ABSTRACT 

 A new generation of hydraulic mine support prestressing 

devices has been developed. These thin-walled steel shells are 

machine-welded and can be inflated with water or any liquid to 

provide prestressing for a wide variety of roof support products 

ranging from Can supports to props and cable bolts. With an 

expansion capability of several inches, depending on the geometry 

and size of the unit, they can also establish roof contact eliminating 

the need for wooden wedges or crib blocks that are commonly 

employed with several standing roof support products. Capable of 

withstanding pressures from 1,000 to 6,000 psi, these cells are able 

to transfer roof loading into the support structure without rupturing. 

A recent development has been the addition of an inexpensive yield 

valve that provides control of the maximum pressure and load 

development on the support. This paper examines the performance 

capabilities of these inflatable prestressing units and the impact 

they have on the performance of various support systems, including 

an evaluation of the overall stiffness of the support system and the 

load control during yielding of the prestressing unit. 

Recommendations are made regarding the design requirements of 

the prestressing unit to optimize the support performance. A 

summary of mine applications using this technology is provided 

with general comments regarding their impact on ground control. 

INTRODUCTION 

 A primary goal of any roof support system is to support the 

mine roof as soon as possible. This goal has led to prestressing 

roof support systems so that an immediate active support load is 

applied to the mine roof. Although the benefit of prestressing 

remains a debatable issue, most mining engineers will agree that 

prestressing generally has inherent advantages over a strictly 

passive roof support system. The active roof loading provided by 

prestressing closes separations in the roof and increases frictional 

restraint along bedding, joints, and other fracture planes, thereby 

enhancing beam building. 

 While pretensioning of intrinsic supports has been practiced 

since the earliest days of roof bolting, prestressing of standing 

supports has not been done nearly as much. And since most 

standing supports are passive, closure of the opening must occur for 

a resistive load to be developed. In the case of timber cribs, some 

5-10% closure is required before the crib provides significant roof 

support capacity and as much as 40% closure before attaining its 

full load capacity (1). This soft response can cause damage to the 

roof beam, dislodgment of key blocks of roof rock, and subsequent 

unraveling of the roof structure with implications for the safety of 

the miners. 

 The development of low-cost, inflatable bladders that can 

function as easy-to-install prestressing units (PSU’s) has provided 

active loading to a wide variety of support systems that previously 

were utilized only as passive supports in many different mining 

operations. Developed in the late 1990’s, this new generation of 

water-filled PSU’s has evolved rapidly for use in the hard rock 

mining industry, especially in South Africa, where blasting is still 

the predominant extraction method, with thousands being used on a 

daily basis. Preloading helps to keep the supports, such as timber 

props and cribs, in place during the dynamic loading associated 

with the blasting. Recently, this technology has been introduced to 

coal mining operations in the United States and Australia. 

 The purpose of this paper is to examine this new generation of 

PSU’s in more detail including: the range of support applications 

using this technology, the performance characteristics of the units 

relative to their expansion capabilities, load capacities, yielding 

characteristics, and their impact on the overall support stiffness. 

Proper installation procedures to ensure the full benefit of this 

technology are also discussed. In-service observations from early 

trials of these units in U.S. coal mines are presented. 

HISTORICAL OVERVIEW OF PRESTRESSING SYSTEMS 

 Preloading of standing roof supports is carried out in 

compression and since most supports are completely passive, the 

preload is generally achieved by incorporating some sort of 

expansive unit within or on top of the support structure. Systems 

for preloading standing supports can be categorized as follows: 

wedging, mechanical devices, air-filled rubber bladders, groutfilled bags, and water-filled metal diaphragms. 

Wedging - Wooden wedges have been used since the earliest 

times to secure a crib or prop support against the mine roof. 

Although generally not known for their prestressing capability, 

driving wooden wedges in place with a 5-10 lb hammer can 

generate 2-4 tons of preload on timber posts or cribs depending on 

the size of wedge used (2). Preloading in this manner is a laborious 

task and because of the low mechanical efficiency of wedges 

(~20%), a lot of effort is required for minimal benefit. 

HYDRAULIC PRESTRESSING UNITS: AN INNOVATION IN 

ROOF SUPPORT TECHNOLOGY 

Thomas M. Barczak, Research Geophysicist 

Stephen C. Tadolini, Section Chief, Geotechnical Engineering

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania USA 

Paul McKelvey, Director--AMSS 

New Concept Mining 

Johannesburg, South Africa 
 Mechanical Devices - Mechanical devices for prestressing 

standing roof supports are relatively rare. The Loadmaster or 

Power Wedge1

 (figure 1) is one example. Using a torque wrench, 

pre-loads of up to 11 tons can be achieved with 300 ft-lbs of 

applied torque (3). Any mechanical device, and wedges in 

particular, suffer from poor efficiency. A screw thread is at best 

25% efficient, and when combined with a wedge the overall 

efficiency may be as low as 5%. This is a disadvantage when the 

pre-stress unit is required to be manually set such as with the 

Loadmaster or Power Wedge prop. 

Air-Filled Rubber Bladders - Prestressing of roof support 

using air-inflated Vetter Bags1

 was carried out in the early 1980’s 

in South African mines. The air bags were placed on top of the crib 

and inflated to preload the support prior to driving in wooden 

wedges. The air pressure was then released and the bag was 

removed and reused on the next support, leaving the wedges to 

sustain the preload. 

Grout-Filled Bags - In 1990, a South African company called 

HLH introduced the Packsetter – a woven polypropylene bag with 

a thermoplastic liner into which a special grout was pumped at 

pressures of up to 60 psi. Depending on the size of the grout bag, 

loads of up to 80 tons have been achieved using a single bag. This 

system was effective, economical and very successful in South 

 

1

Mention of company name or product does not constitute endorsement by 

the National Institute of Occupational Safety and Health. 

African mines. It eliminates the use of wedges and blocking 

material used to fill in gaps between the top of the crib and the roof. 

The main disadvantages were the inconvenience of transporting, 

mixing, and pumping the grout. HLH then developed the 

Propsetter. Strata Products USA has marketed the Propsetter 

system as shown in figure 2 in the U.S. for the past 10 years. This 

system provided up to 10 tons of active loading, but material creep 

typically reduced it to around 5 tons of sustainable preload on 

timber props (4). Until now, the Propsetter has been the most 

commonly used prestressing system on standing roof supports in 

U.S. coal mines. 

Water-Inflated Steel Diaphragms - The use of water as a 

prestressing medium provides advantages over the grout-based 

systems used in the past. First, water is generally available 

underground, eliminating the need to bring in additional materials. 

In addition, prestressing can be repeated if necessary to offset creep 

that may occur in the support structure, particularly timber support 

systems. While initially a controversial idea, the development of 

leak-tight inlet valves and improved fabrication techniques has led 

to greater acceptance of this technology. The use of inflatable steel 

diaphragms to pre-load mine props was first tried at Winkelhaak 

Gold Mine in South Africa in 1978. While the preloads achieved 

were adequate and technically the trial was successful, the idea was 

abandoned because of the high cost of the units at that time. Then 

in 1997, New Concept Mining carried out further development 

resulting in the introduction of the Jackpot which has become the 

most popular design for prestressing of posts and props. 

Subsequent to the development of the Jackpot for prestressing 

props, flat circular and rectangular units (Jackpacks) were 

developed for prestressing crib type supports. The Jackpot and 

Jackpack products are marketed by the Heintzmann Corporation. 

Similar products are also manufactured by other companies in 

South Africa and are being distributed through other vendors in the 

United States. 

PERFORMANCE CHARACTERISTICS OF HYDRAULIC 

PRESTRESSING UNITS 

Hydraulic prestressing units (PSU’s) work on the principle 

whereby a sealed deformable steel diaphragm is inflated with 

pressurized water resulting in the conversion of hydraulic energy 

into active loading onto the mine roof and floor upon installation of 

the roof support. Hydraulic PSU’s have been developed for the 

Figure 2. Propsetter grout bag has been the most commonly 

used prestressing system in the United States thus far. 

Figure 1. Mechanical device used to preload (various) timber 

post support systems. 

3 in
prestressing of post or prop type supports, crib or pack type 

supports, as well as for cable bolt or intrinsic roof anchors. The 

development continues to expand as new support applications are 

introduced. 

Post or Prop Type Supports 

 The Jackpot (figure 3) is a cap-shaped unit designed for proptype supports. It is fabricated in various sizes ranging from 5.5 to 

10.4 inches in diameter to accommodate different size props, 

particularly timber posts. It is designed to fit over the top of the 

post with the lip helping to keep the unit in place. The shape of the 

unit is critical to its performance capabilities. The perimeter 

section that forms the lip of the cap is about 2 inches high and 3⁄4 of 

an inch wide. This shape not only contributes to its strength, but 

also allows for considerable expansion of the unit if necessary 

during inflation. As the unit is pressurized with a gap between the 

top of the support and the mine roof, the bottom interior section of 

the cap expands downward unrolling the interior face of the lip. 

Although the Jackpot can be expanded to a spherical shape, the 

useful expansion capability of the Jackpot is 3-4 inches, and it is 

generally recommended that this expansion be limited to less than 

3 inches to ensure the full loading capability of the unit. At 

expansions of less than 2.5 inches, the unit has an installation 

pressure rating of 2,000 psi with a burst pressure of 6,000 psi. 

Figure 4 shows calibration curves for standard Jackpot production 

units correlating pressure to support load. It is seen from the figure 

that the loading is linearly related to the internal pressure of the 

cell. Since some work is required to deform the metal structure, the 

effective area of the unit is somewhat less than the top surface area 

calculated from the diameter of the unit. For example, the effective 

diameter of the 10-inch diameter unit is 9.5 inches. The cell 

initially requires about 100 psi water pressure to cause it to inflate. 

Crib or Pack Type Supports 

 Since the surface area of these supports is so much bigger than 

those for prop-type supports, a different design is employed. These 

units can be described as “flatjack” constructions where two thin 

(1-2 mm) sheets of cold-rolled steel are machine welded along the 

perimeter to form an encapsulated cell that can be pressurized. The 

units can be fabricated in various shapes (circular, square, 

rectangular) and sizes to accommodate different support 

geometries. Typical Jackpacks PSU's are shown in figure 5. 

 The expansion capability of the Jackpack units can vary 

depending on the size and shape of the unit. Square geometries 

tend to perform better (expand more evenly with less folding of the 

metal) than circular geometries at high levels of expansion. A 

36 x 36-inch unit has been tested at expansions up to 12 inches at 

300 psi of loading. However, the cell is more likely to rupture at 

lower pressures when folding of the metal container is excessive at 

these large expansions. It is recommended that the cell expansion 

be limited to 6 inches or less to ensure the full operating pressure of 

the cell is maintained with a high level of confidence. 

 The burst pressure on the Jackpacks is considerably lower than 

that of the Jackpots. The burst pressure can vary depending of the 

shape and deformation of the cell. Maximum pressures of 

1,000 psi are attainable under full contact loading conditions. 

Loads of 400 tons with an internal pressure of 800 psi have been 

Figure 3. Jackpot prestressing cap on a timber post. 

Figure 5. The Jackpack PSU's can be fabricated in a variety 

of shapes and sizes to accommodate specific support 

applications. 

Figure 4. Calibration curves showing load as a function of 

internal water pressure for Jackpot PSU. 

y = 0.0232x - 7.3464

y = 0.0305x - 7.6371

y = 0.0385x - 10.214

y = 0.0451x - 8.7057

y = 0.0655x - 12.736

0

20

40

60

80

100

120

140

160

180

0 500 1,000 1,500 2,000 2,500

WATER PRESSURE, psi

PSU LOAD, kips

JP140 JP160

JP175 JP190

JP220 JP260

(JP140)

(JP160)

(JP175)

(JP190)

(JP220)

y = 0.0801x - 9.24 (JP260)
achieved under these ideal loading conditions. Because of the 

larger area, Jackpacks are typically fitted with a pressure relief 

valve to control loading and avoid over pressurization. Yield 

settings are typically in the range of 250-350 psi. 

Cable Bolt or Roof Anchor Supports 

 Pretensioning of roof bolts and some cable bolts is achieved by 

tightening the end nut to a predetermined torque. Although this has 

been an effective means of pretensioning conventional roof bolts, 

this approach is more problematic with cable bolts since the wire 

strands twist when the torque is applied and can untwist when the 

torque is removed resulting in a decay of the achieved tension. Nut 

tightening to achieve pretension is also subject to significant 

frictional losses that further reduce the efficiency of this approach. 

These factors lead to a high variability (up to 45%) in the achieved 

pretension load (5). Another approach for pre-tensioning cable 

bolts is to use a hydraulic jack. Tensioning jacks are heavy and 

manually applied and the procedure can be time consuming. In 

addition, barrel and wedge type anchors, often used with these 

cables, are subject to distortion or slippage, resulting in some 

reduction of pretension load following release of the jack (6). 

 Conversely, donut-shaped PSU’s create axial loads in the cables 

without twisting the cables’ strands and have around 5% variation 

in preload. The donut-shaped PSU (figure 6) is fitted underneath 

the roof bolt or cable bolt end plate and applies tension by 

hydraulic inflation. This avoids the common causes of load loss 

associated with both frictional and anchorage effects. Typical load 

capacities for these units are 25 tons and 50 tons for the 6-inchdiameter units and 8-inch-diameter units respectively. The loadpressure relationship for the 8-inch-diameter design is shown in 

figure 7. Tests in the UK using a resin-anchored rock bolt and an 

8-inch-diameter PSU demonstrated ”lock in” of the pretension 

using a two-speed resin system. This is feasible because the 

tensioning procedure using donuts is quick and simple. 

Application of the tension after the fast resin has cured, but the 

slow resin is still liquid, results in retention of this load over most 

of the bolt length, even if the donut is subsequently depressurized 

and removed. 

Pressure Relief Valve 

 The use of water also allows the incorporation of a pressure 

relief valve (PRV) to limit the load development in the PSU. By 

expelling water through the PRV at a designated pressure, the PSU 

will provide a degree of controlled yielding to the support until the 

PSU is fully compressed and no fluid remains. The valve, as 

shown in figure 8, is a simple plastic unit with a spring loaded 

plunger that seats against the body to act as a check valve. The 

yield pressure is controlled by the stiffness of the spring. The 

Jackpack pressure relief is typically set at 250-350 psi, sufficient to 

develop loads comparable to the capacities of larger scale supports 

such as the Can. The smaller Jackpot PSU’s, which are designed 

for posts or prop-type supports, require considerably higher yield 

pressures to develop comparable load to the capacities of those 

support systems. The pressure relief on these PSU’s is typically set 

between 1,500 and 2,000 psi. 

Figure 6. Donut-shaped PSU used to pretension cable bolts. 

Figure 8. Pressure relief valve installed to provide load control 

or prevent over pressurization of PSU. 

Figure 7. Calibration curves for cable bolt PSU. 

y = 0.0345x - 19.766

y = 0.0346x - 21.546

y = 0.0445x - 47.6

0

10

20

30

40

50

60

70

80

90

500 1,000 1,500 2,000 2,500 3,000

WATER PRESSURE, psi

PSU LOAD, kips

13mm Lift

20mm Lift

50mm Lift

50 mm:

13 mm:

20 mm:
 Studies were conducted using a 36 x 36-inch Jackpack to 

evaluate the performance characteristics of the pressure relief 

valve. The tests showed that the load required to produce yielding 

of the support was 317 kips (158.5 tons), which was higher than 

anticipated based on the manufacturer’s specifications. Comparing 

this to the capacity of a Can support, a 30-inch-diameter Can will 

yield at approximately 150 tons, therefore the Can will yield before 

the PSU yields in this particular case. If the goal is to have the 

PSU yield prior to the Can, then a lower rated yield valve or a 

smaller sized PSU would need to be used. This design issue 

illustrates the need to install the proper yield valve for a particular 

support. In the studies conducted on the 36 x 36-inch Jackpack, the 

load dropped to about 150 kips (75 tons) when the load frame 

displacement was held, providing an indication of the resetting 

behavior of the PRV. The yield valve was also cycled several 

times during the tests to confirm the repeatability of the unit over 

several loading cycles. 

GROUND CONTROL PERFORMANCE RELATED ISSUES 

Jackpack Preload Can Vary with Inflation Height 

The Jackpack will change shape as it is inflated and this will 

cause a variation in preload for the same fluid pressure. The cell is 

originally flat. When inflated, it forms a pillow shape and the 

curvature from the perimeter of the cell results in less contact area 

on the mine roof as the inflation thickness increases. Figure 9a 

shows the correlation of inflation thickness to achieve preload for a 

36 x 36-inch PSU over a thickness range from 1 to 6 inches with 

inflation pressures ranging from 0 to 300 psi. Figure 9b shows the 

preload for 24-inch-diameter circular PSU for a relatively constant 

inflation pressure of 115 psi. The following conclusions are 

derived from this laboratory study. 

 

• For a given inflation thickness, equivalent to the gap between 

the support and the mine roof, the preload will increase 

proportionally with an increase in the inflation pressure. 

• The achieved preload for a given inflation pressure will 

decrease proportionally as the inflation thickness increases. 

Using the 24-inch-diameter circular cell as an example, the 

preload at 115 psi inflation pressure under full contact 

conditions decreases by 5.8 kips (2.9 tons) for each inch of 

increased bladder thickness as shown in figure 9b. Likewise, 

the preload decreases by about 25 kips (12.5 tons) for each 

additional inch of bladder thickness for the 36x36-inch PSU 

(figure 9a). 

Preload and Load-Pressure Response Will Vary with 

Contact Conditions 

 A question that has been asked by mines using the PSU’s to 

preload supports is: How much preload is developed when an 

oversized PSU is placed on top of a support such as the Can shown 

in figure 10? Is the preload determined by the pressure acting on 

the larger roof contact area or is it dictated by the smaller area of 

the support, i.e., the Can in figure 10? Since there must be load 

transfer between the PSU and the support, whatever load is acting 

on the roof by pressurization of the cell is also acting on the 

support. Therefore, the water pressure acting on the larger mine 

roof contact surface for the most part controls the preload 

developed on the support. The part of the cell that is not in direct 

contact with the support is transferring the force acting in this 

portion of the cell to the support. In this sense, it is no different 

that having timber blocks on top that are larger than the Can. 

Whatever roof load that is being carried by the timber blocks is also 

being carried by the Can. The stress acting on the timber blocks 

against the mine roof is lower than the stress acting on the Can, but 

the load is the same. So, the preload can be estimated by taking the 

roof contact area times the water pressure. Remember, when 

making this estimate, that the contact area will be a smaller 

percentage of the full un-inflated area of the cell with this 

percentage dependent on the inflation thickness. 

Figure 9a. Variation in load development in 36 x 36-inch 

PSU for inflation thickness ranging from 1 to 6 inches. 

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300

WATER PRESSURE, psi

PSU LOAD, kips

1 in thick 

6 in thick

4 in thick

3 in thick 

2 in thick 

Calculated Load 

36 x 36 area, kips

Figure 10. Oversized PSU inflated on top of Can support. 

PSU

CAN 

Figure 9b. Load as a function of inflation thickness for a 24-

inch diameter circular PSU. 

y = -5.7938x + 46.938

R2

 = 0.983

0

5

10

15

20

25

30

35

40

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

PSU THICKNESS, inches

PSU PRE-LOAD, kips
 As noted, this conclusion is qualified with the comment “for the 

most part”. This is necessary because the PSU is not a rigid 

structure. The PSU will deform when the contact area of the 

support is smaller than the PSU. The resulting shape of the PSU 

will reduce the preload from the full contact condition or rigid 

assumption made in the preceding paragraph. This can be 

explained by analyzing the forces involved. Because fluids cannot 

contain shearing stress, the water pressures act normally (at right 

angles) to the interior surface of the cell. Since the roof contact is 

relatively flat, the water pressure is “directly” used in developing 

active roof loading and preloading of the support. On the bottom 

side where the PSU contacts the support, the PSU will expand or 

swell outward due to the internal pressures in the zones adjacent to 

the smaller support contact area (see figure 10). If the shape 

remained perfectly symmetrical, then shear forces will still not 

exist and the preload would be the same since the resultant vertical 

forces would be the same. However, this ideal condition does not 

exist and the resulting stress in the metal container decreases the 

effective hydraulic pressure of the water. Another way of 

envisioning this is to think that some of the work of the water 

pressure is lost to deform the cell instead of being used to preload 

the support. Another contributing factor is that the empty shell 

itself (no water inside) can carry some load. This component will 

be maximized for full contact conditions as a uniform ring around 

the perimeter will provide a significant load capacity (10 kips or 

more.) The total capacity of the PSU is the hydraulic load plus the 

structural capacity of the cell. 

 Figure 11 is an example for a three-inch inflation thickness on a 

24-inch diameter PSU showing three support contact conditions: 

(1) full top and bottom contact equivalent to having the PSU fully 

sandwiched between the mine roof and the support, (2) a simulated 

17.75-inch-diameter support, and (3) a simulated 14-inch-diameter 

support. The full contact condition is equivalent to having a 

support as large as or bigger than the PSU. Conversely, the other 

two conditions simulate a smaller support than the PSU or an oversized PSU. At this inflation thickness, the contact diameter on the 

mine roof is approximately 19.5 inches. As seen in the figure, full 

top and bottom contact condition provides a slightly larger load 

than what the contact area suggests it should be. The reason for 

this is that the empty cell has some load carrying capability (about 

10 kips in this particular PSU), much like a soft drink (soda) can 

has load carrying capability without the liquid in it or a Can support 

has capacity without the fill material in it. As the support contact 

area decreases, the support load decreases for a given water 

pressure in the PSU. This is expected since the unsupported 

section of the PSU forms a larger rounded shape as the unsupported 

section grows in size. However, the achieved preload will always 

be closer to that determined by the full roof contact area than the 

support contact area. 

Variation in PSU Stiffness Due to Contact Conditions 

This section deals with the stiffness of the hydraulic cell, not 

the system stiffness which includes the combination of the PSU and 

the support element. That topic is discussed in the next section. 

Because as an elastic (bulk) modulus of about 300,000 psi, the 

hydraulic bladder can be a very stiff system. For this to happen, the 

PSU shell must not be severely stressed beyond its elastic limits 

where permanent deformations cause the shell to change shape 

during loading after the cell is inflated upon installation. This 

condition only exists for full top and bottom contact conditions 

where the cell is fully sandwiched between the mine roof and the 

support product. Once the compliance is achieved between the 

contact surfaces of the mine roof and the support, the fluid being 

confined by the cell is compressed by the applied loading providing 

a stiff response. For a 24-inch-diameter circular PSU with an 

inflation thickness of 1 inch, stiffness up to 5,000 tons/in can be 

achieved in this full contact condition. This is an order of 

magnitude higher than any standing roof support structure. 

 If the bladder is not fully confined on the top and bottom during 

inflation, then the unconfined portions of the cell will expand or 

balloon as shown in figure 10. This can temporarily cause a 

reduction in stiffness while the ballooning is occurring. Once the 

cell is fully reformed by sufficient pressure development within the 

cell, it will attain a stiffness comparable to the full contact stiffness, 

providing the metal container maintains a consistent shape and does 

not wrinkle or develop folds in the unconfined ballooned areas. 

This concept is illustrated in figure 12, which compares the 

stiffness of a 3-inch-thick, 24-inch-diameter, circular PSU under 

full top and bottom contact and when the PSU was sandwiched 

between two 17.75-inch-diameter concrete disks. In this particular 

test, about 0.5 inches of displacement occurred transforming the 

cell from a flat section to the donut-shaped structure. As shown, 

the stiffness of cell during this transformation was less than 

50 tons/inch. The range of displacement required to transform the 

cell into a final shape depends on the volume of fluid that must be 

displaced. As shown, the stiffness transitions to a final stiffness of 

1,150 tons/in. It is also seen from this figure that if the cell is 

unloaded and then reloaded, the cell resumes its final stiffness from 

the previous loading and quickly approaches a stiffness comparable 

to full contact conditions. 

 Any folds that develop while the cell is ballooning to a new 

shape can significantly degrade the rigidity of the cell, preventing it 

from maintaining its shape such that the full stiffness capability of 

the cell may not be achieved. Geometric changes that occur when Figure 11. Impact of contact area on pressure-load response of 

PSU. 

VERTICAL FORCE, kips

0 100 200 300 400 500 600 700 800

WATER PRESSURE, psi

Calculated Load for Full 

24 in dia area

Full top and bottom 

contact

17.75 in diameter top 

and full bottom contact

14 in diameter top and 

full bottom contact

0

50

100

150

200

250 Full top and bottom 

contact

17.75 in diameter top 

and full bottom contact

14 in diameter top and 

full bottom contact

14 in

17.75

Figure 12. Transforming of PSU to a ballooned shape 

increases the stiffness of the cell. Note difference between 

first and second loading of the PSU sandwiched between two 

concrete disks. 

0

50

100

150

200

250

300

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

DISPLACEMENT, inches

LOAD, kips

Full top and bottom contact

17.75 in dia x 3 in top and bottom 

contact -- 2nd loading

17.75 in dia x 3 in top and 

bottom contact -- 1st loading

K = 1,150 tons/inch

K = 50 tons/inch

17.75 in 

17.75 in 

PSU 
Figure 13. Stiffness of PSU can be significantly reduced if 

partial contact causes the cell to fold in areas outside of the 

support contact area. 

0

50

100

150

200

250

300

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

DISPLACEMENT, inches

PSU LOAD, kips

Folding of 

PSU

Full top and 

bottom contact

17.75 in dia top 

contact and full 

bottom contact 14 in dia top contact 

and full bottom contact

Folds 

over-sized PSU’s are loaded can produce shear stresses that exceed 

the capability of the thin walled vessel. This causes folds to 

develop, which dramatically reduce the stiffness of the cell. The 

behavior is similar to that of a soft drink (soda) can under axial 

load. As long as the can is free from any wrinkles, then the axial 

loading capacity of the can is quite high (you can stand on it and it 

will support your weight). However, as soon as a wrinkle occurs, 

the can buckles and quickly looses its load capacity. Figure 13 

shows an example of the reduction in stiffness caused by folding 

for a partial contact cell, in this case a 14-inch-diameter simulated 

support contact on a 24-inch-diameter cell. 

Impact of PSU Stiffness on Support System Stiffness 

Since the prestressing unit is used in series with the support 

structure, the overall support stiffness will never be as high as when 

the PSU is not used (see equation 1). However, for full contact 

conditions, the stiffness of the cell is an order of magnitude higher 

than the support, hence, the stiffness of the support system (PSU 

and the support element) will be controlled by the support element 

regardless of what support is used. For partial contact conditions 

where the hydraulic cell is larger than the support, the prestressing 

unit will control the initial stiffness of the support system until the 

cell is reshaped and stressed by the loading application. Beyond 

this point, the support will again control the stiffness of the support 

system since the cell stiffness increases dramatically once it is fully 

ballooned. If sufficient pressure is applied to reform the cell during 

inflation, then the support stiffness and not the cell will again 

govern the support response during roof to floor convergence. The 

caveat to this conclusion is that if the folding occurs, the cell again 

controls the stiffness of the support system. Figure 14 compares 

the load-displacement response for a 24-inch-diameter Can support 

that is preloaded to about 50 kips with water pressure of less than 

100 psi in a 30 x 30 inch PSU compared to a passive support 

response without the PSU. As shown in the figure, the initial 

response of the Can with the PSU is softer prior to yielding than the 

Can without the PSU. The 6-inch thick, over-sized PSU, was 

deformed by the Can as the load developed, causing a reduced 

stiffness. In this case, the PSU was not inflated with enough 

pressure to produce the ballooning of the cell that eventually 

occurred as the loading developed. 

 (1) 

where K = stiffness.

Use of PSU to Control Load on the Support 

The Jackpack PSU’s are fitted with a yield valve to limit the 

pressure from the roof loading. The valve as shown in figure 8, is a 

simple plastic unit with a spring loaded plunger that seats against 

the body of valve to act as a check valve. There are two holes in 

the base of the housing about 1.0 mm in size (about the size of a 

mechanical pencil lead) through which the fluid must flow to 

relieve the pressure in the diaphragm. Obviously this small of an 

opening has a relatively small flow rate that limits the capability of 

the PSU to control pressure increases in the cell or support load 

development from active roof loading. Figure 15 compares a 

36x36-inch PSU response to load frame displacement rates of 0.01, 

0.08, 0.16, and 0.32 inches per minute. At rates equal or below 

0.08 inches per minute the PSU can limit the load development on 

the support, in this case at approximately 300 kips. However, at 

rates between 0.16 and 0.32 inches per minute, the load continues 

to increase beyond the yield valve rating to the point where the 

valve fully opens and begins to exert control on the load 

development, although at a higher load. As seen in figure 15, at 

slow loading rates the yield valve opens at a load of about 320 kips 

and is able to maintain this load by opening and closing as needed. 

However, at a rate of 0.16 inches per minute, the load reaches a 

level of about 400 kips before the valve fully opens. Here the valve 

remains fully open and the load continues to build slowly, reaching 

a load of nearly 500 kips after 1 inch of convergence. At 

0.32 inches per minute, the load builds to about 950 kips after 1 

inch of convergence. At a loading rate of 0.5 inches per minute, 

the load continues to build at a rate of 3,200 kips per inch of 

convergence even while the yield valve is fully open. 

 Therefore, the capability of the PSU to limit load development 

on the support (assuming the support is not yielding and providing 

this function) will depend on the loading rate and to some extent on 

the size of the unit. Loading rates can vary significantly in 

different mining conditions. However, the threshold rate of 0.08 

inches per minute, when the valve can no longer limit the load 

development, is a significantly faster loading rate than is likely to 

occur in nearly all mining conditions. The exception being bump 

or bounce prone situations that produce dynamic loading. Recent 

studies in a longwall tailgate at a western PA mine operating in the 

Pittsburgh seam found outby convergence rates in the abutment 

zone to be on the order of 0.0005 inches per minute and 

0.005 inches per minute just inby the face (7). Even the 

convergence rate inby the face is an order of magnitude slower than 

what the PSU is capable of handling, so it is concluded that the 

PSU can be an effective support load control device in longwall 

tailgate applications. The support performance characteristics are 

Figure 14. Comparison of pre-load Can support and a 

passive Can support. 

0

50

100

150

200

250

0 2 4 6 8 10

DISPLACEMENT, inches

SUPPORT LOAD, kips

Pre-loaded Can with 30x30 in 

PSU -- 6 inches thick

Passive Can Response

10 inches

200 kips

Initial 

preload 

Support PSU

Support PSU

System K K

K K

K

+

× =
also relevant to this discussion. If the support has a yielding load 

below that of the PSU, and assuming this yielding behavior is not 

rate dependent, then in theory, the PSU will never reach the 

pressure relieving state since the support yielding capacity will 

limit the load. In this case, the yield valve is not really necessary. 

Operation in Bump or Bounce Prone Areas 

No dynamic testing was conducted on the PSU’s at NIOSH. 

Although the threshold-loading rate of 0.08 inches per minute is 

adequate for most mining conditions, dynamic or impact loading 

will cause pressure intensification beyond the yield valve rating. 

Whether this causes the PSU to rupture will depend on the level of 

loading and the condition of the cell prior to the dynamic loading. 

Under ideal conditions, the Jackpack PSU has a pressure rating of 

1,000 psi. If the static pressure is limited to 250 psi by a pressure 

relieve valve, it is likely that the PSU would survive the dynamic 

loading. On the other hand, if the PSU is highly deformed or 

loaded unevenly, then it is much more likely that the PSU could 

rupture from the dynamic loading. Therefore, the capability of the 

PSU to function in this environment may be dependent upon the 

yielding characteristics of the support, which can vary widely for 

the various standing supports that are now available. It should be 

noted that the yield load of the support is also likely to increase for 

dynamic loading. 

PROPER INSTALLATION PROCEDURES 

 Basic installation requirements for both crib and prop PSU’s are 

similar. The support should be set perpendicular to the roof on a 

clean floor. Although the units have considerable expansion 

capability to establish roof contact, the optimum performance will 

be realized when the contact distance between the support and the 

uninflated PSU is minimized. This will maximize the contact area 

and the resulting preload will be maximized. 

 The contact area at the mine roof or floor should be free of 

obstructions such as roof bolts, channels, or steel straps. Beyond 

these, the PSU will conform reasonably well to roof or floor rock 

surfaces. Meshing will not cause a problem. If the unit is to be 

installed directly on Can supports, the PSU should be placed on the 

closed end of the Can. Tests have shown the weld sections on the 

open end of the Can can puncture the PSU on occasion when the 

Can deforms. The PSU needs to be big enough to have access to 

the inlet port for inflating the PSU. As the PSU inflates the inlet 

port orientation may rotate if the inflation thickness is large, and 

this needs to be taken into account in the sizing of the PSU as well. 

The inlet port and yield port should not be in direct contact with the 

support or mine roof or floor. 

 Selecting the size of the PSU brings up several interesting 

points. There are several standard Jackpot and Jackpack sizes 

currently available. Custom sizes can be made if the need is 

justified. Selecting an oversized PSU (i.e. larger than the support) 

will provide a higher preload for a given inflation pressure and will 

provide a higher yield load for the same yield pressure. These can 

be considered advantages of using an oversized bladder. The 

disadvantage of using an oversized bladder is that it can reduce the 

initial stiffness of the support if it is not fully inflated under 

pressure to a permanent deformed (ballooned) shape. Another 

disadvantage of an oversized PSU is that the irregular shape caused 

by ballooning will most likely significantly increase the probability 

of premature rupture. A similar argument can be made in reference 

to the contact surface between the support and the PSU. If the 

support surface is irregular such as that of a wood crib product 

including Link-N-Lock, Hercules crib, Tri-Log crib, etc, then 

providing a full contact surface with the PSU by adding a full layer 

of timber, would provide for more efficient load transfer though the 

PSU. In addition to these performance issues, the cost of the PSU 

increases with increase in size, providing another incentive not to 

use over-sized units. 

 The amount of preload that is needed can also raise some 

interesting questions, particularly since the preload that can be 

developed by these units can exceed the yield load rating of many 

supports. This discussion should begin with why the preload is 

used in the first place. The primary justification for preloading 

standing supports is to: (1) increase the overall stiffness of the roof 

support, for example, by “closing gaps” in multi-piece units such at 

timber cribbing, (2) closing gaps in the immediate roof structure to 

enhance beam building, much the same way as pre-tensioning roof 

bolts, and (3) securing the support to offset timber shrinkage, creep, 

or any other issue that may cause the support to loose contact with 

the mine roof prior to any roof loading. Since these are all loaddependent functions, then the amount of preload needed can be 

examined based on completing these tasks. Preloading beyond 

these requirements depends on whether the mining induced stresses 

can be controlled by the additional preloading. This is a complex 

question upon which research continues, but there is most likely a 

diminishing benefit for preloading beyond this level. 

RANGE OF SUPPORT APPLICATIONS 

As noted in the introductory comments, water-filled PSU’s 

have been extensively used in South African hard rock mines on 

various timber packs and timber posts support systems. Coal 

mining applications in the U.S. and Australia have begun during 

the past year. The majority of these applications have been on the 

Can support and the Pencil Prop. Some preliminary trials have 

been made with the following supports (1) MX props, (2) Spider 

Props, (3) Wedge Props, (4) conventional timber posts, (5) Link-NLock, (6) Hercules cribs, (7) Sand Props and (8) Bolt Props. 

OBSERVATIONS FROM APPLICATIONS IN U.S. COAL 

MINES 

The applications in U.S. and Australian coal mines have been 

primarily longwall tailgate installations and shield recovery 

operations. At this point, these have been primarily trial 

evaluations, but results to date have been positive. The tailgate 

Figure 15. Yield valve performance for different loading 

rates. 

0

100

200

300

400

500

600

700

800

900

1000

0.00 0.20 0.40 0.60 0.80 1.00 1.20

DISPLACEMENT, inches

PSU LOAD, kips

0.01 in/min load rate

0.08 in/min load rate

0.32 in/min load rate

0.16 in/min load rate

Yield valve opens

Yield valve closes
applications generally report improved ground conditions when 

prestressing is applied. The shield recovery operations provide 

another interesting perspective. In one case, the mine was using 

Can supports as part of the shield recovery operation. Cans were 

normally installed once a shield was removed to provide ground 

control and safety to the mine workers in this critical area. In 

addition to the benefits of providing an active roof load to minimize 

ground movements, the mine felt that the fast installation of the 

PSU’s compared to the more laborious process of topping off the 

Can’s with crib timbers was a significant safety advantage to the 

mine workers. 

 The problems and concerns expressed by the mines are 

summarized as follows. The inlet check valve was damaged on 

several occasions when high flow was used to fill the PSU’s in a 

matter of a few seconds and the check valve was on occasion 

destroyed by the high flow rate. A different check valve design is 

currently being pursued to help alleviate this problem or a flow 

control device can be used to limit the flow from the source. The 

problem can also be avoided by using the hydraulic pump supplied 

by the manufacturer. The air pump also has the advantage of being 

able to control the preloading pressure. The problem generally 

occurred on the longwall recovery operations where a very high 

flow rate is available. The other primary concern expressed by 

several western mines was what would happen in bounce prone 

conditions where an impact or dynamic load would be applied to 

the support system. 

CONCLUSIONS 

Standing supports have traditionally been totally passive 

support systems, meaning entry convergence must occur before the 

support develops any load carrying capacity. This is no longer the 

case. The PSU can economically turn almost any passive standing 

support into an active support that can apply considerable 

immediate load to the mine roof and floor. This represents a major 

change in standing support design. In addition to functioning as a 

preloading device, the development of an inexpensive yield valve 

also allows the unit to function as a yielding or load control device. 

By correctly designing the yield pressure in relation to the load 

capacity of a support, the PSU can then turn a non-yielding support 

into a yielding support at least through a few inches, depending on 

the PSU installed. This can significantly improve the capability of 

very stiff, but brittle supports, such as conventional concrete 

cribbing, cuttable cribs used in longwall recovery operations, and 

pumpable cribs used in longwall tailgate applications; just to name 

a few. 

 Another advantage of the PSU is that it can fill the gap between 

the top of the support and the mine roof. Most standing support 

systems, like the Can, must be topped off with a secondary support 

material to fill the gap between the top of the support and the roof. 

The Jackpack PSU can perform this function and preload the 

support as well. When inflated, the PSU will conform to the roof 

surface and provide more uniform loading on the support than 

timber blocks or wooden wedges where full support coverage is not 

typically achieved. Since the PSUs are transported into the mine in 

a collapsed form, they also reduce the volume of material that must 

be brought into the mine to construct the support systems. 

It is important to understand that the preload is primarily 

determined by the overall size of the PSU, not the size of the 

support. This might be important if the goal is to avoid putting the 

support into yield with the preload. The use of over-sized PSU’s 

relative to the support size can provide for larger preloading 

capability, but the oversized PSU can also decrease the initial 

stiffness of the support if it is not fully pressurized to a permanent 

shape. Over-sized PSU’s are also more likely to fail prematurely 

than ones that are fully sandwiched between the mine roof or floor 

and the roof support structure. 

 The application of this technology in underground coal mining 

is currently limited, but several trials are underway in the United 

States in longwall applications, both in tailgates and shield 

recovery operations, and the technology has a proven history in 

mining applications in countries around the world. A few concerns 

have been identified from the trials thus far, but overall the 

response had been very positive. Modifications to improve the 

inlet check valve to avoid damage to high flow rates are already 

underway. As experience is gained with their use, additional 

designs that are specifically suited to a particular support are likely 

to follow. 

REFERENCES 

1. Barczak, T.M. and Gearhart, D.F. Engineering Methods for 

Design and Employment of Wood Cribs. U.S. Bureau of Mines 

Information Circular 9361, 1993, 34 pp. 

2. Tests in NIOSH’s Mine Roof Simulator, June 1995. 

3. Tests in NIOSH’s Mine Roof Simulator, August 1997. 

4. Barczak, T.M. and Gearhart, D.F. Renewed Interest In Prop 

Supports As A Replacement For Wood Cribs. Proceedings, 

14th International Conference on Ground Control in Mining, 

Morgantown, WV, Aug. 1-3, 1995, pp. 149-158. 

5. Rataj, M. and Thomas, R. New Methods and Technologies of 

Roof Bolting in Australian Coal Mines. Proceedings, 16th 

International Conference on Ground Control in Mining, 

Morgantown, WV, Aug. 5-7, 1997, pp149-157. 

6. Rataj, M. Improvement in Pre-Tensioning of Strand Bolts in 

Australian Coal Mines. Proceedings, 21st International 

Conference on Ground Control in Mining, Morgantown, WV, 

Aug. 6-8, 2002, pp. 145-149. 

7. Barczak, T.M., Chen, J. and Bower, J. Pumpable Roof 

Supports: Developing Design Criteria by Measurement of the 

Ground Reaction Curve. Proceedings, 22nd International 

Conference on Ground Control in Mining, Morgantown, WV, 

Aug. 5-7, 2003, pp. 283-293.