Innovation awards 2nd place – Oaky Creek Coal – Oaky North Underground Mine
IMPROVING THE RIB SUPPORT SYSTEM AT OAKY NORTH – THE DEVELOPMENT OF THE “LOAD INDICATOR PLATE”
THE PROBLEM
Late in 1999 a slab of rib coal weighing approximately 1.5 tonnes fell injuring a contract employee at the Oaky North Underground Mine. The piece of rib coal was supported by 3 fibreglass rib dowels and the combined load bearing (shear) capacity of the bolts should have prevented this piece of coal from falling.
The subsequent investigation into the cause of the incident identified the fact that all 3 of the rib bolts had failed in sher and the cause of this failure was due to excessive longitudinal fracturing of the fibreglass dowel.
Further investigation and trial identified the fact that during the installation of fibreglass rib bolts using hydraulic bolting rigs it is very easy for the operator to overtorque the bolt which essentially twists the bolt along its length resulting in longitudinal fracturing. In cases where the overtorquing is excessive the tensile and shear properties of the fibreglass dowel can be significantly reduced or even destroyed.
An audit of the integrity of the rib bolts previously installed throughout the mine highlighted the fact that a significant problem existed as a high percentage of these bolts had little or no inherent strength.
Due to the significance of the problem a rib support review workshop was convened by Oaky North to highlight an review issues/concerns associated with the use of cuttable rib dowels and to determine a course of action to minimise the risk of injury from rib failure. This workshop was attended by a cross-section of the Oaky North and Oaky No. 1 workforce as well as technical support, suppliers and manufacturers.
When installing fibreglass rib bolts using hydraulic bolting rigs it became obvious that the rib bolting system in place at the mine was enabling the bolts to be easily overtorqued (over tightened), well beyond the shank torque capacity of the bolt.
An initial trial was conducted to determine whether it was possible to compromise the strength of a fibreglass dowel due to twisting along its axis. This trial was conducted in workshop conditions using a vice to fix one end of the dowel and using a set of stillsons to twist the free end of the bolt.
Table 1 details the results obtained from twisting a free length 300-400mm of fibreglass bolt along its axis.
Table 1: Result of twisting a 300-400mm length of fibreglass bolt along its axis.
Twist Applied to Bolt
/Observations
180 degrees / The bolt returned to normal – no permanent deformation observed.270 degrees / Slight deformation observed – the free end of the bolt could be broken using some physical effort.
360 degrees / The free end of the bolt had been permanently deformed – the free end could be easily broken by hand – significant reduction in both shear and tensile strength.
To overcome the problem every component of the existing rib support system was reviewed and evaluated. The result was the development of a new rib support system for the mine which significantly reduced the possibility for a fibreglass rib bolt to be twisted beyond its Shank Torque Capacity during the installation process.
Figure 1: Rib Bolt Installation
This diagram highlights the variable (un-encapsulated) length of a rib bolt. As the un-encapsulated length of the bolt increases the twisting and resultant damage from a given applied torque is also significantly increased.
Figure 2: Sample failed rib bolts – Note the permanent deformation and longitudinal fracturing.
SOLUTION
The encapsulated length of the bolt plays a significant role in the final integrity of the bolt due to the free length of bolt available to be twisted during installation. Therefore is it beneficial to minimise the annulus between the bolt and the hole by making either the bolt larger or the hole smaller.
The components of the Rib Support System, both previous and proposed are listed in Table 2.
Table 2: Components of the Rib Support System.
System Component
/ Previous Support System / Proposed Support SystemBolt Diameter/Length / 19.5mm-20.0mm/1800mm / 22.0mm/1800mm
Chemical Length/Type / 1000mm/duo-speed / 1000-1200mm/medium-set
Drill Bit Diameter / 27mm / 26mm
Drill Rigs / Hydramatic – Series 55 & 5500
Delivered torque = 240-305ftlb / Hydramatic – Series 55 & 5500
Delivered torque – 240-305ftlb
Rib Plates / RRS HIPP-Flat / RRS HIPP-LIP
Bolt/Diameter/Length:
The rib support system now requires the use of 22.0mm diameter bolts. In addition to being a much strong bolt the increased diameter and resulting annulus between the bolt and the hole assists in achieving full encapsulation. Table 3 details the characteristics of the two types of bolts.
Table 3: Characteristics of the Fibreglass Rib Bolts.
Specifications
/ AROA & RRS19.5 / CT30T/22 & RRS22.5Length / 1800mm / 1800mm
Nominal Diameter / 19.5-20.0mm / 22.0mm-22.5mm
Ultimate Tensile Strength (rod) / 150-195kN / 300-350kN
Shear Strength (rod) / 50-70kN / 75-100kN
Shank Torque / 35-50ft/lb / 70-85ft/lb
Chemical Length/Type:
The potential benefit of replacing the duo-speed chemical with a full medium set chemical was the elimination of the point anchoring effect. Point anchoring the bolt aty the back of the hole and then tightenining the nut prior to the remaining length of chemical “going off” creates a free length of bolt able to be twisted along its axis.
Following trial it was decided to continue to use the duo-speed chemical for the following reasons:
- Continued use of duo-speed chemicals for roof bolt installation would necessitate the carrying of additional consumables on the continuous miner;
- If full encapsulation length is not achieved using the medium set chemical then twisting may occur; and
- The introduction and correct use of the polypropylene “load indicator plate: provides the operator with a visual indication that the bolt is being tensioned. Using this plate, correct bolt installations can be achieved independent of the chemical type being used or the resultant encapsulation length.
Drill Bit Diameter:
The use of the smaller diameter drill bit provides reduced free space between the bolt and the side of the hole. For a given bolt diameter and chemical length the use of a smaller drill bit improves the encapsulation length of the bolt.
Full encapsulation is currently being achieved using minimum 22.0mm diameter bolts with a 1000mm chemical in a 27mm diameter hole. Therefore, in current mining environment there is no need to reduce the drill bit diameter.
Drill Rigs:
Hydramatic Series 55 and Series 5500 hydraulic motors are fitted to the rib bolting rigs on the Joy 12CM12 continuous miners in use at the mine. The Series 55 and Series 5500 drill motors deliver a maximum torque of 305ftlb and 240ftlb respectively during the installation of a rib bolt.
The option of reducing the delivered torque of the drill motors was not pursued due to the need for increased torque during the installation of steel rib bolts.
Rib Plates:
The rib plate currently in use at the mine is a 235mm diameter polypropylene mine washer supplied by Roadway Reinforcement Services. The rib plate has a flat surface and provides little indication when the bolt is being overtensioned.
Figure 3: Examples of Rib Plates which are used at present.
It is worth noting that changing the aforementioned components of the rib support system does not eliminate the risk of the bolt being overtensioned during installation as the hydraulic bolting rigs are quite unforgiving and the operator is not provided with any indication that excessive tension is being applied.
In order to address this issue and provide the operators with a visual indication as tension is applied to the bolt a concave shaped rib plate was proposed. As the bolt is tightened the centre of the plate will be drawn toward the rib tending to flatten the plate.
The concept of a concave rib plate was discussed with Mr Ghassan Skybey of Roadway Reinforcement Services who assisted in the development and supply of the prototype rib plate.
Figure 4 shows the prototype load indicator plate pre and post installation.
Figure 4: Prototype “Load Indicator Plate”
The prototype rib plates have been trialed in the mine to prove the concept and identify any deficiencies with the product. The trial proved successful with good feedback received from the operators. An injection mould is presently being manufactured to enable full production of the “Load Indicator Plate” to commence.
BENEFITS:
With the introduction of polymer products the use of timber rib plates has been phased out of the industry. One shortcoming of the current polymer rib plate designs is that unlike their predecessor they fail to provide operators with a visual and audible warning as the bolt is being tensioned.
The benefit of using the load indicator plate in conjunction with fibreglass rib bolts is that during the tensioning stage of the bolt installation the operator is given a visual indication that tension is being applied as the plate will begin to flatten against the rib.
The load indicator plate has the potential to prevent the fibreglass rib bolts form being overtorqued and will complement all rib bolting systems in use throughout the mining industry, even where the bolts have been point anchored. Provided the operator does not fully flatten the plate against the rib there will be minimum torsional stress applied to the shaft of the fibreglass bolt preventing any damage to the bolt which in turn will reduce the risk of rib failure and injury to mine personnel.
COST:
The cost of manufacturing one (1) injection mould to produce the polypropylene rib plate is $15 180.00. This cost will be bourne by the supplier, Roadway Reinforcement Services.
Roadway Reinforcement Services will be supplying the “Load Indicator Plate” to Oaky North at the same cost as the previous flat plate design.
Although difficult to quantify, substantial savings will be afforded through significantly reducing the amount of backbye bolting work required to replace the rib bolts damaged during installation.
APPLICABILITY TO OTHER SITES:
There is a significant number of underground coal mines installing fibreglass rib bolts to provide rib support. This form of rib support is typically installed in areas where the coal being supported by the bolts is to be mined in the future.
Given the extremely weak nature of the coal ribs at the Oaky North Mine the fact that the rib bolts were being damaged during installation was recognised following an investigation into the cause of failure of a large block of rib coal.
Wherever fibreglass rib bolts are being installed using hydraulic bolting rigs there is a chance that bolts could be over tightened during installation. In mines with stronger ribs the impact and significance of the damaged rib bolts may not yet have realised, however an unacceptable residual risk may exist.