Abstract 

 Western mines have had limited choices for standing support. 

Wood cribs were often too soft and unstable. The introduction of the 

Can support in the early 1990’s provided a very effective alternative 

and remains the dominant form of tailgate support. Water-filled 

prestressing cells are now used to cap the Can and preload it to 

provide an active roof load. The Cluster Prop, consisting of three 

timber wedge props bundled together, provides more capacity than 

an equivalent sized Can support and improves transport efficiency. 

However, the Cluster Prop is less stable and does not maintain a 

consistent load throughout its loading profile. Pumpable roof 

supports are another alternative support, but they have not been 

proven in high deformation environments. Alternative supports are 

also being used in longwall recovery operations. These include the 

Rocprop and Omni Prop, which are extendable support systems. 

They also provide active roof loading which can be beneficial in 

these applications. Specialty supports, such as the Sand Prop and 

Spider Prop, have also been introduced into western longwall 

operations. This paper compares the performance characteristics of 

these various support systems. 

Introduction and Historical Perspective 

of Western Support Applications 

Gateroads and longwall recovery rooms require proper support 

to ensure successful longwall operations regardless of geographic 

region, but various circumstances make this particularly difficult for 

western U.S. longwalls. Nowhere in mining is the compatibility of 

secondary supports more critical than in western mines where 

convergence is often an order of magnitude greater than in most 

eastern mines. In particular, the stiffness of the support must be 

compatible with the ground reaction to avoid premature support 

failure or unintentional damage to the roof and floor when the 

support is too stiff and catastrophic failure of the roof when the 

support is too soft (figure 1). 

 Historically, timber cribs and posts have been the dominant form 

of secondary support. However, the demands placed on these 

supports by longwall mining, coupled with the low-strength, short 

supply, and increasing cost of mine timber in western U.S. mines, 

have necessitated the development of alternative support systems. 

Beginning in the mid 1990’s, the National Institute for Occupational 

Safety and Health (NIOSH) in conjunction with various support 

manufacturers promoted the development of innovative support 

systems. The goal was to (1) design supports that are more 

Standing Support Alternatives in Western Longwalls 

Thomas M. Barczak 

Stephen C. Tadolini

NIOSH 

Pittsburgh, PA

Figure 1. The compatibility of standing support with the 

loading conditions is essential for western longwall 

operations. 
compatible with ground behavior, particularly the high deformation 

conditions found in western mines, and (2) develop cost-effective 

alternatives to conventional wood cribbing that reduce material 

handling injuries and provide equally or more effective ground 

control. A report published in the 15th International Conference on 

Ground Control in Mining documented these initial support 

developments (Barczak, 1996). 

 The first alternative to the wood crib in western mines was the 

Confined Core Crib or 3C support (figure 2). The 3C support was 

developed by John Frederick at Southern Utah Fuel Company 

(SUFCO)1

 and successfully employed for years in the SUFCO mines 

(Frederick, 1994). The 3C support is a corrugated steel container 

filled with minus 3-inch (76.2 mm) pumice rock that was readily 

 

1

Mention of any company name or product does not constitute endorsement 

by the National Institute for Occupational Safety and Health. 

available at the Utah mine site. The steel container provides 

confinement to the pumice rock as it was loaded by the convergence 

of the mine entry. Initially the 3C is a very soft support due to the 

void ratio of the pumice fill, requiring over a foot of convergence to 

provide a reasonable degree of load resistance. However, despite 

this soft response, the support performed adequately in the twoentry yield pillar tailgates in the SUFCO mines where moderate floor 

heave, occurring prior to the longwall panel front abutment, caused 

sufficient convergence to compact the fill material and thereby stiffen 

the support for improved performance as the face approached. The 

3C support was the predecessor of the Can support. The Can 

support, as shown in figure 3, has replaced wood cribbing and 

continues to be the dominant form of standing support in western 

mines today. 

 The previously cited paper in the 15th Ground Control 

Conference also discussed several other developments in timber 

supports designed to enhance the stiffness of timber cribbing. 

These included the Hercules and Link-N-Lock wood crib 

developments by Strata Products USA (figure 4). Although the 

improvements in stiffness and stability were beneficial to 

conventional crib designs, the lack of good quality timber still made it 

impractical to utilize this technology on a consistent basis in western 

mines. A few trials and some spot applications of these supports 

continue today, but only in relatively small numbers compared to the 

Can support. The Propsetter support (figure 5) was also developed 

in the mid-1990’s by Strata Products and has become one of the 

Figure 2. Confined Core Crib (3C) support was the first viable 

alternative to wood cribbing in western longwall tailgates. 

Figure 3. The Can support has replaced wood cribbing and is 

now the dominant secondary support used in western tailgates.
most commonly used longwall supports in eastern mines, but its use 

in western mines has been limited, in part due to its limited yield 

capability and stability in the 10+ ft (3.05+ m) mining height 

applications common in western mines. Other support technologies 

have not proven to be effective support solutions and are currently 

not utilized at all. These include: (1) the Mega Prop, (2) Variable 

Yielding Crib, and (3) Power Crib. 

 The purpose of this paper is to discuss enhancements to the 

Can support technology and recent support innovations that provide 

alternatives to the Can support for western tailgate applications. 

Specifically, the paper will address the use of a new prestressing 

technology for Can supports that will maximize its stiffness and help 

to offset the deficiencies caused by topping the Can with timbers. A 

new timber support called the Cluster Prop has been developed by 

Strata Products USA and has been successfully utilized in several 

eastern mines as an alternative to the Can support. Currently, trials 

of the Cluster Prop are underway in a few western mines. A 

comparison of the performance capabilities of the Cluster Prop with 

the Can support is included in this paper. Pumpable roof supports is 

another new technology that has performed well in eastern longwall 

tailgates and has recently been introduced to western mines, but the 

evaluation is incomplete on how well these supports can perform in 

high deformation environments. The paper also discusses a few 

new support technologies that can be used for longwall recovery 

operations and provides some insight into other new support 

technologies under development for western longwall operations. 

Improving the Performance of the Can Support 

 The Can support remains a fundamentally sound support design 

with an excellent performance record. No other support currently on 

the market can match the stability and high yield performance of the 

Can support. It has performed well in both high mining heights and 

high deformation environments that include 2-3 feet (0.61-0.91 m) of 

floor heave that produces large lateral displacements of the base of 

the Can relative to the roof contact (figure 6). The Can is installed 

by a machine which eliminates much of the material handling 

required for the support installation, and thereby, has been shown to 

dramatically reduce material handling injuries compared to wood 

crib construction. From a design perspective, it has only one 

significant drawback, it has to be topped off to establish roof contact. 

Normally, this is done with conventional wood crib blocks. 

Figure 4. Advancements in timber supports, such as the Hercules crib (left) and Link-N-Lock crib (right) have 

overcome some of deficiencies in timber supports, but their application in western mines is likely to remain limited. 

Figure 5. The Propsetter support has provided a yielding 

timber prop, and although it is one of the most commonly used 

supports in eastern longwall tailgates, its application in western 

mines is limited due to its yield capability and stability in the 

higher mining heights. 

Figure 6. The Can is the most stable support presently 

available. It can accommodate large roof-to-floor 

convergence as well as large lateral displacements. 
 A closer examination of the impact of the crib blocks on the 

performance of the Can reveals just how important this material is to 

preserving the performance potential of the Can. The first point to 

understand is that the wood crib blocks will always reduce the 

stiffness of the support. The wood crib blocks stacked on top of the 

support act in series with the Can. The equivalent stiffness of the 

complete system can be theoretically determined from equation 1. 

Hence, if the wood cribbing on top of the Can had the same stiffness 

as the Can itself, then the overall stiffness of the support system 

would be reduced by 50 pct. In other words, the Can would be twice 

as stiff if it was not topped off with wood cribbing, assuming the two 

components had equal stiffness. 

 (1) 

 Table 1 shows the estimated initial stiffness of a 36-inch 

(0.91 m) diameter; 6-ft (1.83 m) tall, Can topped off with 2 to 4 layers 

of a 9-point or 16-point wood crib structure made from 8x8x36-inch 

(0.20x0.20x0.91-m) Lodgepole pine timbers. The stiffness values 

shown in this table have been derived from full-scale tests of these 

structures conducted in the NIOSH Mine Roof Simulator. As seen 

from this table, the initial stiffness of a 36-inch (0.91 m) diameter 

Can is reduced from 400 tons/inch (143 metric ton/cm) to 

99 tons/inch (35 metric ton/cm) if the Can is topped off with a 9-point 

crib structure that is 4 layers tall (32 inches [0.81 m]). 

 A full wood timber contact layer should always be used on the 

top of the Can even if fewer timbers are used above the immediate 

layer. Without a full immediate contact layer, the crib blocks will 

punch into the Can and initial capacity of the Can will be reduced to 

that provided by the contact area of the timbers instead of the full 

contact area of the Can (figure 7). In addition, the yield capacity of 

the cribbing structure should always be greater than the yield 

capacity of the Can. The NIOSH Support Technology Optimization 

Program (STOP) software can be used to estimate the yield 

capacity of the wood crib structure used on top of the Can (Barczak, 

2000). The yield capacities of the Can can also be determined from 

STOP, which will provide the following approximations: (1) 36-inch 

(0.91 m) diameter Can – 200 tons (181 metric tons), (2) 30-inch 

(0.76 m) diameter Can – 125 tons (113 metric tons), and (3) 24-inch 

(0.61 m) diameter Can – 100 tons (91 metric tons). For example, a 

9-point lodgepole pine wood crib has a yield capacity of about 

100 tons (91 metric tons). Therefore, if a 9-point crib would be 

constructed on top of a 36 inch (0.91 m) diameter Can, the 9-point 

crib would limit the capacity of the Can through the first 6+ inches 

(152+ mm) of convergence. Only after the wood crib has gone 

though several inches of plastic deformation (assuming it remains 

stable during this transformation) is it likely to gain sufficient stiffness 

to transfer load fully to the Can and cause the Can to yield. Another 

design consideration is the size of the Can for a particular seam 

height. The recommended height-to-diameter aspect ratio for 

application of the Can support is 5.0 to 1. For example, in a 10-ft 

(3.05 m) seam height, the minimum Can diameter would be 

24 inches (0.61 m). 

 Recently, a new prestressing technology has been introduced 

that can eliminate a lot of these issues. Thin-walled steel 

diaphragms that can be inflated with water have been designed for 

prestressing a wide variety of roof support products including the 

Can (Barczak et. al., 2004). The prestressing units (PSU’s) 

designed for the Can support are generally square in shape and can 

be fabricated in different sizes to accommodate different sized Can 

supports (figure 8). These units can be described as “flatjack” 

constructions where two, thin 0.039-0.079 inches (1-2 mm), sheets 

of cold-rolled steel are machine welded along the perimeter to form 

an encapsulated cell that can be pressurized. The PSU can be 

expanded up to about 6 inches (152 mm) after it is placed on top of 

the Can to close the gap between the Can and the mine roof. If the 

Can support can be sized to within 6 inches (152 mm) of the 

installation height, then wood cribbing can be eliminated. In this 

configuration (no wood cribbing), the PSU can be inflated with 

sufficient pressure to cause any size Can to yield. This eliminates 

all the stiffness reductions typically associated with wood cribbing on 

top of the Can and allows the full capacity of the Can to be applied 

immediately to the mine roof and floor upon installation. Since the 

Can has sufficient yield capability in itself, preloading the Can to its 

yield state will not have any negative impact on the subsequent 

support performance, and should enhance overall roof control by 

helping to build a more competent roof beam. If a moderate amount 

of timber is used in conjunction with the PSU, i.e., a full timber 

Can WoodCrib

Can WoodCrib

System K K

K K K

+

× =

Table 1. Impact of wood cribbing on overall stiffness of the Can support system. 

Can diameter, inches 

(m) Wood crib Wood crib stiffness, tons/in 

(metric ton/cm) 

Can stiffness, tons/in 

(metric ton/cm) 

System stiffness, tons/in

(metric ton/cm) 

36 (0.91) 9 pt 8x8x24 - 2 layers 210 (75) 400 (143) 138 (49) 

36 (0.91) 9 pt 8x8x24 - 3 layers 164 (59) 400 (143) 116 (41) 

36 (0.91) 9 pt 8x8x24 - 4 layers 131 (47) 400 (143) 99 (35) 

36 (0.91) 16 pt 8x8x24 - 2 layers 320 (114) 400 (143) 178 (64) 

36 (0.91) 16 pt 8x8x24 - 3 layers 287 (103) 400 (143) 167 (60) 

36 (0.91) 16 pt 8x8x24 - 4 layers 227 (81) 400 (143) 145 (52) 

0

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DISPLACEMENT, inches

SUPPORT LOAD, tons

Figure 7. A full layer of timbers should be used for the first 

layer. The photo and graph shows the consequences of not 

using a full timber layer. 
contact layer or header board, then the PSU will still have sufficient 

inflation expansion to generate yield loading in most applications. If 

the PSU is used without any timber, it should be placed on the 

closed end of the Can. Tests have shown that the welded seam 

sections on the open end of the Can will puncture the PSU on 

occasion when the Can deforms. The PSU can also be placed on 

the floor and the Can set on top of the PSU. There will still be plenty 

of pressure to lift the Can and preload the support in this 

configuration. 

 These PSU’s can also be equipped with a hydraulic yield valve 

to control the load development on the Can or avoid overpressurization of the PSU to prevent premature rupture. The yield 

valve is a simple spring-loaded system that is incorporated internally 

into the inlet check valve. Some older models may have a separate 

port for the yield valve. Different yield ratings can be obtained, but 

are typically 250 or 350 psi (1.7 or 2.4 MPa, respectfully) for the Can 

support applications, however, they can be customized if needed. If 

the PSU is not properly sized, the Can would yield before the PSU 

would yield. In this case, a yielding PSU may not be needed. For 

long-term applications of a year or more, it might be desirable to 

have the PSU yield prior to the Can yielding. This will help to reduce 

the risk of premature failure of the PSU due to corrosion. 

 If an oversized PSU is used as is illustrated in figure 8, which 

would be typical for Can applications, the PSU should be inflated 

with sufficient pressure to cause the PSU to fully balloon around the 

Can support. If this is not done, then the PSU will reduce the initial 

stiffness of the support as it is reshaped from the roof loading. The 

amount of pressure it takes to fully reshape the PSU during the 

inflation depends to some degree on the thickness and size of the 

PSU, but as a rule, a minimum setting pressure of 200 psi (1.4 MPa) 

will be adequate in most cases. The reshaping can be visually seen 

and the operator will not have a problem in knowing that the unit is 

properly inflated. The key point is that the unit should be used to 

preload the Can and not just to fill the gap between the Can and 

mine roof. The operator should also be aware that the preload is 

governed by the larger roof contact area and not the area of the 

Can. This may be important if the operator is trying to achieve a 

certain amount of preload on the support. 

 Some caution should be exercised in selecting the size of the 

PSU. If the PSU is too large, folds may occur in the ballooned areas 

around the Can. These folds can reduce the rigidity of the PSU and 

substantially reduce its stiffness to the point where the PSU and not 

the Can is controlling the stiffness. The folds will also increase the 

probability of premature failure of the PSU. A new development is 

underway to prevent this problem. The PSU has been reshaped in 

this new design to fit as a cap over the end of the Can. Figure 9 

shows a drawing of this new design. Prototype testing is expected 

to begin soon. 

 Although the Can has considerable capacity to absorb lateral 

displacements induced by floor heave, a study of this performance 

conducted by NIOSH through full-scale testing in the Mine Roof 

Simulator showed that this capability can be enhanced by proper 

fabrication of the Can. Cans in excess of 6 ft (1.83 m) in height are 

constructed in two or more sections that are welded together at 

horizontal seams. The welds tend to create a stress concentration 

due to the annealing that occurs during the welding. The stress 

concentration can cause a fold to occur near the weld during the 

early phases of the loading cycle. When this occurs and horizontal 

displacements are applied during testing to simulate floor heave 

conditions, the deformed area near the welded joint may act as a 

hinge allowing one section of the Can to tilt, while the other section 

remains more vertically oriented (figure 10). This hinge action will 

cause the Can to shed load throughout the loading cycle once the 

joint forms and horizontal displacements occur. The amount of load 

shedding can vary depending on how much rotation is occurring at 

the joint. In the example shown in figure 11, about 80 kips (40 tons 

Figure 8. A water-filled prestressing unit (PSU) is now 

available for topping off the CAN and applying a substantial 

active loading to the mine roof and floor. 

Figure 9. Diagram of a new PSU design for Can supports. 

This unit fits over the top of the Can like a cap and avoids 

issues caused by an oversized square PSU. 
[36 metric tons]) of load was lost through the 21 inches (0.53 m) of 

convergence and 14 inches (0.36 m) of lateral movement which 

occurred after the peak loading was reached. The load shedding is 

much less likely to occur if the deformation is confined to the top and 

bottom areas of the Can (figure 12). 

 Based on this observation, Can constructions where pieces are 

added onto both ends of a center, main body, section will provide 

optimum performance. Figures 13 shows a 24-inch Can that has 

gone through 25 inches (0.64 m) of both vertical and horizontal 

displacement without failure or instability that results in significant 

load shedding. Testing showed that 25 inches (0.64 m) of lateral 

displacement was the limit in this test configuration. Lateral 

displacements larger than that caused the Can to rip open. 

 In summary, the key to using the Can support successfully is to 

preserve the stiffness capability of the Can by avoiding too much 

wood cribbing on top. Since the wood acts in series with the Can, 

any amount of wood will degrade the initial stiffness of the Can, but 

the softer the crib structure used on top of the Can, the softer the 

Can response prior to yield will become. The hydraulic PSU 

technology can overcome this problem by preloading the Can to its 

yield rating so that this load is immediately applied to the mine roof 

and floor upon installation of the support. 

Figure 10. Buckling of Can caused by welding of sections 

together can create a hinge point during lateral displacement 

that may cause load shedding during yielding. 

0

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VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, kips

CAN Support (H) -- 24 in dia -- 8 ft height -- Single

timber layer -- 1.5:1 Vertical/Horz disp

Figure 11. Load shedding during yield is caused by the welded 

joint as shown in this full-scale performance test in the NIOSH 

Mine Roof Simulator. 

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VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, kips

CAN Support (O) -- 24 in dia -- 9 ft height --

Single timber layer -- 1.5:1 Vertical/Horz disp

Figure 12. Load shedding is less likely to occur if the folding 

can be confined to the top and bottom sections as shown in 

this test. 

0

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VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, kips

Can Support (B) -- 24 in dia -- 8 ft height (6

ft and 2 ft sections) -- 1:1 vert/horz disp

Figure 13. The limit of lateral displacement is shown where 

25 inches of lateral displacement occurred prior to the Can 

ripping open. 

Figure 14. The Cluster Prop is three Wedge Props bundled 

together as shown in this longwall gateroad application. 
Cluster Props as Alternative Tailgate Support 

An alternative to the Can support is the Cluster Prop. The 

Cluster Prop, as shown in figure 14, is based on the Propsetter 

support, which relies on the wedges cut into the bottom section of 

the timber post to control yielding of the prop. Three Wedge Props 

are bound together with three strong steel bands positioned at the 

top, middle, and bottom section of the props. This allows the three 

props to perform in unison and provide a stiff, high capacity, yet 

yielding support system. Currently, two Wedge Prop sizes, 8.5-inch 

(0.22 m) diameter and 10-inch (0.25 m) diameter, are used in the 

Cluster Prop design in western mines. The maximum 

recommended operating ranges for the 8.5-inch (0.22 m) Cluster 

Prop is 6 to 9 feet (1.83 to 2.74 m) and 6 to 12 feet (1.83 to 3.66 m) 

is the recommended operating range for the 10-inch (0.25 m) 

Cluster Prop. Each size is available from the manufacturer, Strata 

Products USA, in 6-inch (0.15 m) length increments. 

 The performance curves for the 8.5-inch (0.22 m) and 10-inch 

(0.25 m) Cluster Props as measured during full-scale testing in the 

NIOSH Mine Roof Simulator are shown in figure 15. Figure 16 

includes the 24, 30, and 36-inch (0.61, 0.76, and 0.91 m, 

respectfully) diameter Can performance. These conclusions are 

drawn from these performance curves. 

1. The 8.5-inch (0.22 m) Cluster Prop is rated at an average 

yield capacity of 150 tons (136 metric tons) and the 10-inch 

(0.25 m) Cluster Prop at an average yield capacity of 

225 tons (204 metric tons). Overall, the equivalent Cluster 

Prop provides support capability similar to that of the Can 

support. The equivalent support would be a 10-inch (0.25 m) 

Cluster Prop in comparison to a 36-inch (0.91 m) diameter 

Can and a 8.5-inch (0.22 m) Cluster Prop in comparison to a 

30-inch (0.76 m) diameter Can. 

2. The Cluster Prop will be slightly stiffer than the Can as a 

passive support, particularly when a lot of wood cribbing is 

used in conjunction with the Can. The 10-inch (0.25 m) 

Cluster Prop is stiffer than the 8.5-inch (0.22 m) Cluster Prop 

due to its larger cross sectional area. 

3. The yield behavior of the Can is more controlled with less 

load shedding and more consistent than that of the Cluster 

Prop. This is because the Cluster Prop relies on deformation 

of wood that can be erratic, while the Can relies on the 

confinement and folding of the steel container to control the 

post yield load. 

4. The Can has larger yield capability than the Cluster Prop. 

Although not shown in figure 16, full-scale tests have 

confirmed that for vertical loading only, the Can can yield 

through 50 pct strain. The Cluster Prop has been shown 

through full-scale testing to yield up to 20 inches (0.51 m) 

without headboards or footboards, provided the wedge props 

are yielding in unison. The yield capability of the Cluster 

Prop is less consistent and can be impacted by the loading 

conditions. It will be maximized when the props yield in 

unison and will be minimized if the props act independently. 

Hence, the steel strapping must maintain sufficient 

confinement to provide uniform behavior. 

 Installation issues can also be critical to optimizing the 

performance of the Cluster Prop. The Cluster Prop will perform best 

when used with no more than a rugged, 4-inch (102 mm) thick foot 

board or headboard, and can be used without any headboard or 

footboard. Like the Can support, any additional material in series 

with the prop will soften the overall response of the support. Its yield 

range is limited to the yielding of the Wedge Props. Attempting to 

extend the yield range by adding soft crib block timbers on top of the 

support (figure 17) is not recommended. The wood crib timbers, in 

addition to softening the support response, create a hinge point that 

can severely degrade the stability of the support, particularly in floor 

heave conditions where the base of the prop is moved laterally from 

the roof contact. The Cluster Prop can be installed with either end 

up. Traditionally, the prop is installed with the wedged end of the 

props down, but from a performance perspective this orientation 

does not matter. If used without a footboard or headboard, it may 

make more sense to install the wedged end against the roof if the 

floor is too soft to handle the bearing load when installed with the 

wedged end on the floor. Since the prop performance will be 

optimized when the wedge formation and crushing of the individual 

props act in unison, the orientation of the props with respect to the 

lateral movement or any condition that is likely to cause uneven 

loading of the individual props may help to determine how the prop 

is installed. As a rule, the Cluster Prop should be installed as 

vertically as is practically possible, although a 5-degree tilt was 

determined not to have a dramatic effect on the prop performance 

during full-scale laboratory testing. 

 The Cluster Prop, like the Can support, is installed with a 

mechanical aid. Unlike the Can system that utilizes a hydraulically 

powered clamp, the Prophandler is simply a mechanical clamp and 

a static tray from which the Cluster Props are slid into position. One 

advantage of Cluster Prop over the Can support is an improvement 

in haulage productivity since more units of equivalent capacity can 

be transported on the same supply car due to their smaller size and 

physical shape. Up to 100 of the 10-inch (0.25 m) Cluster Props can 

be transported in the same load compared to 35 Cans that are 

36-inches (0.91 m) in diameter. 

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DISPLACEMENT, inches

SUPPORT LOAD, Tons

Cluster Prop 8.5 inch

Cluster Prop 10 inch

Figure 15. Full-scale performance test data for the Cluster 

Prop illustrating a load-displacement performance for 8.5-inch 

and 10-inch models. 

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DISPLACEMENT, inches

SUPPORT LOADING, tons

Cluster Prop 8.5 inch

Cluster Prop 10 inch

Can 24 inch dia

Can 30 inch dia

Can 36 inch dia

Figure 16. Comparison of Cluster Prop to the Can support 

performance. 
Pumpable Roof Support Technologies 

 Pumpable roof support technologies have advanced with new 

developments in the late 1990’s. Heitech (part of Heintzmann 

Corporation) has led the development of a two-component, quick 

setting, grout system that can be pumped over 3 miles (4.83 km) 

through a surface borehole, into a fabric bag hung from the mine 

roof (figure 18). The bag not only provides a structure to form the 

support, but also provides confinement to the fractured grout once 

its peak capacity is exceeded. The support can be sized to satisfy 

specific loading conditions, a 24-inch (0.61 m) and 30-inch (0.76 m) 

diameter are the two standard sizes used in eastern tailgates. The 

performance curves for these two supports are shown in figure 19. 

As seen in these figures, the pumpable support provides a very stiff 

response, considerably stiffer than the Can support. But unlike the 

Can which utilizes an air-entrained material that can be 

volumetrically crushed and a steel container that can sustain its 

peak support capacity during yielding, the pumpable support sheds 

considerable load during its post peak behavior. This is because the 

fabric bag does not have the rigidity of the steel Can container, and 

cannot provide sufficient confinement to prevent this load shedding. 

A residual load of about 100 tons (91 metric tons) can be maintained 

through several inches, but the pumpable crib is never going to have 

the yield capability of a Can support. These supports have been 

successfully employed in a few eastern longwall tailgates and have 

performed very well for these applications. Successful trials of this 

support technology have also been conducted in bleeder entries in 

at least one western longwall with nearly a foot of floor heave, but 

the floor material moved around the support and did not induce 

much (probably less than 2 inches [51 mm]) deformation within the 

support. It is still unknown as to whether this technology can 

perform well in a high deformation environment where the 

convergence cannot be controlled by the support capacity. Hence, 

its application potential in yield pillar tailgates or areas of excessive 

floor heave remain unknown. 

New Support Technologies for Longwall Moves 

 Longwall moves have always been a critical element in longwall 

operations. When shields are removed from a completed longwall 

panel, some form of standing support is usually installed to provide a 

stable environment to protect workers. Historically, this has been 

done with conventional wood cribbing. However, transporting 

heavy, bulky crib blocks along a longwall face can be difficult and 

time consuming. Further, constructing the crib takes considerable 

time and exposes miners to hazard during support construction. 

Some western longwall operations have also used the Can for 

support in these longwall recovery operations. Although the Can is 

still bulky, it can be transported as a unit and installed quickly with a 

machine. At least one western longwall operator is also using the 

hydraulic PSU’s discussed in the previous section to preload the 

Can and actively transfer load to the mine roof for longwall recovery 

operations. In this application, the PSU with a yield valve installed, 

is placed underneath the Can on a layer of wood cribbing and 

Figure 17. Adding crib timbers to the top of a Cluster Prop is 

not advisable. As shown in this figure, this can create a hinge 

point that can severely degrade the stability of the support in 

high deformation environments. 

Figure 18. Pumpable roof supports offer an alternative 

support system whereby the support material can be pumped 

from the surface or a remote underground location. 

Figure 19. Performance assessment of Pumpable Roof 

Supports from Heitech showing load-displacement curves for 

24 and 30 inch diameter supports typically used in longwall 

tailgate applications. 

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0123456789

VERTICAL DISPLACEMENT, inches

SUPPORT LOAD, tons

Heintzmann Pumpable Crib - 30 in dia

Heintzmann Pumpable Crib - 24 in dia
inflated to the yield pressure. Once the adjacent shield is removed, 

the PSU yields fluid as the additional roof weighting occurs. The 

controlled yielding provided by the yield valve ensures a controlled 

transfer of load from the mine roof to the standing support to 

maintain a stable roof condition during the shield removal process. 

 Other mines have moved towards smaller standing support units 

in order to alleviate the transportation requirements for the bulkier 

Can support. Various forms of steel and timber props have been 

used in this capacity in the past including engineered supports such 

as the Propsetter, but recently two new products have shown 

promise in this application. These are the Rocprop and the Omni 

Prop. 

 The Rocprop, marketed by Strata Products USA, is a hydraulic 

cylinder that is extended against the mine roof and floor by pumping 

water or some other hydraulic fluid into the cylinder. Once the 

cylinder is set, a locking collar transforms the cylinder into a 

mechanical prop where the steel tubing is deformed in a controlled 

manner. A 35 and 50 metric ton unit (yield load rating) is available 

for 10-ft (3.05 m) mining heights. As a mechanical prop, Rocprop 

provides a very controlled yield capability which is beneficial, but its 

main advantage in the longwall recovery operation is that it can be 

set to various heights very easily and can provide an active load to 

the mine roof during installation. The performance curves as a 

passive support (no preloading) for the 35 and 50 metric ton unit are 

shown in figure 20. 

The Omni Prop is a South African product that has recently been 

introduced into the American market by Heintzmann Corporation of 

Cedar Bluff, VA. The Omni Prop is essentially a single stage 

hydraulic cylinder. Water is pumped into the prop using shield 

emulsion or water. The fluid causes the end of the prop to extend to 

the roof line. As pressurized fluid continues to enter the prop, an 

active roof load up to the yield load rating of the prop can be applied 

to the mine roof. Up to this point, the Onmi Prop functions basically 

the same way as the Rocprop. However, unlike the Rocprop, the 

Omni Prop remains a hydraulic prop. Once the setting phase is 

completed and the hose is removed from the inlet port, a check 

valve in the bottom of the prop maintains the setting load. As the 

roof loading increases and overcomes the setting force, the 

captured water in the prop increases in pressure, functioning just 

like a hydraulic cylinder. The piston in the cylinder includes a yield 

valve, which controls the pressure in the cylinder. Once the yield 

pressure is reached, water is relieved though the piston and into the 

hollow section of pipe on the opposite end of the prop. If the prop is 

installed with the pressurized cylinder against the mine roof, the 

relieved water will flow into the bottom section and out a small slit in 

the base of the prop. This is a visual sign that the support is 

yielding. The performance curve for the Omni Prop is shown in 

figure 21. As seen in the figure, the support provides a consistent 

yield load of slightly more than 50 tons (45 metric tons). 

Other Support Technologies 

 The Sand Prop, marketed by Strata Products USA, incorporates 

a novel approach to developing an extendable support. The support 

consists of two telescopic sections of pipe. It is preassembled and 

shipped in a collapsed state. The top section is pre-filled with a 

“sand-like” or granular material. As the upper section is lifted into 

place, the granular material flows from the top section into the 

bottom section to form a column of granular material upon which the 

upper section rests. Figure 22 shows the performance curve for the 

Sand Prop and an underground installation. It has a load rating of 

50 tons (45 metric tons). A yielding version of the Sand Prop is 

called the Bolt Prop. The Bolt Prop includes a collar that is threaded 

with bolts which are inserted to a specified torque and shear into the 

face of the inner pipe as yielding occurs. The yield rating of the Bolt 

Prop is 40-50 tons (36-45 metric tons), respectfully, but can be 

inconsistent during yielding, especially in the presence of eccentric 

loading. 

 Another support that has recently been introduced into western 

longwall operations is the Spider Prop, marketed by Heintzmann 

Corporation. The Spider Prop has a unique design that provides 

extension capability to a conventional timber post or steel prop. A 

photo of the Spider Prop is shown in figure 23. In this version, a 

steel cap is fitted over a conventional timber post. A specially 

designed steel endplate is secured to the end of the timber post. 

Cutters are included in this end piece as shown in the figure. A wire 

spring holds the cutters in place as the metal cap is lifted to the roof 

height. Then the cutters engage the inside of the metal cap and 

0

10

20

30

40

50

60

70

0 4 8 12 16 20

DISPLACEMENT, inches

SUPPORT LOAD, tons

Rocprop - 35 metric ton

Rocprop - 50 metric ton 

Figure 20. Performance assessment of the Rocprop support 

showing 35 and 50 metric ton designs. 

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10 12 14 16

VERTICAL DISPLACEMENT, inch

VERTICAL LOAD, Tons

OmniProp

Figure 21. Performance assessment of the Omni Prop, a 

water filled hydraulic cylinder that provides active roof loading 

with controlled yielding. 

0

10

20

30

40

50

60

0.0 0.5 1.0 1.5 2.0 2.5 3.0

DISPLACEMENT, inches

LOAD, tons

Sand Prop Model 9-11 ft

Figure 22. Performance curve for the Sand Prop from fullscale testing in the NIOSH Mine Roof Simulator and an 

underground installation. 
shear through it as the roof loading begins. The cutters then peel 

open 1/4-inch (6.25 mm) slots in the metal cap to provide controlled 

yielding. The performance curve is shown in figure 24. 

Summary and Conclusions 

 Secondary support in western U.S. mines has evolved as Can 

supports replaced conventional wood cribbing. The Can continues 

to be an effective design with a very successful performance record. 

The key is to maintain the performance capability of the Can by 

proper capping practice, which historically has been done with crib 

timbers. Poor practice in this regard can significantly soften the 

support response and degrade its stability as well. Recently, a 

water-filled diaphragm has been developed as a prestressing unit 

(PSU) for Can applications. The PSU can eliminate the need for 

wood cribbing if the Can is sized to within 6 inches (0.15 m) of the 

installation height. The PSU can also preload the Can to its yield 

capacity and provide a substantial active load to the mine roof and 

floor. Prestressing secures the Can in place and prevents it from 

being dislodged, unintentionally by pillar sloughage or other events, 

once it is set in place. The PSU can also be used to control load 

development by utilizing a yield valve in the PSU. 

 Other support concepts have been introduced in the past 2 or 3 

years that can provide an alternative to the Can. These include the 

Cluster Prop and the pumpable roof support. The Cluster Prop is a 

timber support that utilizes three Wedge Props to provide a support 

system with equivalent capacity to the Can. It is slightly stiffer than 

the Can prior to yielding and can yield through about 20 inches 

(0.51 m) of convergence, but its yield load is less controlled than 

that provided by the Can support. Its primary advantage is improved 

haulage productivity since more units of equivalent capacity can be 

transported on the same supply car due to their smaller size and 

shape. Pumpable roof supports continue to perform well in eastern 

longwall tailgates and at least one successful trial has been 

achieved in a western longwall bleeder entry application. The 

pumpable roof support is significantly stiffer than the Can prior to 

yielding, but the fabric bag does not have sufficient rigidity to sustain 

the peak loading, and this is the primary performance difference 

between the pumpable roof support and the Can. The pumpable 

roof support’s capability to perform in a high deformation 

environment, where the convergence cannot be controlled by the 

support capacity, is doubtful but extensive trials in such applications 

have not yet been done. 

 Other advancements have been made in support applications for 

longwall support recovery. Here too, the Can in combination with 

the hydraulic PSU’s, have been successfully employed. Other 

supports that show promise include the Rocprop and Omni Prop. 

The Rocprop has some proven history in this area in U.S. mines 

both in the East and West, while the Omni Prop has just recently 

been introduced to the U.S. market. Although the Onmi Prop has a 

successful history in South African mines, it has not been installed 

underground in the U.S. to date. Both of these products provide a 

relatively small, easily transported, support that can be quickly set 

and provide a substantial active roof load. They also provide very 

controlled yielding. These performance characteristics make them 

well suited for longwall recovery operations, and are often used in 

conjunction with roof beams to help control the span in the 

immediate working area during shield recovery. 

 Finally, specialty products now exist in standing supports much 

like they do in roof bolts. Two such products are the Sand Prop and 

Spider Prop. The primary features of both products are their 

extendibility, and therefore, ease of installation. The Sand Prop 

transfers granular material from one end to the other to provide easy 

height adjustment during installation. The Sand Prop is considered 

a “non-yielding” support with only 1-2 inches (25-50 mm) of yield 

capability. A variation of this design, called the Bolt Prop provides a 

yielding support product. The Spider Prop, on the other hand, uses 

a rather sophisticated mechanical cutter assembly to allow height 

adjustment of a timber post or steel prop. These products have 

limited capacities and stability, and therefore are more likely to be 

used in areas along belt lines or other applications where space is 

limited and is a primary factor in support selection. 

 In summary, because of these innovations in support 

technology, there are now a wide variety of support systems to 

assist in gateroad and shield recovery ground control. Each system 

has its advantages and disadvantages. There is not an ideal roof 

support for all conditions. However, these new support systems 

provide mine operators with several alternatives to the conventional 

wood cribs that have historically been used for secondary roof 

support and the more recent Can support applications. 

Performance data for these support systems, presented in this 

paper, will help to ensure their safe utilization. 

References

1. Barczak T.M. and G.M. Molinda (1996), "Innovative Secondary 

Support Technologies for Western Mines", Published of the 15th 

International Conference on Ground Control in Mining, Golden 

CO, August 13-15, pp 349-364.

2. Frederick, J.R. (1994), "The 3C Support: A Survivable 

Alternative to Wood Cribbing", Published of Longwall USA, June 

7-9, pp 11-21.

3. Barczak, T.M., S.C. Tadolini and P. McKelvey (2004), "Hydraulic 

Prestressing Units: An Innovation in Roof Support Technology. 

Published in the 23rd International Conference on Ground 

Control in Mining, Morgantown WV, August 3-5, pp. 286-294. 

4. Barczak, T.M. (2000), "Optimizing secondary roof support with 

the NIOSH Support Technology Optimization Program (STOP)", 

Published in 19th International Conference on Ground Control in 

Mining, Morgantown WV, August 1-3, pp. 74-84. 

Prestressing bladder 

Steel cap with protective 

rubber shroud 

Cutters 

Timber post 

Figure 23. Photo of the Spider Prop, a unique approach to 

developing an extendable and yielding timber prop. 

0

10

20

30

40

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

VERTICAL DISPLACEMENT, inches

VERTICAL FORCE, tons

Figure 24. Performance curve for the Spider Prop based on 

full-scale testing in the NIOSH Mine Roof Simulator.