Performance Evaluation of Flexibolt Flexible Roof Bolts

The FLEXIBOLT flexible roof bolt was initially developed under NERDDC Project 1334. A number of collieries expressed interest in further development of the FLEXIBOLT so it could be applied as either long (>3m) or short tendons in the production environment.  To this end the project involved two major field investigations; the first at Ellalong Colliery which investigated the potential to use a lower density of FLEXIBOLTS compared to the required density of conventional rigid bolts and the second, at Angus Place Colliery, which investigated the ability of long (4.1m) FLEXIBOLTS to control the deformation of a weak, thick coal roof adversely affected by geological structure.  The project has also included laboratory testing to provide further knowledge on the reinforcement properties of the FLEXIBOLTS with different grouts and with a deformed strand profile. 

Trial methodology

Following on from the successful outcome of the trial conducted under the NERDDC project a second large scale trial was conducted. This aimed to evaluate the impact on roof deformation and bolt loading of using a reduced density of FLEXIBOLTS compared to the standard pattern of X-bar bolts. The trial methodology was to install FLEXIBOLTS at various densities in a cut-through (Cut-through 12, Longwall 10), to monitor the roof deformation and to compare the results with monitoring data from the adjacent cut-through which was supported with the standard pattern of X-bar bolts.Cut-throughs were chosen for the evaluation because they were in the least favourable orientation with respect to the stress field and hence maximum roof deformation was expected in this situation. The instrumentation was installed progressively with development in order that the ground response from both development and longwall induced stress could be monitored. 

Results of Ellalong Colliery Trial

• FLEXIBOLTS, at a reduced density, restricted roof movement in the bolted horizon to lower levels than X-bar bolts; 

• the restraint of roof movement in the bolted horizon with FLEXIBOLTS is believed to be the reason for less deformation above the horizon in the region supported with FLEXIBOLTS compared to the region supported with X-bar bolts; 
• the level of stability achieved with 4 or 5 FLEXIBOLTS per strap was at least equivalent to that achieved with 7 X-bar bolts per strap; and 
• the roof deformation that should occur with FLEXIBOLTS installed in the maingate direction (the preferred direction in terms of horizontal stress) should be lower than that in the less favourable cut-through direction.  

Results of Angus Place Colliery Trial

• Bolts anchored in the upper horizon (stone) achieved peak loads 20% higher than bolts anchored in the lower, coal horizon. 

• The pullout stiffnesses are very similar and are probably a function of hole size rather than strata type. 
• In all tests at the same horizon, the tests in 28mm holes had a 40% higher peak pullout load and a 30% higher pullout stiffness than tests in 29mm holes. 
• In all tests FLEXIBOLTS maintained a pullout resistance at or near peak load with further pullout.

The overall conclusion from this series of tests was that a point anchored FLEXIBOLT could perform as a yielding support system at Angus Place. The yield load and post-yield behaviour could be controlled by selecting an appropriate cartridge length and hole diameter for the given roof horizon.

This was very important for Angus Place as extensometer results showed that the freelength of the FLEXIBOLT could be subject to strain levels of up to 4% which approaches the strain limit of the FLEXIBOLT strand.

Hence, by designing a point anchor with a capacity lower than that of the strand the support system could be designed so that the point anchored FLEXIBOLTS could yield when strain levels in the freelength approached 4%, rather than inducing failure of the strand. It is believed that this is the first time that a high capacity point anchored support system has been able to be designed this way. 

Conclusions

The principle conclusions of the project are:

• Reduced densities of FLEXIBOLTS provide equivalent roof support performance to the previously required densities of rigid bar bolts in roof subject to significant shear movement. 

• Reduced bolting density provides significant improvements in roadway development productivity. 
• Thick, weak roofs subject to high deformation can be controlled with long, point anchored FLEXIBOLTS installed at the face to provide continuous production. Long term stability can be achieved by either post-grouting the freelength of the FLEXIBOLT and/or installing conventional secondary support as an outbye process. 
• FLEXIBOLTS offer high reinforcement potential against both axial and shear deformation with both resin and cementitious grouts. Deforming the strand of the FLEXIBOLT allows enhanced reinforcement performance to be achieved in holes with diameter significantly larger than the strand.  

Recommendations

The principal recommendations from the project are:

• Comparative in-situ trials are necessary before transferring to a lower density of FLEXIBOLTS as a primary support system in place of current densities of rigid bar bolts. The benefits from improved shear reinforcement, the principal advantage of the FLEXIBOLT, can only be assessed in this way. 

• Extensive roof displacement monitoring is necessary to understand the roof behaviour, before and during the implementation of FLEXIBOLT systems. This allows an appropriate FLEXIBOLT system to be selected and subsequently refined if necessary. Most importantly it also allows any operational limits of the system to be recognised so that control can be maintained. 
• Installation systems and resins should be developed to allow, in normal operations, full encapsulation of 4m long FLEXIBOLTS with a standard form in 28mm holes or 6m long 'caged' FLEXIBOLTS in 36mm holes.