Post Grouting Technology to Reduce Bolting Cycle Times

This report details the results of a roof bolt trial utilising the Ingersoll Rand marketed CT-Bolt at a number of collieries in the Sydney and Bowen Basins. The project was conducted under the guidance of ACARP to assess the potential for an increase in underground roadway development productivity through modification of the current roof bolting techniques generally utilised in the coal mining industry. The trial conducted involved the use of current coal mining bolting methods and equipment in conjunction with the post-grouted roof bolting technology.

The CT-Bolt is a rock bolt from ?rsta St?lindustri AS, Norway. It is a typical combination bolt, designed to be installed at the work face as a mechanical point anchor support that can later be grouted to form an effective permanent support system. Civil and metal mining support practices have utilised cement-grouted reinforcement extensively for a number of years. The purpose of this project has been to make an assessment on the applicability of the post-grouting technology to current coal mining activities, its impact on the production cycle and potential for reducing costs and improving productivity.

The project was initiated in October 1996 with a project startup date of February 1997. The original project specification was to develop a post grouted bolting cycle using resin grout. This required a reduction in hole size from the 45 mm diameter used in the metal and civil industries in order to reduce the cost of resin and maintain the viability of the project. Trial installation of the standard 45mm diameter cement grouted bolt was undertaken at South Bulli Colliery on 5 February 1997. At the progress meeting held on that date it was decided to recommend to the monitors that rather than concentrate on developing a new product, the coal industry could be better served by proving a role for an existing product used in the metal and civil industry. The monitors approved this change.

Fieldwork consisted of pull out tests of the CT-Bolts at a number of locations, followed by the successful completion of the trial in October 1998 at Oakdale Colliery. The field trial involved the installation of 1.8m CT-Bolts in a Bulli Seam development driveage between 37 and 38 cut-throughs in the D Heading of the 500 Extended Panel.

The trial of the CT Bolts was successful in terms of utilising available coal mining equipment and skills to transfer the concept of post grouting technology into the industry. The main findings of the project include that:

• Civil and metal mining support practices have successfully utilised the mechanically pointed anchored and cement grouted reinforcement for a number of years. This system of support tested in a production cycle at the Oakdale Colliery produced reasonable rates of advance with the most basic equipment, demonstrating that the technology can be readily installed in the coal industry using current equipment and work systems with minimal impact to current work procedures.
• The use of post grouting was able to be undertaken with both bolt size and density to be reduced in comparison with resin grouting bolts, whilst still maintaining roof stability. Further reductions in bolt density are considered to be achievable as the database is further compiled and installation skills are acquired.
• It is assessed that post grouting is applicable to ground support in roadway development at a competitive cost to current practice in difficult ground conditions. The unit rate comparison for the trial was $110.90 per metre consumable cost with scope to reduce to around $80 based on observations of the roadway conditions post trial. The current consumable cost per metre of advance in this area of the mine using resin bolting techniques is approximately $84.
• The system is particularly applicable to a multiple heading or place changing operation that allows campaign grouting to occur without restricting the production cutting cycle. This method would allow coal to be produced whilst completing the bolting cycle thereby increasing advance rates and further offsetting costs.
• The methodology has the potential to reduce bolt density in general driveage as seen in the trial results and therefore the potential for significant cost savings. This process could also be applied to longwall installation and salvage operations where resin loss in current bolting practice reduces the installed capacity of the roof support and can lead to premature roof failure and expensive remedial processes.
• Significant cost savings in both primary and secondary supports are achievable by guaranteeing a quality installation through the grouting process where grout quantities can be measured.

It is assessed that the use of mechanically pointed anchored, cement grouted bolts provides improvements over the conventional resin grouted bolts in coal mining applications in the following ways:

• Bond strength from the mechanical anchor is available immediately on installation without the potential for introducing grout into open voids;
• Bed separations in roof strata will tend to be closed up under the pre-tension applied to the point anchor prior to the injection of grout. This reduces the potential for grout to hold the separations in the roof open;
• The pull out resistance of the cement grout bolt is greater because of the combined effects of a mechanical point anchor and the increased contact resistance arising from the larger hole diameter used for post grout bolts. This can be a significant advantage in poor ground as the length of bolt required to develop the design anchorage can be reduced;
• The flexural stiffness of the composite bolt/grout column is higher than for resin grouted bolts due to the greater hole diameter. It is considered that the increase in flexural stiffness will reduce the magnitude of initial roof movement.

The purpose of this project has been to make an assessment on how this practice can be applied to current coal mining activities. The relationship between vertical and lateral movement and the effect of stiff reinforcement are functions that require additional research to quantify. Formulae currently in use in civil foundation design can be readily applied to calculate the bending of a cement grouted bar or tube. However, field trials in controlled circumstances to monitor and measure ground movement will be required to assist in calibration of the available design tools. The research program has also identified a number of issues to improve the underground installation of the CT-Bolts, based on current coal mine roof support systems.