traditionally accepted in mine controls, which for block stoppings is generally accepted as 

39 psf as referenced in the preamble to the CFR standard. This measure is based on physical testing of a 

freestanding wall in accordance with ASTM E 72 specifications, where our past research has shown that the 

dominant parameter is the tensile strength of the sealant. A new protocol based on rigid arch loading of the 

structure is proposed to determine the true transverse load capacity of block stoppings. Arching is achieved by the 

restraint of the stopping against the mine roof and floor, whereby compressive forces are developed within the 

wall. A laboratory procedure using the NIOSH Mine Roof Simulator (MRS) to simulate rigid arching of block 

stoppings was developed and results verified through full-scale in-mine tests. The rigid arch tests have 

demonstrated transverse load capacities more than an order of magnitude higher than the ASTM E 72 evaluation 

method from which the 39-psf requirement is derived. More importantly, the rigid arch tests have shown that the 

load capability is dependent on the physical properties of the block and geometric properties of the wall and not 

the sealant. 

Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily represent 

the views of the National Institute for Occupational Safety and Health. 

Development of new protocols to evaluate the transverse loading of 

mine ventilation stoppings 

T. M. Barczak and T. J. Batchler 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA 
Roof Simulator (MRS), rigid-arch conditions for 

stoppings are simulated in the laboratory. Verification 

of the procedure has been done through full-scale 

testing of stoppings in a pressure chamber in the 

NIOSH Experimental Coal Mine and in the 

underground Longwall Gallery from stopping failure 

data gathered during full scale explosion testing at the 

NIOSH Lake Lynn facility. Ultimately, this should 

lead to more appropriate design criteria and allow 

regulatory statutes to be developed that will ensure a 

more accurate evaluation of stopping transverse 

loading capability. 

2 CURRENT CFR CRITERIA FOR STOPPINGS 

Part 75.333 Ventilation Controls of the CFR requires 

that stoppings be tested in accordance with ASTM E 

72 specifications. 

2.1 ASTM E 72 Test Specifications 

The procedure requires testing of a nominal 4x8-ft 

section of wall (ASTM E-72, 1981). The wall is to be 

constructed in the manner it will be used in the mine, 

including the application of sealant when specified. 

The test apparatus is shown in the diagram illustrated 

in figure 1. The wall is placed on a steel channel which 

rests on a cylindrical roller to prevent restrained end 

conditions. The axis of the roller is parallel to the face 

of the wall, allowing rotation to occur without 

restraint, as the wall is flexed from the application of a 

transverse load. Two reaction rollers and contact plates 

positioned at the top and bottom of the wall allow the 

wall to flex under the application of a transverse load 

from the opposite face. Again, rollers are utilized to 

prevent longitudinal restraint as the wall flexes. 

Transverse loading is applied across the width of the 

wall through a steel contact plate at quarter-height 

points of the wall. Rollers in the form of a steel pipe 

are again used to transfer the load from a central Ibeam through the contact plates, again to prevent any 

rotational restraint from occurring. As the load is 

applied, it is required that the load be recorded as a 

function of the displacement at the mid-span of the 

wall height. The maximum load normalized to the 

square foot area of the wall is then defined as the 

transverse load capacity for the wall. It is also required 

that three separate walls be tested. The average 

transverse load capacity from these three tests must 

exceed 39 psf to comply with the CFR statute. 

2.2 Inadequacies of Current CFR Specifications 

Examination of the mechanics of the wall response to 

transverse loading reveals the inadequacies of the CFR 

test procedure. First, it is seen that great care is taken 

to ensure that there is no longitudinal restraint 

provided to the wall as the load is applied. Essentially, 

the wall is considered freestanding and unrestrained 

from vertical movement as it flexes or bends from the 

application of the transverse load. The objective of the 

test is to evaluate the flexural strength of the wall. Any 

structure that is subject to bending produces tensile 

stresses on one side of the structure and compressive 

stresses on the opposite side of the structure. Typically, 

the tensile strength of the material, being weaker than 

the compressive strength, controls the capability of the 

structure to withstand loads that produce bending. 

Concrete has relatively low tensile strength, but a drystacked block stopping has no effective tensile strength 

because the joints are not bonded. Theoretically, the 

transverse load capacity of a freestanding, dry-stacked 

stopping would be provided only from the weight of 

the block, which acts to provide a superimposed 

vertical load on the structure. Even the heaviest typical 

blocks, which weigh about 55 lbs, would not provide 

enough axial loading to meet the 39-psf criteria. 

The tensile strength for stoppings constructed in the 

mine is actually provided by the application of sealant 

to the face of the wall. First, this is obviously not the 

primary function of the sealant. In order for the sealant 

to be effective for controlling the transverse loading, it 

must be applied to the anticipated low-pressure side of 

the stopping or the face opposite the side where the 

transverse load will be applied. If the ventilation 

pressure could be reversed either intentionally or 

unintentionally, then the sealant should be applied to 

both sides of the stopping under these criteria. Since 

several sealants are available, each with different 

material properties, then the stopping should only be 

certified with the specific sealant used in the 

evaluation test. Furthermore, for a given sealant, the 

thickness of the sealant contributes significantly to the 

effective tensile strength and resulting transverse load 

capacity of the stopping. How thick the sealant is 

applied in the test program compared to the thickness 

normally applied to such stoppings in the mine is 

another issue of concern. The test program should 

exclude abnormally thick sealant applications, since inFigure 1. Diagram of test apparatus for conducting E-72 testing of stopping walls. 

SUPPORT ROLLER 

SPONGE AND PLATE 

RUBBER 

LOADING ROLLER AND PLATE 

ROLLER 

REACTION 

FRAME 

TRANSVERSE LOAD 
mine constructions are not likely to apply the sealant 

any thicker than is necessary to prevent air leakage. 

3 RIGID ARCH LOADING MECHANISM 

In the mine, stopping walls are not freestanding 

structures as evaluated by the ASTM E 72 test 

referenced in the current CFR criteria. Stoppings, as 

constructed in the mine, bridge the distance between 

the mine floor and the mine roof and are typically 

wedged in place at the roof interface to provide a tight 

fit during installation. They also span the full entry 

width, butted against and typically trenched into the 

un-mined pillars on both sides. Hence, if the mine 

stoppings are restrained by the mine roof and floor and 

pillars, this restraint allows for a completely different 

loading mechanism to occur, namely arching. 

3.1 Description of Arching 

Arching is the mechanism that occurs when the 

curvature of the stopping, specifically the extension of 

the tension face of the stopping, as it bends under the 

application of transverse loading is prevented by the 

rigid contacts of the mine roof and floor. This arching 

of the wall produces a thrust force that acts at the mine 

roof and floor interface, and produces compressive 

forces within the wall that can dramatically increase 

the transverse load capacity of the wall compared to a 

freestanding condition. In the unloaded or minimally 

transverse loading condition, the ends of the wall are in 

full contact with the mine roof and floor and the 

individual horizontal joints between the courses of 

block are in full contact with each other. As the 

transverse loading increases, the wall will begin to flex 

or bend. Associated with the bending will be the 

opening of the block joints along the mid height span 

of the wall (location of the maximum positive 

moment), and the opening of the interfaces between 

the blocks and the mine roof and floor (location of the 

maximum negative moment). A three-hinged arch is 

formed where the external moment caused by the 

transverse loading (w x ρ x L2

/8 term in equation 1) is 

resisted by the internal force couple (P x r), where r is 

defined as the width of the arch and P is the thrust 

force generated by the arching. This condition is 

illustrated in the free-body diagram in figure 2 and 

expressed mathematically by equation 2. This equation 

can then be solved for the transverse pressure (ρ) as 

shown in equation 3. 

(1) 

 (2) 

 (3) 

Where ρ = transverse load, psi, 

 L = height of the wall, in, 

 w = width of the wall, in, 

 P = resultant thrust force at the hinge points, 

lbs, and 

 r = width of the arch, in. 

If it is assumed that the arching thrust (P) is controlled 

by the compressive strength of the block material and 

the “crush zone” is acting over an area of the block 

equal to 2/10 the thickness of the wall (see figure 3), 

then an expression for P can be derived as given in 

equation 4. As shown in figure 3, this assumption also 

results in the width of the pressure arch (r) being equal 

2 Lw

rP8

×

× ×

=ρ

ρ 8

Lw

ρ 2

y yw ρ wdy

2L/ 2

0

2 2L/

0

×

×

=××=××× ∫

P

P

HF

L/2 r

L/2

L/2

ρ

t t

Full-wall Half-wall 

ρ

Figure 2. Three hinge arch illustration and free body diagram

of half-wall. 

rP

8

w ρ L2

×=

××

y

Figure 3. Examination of the thrust force developed in rigidarching loading conditions. 

L/2

0.2t

a

P

0.2t

Half Wall ρ L/2

ρ L/2

P

P

L/2

a

P

Half Wall ρ L/2

ρ L/2

P

P

P
2

c L

t f28.1 ⎟

⎠

⎞ ⎜

⎝

⎛ ××=ρ

to 0.8 x t. Substituting this expression for r and the 

expression for P from equation 4 into equation 3 yields 

a solution for determining the transverse load capacity 

of a stopping wall (equation 5). 

 (4) 

Where ρ = transverse load, psi, 

 P = arching thrust, lbs, 

 t = thickness of wall, in, 

 L = wall height, in, and 

 fc = compressive strength, psi 

 (5) 

An example is considered using a 6-in-thick wall 

that is 72 in high and constructed from concrete blocks 

with a compressive strength of 1,000 psi. The term 

fc X (t/L)2 equates to 6.94 psi for this example, which 

computes a predicted transverse load capacity of 8.9 

psi or 1,279 psf. 

Using these same relationships, it can be shown that 

the transverse pressure acting on a full-scale stopping 

can be computed from the measured horizontal force 

(HF) at the base of a half-wall as used in the MRS 

laboratory testing from equation 6, where w is the 

width of the wall and (L/2) is the half-wall height. 

 (6) 

3.2 Implications of Rigid Arching To Stopping Design 

Rigid arching indicates that the physical properties of 

the block and the size of the mine opening must be 

considered to evaluate the transverse loading capability 

of a stopping for design purposes. Intuitively, higher 

strength block will provide greater transverse loading 

capability, but increasing the thickness of the block, or 

constructing the stopping with the wide side of the 

block as providing the contact area, can also greatly 

increase the transverse load capacity of the stopping. 

Likewise, it is important to recognize that, for given 

design parameters, the transverse load capacity will 

decrease as the entry height increases. These 

relationships are illustrated in figure 4 for conventional 

concrete masonry units that have historically been used 

to construct stoppings. These solid blocks measure 

nominally 6x8x16 inches and have a unit block 

compressive strength of 1,330 psi. As seen in the 

figure, using the 8 in-wide construction nearly doubles 

the transverse load capacity of the stopping for a 

specific construction height. For comparison, for a 

stopping constructed in a 16 ft-wide by 8 ft-high 

opening, the wide-side construction would require 192 

blocks while the narrow-side construction would 

require 144 blocks. In other words, for a 33 pct 

increase in the number of block, the transverse load 

capacity can be increased by 100 pct. Also, note that 

the transverse load capacity would drop by a factor of 

four if the construction height were doubled. 

4 SIMULATING RIGID ARCHING THROUGH 

BIAXIAL LOADING IN THE MINE ROOF 

SIMULATOR 

NIOSH has a unique load frame that was designed to 

simulate the behavior of rock masses in underground 

mining operations. It is called the Mine Roof Simulator 

(MRS). This unique facility’s capabilities provide an 

ideal framework in which to conduct rigid-arch testing 

of stopping walls. 

 Since the load frame platens are 20 ft x 20 ft and 

with a maximum vertical opening of 16 ft, the MRS 

can accommodate full-scale stopping constructions. 

The MRS is capable of providing controlled biaxial 

loading in the vertical as well as one horizontal axis. 

Up to 3 million lbs of vertical force can be applied 

through a 24-in vertical stroke of the lower platen and 

up to 1.6 million lbs of horizontal force through a 16-in 

horizontal stroke of the lower platen. The loads or 

displacements in these two axes can be applied 

individually or simultaneously if desired. 

4.1 Test Protocol for Simulating Rigid Arching 

In order to simulate rigid arching, a half-height section 

of a stopping wall was placed in the load frame in a 

typical vertical orientation as it would be in the mine. 

The upper platen position was adjusted to the height of 

the block column and was hydraulically clamped to 

)2/L(w

HF2

×

×

=ρ

c ×××= wft2.0P

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

40 60 80 100 120 140

HEIGHT OF WALL, inches

TRANSVERSE PRESSURE, psf .

Narrow (6 in) width Wide (8 in) width

Narrow Width

Wide 

Figure 4. Impact of wall thickness and height on transverse

load capacity of a conventional concrete block stopping. 
maintain its position. The vertical position of the lower 

platen was commanded to remain constant. Hence, the 

fixed positions of the upper and lower platen allowed 

them to act as rigid restraints. The lower platen was 

then moved horizontally at a constant velocity of 0.5 

in/min, causing the wall to rotate (figure 5). As the 

base of the wall was forced to move horizontally, crush 

zones were created at the ends of the wall on opposite 

sides, consistent with the rigid-arch loading 

mechanism. The horizontal force applied to the base of 

the half-wall by the MRS was measured and was 

equated to the transverse pressure acting on a stopping 

wall using equation 6. 

4.2 Transverse Load Determinations from MRS HalfWall Testing 

An example of a transverse loading test is shown in 

figure 61

. The test consisted of a single column of 

lightweight, autoclaved concrete block, stacked four 

blocks high with the narrow side contact between 

blocks. This block measures 5.875 x 8.375 x 17.250 in 

with a density of 42.5 lbs/cu ft resulting in a unit block 

weight of approximately 21 lbs. Tests conducted on an 

individual block indicated that the compressive 

strength was 546 psi. The computed transverse 

pressure determined from the measured horizontal 

force during this test was 834 psf as computed by 

equation 6. 

The measured vertical force is equivalent to the 

arching thrust (P) as defined in equation 4. The arching 

thrust is a function of the elastic properties of the 

concrete block and the contact area that develops as the 

wall rotates. The kinematics of the wall suggests that 

the contact area will decrease with increasing lateral 

displacement. This essentially causes an increase in the 

stress acting on the crushing zone until the 

 1

Mention of company name or product does not constitute endorsement by the National Institute for Occupational Safety 

and Health. 

compressive strength of the material is reached. Once 

the compressive (or shear strength) of the material is 

exceeded, the thrust force will decrease and the 

transverse pressure capacity of the stopping will 

decline. 

Tests conducted on a similar half-wall constructed 

from block made from conventional Portland cement, 

sand, and aggregate material with a compressive 

strength of 1,330 psi produced a maximum transverse 

loading of 2,134 psf, or 2.6 times that of the autoclaved 

block used in the previous test, which is consistent 

with the difference in material strength. This provides 

additional validation for the application of arching 

theory to stopping wall behavior. 

5 FULL-SCALE LOAD VERIFICATON TESTING 

In order to confirm that arching was the proper loading 

mechanism controlling the transverse load capacity of 

mine ventilation stoppings and to verify the MRS halfwall rigid-arch testing protocol, full-scale tests of 

stopping walls were also administered. These tests 

were conducted in the NIOSH Experimental Coal 

Mine at the Pittsburgh Research Laboratory. Test data 

of full-scale stoppings was also analyzed from 

NIOSH’s explosion testing at the Lake Lynn 

Laboratory. 

5.1 NIOSH PRL Experimental Coal Mine Tests 

An air pressure chamber was constructed in one of the 

crosscuts in the mine to provide a facility for static 

loading of mine ventilation stoppings. Stoppings were 

constructed in a crosscut measuring approximately 16 

ft wide and about 80 in high. The test wall was 

constructed in a normal dry-stacked fashion. The top of 

the wall was tightened with wood wedges and the gaps 

were filled with mortar and sealant to prevent air 

leakage. 

MRS UPPER PLATEN

θ

CRUSH 

ZONE

CRUSH 

ZONE

Figure 5. Simulating rigid arching in the NIOSH Mine

Roof Simulator. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.0 0.5 1.0 1.5 2.0 2.5 3.0

LATERAL DISPLACEMENT, inches

HORIZONTAL FORCE, kips . 

0

3

6

9

12

15

18

THRUST FORCE, kips .

MRS Half-Wall Test (Horizontal Force )

MRS Half-Wall Test (Thrust Force )

Thrust 

Force

Horizontal 

Force

Figure 6. Example test result of 4 course half-wall arch test 

of Kingsway block stopping wall in the MRS. 
Pressurized air was injected into the chamber 

between the barrier and the test wall through the air 

intake port. The air pressure was increased gradually in 

increments by adjusting a control valve on the pressure 

line. This process continued until the air pressure in the 

chamber blew out the wall. Three displacement 

transducers were utilized to measure the lateral 

displacement at the mid and quarter point heights of 

the wall as the pressure was applied. 

5.2 Test Results and Comparisons to Half-Wall MRS 

Tests 

Two full-scale wall tests were conducted in the NIOSH 

Experimental Coal Mine. For direct comparison 

purposes, these were constructed using the same block 

materials that were utilized in the MRS laboratory 

tests. The first test utilized the lightweight autoclaved 

concrete blocks, and the second test was a wall 

constructed from the conventional solid concrete 

aggregate block. Comparisons of the MRS half-wall 

rigid-arch test to the full-scale mine test are shown in 

figures 7 and 8. Figure 7 shows that the peak 

transverse loading for MRS half-wall test was higher 

than that observed in the full-scale mine test. 

Examining figure 7, it is seen that the peak transverse 

pressure for the MRS test occurred at a larger lateral 

displacement, suggesting that the full-scale mine test 

may have failed prematurely compared to that 

observed in the MRS. The full-scale mine stopping 

also exhibited a stiffer response than the MRS test. 

This may be caused by loading of the wall from 

convergence of the mine entry or by a shorter arching 

height than was considered in the MRS test. As shown 

in figure 8, only a 4-course and 6-course-high half-wall 

was tested in the MRS for the conventional concrete 

block, while the in-mine test was constructed with 10 

courses (full-height). Therefore, a direct comparison 

was not provided. However, it is seen from the figure 

that, as expected, the full-scale mine test fits between 

the two laboratory tests. It is concluded that the MRS 

laboratory half-wall tests reasonably predict the fullscale mine tests in both cases. 

These results are also consistent with the arch 

mechanics theory presented in the previous section, 

which indicate that higher walls will have less 

transverse load capacity than shorter walls and that 

weaker block materials will provide less transverse 

load capacity than higher strength block materials. 

These relationships were expressed mathematically by 

the fc X (t/L)2

 term (compressive strength of the 

material times the square of the ratio of the wall 

thickness to the wall height) in equation 5. Figures 9 

and 10 plot the measured transverse pressure for both 

the half-wall laboratory tests and full-scale mine tests 

as a function of the fc X (t/L)2

 term. The chart shows a 

strong correlation of the half-wall transverse pressure 

measurements to this term. Also shown on the chart 

are the full-scale mine tests, and again, it is seen that 

the mine tests also fit this correlation very well. The 

theoretical design curve produced from equation 5 

over-predicts the measured responses for the lower fc X

(t/L)2

 values, and more accurately predicts the 

transverse pressure as the value increases. It is believed 

that the error lies in the arch thrust moment. The 

current formulation does not include the lateral 

displacement of the wall, which occurs throughout the 

loading history. Lateral displacement reduces the arch 

thrust moment and the resulting force couple. This 

topic is currently being addressed in the continuing 

research at NIOSH. 

Additional full-scale transverse loading tests of 

stoppings were conducted at the NIOSH Lake Lynn 

Laboratory (Sapko, 2003 and Weiss, 2004). Both 

hollow-core and solid, high-strength, concrete block 

stoppings were evaluated in this study. The hollowcore block had an average material compressive 

0

100

200

300

400

500

0 0.5 1 1.5 2 2.5

LATERAL DISPLACEMENT, inches

TRANSVERSE PRESSURE, psf .

MRS Half-Wall Test (Kingsway)

Expermental Mine Full-Scale Test (Kingsway)

Expermental 

Mine Test

MRS 

Test

Figure 7. Comparison of MRS laboratory test and full-scale 

mine test of low strength block stopping. 

0

400

800

1,200

1,600

2,000

2,400

0.0 0.5 1.0 1.5 2.0 2.5 3.0

LATERAL DISPLACEMENT, inches

TRANSVERSE PRESSURE, psf .

Exp. Mine Full-Scale Test -- 10 course high

MRS Half-Wall Test -- 4 course high

MRS Half-Wall Test -- 6 course high

Theoretical Prediction -- 5 courses high

4 course 

high test

Theoretical 

Prediction

Exp. Mine 

Test

6 course high test

Figure 8. Comparison of MRS laboratory test and full-scale 

mine test of high strength block stopping. 
strength of 1,456 psi and the solid block an average 

material compressive strength of 1,900 psi. Typically, 

the block strength achieved in a unit block or column 

of block measurement is considerably less than the 

material strength tests that are conducted under 

ASTM specifications on small scale cylinders or 

cubes. As shown in the chart in figure 10, the mine 

test data is closer to the MRS laboratory tests when 

the compressive strengths are reduced to from 1,900 

to 1,500 psi. The stopping walls constructed in the 

crosscuts were 12 courses high (7.5 ft), 6-in thick, 

and approximately 20 ft in length. 

y = 229.43x - 179

R2

 = 0.98

0

100

200

300

400

500

600

700

800

900

1,000

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Fc x (t/L)2

TRANSVERSE PRESSURE, psf .

MRS Half-Wall Test Data (Kingsway)

Experimental Mine Full-Scale Test (Kingsway)

Rigid Arching Theory Predictions Trendline

Experimental 

Test

Figure-9. Comparison of MRS laboratory test, full-scale test, and theoretical predictions of transverse load capacity of various

stopping constructions using low strength cellular concrete block. 

y = 233x - 681

R2

 = 0.99

0

500

1,000

1,500

2,000

2,500

3,000

0 2 4 6 8 10 12 14 16 18

fc x (t/L)2

TRANSVERSE PRESSURE, psf.

MRS Half-Wall Test Data (Klondike 1,330 psi)

MRS Half-Wall Test Data (Klondike 1,727 psi)

Experimental Mine Full-Scale Test (Klondike 1,330 psi)

Lake Lynn Full-Scale Test (Conventional Solid Block 1,900 psi)

Lake Lynn Full-Scale Test (Modified Block Strength 1,500 psi)

Lake Lynn Full-Scale Test (Hollow Core Block 1,456 psi)

Rigid Arching Theory Prediction Trendline

Figure 10. Comparison of MRS laboratory tests, full-scale tests, and theoretical predictions of transverse load capacity of

various stopping constructions using conventional solid and hollow core block. TRANSVERSE PRESSURE, psf
6 SUMMARY AND CONCLUSIONS 

Stoppings are a key component of underground mine 

ventilation systems. Permanent stoppings are often 

constructed from some form of concrete block, 

typically dry-stacked to form a wall, equal in thickness 

to the narrow or wide dimension of the block, and 

bridging between the mine roof and floor and pillar 

ribs. The criteria for approval of block stoppings to be 

suitable for coal mine use in the United States is 

generally 39 psf of transverse load capacity in a 

freestanding loading condition. This specification is 

based on ASTM E 72 testing requirements. Based on 

the work presented, this standard does not provide an 

accurate representation of the loading conditions that 

occur in the mining situation if the restraint of the mine 

roof and floor is considered. For dry-stacked stopping 

constructions, the transverse load capacity under the 

ASTM E 72 criteria is primarily determined by the 

tensile strength of the sealant. Any block material, 

regardless of its physical properties, can be made to 

pass this test criterion for use in underground coal 

mines provided the sealant is strong enough and can 

adhere to the surface of the block. 

The restraint provided by the mine roof and floor and 

coal pillars allows the stopping wall to arch between 

these abutments as the wall flexes and bends from the 

application of transverse loading. Arching has long 

been recognized as a valid loading mechanism that can 

more realistically show the increased capability of 

jointed structures to resist loading induced by bending 

when end restrained. Arching relies on compressive 

forces within the wall structure to offset the bending 

moment induced by the deflection of the wall from the 

application of transverse loading. For dry-stacked 

stopping constructions, which have no tensile strength 

across the joints except for the sealant on the face of 

the joint, these compressive forces can increase the 

transverse load capacity of a stopping by two orders of 

magnitude compared to the freestanding condition. 

A static analysis of the arching condition was 

examined to develop a theoretical relationship for 

predicting the transverse load capacity of a stopping. 

The analysis indicates that both the compressive 

strength (fc) of the construction material and the 

thickness (t) of the wall have a significant impact on 

the transverse load capacity of a stopping. Increasing 

the thickness of the wall will cause an increase in the 

force couple developed by the resultant compressive 

forces acting on the wall, and thereby increase its 

transverse loading capacity. Conversely, increasing the 

height (L) of the stopping will reduce the force couple, 

and thereby reduce the transverse load capacity of a 

stopping. None of these factors is considered in the 

current criteria for mine ventilation stoppings. A 

theoretical design equation to predict the transverse 

loading was developed using these key factors, 

expressed by the term fc X (t/L)2

. 

A laboratory testing protocol to simulate rigid 

arching of stopping walls by biaxial loading in the 

NIOSH MRS was developed. This process is most 

easily simulated in the MRS by testing a half-height 

section of wall. The wall is restrained vertically by the 

fixed vertical position of the load frame platens, 

thereby acting as rigid end restraints simulating the 

mine roof and floor. The lower platen is then moved 

laterally, causing the base of the wall to displace with 

the platen and causing the wall to rotate accordingly, 

similar to the three-hinge theory. Crush zones are 

created at edges of the half-wall in the areas where 

these two hinges would occur in a full-height wall. By 

measuring the horizontal forces applied to the base of 

the wall by the MRS, the transverse load capacity of 

the wall can be determined. 

Tests were conducted in the MRS on two different 

block types using this testing protocol. The two block 

types were selected for study because of their differing 

physical characteristics. One block was made from a 

low-density, autoclaved concrete material (546 psi 

compressive strength) while the other block was made 

from a more conventional material - Portland cement, 

sand, and aggregate (1,330 and 1,727 psi compressive 

strength). Tests were conducted on several wall heights 

ranging from 5 to 10 ft. For these tests, the transverse 

load capacity ranged from 138 to 834 psf for the 

lightweight block and 96 to 2,136 psf for the 

conventional block. Comparing these results to the 39-

psf requirement clearly shows the disparity caused by 

the assumptions made in the boundary conditions, 

freestanding as considered in the ASTM E-72 test 

specification and the rigid arch conditions being 

proposed here as a more accurate representation of the 

actual in mine conditions. 

The MRS half-wall rigid-arch testing methodology 

was verified with two full-scale tests of stopping walls 

in the NIOSH Experimental Coal Mine at the 

Pittsburgh Research Laboratory and two full-scale 

explosion tests at the NIOSH Lake Lynn Laboratory. 

Overall, good agreement was shown between the MRS 

tests and the full-scale mine tests. 

In conclusion, rigid arch stopping design would be 

a departure from the current freestanding wall design 

assumed in the ASTM E-72 specifications cited by the 

CFR. For arch loading conditions, the physical 

properties of the block and the size of the mine 

opening should be examined to determine the proper 

design for a stopping application. The sealant would no 

longer be considered to impact the transverse load 

capability of the stopping. This approach, which will 

be presented and discussed with MSHA and the 

ASTM, could lead to a safer mine environment for 

mineworkers in underground coal mines by 

distinguishing the transverse load capacity of different 

stopping designs; opposed to the current system that 

permits stoppings of widely varying transverse loading 

capabilities to be employed in the same environment. 
7 REFERENCES 

ASTM Designation E 72-80. Standard Methods of 

Conducting Strength Tests for Building 

Construction. Annual Book of ASTM Standards, 

Vol. 04.07, March, pp. 283, 293, 1981. 

Code of Federal Regulations, Title 30 Mineral 

Resources, Part 75 Mandatory Safety Standards – 

Underground Coal Mines, Subpart D – Ventilation, 

Section 75.333 Ventilation Controls, pp. 471-473, 

July, 2005.

Sapko, M. J, E. S. Weiss and S. P. Harteis. Alternative 

Methodologies for Evaluating Explosion-Resistant 

Mine Ventilation Seals. Published in the 30th

International Conference of Safety in Mines 

Research Institutes, South African Institute of 

Mining and Metallurgy, 2003.

Weiss, E. S., S. P. Harteis and K. L. Cashdollar. 

NIOSH-PRL Evaluation of the Kennedy and SolidConcrete-Block Stopping Designs for MSHA 

during LLEM Explosion Tests, Sept., 40 pp, 2004.