Geotechnical Design of Roof Support and Reinforcement

The aim of this project was to develop an engineering design methodology for roof support so that alternative bolting strategies can be compared qualitatively in terms of productivity, cost, safety, and risk. Such a design methodology will provide a framework to assess development rate requirements (ie metres/shift) in terms of operational constraints, geological conditions and practicality.

Scope

Design in geotechnical engineering can be considered as a step process requiring:

• a precise statement of the problem in terms of operational requirements (ie metres/day, design life, dimensions etc);

• recognition of the constraints to the solution e.g., mining method, costs, regulations etc.;
• data collection, formulation and analysis of geotechnical models;
• identification of alternative solutions;
• evaluation and selection of appropriate strategy;
• implementation and monitoring.

The Australian coal industry does some of these steps very well but the fact that a development constraint exists and that bolting patterns are remarkably similar throughout the industry suggests that the full design process is not being implemented. At the outset this project recognised that the lack of simple analytical tools was a major limiting factor to the application of a rigorous design approach. In particular the project recognised that there was a requirement to:

• understand and analyse the behaviour of individual bolts in shear and tension;

• from the above, be able to specify the number and pattern of bolts to be installed, and to
• identify other support strategies that may have application.

Referring to individual bolts, most roof bolts installed in Australian coal mines are untensioned ie. installed as dowels. In most rock types, and with resin grouts, fully encapsulated dowels reach their peak axial load capacity (ie yield) at less than 2.5 - 5mm of bed separation.

The peak load is controlled by the tensile strength of the steel.

This needs to be compared with the behaviour in shear whereby extra high strength steel dowels (e.g. AX bar) offer a maximum of 2 - 7 tonnes of shear resistance (compare 30 tonne axial capacity) depending on the strength of the rock which is installed. Pretensioning or angled installation are required to better mobilise the full capacity of the steel when bolts are required to resist shear.

Results

Two design procedures for specifying bolting patterns have been developed.

In laminated roof and under moderate horizontal stresses, the design procedure is based on resisting shear movements along bedding surfaces. Some movements are allowed that are compatible with a specified centreline definition. It is noted that implicit in the use of dowels is the inevitability of roof movement so that the bolts can load up. The design procedure is amenable to the development of design charts. Preliminary analyses suggest that the bolting intensities are similar to those currently installed and highlight the advantages of pretensioning and angled bolting.

In highly stressed environments, the roof beam may undergo compressive failure. The role of roof bolts changes from resisting shear to the support of fractured rock. In this model the key bolt parameter changes from shear strength to axial capacity. Simple design tools have been developed to estimate the height of softening and the bolting intensity required to prevent the rotation of failed roof beams out of the roof. Good agreement with bolting patterns installed in highly stressed ground has been obtained.

Whilst the analyses and designs need calibration and verification, they do give some insight into possible improvements in current bolting practice.

Pretensioning

Pretensioning is considered to have a major application in laminated roof/moderate stress environments where the design issue is the resistance to shear. Pretensioning may have a lesser application in high stress environments as the pretensioning reduces the available axial capacity to resist separation as failed blocks begin to rotate out of the roof.

Point Anchoring

Point anchoring may have an application in allowing easier automation of the bolting cycle. It is assessed that the perceived disadvantages with these bolts can be addressed with minimal engineering design effort. Point anchors result in larger loads being applied to the collar and anchor. Satisfactory anchorages are believed to be available but a method of engineering a yielding collar assembly will need to be developed.

Off Centre Line Bolting

In laminated roof/moderate stress environments, bolts should be orientated away from the centreline. This may be an alternative to pretensioning.

Detection of Shear

The spacing of strain gauges on instrumented roof bolts will need to be reduced to about 50mm if shear is to be detected.

Conclusion

The project was formulated to examine and outline design procedures. The comparisons between field data and our designs are very encouraging and suggest that the procedures have great potential. More work is required to calibrate the procedures and to produce a routine design tool that can be used by mine engineers. In addition, the research to date already points to several opportunities to reduce bolting intensities and cycle times through a better matching of roof deformation patterns to known bolt behaviour.

Further Research

There is an urgent need to verify and calibrate the analytical methods developed during this research. This was recognised in the initial proposal whereby funding was sought only to investigate the possibility of developing simple analytical techniques and not to produce a verified and calibrated design technique for general release.

Verification will require better knowledge of the distribution and nature of defects within the bolting horizon, in particular the extent and direction of shear movements along defects.

A number of practical installation options are suggested by the analyses in Chapters 2 & 3 of the final report such as:

• Outward inclination of bolts. It is recommended that the practical advantages of an inclined bolt pattern be established. Figures suggest that outward bolt inclination offers the opportunity to significantly increase the effective installed capacity of roof bolts to levels beyond even those of pretensioning. Conversely, inward orientation of bolts is shown to be counterproductive. Properly designed machine-mounted drill rigs may offer the opportunity to install high angled bolts. 

• Point anchored bolts. These offer the potential of reduced installation times as a result of the removal or time required for the resin insertion step. However there is a need to upgrade the strength and yielding capacity of the anchors and in particular the collar. Point anchored bolts offer easier pretensioning. Outbye grouting of the hole annulus may be required to prevent long-term deterioration of the rock. 
• Pretensioning - offers substantial benefits for beam building but reduces the capacity of the bolt to resist any subsequent movement. A mix of pretensioned and untensioned bolts may be more appropriate. Recognising the distribution of shear and vertical movements in the roof, pretensioning could be restricted to the outer bolts where shear is dominant whilst dowels could be installed closer to the centreline where bed separation is more likely should the beam start to delaminate.