New Support Concepts for Hard Rock Mining Applications 

Thomas M. Barczak 


NIOSH – Pittsburgh Research Laboratory 


Stephen C. Tadolini 


NIOSH – Pittsburgh Research Laboratory
 

Abstract 

A critical safety component for all underground mining is intrinsic and standing support systems. Specifically, 

tabular hardrock mining exploits a wide variety of deposits that require various methodologies to safely mine the ore 

material. Several new support systems have been developed in recent years for hard rock applications which have 

been modified or applied in a variety of ground control applications. These include prestressing devices, improved 

cribs, and free standing supports. Various standing support and prop-type systems have been designed for hard rock 

applications with seismic loading conditions to accept prestressing. The prestressing, using water-filled cells, 

creates an active load upon installation and is considered essential to maintain support during and after the blasting 

of the mine faces. Heavy seismic activity is also present in these mines and the prestressing units can provide some 

energy absorption capability to help preserve the integrity of the support. This paper presents an overview of these 

innovations in support technology. 

Introduction 

For tabular deposits, a wide variety of standing support and cementitious packs are used with 

mining methods such as longwall mining that are similar to coal mining practices. In fact, timber 

props and wooden cross members were some of the earliest forms of standing support as shown 

in figure 1.

Figure 1: A historical photograph of miners setting timber posts and beams. 

 Although these support products would have limited application in vein deposits 

common in North America, some of these technologies, particularly the prestressing systems, 
can be beneficial to stope support that commonly uses a variety of timber posts and headers. A 

typical hard rock installation of timber post and cross-members used in underground mines is 

shown in figure 2. 

Prestressing Devices for Roof Bolts and Standing Supports 

Figure 2: Traditional timber sets for hardrock applications. 

A wide variety of prop-type supports has been developed that provide both 

non-yielding and yielding behavior. Standing support systems use has also been limited due to 

stability problems at operating heights beyond 2.5 m. Here too, improvements in support 

technology have been made in recent years, including improvements in timber crib systems 

which have operated in heights up to 4 m. The performance capabilities of the described systems 

will be addressed recognizing that they may not be applicable in all conditions in North 

American hardrock mines. 
The benefit of pretensioning of tendons is to change the state of stress in rock formations or to 

provide confining forces that resist movement along fracture planes has been commonly used in 

mining engineering fields for many years. Pretensioning of long cable bolts, which are a 

common form of support in hardrock mining, has been particularly difficult because mechanical 

approaches typically apply a torque to the cable strands creating a spring back effect that reduces 

the tension after the external torque is removed. Recently, an inflatable metal bladder has been 

developed that can provide a direct axial pretensioning force to the cable through hydraulic 

pressure without inducing any torque into the bolt. The inflatable metal bladder is placed 

between the roof bolt plate and the head of the bolt as shown in figure 3. 

Figure 3: Water inflatable prestressing bladder used to pretension roof (cable) bolt. 

 Laboratory and field 

tests in cable bolts indicated that the bladder, when simply inflated with water pressure, can 

create up to 10 tons (89 kN) of preload. As shown in laboratory experiment in figure 4, a quick 

connect hose is placed on the bladder and filled using air or hydraulic pressure. 
Figure 4: Laboratory installation of pretensioning bladder used in conjunction with a cable bolt. 

This same concept has been applied to prestress standing roof support systems that bridge the 

mine opening from the floor to the mine roof (Barczak et al., 2004). Historically, these supports 

have been mainly passive supports that generate their load carrying capacity only through the 

closure of the mine opening, i.e. through roof movement. For timber posts with header boards or 

cross-timbers, this can mean a few centimeters of convergence will occur before the support 

generates significant load resistance. Application of a prestressing cell as shown in figure 5 can 

create an immediate active force against the mine roof and floor. These units can be inflated 

with up to 12 MPa of water pressure using a portable pressure intensifier to create preloads in 

excess of 50 tons on a 220 mm cell. The units can be sized to fit any timber post diameter with 

standard sizes of 140, 160, 175, 190, 220, 260 mm, or fabricated in geometries that can 

accomated timber sections directly as shown in figure 6. They can also be equiped with 

headboards to further distribute the load to the mine roof. 
Prestress

cell

Figure 5: Water inflatable prestressing bladder used to preload a timber post and provide active 

roof loading to a traditionally passive support. 

Figure 6: Water inflatable prestressing bladders can be fabricated in a variety of shapes to 

accommodate different supports or roof header materials. 
In addition to improving roof control, prestressing of these props can be beneficial in ensuring 

that the props are able to withstand ground reactions and air blast from blasting operations when 

used in the immediate vicinity of the face, as shown in figure 7. 

Figure 7: Prestressed props can be employed along face line as they can withstand the ground 

reaction and force from the blasting at the face. 

A purely passive support or one 

that is only lightly preloaded from wood wedges is likley to become dislodged and fall over 

during nearby blasting operations. Another design of these prestressing bladders can also be 

enlarged to fit a variety of crib or pack type supports. In this system, two flat sections of metal 

are welded along the perimeter to create a large cell that with relatively little water pressure can 

create large preloads. In South African gold mines, these systems are used on timber packs as 

shown in figure 8. In addition to providing a substantial active force to the mine roof, these 

devices can be beneficial in prestressing the support structure to remove any initial softness due 

to construction whereby timber dimensional tolerances or some other issue create a disjointed 

structure (Barczak, 2005). 
Figure 8: Application of prestresssing bladder on a timber pack. 

Summary and Conclusions 

Roof support is a fundamental requirement for all underground mining operations. Hard rock 

mining operations can vary widely depending on the nature of the deposit and require varying 

degrees of ground support to provide a safe working environment. Blasting and seismic loading 

can create additional hazards for the rock mechanics engineer who must design an effective 

support system for these conditions. Nonetheless, the fundamental aspects of rock support 

remain the same, keep the rock from moving when possible and maintain support as the rock 

deforms when it is not possible to achieve complete equilibrium. 

Several advancements in roof support technology have been made in the past 10 years, providing 

a host of new products that improves all three measures of support design; namely strength, 

stiffness, and stability. The cross pollination of support applications has also grown, with 

supports developed for gold mine applications transformed to coal mine roof support systems 

and vice versa. However, there is no and never will be a universal support that will be effective 

in all conditions. The goal remains to match the support performance characteristics with the 

ground response - that will always require a site-specific design to achieve support optimization. 

The purpose of this paper is simply to summarize recent developments in support technology and 

to introduce hard rock mining engineers to support concepts that they have not been exposed to 
in the past and is not intended to provide a technical assessment of rock mechanics or support

performance. Obviously, these concepts will have varying degrees of application, success, or

even failure. However, knowledge of their existence may release the ingenuity that all mining

engineers possess which may help to create safer conditions for miners everywhere.

References 

BARCZAK, T. M., TADOLINI, S. C., and McKELVEY, P., 2004. Hydraulic Prestressing Units: An Innovation in

Roof Support Technology In: Proceedings of 23'd International Conference on Ground Control in Mining,

Morgantown, WV, p. 286-294.

BARCZAK, T. M., 2005. An overview of standing roof support practices and developments in the United States.

In: Proceedings of the Third Southern African Rock Engineering Symposium (Johannesburg, Republic of South

Africa, October 10--12, 2005). Johannesburg, Republic of South Africa: South African Institute of Mining and

Metallurgy, p. 301-334.