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HYDRAULIC PRESTRESSING UNITS: AN INNOVATION IN ROOF SUPPORT TECHNOLOGY


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 (PSUs) 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 1990s, this new generation of
water-filled PSUs 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
PSUs 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, grout-
filled 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 Wedge
1
(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 Bags
1
was carried out in the early 1980s
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 prestres