Since coal mining began the roof in a mine was supported to ensure the rock strata over our heads didn’t collapse onto our heads. This was initially achieved by setting wooden props from the floor to the roof to stop the strata moving, fracturing and collapsing, often with catastrophic results. For hundreds of years this simple system was used often in conjunction with wooden bars set over the props to span roadways.
Prop and bar
Steel support joist, known as girders or arches were introduced due to their greater inherent strength. Many types of mining arches, square section girders and joists were developed for different seams, conditions, weight, speed of setting and cost. The roof above the girders were usually covered with corrugated steels sheets to stop rock falling onto the men. The Selby Coalfield, which worked the gassy Barnsley seam was slightly different in that open steel mesh was the only coverings allowed above the supports. This ensured full access to the roadway and ensured it could tested for methane without hidden pockets behind steel sheeting.
Gascoigne Wood North Spine tunnel
Gascoigne Wood South Spine tunnel
Steel arches in H504s T/G at Riccall Mine
Using roof bolting as a primary support system for strata control was developed in the U.S. during the 1940s and into the 1950s. They were initially used to replace wooden props and bars used in the American mines and was thought as a more effective and safer system of roadway support. Roof bolting as a secondary support system to supplement flat top girders was used with great success at Hartley Bank Colliery at Netherton, Wakefield as early as 1953 on both headings and longwall roadways.
Legislation stated that rockbolts may only be used as a principal support system in a coal mine if the H.S.E. has granted an exemption from the requirements to set recognised authorized supports. An exemption was only given if the roofbolting system was tested and proven at each individual mine to be safe and a geotechnical assessment and site investigation was carried out by a suitably qualified and competent person.
Below are the requirements to be taken into account when carrying out the site investigation;
Geology: including the strata section, rock properties, faults, cleat, parting planes, presence of water or any substance likely to flow, borehole information and gradients. All factors need to be correlated relative to the mining horizon;
Stress: the direction and magnitude of the stress field components for pre mining, mining induced conditions and interaction;
Pillar design and effects: the assessment needs to include drawings and diagrams to illustrate potential risk areas;
Environmental effects: the effects of ambient temperature, mine water and associated impurities.
Bond strength: measured by short encapsulation pull tests using the proposed rockbolting materials and components. The tests need to be carried out for all major roof horizon changes within the length of the proposed rockbolt and the effects of wet flushing or alternative dust control system on the bond strength determined.
Standup time: dilation related to distance from the face.
Information taken from H.S.E. guidance on the use of rockbolts to support roadways in mines.
In the early 1980s Allerton Bywater Colliery used roof bolts in the Middleton Little seam on 56Bs district, as part of a trial, when mining small panels of remnant coal referred to as finger panels and proved a great success. A consequence of the trial and the technical information gained, they became the recognised experts and leaders of roof bolting in the U.K. The introduction of totally rockbolted supports was introduced on 56As which was the next panel to be worked.
This system of roof support was used in the Selby Coalfield in early 1986 when HO2DRs, one of the first two faces at Riccall Mine were developed. D2s tailgate was a trial using rockbolts in conjunction with flat profile arches using a Lee Norse LN800 1TT continuous miner.
Lee Norse LN800 1TT continuous miner.
Rockbolting was introduced as primary supports at Wistow Mine in 1990 due to difficulties with the geology at the mine.
During the development of the Selby Coalfield new technologies were introduced very quickly as they became available. Roof Bolting was one of these system to revolutionise the speed, cost and safety of roadway developments initially on coal face access roadways. As the technology and monitoring was improved, main lateral roadways and face support salvages, which were previously supported by girders, became supported by roof bolts and cable bolts.
Roofbolting used for face support salvage at Riccall Mine.
Stopping roof collapses in a coal mine was always seen as spanning the gap created when coal or rock was removed by setting a support in this void therefore holding the roof up from below. Roof bolting and cable bolting are a completely different way of looking at this problem. Roof bolting uses the inherent strength of the strata situated above the roadway as a support. A hole is drilled to specified length and diameter into strata which has the required strength passing through the weaker layers of strata. Resin adhesive capsules are entered into the drilled hole and a steel bolt is then inserted. When the bolt is rotated and forced into the hole, by action of the rock drill, the resin capsules and hardening agent are mixed and fill the gap between the bolt and strata, known as the annulus. This creates a solid bond between the bolt and the surrounding strata. When the bolt is fully entered the nut is tightened onto a flat plate against the roof and tensions the bolt which holds the strata together. If this process is carried out quickly after the roof is exposed during mining process, the bolts and resin hold the strata together ensuring no bed separation of the rock and no roof falls occur.
The roof bolts used at Riccall Mine were threaded high tensile steel with a diameter of 22mm and were 2.4 metres in length. The roof bolts were installed into a 27mm drilled holes using two part high strength polyester resin. The resin capsules used were colour coded red for quick setting and green for slow setting. Two slow and one quick setting capsules were inserted into the holes and the bolt was drilled into the hole using a compressed air drill known as a ‘Gopher’. The drilling action and the sharp wedge tip of the bolts punctured and mixed the resin capsules which created a very powerful bond between the bolt and the surrounding strata. Once the bolt and resin are set to an adequate strength the 24mm nut is tightened on to a conical insert and plate up to the roof.
This system of rockbolting became standard throughout the Selby Coalfield and was used in conjunction with either W bars or flat straps with 1m spacing holes across the section of roadway. Each support was set at a 1.2m spacing. Steel or plastic mesh was used as part of the support.
To apply for an exemption to use fully rockbolted roadways many test were carried out. Roof / Strata movement measurement, rockbolt strength tests, rockbolt pull test and strain gauge tests which were all designed to check the performance of the rockbolt /resin/ rock system. Below are the areas of a mine where rockbolts may provide principle support;
Development headings and junctions;
Coal face development drivages;
Retreat district gate roads including the face ‘Tee’ junction;
Room and pillar coal production districts;
Special purpose drivages, eg to house equipment.
Four examples of places which may not be suitable are:
Goaf scours;
Gate roads serving advancing faces;
Cross measures drifts;
Headings formed by shotfiring off the solid.
Once a safe system has been designed and tested through the four stages of acceptance, the exemption to use rockbolts as a primary support is granted. The Mine Manager and Rockbolting Co-ordinator were required to instigate a system of routine monitoring and recording called the’ Scheme for the routine monitoring of roadways’
The system of monitoring included visual indentification of roof movement called dual height telltales.
Dual height telltale showing general assembly
The telltales were installed in the rockbolted roadway at intervals not greater than 20m apart. They were drilled to a height of at least twice the height of the rockbolts and basically monitored the movement of the strata above the roadway. Two spring loaded wires were anchored into the strata above the roadway and were given a copper tube set at the roof height as a reference point. If the strata moves downward the telltale will move upward and gives an indication of movement in millimetres. The telltales are monitored on a shift basis and recorded by the district official. Any movement is recorded. Excessive movement (25mm) must be reported to the senior official and action taken to remediate the movement should be carried out quickly.
Multiwire-extensometers were used and acted in a similiar manner to dual height telltales. This system used a four wire system set at four levels in the strata in a 7m hole and were set at 200m intervals in a roadway. This system is recorded by suitably trained people and could be tested manually or by remote electronic systems and was part of the ongoing roadway monitoring system.
Multiwire Extensometer
Information taken from H.S.E. guidance on the use of rockbolts to support roadways in mines.
Fully rockbolted headings became very common in face developments in the early 1990s, as different types of continuous miners which were ideal for rectangular section roadways were introduced throughout the Selby Coalfield. BJD(Jeffrey) Heliminers were used at Wistow and Stillingfleet Mine’s. Whitemoor Mine introduced Joy CM12s with North Selby and Riccall Mine’s used Lee Norse LN800 machines.
Fully rockbolted headings were introduced at Riccall Mine in July 1991 on H474s and June 1992 on H430s. During the H430s face development in mid 1992, the teams set a European record of 180m of development in a week using a Lee Norse LN800 continuous miner using the fully roofbolted system. This record was surpassed many times at various mines throughout the complex as the system was perfected.
H473s tailgate showing rockbolted heading with arches as primary support
H474s tailgate showing fully rockbolted heading and steel mesh
Stanley Main SM501s tailgate showing roofbolting and plastic side meshing. Glenn Bryan is carrying out methane boring using an EDECO mobile drill rig( Moonbuggy) in the photo.
When a roof support rockbolt is used for lifting or slinging the load must not be greater than 1 tonne.
If bolts are needed for lifting of greater loads, specially installed bolts called anchor bolts (we called them spot bolts at Riccall Mine) along with suitable colour coded lifting shackles should be used. They must be identified as lifting bolts with safe working loads shown.
Selby Superpit 