Comparison of the transverse load capacities of various block 

ventilation stoppings under arch loading conditions 

T. M. Barczak & T. J. Batchler 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh PA 

ABSTRACT: Stoppings are required to resist lateral forces on the face of the structure to control pressure 

differentials created by ventilating air. The design criteria in other parts of the world, including Australia, exceed 

this requirement by specifying over pressurization control to as much as 14 kPa (2 psi) in the active sections and 

35 kPa (5 psi) in the main ventilating control areas. This is done to provide protection to the mine workers from 

explosive or air blast events within the mine that can create this over pressurization. The National Institute for 

Occupational Safety and Health (NIOSH) has been conducting research to develop a new testing protocol for 

rating mine ventilation stoppings. The premise of this work is that arch-loading conditions more accurately 

reflects the transverse load capabilities of stoppings in underground coal mines than the freestanding flexural 

strength test specified in the current Code of Federal Regulations (CFR). This work clearly shows that the 

transverse load capabilities of dry-stacked, mine ventilation stoppings are dependent upon the material strength of 

the block and the height and thickness of the wall, none of which are part of the current CFR criteria based on 

ASTM E-72 specifications. This paper compares the transverse load capabilities of several block materials and 

wall dimensions commonly used in stopping constructions based upon simulated three-hinge, rigid-arch loading 

tests of half-wall constructions in the NIOSH Mine Roof Simulator. The results indicate a wide-range of 

transverse load capacities, ranging from as low as 21 kPa (0.3 psi) to as much as 117 kPa (17 psi) for the 

conditions evaluated in this study. The current CFR criteria do not distinguish between applications of these 

stopping designs despite their wide variance in transverse load capabilities, which is not conducive to employing 

ventilation control strategies that isolate certain sections of the mine to prevent injury to mine workers. 

1 Introduction 

Stoppings are required to resist lateral (transverse) forces 

on the face of the structure to control pressure differentials 

created by ventilating air. These pressures can range from 

as low as 2 kPa (0.25 psi) in the working sections of the 

mine to over 7 kPa (1 psi) near the area of a bleeder fan. 

The design criteria in other parts of the world, including 

Australia, exceed this requirement by specifying over 

pressurization control to as much as 14 kPa (2 psi) in the 

active sections and 35 kPa (5 psi) in the main ventilating 

control areas (Lyne B, 1996). This is done to provide 

protection to the mine workers from events such as large 

roof falls, face ignitions or other explosive events within 

the mine. Although these requirements appear to be 

somewhat arbitrarily chosen, they are consistent with 

forces that can cause harm and serious injury to a human 

being. The force from a 14-kPa (2-psi) blast of air will 

cause a standing person to be thrown hard enough to cause 

incapacitating injuries and above 35 kPa (5 psi) fatalities 

are likely (>50% fatality rate). A 95% fatality rate is 

expected with pressures of 48 kPa (7 psi) (Cornwell and 

Marx, 1997). 

There are no full-scale tests required for stoppings to 

determine their load capacity under U.S. mining 

regulations. The current Code of Federal Regulations 

(CFR) requirement is to test 1.2 x 2.4-m (4 x 8-ft) sections 

of freestanding walls in accordance with ASTM E-72 

specifications (30 CFR, Part 75.333, July 2007). For drystacked stopping constructions, the transverse load 

capacity under the ASTM E-72 criteria is primarily 

determined by the tensile strength of the sealant. Under the 

freestanding load condition, 2 kPa (39 psf) is designated in 

the preamble to the CFR as the minimum required 

transverse load capacity. 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. This test does not determine the transverse load 

capacity of actual in-mine stopping constructions when the 

mine 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. 

A protocol to evaluate the transverse load capacity of 

block stopping constructions under arch-loading conditions 

using the unique biaxial loading capabilities of the 

National Institute for Occupational Safety and Health’s 

(NIOSH) Mine Roof Simulator (MRS) has been developed 

and verified with in mine experiments (Barczak and 

Batchler, 2006). Using this protocol, this paper documents 

the transverse load capacities of stoppings constructed 

from various block materials that have been acceptable for 

use in U.S. underground mines. It is shown that a wide 

range of transverse loading capacities exist, not only 

because of the introduction of light weight, low strength 
 

 

 

 

 

block materials, but also due to the impact of mining 

height and wall thickness. It is imperative that the true 

transverse load capability of the stoppings be determined if 

the design requirements are to extend beyond the loading 

levels induced by ventilating air pressure differentials to 

provide protection to mine workers in the event of over 

pressurization from roof fall blasts or explosive events 

within the mine. 

2 Review of Test Protocol for Arch-Loading 

Evaluation 

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 

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 threehinged arch is formed, which is simulated in the NIOSH 

MRS load frame by examination of the half-wall section as 

illustrated in Figure 1. 

The half-height section of a stopping wall is placed in 

the load frame in a typical vertical orientation, as it would 

be in the mine. The upper platen position is adjusted to the 

height of the block column and is hydraulically clamped to 

maintain its position. The vertical position of the lower 

platen is commanded to remain constant. Hence, the fixed 

positions of the upper and lower platen allow them to act 

as rigid restraints. The lower platen is then moved 

horizontally at a constant velocity causing the wall to 

rotate (Figure 1). As the base of the wall is forced to move 

horizontally, crush zones are created at the ends of the wall 

on opposite sides, consistent with the three-hinge-arch 

loading mechanism. The horizontal force (HF) applied to 

the base of the half-wall by the MRS is measured and is 

equated to the transverse pressure (ρ) acting on a stopping 

wall using equation 1, where (w) is the width or thickness 

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

2× HF

ρ = (1) 

w×(L / 2) 

C 

o 

m 

pr 

e 

s 

i 

Sealant 

Stopping 

Wall MRS UPPER PLATEN 

θ

Transverse 

presssure 

Flexure of wall Three-hinge arch MRS simulation 

Figure 1. Simulating rigid arching in the NIOSH Mine Roof Simulator (MRS). 

3 Stopping Materials Examined in this Study 

Using the established protocol, the transverse load capacity 

of various stopping constructions under arch-loading 

conditions was determined through a series of half-wall 

tests in the NIOSH Mine Roof Simulator (MRS) load 

frame. The block materials chosen for the study represent 

the full spectrum of materials that have been determined as 

suitable for mine use by the Mine Safety and Health 

Administration (MSHA) using the current CFR 

freestanding wall assessment criteria. The block materials 

can be categorized as follows. Table 1 documents the 

relevant material properties for the blocks used in this 

study. 
 

 

 

 

 

 

 

 

 

 

 

 

Table 1. Summary of the properties of the various stopping block materials evaluated in this study. 

Block Type Dimensions, cm (in) 

Material 

Density, kg/m3 

(lbs/ft3) 

Unit Block 

Weight, kg 

(lbs) 

Unit Block 

Compressive 

Strength, kPa (psi) 

Standard CMU 15 x 20 x 40 (5-5/8 x 8 x 16) 2,082 (130) 24 (54) 9170 – 12,273 (1,330 

– 1,780) 

Lightweight Aggregate CMU 15 x 20 x 40 (5-5/8 x 8 x 16) 1,602 (100) 20 (43) 14,955 (2,169) 

Hollow Core Block 15 x 20 x 40 (5-5/8 x 8 x 16) 2,082 (130) 15 (32) 6,254 (907) 

AAC Materials 

• Block A 15 x 30 x 60 (6 x 12 x 24) 561 (35) 16 (35) 2,903 (421) 

• Block B 20 x 20 x 60 (8 x 8 x 24) 545 (34) 14 (30) 4,861 (705) 

• Block C 15 x 21 x 39 (5-7/8x8-3/8x15-1/4) 689 (43) 9 (19) 3,765 (546) 

Foamed Cement 20 x 40 x 60 (8 x 16 x 24) 352 (22) 18 (40) 579 (84) 

Extruded Foam Cement 15 x 40 x 60 (6 x 16 x 24) 609 (38) 23 (50) 593 (86) 

3.1 Standard Concrete Masonry Unit (CMU) 

This block is made from conventional Portland cement and 

standard aggregate. These blocks can vary in compressive 

strength depending on the amount of cement used in the 

mix from 7 to 21 MPa (1,000 to 3,000 psi). They generally 

weigh about 23 kg (50 lbs). 

3.2 Lightweight Aggregate CMU 

In order to reduce the handling effort, a lighter weight 

block is manufactured by using lightweight aggregate. 

Lightweight aggregate is made by heating up certain kinds 

of shale. Gases in the shale expand, causing the shale to 

bloat, producing bubbles, making it lighter than normal 

aggregates. Lightweight aggregate, with a specific gravity 

of about 1.7 reduces the weight of the block by as much as 

20 pct. 

3.3 Hollow Core Block 

Another way to reduce the weight is to use hollow core 

block. The block is made from conventional Portland 

cement and standard aggregate with the same basic 

formulation that is used to make the solid blocks. The thin 

webs and facing contribute to the lower strength when a 

full block is tested, typically less than 7 MPa (1,000 psi) 

compressive strength. 

3.4 Autoclaved Aerated Concrete (AAC) Materials 

Another approach to provide a low-density material for 

stopping block construction is the use of aerated cement to 

create pockets or air voids within the concrete mix. To 

manufacture AAC, Portland cement is mixed with lime, 

silica sand, or recycled fly ash (a byproduct from coalburning power plants), water, and aluminum powder or 

paste and poured into a mold. The reaction between 

aluminum and concrete causes microscopic hydrogen 

bubbles to form, expanding the concrete to about five times 

its original volume. After evaporation of the hydrogen, the 

now highly cellular concrete (see Figure 2) is cut to size 

and formed by steam curing in a pressurized chamber (an 

autoclave). The concrete has a homogeneous cell structure, 

visible to the eye, not to be confused with air-entrained 

concrete, in which air bubbles are microscopic in size. 

Several AAC materials have been developed for stopping 

applications in coal mines. 

Figure 2. Cellular foam block showing void structure that 

contributes to light weight. 

3.5 Foamed Cement 

Foamed cement normally has a density of between 400 and 

640 kg/m3

 (25 and 40 lbs/ft3), compared with about 2,000 

kg/m3

 (125 lbs/ft3) for ordinary concrete. It is made of a 

cementitious material, filler or aggregate, and an aerated 

foaming agent. It is also known as cellular concrete. 

Foamed cements typically provide compressive strengths 

of less than 700 kPa (100 psi). The block is brittle and 
 

 

 

 

 

 

 

 

 

susceptible to damage from handling. Small fiberglass 

fibers are sometimes imbedded in the mix to help hold the 

material together and slightly improve its post failure 

loading characteristics. 

3.6 Extruded Foam Cement 

An alternative approach to creating a low density material 

is to imbed styrofoam pellets in the concrete mix, which 

are easily seen as part of the block construction as shown 

in Figure 3. Extruded foam cement has a compressive 

strength of less than 1,400 kPa (200 psi) with a fairly wide 

range depending on the volume and dispersion of the foam 

pellets within the concrete structure. The low density of the 

material allows a large size block of reasonable weight to 

be manufactured. 

Figure 3. Extruded foam block showing close up of foam 

pellets (white colored areas) imbedded in a stopping block. 

4 Examination of the Transverse Load Capacities 

Figure 4 depicts the computed transverse load capacity of a 

2.3- to 2.4-m-high (7.5- to 8-ft-high) stopping wall for the 

various types of block that were examined in this study 

based on the half-wall, arch-loading laboratory tests 

conducted in the NIOSH MRS load frame. The first point 

to be made is that all of the transverse loading is above the 

2-kPa (39-psf) requirement established under ASTM E-72 

freestanding wall test. The standard (solid block) concrete 

masonry unit and lightweight aggregate (solid block) 

masonry unit constructions exhibited transverse load 

capacity of over 19 kPa (400 psf), which is about 3 times 

that provided by a hollow block construction. Three 

different autoclaved aerated concrete (AAC) block, each of 

different dimensions and compressive strengths, were 

tested. The 20-cm-thick (8-in-thick) cellular block with a 

compressive strength of 4.9 MPa (705) psi provided the 

highest transverse load with a capacity of 32 kPa (666 psf). 

This is largely due to the relatively greater thickness of this 

block. The other two AAC blocks were of similar size to 

the masonry block units, nominally 15 cm (6 in) thick, but 

with lower compressive strengths of 2.9 and 3.8 MPa (421 

and 546 psi) compared to the standard masonry units with 

a 9.2 MPa (1,330-psi) compressive strength. The 2.9-MPa 

(421-psi) block provided 8.6 kPa (179 psf) of transverse 

load capacity, while the 3.8-MPa (546-psi) block provided 

21 kPa (429 psf) of transverse load capacity. In contrast, 

the weaker foam cement and extruded foam cement, both 

with compressive strengths less than 700 kPa (100 psi), 

provided only 3 and 2 kPa (63 and 50 psf) of transverse 

load capacity, respectfully. 

0 

100 

200 

300 

400 

500 

600 

700 

TRANSVERSE LOAD CAPACITY, psf 

0.0 

4.8 

9.6 

14.4 

19.2 

23.9 

28.7 

33.5 

TRANSVERSE LOAD CAPACITY, kPa 

Current ASTM criterion 

(freestanding wall evaluation) 

Results shown for 2.4 m 

(8 ft) wall height 

Standard Lightweight Hollow Autoclaved Foamed Extruded 

Concrete Aggregate Masonry Unit Aerated Cement Foam Cement 

Masonry Unit Masonry Unit Concrete 

(AAC) 

BLOCK TYPE 

Figure 4. Transverse load capacity under arch loading 

conditions for various types of concrete block approved 

for stopping construction. 

Several factors influence the transverse load capacity 

under arch loading conditions (Barczak and Batchler, 

2006). 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. Figure 5 illustrates 

the impact of both wall thickness and wall height on the 

transverse load capacity of the stopping. The graph shows 

wall heights of approximately 1.5, 2.3, and 3.0 m (5, 7.5 

and 10 ft) comparing 15-cm-thick (6-in-thick) and 19-cmthick (7.5-in-thick) wall constructions from the same type 

of block (standard CMU). The transverse load capacity 

dropped by nearly an order of magnitude as the wall height 

doubled from 1.5 to 3.0 m (5 to 10 ft). Increasing the wall 

thickness from 15 to 19 cm (6.0 to 7.5 in), by simply using 

the wide side of block instead of the narrow side of the 

block for contact, increased the transverse load capacity by 

factors of 1.8, 2.6, and 3.2 for wall heights of 1.5, 2.3, and 

3.0 m (5, 7.5, and 10 ft), respectively. The strong 

correlation between the three critical parameters: 

compressive strength (fc), wall thickness (t), and wall 

height (L) is shown in Figure 6, which represents data from 

73 tests including eight different block materials, 4 

nominal heights, 4 nominal thicknesses, and 10 

compressive strengths. The term fc X (t/L)2 is derived from 

the moment equilibrium requirements of the half-wall 

loading (Barczak and Batchler, 2006). 
 

 

 

 

 

WALL HEIGHT, m 

1.7 2.3 3.1 

0 

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

4,000 

4,500 

TRANSVERSE LOAD CAPACITY, psf 

0 

24 

48 

72 

96 

120 

144 

168 

192 

215 

TRANSVERSE LOAD CAPACITY, kPa 

Standard Concrete Masonry Unit - 15 cm (6.0 in thick) 

Standard Concrete Masonry Unit - 19 cm (7.5 in thick) 

5.6 7.5 10.3 

WALL HEIGHT, ft 

Figure 5. Impact of wall height and thickness on 

transverse load capacity for conventional concrete 

masonry block constructions. 

fc x (t/L)2

, kPa 

0 34 69 103 138 172 207 241 276 310 345 

12,000 575 

y 219.67x - 423.74 

R2

 0.95 

TRANSVERSE LOAD CAPACITY, psf 

TRANSVERSE LOAD CAPACITY, kPa 

10,000 479 

8,000 383 

6,000 287 

4,000 192 

2,000 96 

0 0 

0 10 20 30 40 50

fc x (t/L)2

, psi 

Figure 6. Correlation between critical design parameters 

(block compressive strength, wall height and wall 

thickness) and transverse load capacity. 

Another factor that can influence the transverse load 

capacity of a stopping is the convergence of the mine roof 

and floor, which creates vertical pressure on the wall 

(Barczak, 2005). The convergence causes increased thrust 

on the wall that makes it more difficult for the transverse 

pressure to offset the moment induced by the thrust force 

to cause the wall to deflect outward, thus resulting in 

higher transverse load capacity. However, the block 

integrity is still limited by the stress developed from the 

overall loading. Therefore, in this case, the transverse 

pressure and stress induced from the convergence 

superimpose, so the benefit of the convergence will be lost 

once the block strength is reached. Since the two stresses 

superimpose, the wall will fail before the stress induced 

from the convergence reaches the compressive stress of the 

block. The example shown in Figure 7 will help to clarify 

these issues. Shown in the Figure are the transverse load 

responses from a series of half-wall tests of standard 

masonry block constructions in which the preload, 

representing the stress induced from convergence, was 

incrementally increased from a nominal 345 kPa (50 psi) 

up to 5,171 kPa (750 psi) for three half-wall heights with a 

constant wall thickness of 15 cm (6 in). Examining the 1.2-

m (46-in) half-wall height data, it is seen that the 

transverse load increased from 22 kPa (450 psf) to nearly 

72 kPa (1,500 psf), reaching a maximum at a preload of 

about 4.8 MPa (700 psi) on average, with some walls 

reaching maximum transverse loading at less than 4.1 MPa 

(600 psi). This represents about half the strength of the 

block. The amount of convergence required to produce this 

level of preload will depend on the elastic modulus of the 

block and the compliance between the block layers and the 

roof and floor contact with the ends of the wall. Tests on a 

1.2-m-high (4-ft-high) half wall constructed from standard 

(solid) masonry block units required only 2.5 mm (0.10 in) 

of displacement to produce 2.4 MPa (350 psi) of stress and 

3.0 mm (0 12 in) to cause 4.1 MPa (600 psi) of preload, so 

the amount of convergence required to produce 

considerable preload is relatively small. 

PRELOAD, kPa 

0 689 1,379 2,068 2,758 3,447 4,137 4,826 5,516 

3,500 168 

30-in half-wall height 

y -0.0035x2

R

 + 3.9991

2

 0.62 

x + 2003 

y -0.0022x2 + 3.2112x + 2514 

R2

 0.87 

46- in half-wall height 

60-in half-wa l height 

y -0.0017x2 + 1

R2

 0.77 

.89x + 60 

3,000 144 

TRANSVERSE LOAD, psf 

TRANSVERSE LOAD, kPa 

2,500 120 

2,000 

1,500 

1,000 

500 

0 

24 

0

0 100 200 300 400 500 600 700 800 

PRELOAD, psi

48 

72 

96 

Figure 7. Impact of convergence expressed as vertical 

preload pressure on stopping wall on transverse load 

capacity of stopping walls. 

Up to this point, the analysis has assumed a rigid arch 

condition whereby the abutments do not deform. Under 

rigid arch conditions, the lateral displacement of the wall is 

controlled by the stiffness and elastic response of the block 

wall. The transverse load capacity will decrease as the wall 

stiffness decreases since more lateral displacement will 

occur. The increase in lateral displacement reduces the 

force couple provided by the arching thrust and this causes 

a decrease in the transverse load capacity of the stopping. 

If the abutments are not rigid, then the lateral displacement 

will increase further, resulting in a further reduction in the 

transverse load capacity of the stopping. The problem can 

be analyzed in terms of the stiffness of the system 

(Barczak, 2005). The system consists of both the wall and 

the abutments. Since the wall and the abutments act in 

series with one another, the system stiffness can be 

expressed by equation 2. 

K wall abutment × K KSystem = (2) K wall abutment + K 

Where Ksystem = system stiffness, 

Kwall = wall stiffness, and

 Kabutment = abutment stiffness. 
 

 

 

 

If the stiffness of the abutment is infinity (perfectly 

rigid abutment), then the wall stiffness will control the 

lateral displacement associated with the arching thrust 

through the deformation of the block as described in the 

previous section. However, examining equation 2 shows 

that if the abutment stiffness was equal to wall stiffness, 

the system stiffness would be reduced by 50 pct. 

Therefore, a small change in the abutment stiffness can 

cause significant changes in the arching capability and 

transverse load capacity of a stopping. 

Figure 8 shows the impact of the reduction in system 

stiffness to 25, 50, and 75 pct of the rigid boundary 

condition at 3 different wall heights. This assessment is 

made by relating the lateral wall displacement and arching 

thrust forces to the system stiffness (Barczak, 2005). It is 

seen from this Figure that the impact of reductions in 

boundary stiffness will have a greater impact in terms of 

absolute reductions in transverse pressure for shorter walls 

than it will for taller walls. For the example shown in 

Figure 8, the transverse pressure for the 76-cm (30-in) halfwall height was reduced from 108 kPa (2,256 psf) for the 

rigid boundary condition to 45 kPa (940 psf) when the 

boundary stiffness is one third of the wall stiffness, thereby 

reducing the system stiffness to 25 pct of the rigid 

boundary condition. This represents a 58 pct decrease in 

the transverse pressure capacity of the stopping. 

500 

2,500 120 

30-in half-wall height 

45-in half-wall height 60-in half-wall height 

TRANSVERSE LOAD, kPa 

2,000 96

TRANSVERSE LOAD, psf 1,500 72 

1,000 48 

24 

0 0 

0% 20% 40% 60% 80% 100% 120% 

REDUCTION IN SYSTEM STIFFNESS RELATIVE TO RIGID BOUNDARY 

CONDITIONS, pct 

Figure 8. Impact of reduction in boundary stiffness 

expressed as decrease in system modulus on transverse 

load capacity of stopping walls. 

Figure 9 shows the impact of reductions in system 

stiffness for a 76-cm (30-in) half-wall height as a function 

of preload. The Figure indicates the reductions in 

transverse pressure as a result of reduction in boundary 

stiffness are reduced as the preload increases. Using the 

76-cm-high (30-in-high) half-wall as an example, the 58 

pct decrease in transverse pressure which occurred by 

reducing the system stiffness to 25 pct of the rigid 

boundary condition, drops to a 7 pct reduction at a preload 

of 3.9 MPa (567 psi). 

Figure 9. Impact of reduction of boundary stiffness when 

convergence causes an increase in vertical pressure on 

stopping wall. 

96 

120 

144 

168 

PRELOAD, kPa 

0 689 1,379 2,068 2,758 3,447 4,137 

4,500 215 

4,000 192

TRANSVERSE LOAD, kPa 

3,500 

TRANSVERSE LOAD, psf 3,000 

2,500 

Rigid 

2,000 75 

1,500 

50 

72 

1,000 25 48 

500 Rigid Boundary Conditions 75% reduction in system stiffness 24 

0 

50% reduction in system stiffness 25% reduction in system stiffness 

0 

0 100 200 300 400 500 600 

PRELOAD, psi 

5 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 block 

stoppings to be suitable for coal mine use in the United 

States is a minimum of 2 kPa (39 psf) of transverse load 

capacity in a freestanding loading condition using the 

ASTM E-72. The authors do not believe that this standard 

provides an accurate representation of the loading 

conditions that occur in the mining situation. For drystacked 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 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 across the outer 

edge of the joint, these compressive forces can increase the 

transverse load capacity of a stopping by more than an 

order of magnitude compared to the freestanding condition. 

A laboratory testing protocol to simulate rigid arching 

of stopping walls by biaxial loading in the NIOSH MRS 

was previously developed, verified through field 

measurements of stopping failures, and used in this study 

to evaluate several blocks used for mine ventilation 

stopping construction. The results confirm the theoretical 

analysis that both the compressive strength of the 

construction material and the thickness of the wall have a 

significant impact on the transverse load capacity of a 

stopping. Therefore, since most blocks are dimensionally 

anisotropic, constructing the wall with the wide side of the 
 

 

 

 

 

 

 

block in contact with the adjacent block layer during wall 

construction can significantly increase the transverse load 

capacity of the stopping. For example, increasing the wall 

thickness from 15 to 19 cm (6.0 to 7.5 in) increased the 

transverse load capacity of a 2.3-m-high (7.5-ft-high) wall 

by a factor of 2.6. Conversely, increasing the height of the 

stopping will reduce the transverse load capacity. Here, the 

impact can be even more dramatic. The transverse load 

capacity dropped by nearly an order of magnitude when 

the 15-cm-thick (6-in-thick) wall doubled in height from 

1.5 to 3.0 m (5 to 10 ft). Despite their significance, none of 

these factors is considered in the current criteria for mine 

ventilation stoppings. 

The full arching potential for any stopping will be 

realized for rigid boundary conditions, which will then 

establish the maximum transverse loading capacity for the 

stopping. Under rigid arching conditions, the lateral 

displacement of the wall is controlled by the stiffness and 

elastic response of the block wall. The transverse load 

capacity will decrease as the wall stiffness decreases since 

more lateral displacement of the wall will occur. If the 

abutments are not rigid, then the lateral displacement will 

increase further, resulting in a further reduction in the 

transverse load capacity of the stopping. Therefore, a small 

change in the abutment stiffness can cause significant 

changes in the arching capability and transverse load 

capacity. A theoretical assessment of the impact of the 

boundary stiffness was made by varying the system 

stiffness, which is the equivalent stiffness of the wall and 

the roof and floor acting in series with one another. The 

system stiffness was reduced to 75, 50, and 25 pct of the 

rigid boundary condition, and the transverse load capacity 

determined using arching mechanics formulations. An 

example was given where the transverse pressure for a 1.5-

m-high (5-ft-high) wall was decreased by 58 pct by 

reducing the system stiffness to 25 pct of the rigid 

boundary condition. 

Another important factor in considering the transverse 

load capacity of a stopping is the axial loading induced 

from the ground pressures. Even without arching, a 

superimposed axial or vertical load acting on a stopping 

wall can greatly increase the transverse load capacity of the 

stopping by resisting the moment induced by the transverse 

pressure. For arching conditions, the superimposed axial 

loading will act to strengthen the force couple created by 

the arching thrust. The result of the superimposed axial 

pressure will be that the transverse load development will 

occur at smaller lateral displacements of the wall, which 

results in higher transverse loading capacities. Increases in 

transverse loading by a factor of 5 can be attained with a 

2.3-m-high (7.5-ft-high) wall constructed from 

conventional solid concrete block materials when the 

ground pressure is increased from 0 to 4.1 MPa (0 to 

600 psi). 

In conclusion, arch stopping design would be a radical 

departure from the current freestanding wall design with 

respect to the ASTM E-72 specifications cited by the CFR. 

The physical properties of the block and the size of the 

mine opening would need to be examined to determine the 

proper design for a stopping application. The sealant would 

no longer be considered to affect the transverse load 

capability of the stopping. Since the actual transverse load 

capacity of the stopping can be determined, the stopping 

can be designed based on the required transverse load 

capacity for a specific set of conditions in the mine, as 

opposed to the current system that permits stoppings of 

widely varying transverse loading capabilities to be 

employed in the same environment. This approach should 

lead to a safer mine environment for the tens of thousands 

of mine workers in underground coal mines. 

Disclaimer: The findings and conclusions in this report 

have not been formally disseminated by the National 

Institute for Occupational Safety and Health and should not 

be construed to represent any agency determination or 

policy. 

References 

ASTM Designation E 72-80, 1981. Standard methods of 

conducting strength tests for building construction in 

Annual Book of ASTM Standards, Vol. 04.07, March, 

pp. 283, 293. 

Barczak TM, 2005. Evaluation of the transverse load 

capacity of block stoppings for mine ventilation 

control (Ph.D Dissertation, West Virginia University). 

Barczak TM and. Batchler TJ, 2006. Development of new 

protocols to evaluate the transverse loading of mine 

ventilation stoppings. Proceedings of the 11th 

U.S./North American Mine Ventilation Symposium, 

June 5-7 (eds Jan M. Mutmansky and Raja V. Ramani, 

The Pennsylvania State University, University Park, 

Pennsylvania). 

Code of Federal Regulations, 2007. Title 30 Mineral 

Resources, Part 75 Mandatory Safety Standards – 

Underground Coal Mines, Subpart D – Ventilation, 

Section 75.333 Ventilation Controls, pp. 487-489, 

July. 

Cornwell, J B and Marx JD, 1997. The significance of 

hazard end points in quantitative risk analysis, The 

Quest Quarterly 2(3). 

Lyne B, 1996. Approved standard for ventilation control 

devices, including seals and surface airlocks. 

(Brisbane, Queensland, Australia: Queensland 

Department of Mining and Energy, Safety and Health 

Division, Coal Operations Branch, QMD 96 7396).