Cablebolts - a market review

They have an increasingly important support role to play. It seems cablebolts are here to stay. By Ron McKenzie *

Staff Reporter

Cablebolts were first used as a form of rock reinforcement in the underground mines in Canada and South Africa during the early 1960s.

 

Extensive development of cablebolt technology took place during the 1970s in Australia, Canada and South Africa. This development was originally done in the metalliferous mines for rock reinforcement in the cut-and-fill stopes and large non-entry stopes. The original cablebolts were generally made from discarded winder rope with the familiar seven wire strand cablebolt being first used at Broken Hill in the mid 1970s.

 

The first use of cablebolts in the coal mines took place at Tahmoor Colliery in 1983. These consisted of plain single strand 15.2mm seven wire cablebolts untensioned and with no collar fittings. Birdcage, bulbed and ferruled strand cablebolts were developed to overcome the problem of rock stripping off the cable because the bond between the grout and the cablebolt failed. Collar fittings such as the barrel and wedge anchor with large bearing plates also improved cable performance. A further development was the introduction of the cable truss, which is particularly useful in broken, “baggy” ground.

 

In 1993 geotechnical consultants BFP and ANI﷓Arnall developed the Flexibolt. This bolt could be point anchored using a chemical resin anchor with a limited tension applied via a nut fitted beneath a barrel & wedge anchor. Cable tendon technology has since developed to the extent that pretensioned groutable bolts with load capacities as high as 200 tonnes are now available.

 

The current cable tendons on the market today come from a variety of suppliers. They are available in plain, cable-point-anchor configuration, through to high-tensioned, fully-grouted cables in 4-10m lengths. The table below gives a summary of the cable tendons currently available.

 

Cablebolts are manufactured from stress relieved, low relaxation, seven-wire steel strand. They can be plain strand, bulbed or birdcaged, although the bulbed type is generally more commonly used for improved load transfer and minimum hole diameter. The cablebolt is generally not pre﷓tensioned and is installed as a dowel. Tensioning of cablebolts after grouting is often done to fix the bearing plate hard against the rock face, with a limited tension being imparted onto the lower portion of the cable. B&W anchors retain the bearing plate.

 

The Celtite GXT Tensionable Cable Bolt is a seven-strand dyformed, resin point anchor bolt, bulbed over the anchor length. The GXT bolt is manufactured from a 22mm-diameter cable that has been drawn through a dye reducing the diameter to 18.5mm. This tends to flatten the outer wires resulting in the wires having a distorted trapezoidal shape. It also allows the bulbing to be kept to 24mm and for the bolt to be installed in a standard roofbolt hole of 27-29mm diameter. Tension of 10-25t is applied using a hydraulic ram. A drive dolly utilising a cam locking principle is used to install the bolt.

 

Jennmar manufacture the Super Strand Bulbed (SSB) and the plain Super Strand (SS). Both are made from the same 19-wire strand, however the SSB is bulbed for a length of approximately 1m with different bulb diameters being available. The SS is installed in a standard roofbolt hole and is thus generally point anchored. Tension of up to 20t is applied using a hydraulic ram. The SSB can be post grouted using 16mm-diameter grout tube and a bearing plate with holes to allow the insertion of the grout tube. A drive dolly with a female profile of the cable is used to install both type of bolts.

 

The Hi-Ten Strand Bolt and the Flexibolt are manufactured by DSI-Arnall from 21-wire strand. Both are designed to be installed in the standard roofbolt hole of 28mm diameter and are generally point anchored. The Flexibolt has a rolled thread on the collar end with a B&W anchor fitted above a break-out nut. Installation is virtually identical to that of a roofbolt with the nut being tightened against the B&W anchor to obtain a bolt tension of up to 5t. The B&W anchor is fitted to take the bulk of the bolt loading. A square spiggot is fitted to the collar end of the Hi﷓Ten to accommodate a mixing dolly. Tensioning of the Hi﷓Ten is done using a hydraulic ram.

 

Megabolts consist of multiple 6.9t, high-tensile, deformed, reinforcing wires configured in continuous birdcaged lengths. A grout injection/breather tube runs internally through the full length of the bolt which has a machined, hardened, threaded head. The bolt is point anchored using resin and can be grouted after tensioning by injection of grout through the bolt head. The centre tube is used as an injection tube for toe-to-collar grouting using low slump grout or as a breather tube for collar-to-toe grouting with liquid grout. A hexagonal shaped end on the head of the bolt allows a standard roofbolt dolly to be used for installation. Tensioning is performed using a threaded coupling and extension bar passing through a hollow centre hydraulic ram. Installed loads of 60t are possible with the 80t bolt.

 

A number of features of cable reinforcement have an impact on the performance of the system. These features are, quality of the point anchor, strength of the tendon, stiffness of the system and integrity of the collar fittings. Other than the cablebolt most cable dowels currently being used have a chemical resin as a point anchor. The issues associated with resin anchors such as hole size to bolt diameter ratio, loss of resin into broken ground, resin mixing and hole depth are well known. Tendon strength along with support density are usually specified by strata control or geotechnical engineers.

 

In most coal mine roofs the geotechnical advice is that the roof reinforcement should be as stiff as possible. Stiffness is obtained by tensioning and then grouting the cable. Cables that are installed in the standard roofbolt hole of 27-29mm, such as the GXT, Flexibolt, Hi-Ten and SS are virtually impossible to post grout because of the lack of clearance between the cable and the hole wall to insert a grout or breather tube. These bolts are tensioned and used as point anchor tendons. Their advantage is that they can be installed by miner mounted rigs during the normal bolting cycle.

 

For these bolts to obtain full column grout coverage, up to 3m (for a 6m bolt) of extra slow set resin is inserted beneath the quick set anchor. Generally it is beyond the ability of the burliest miners to push a bolt through this much resin and get the end of the bolt to the top of the drill chuck of a hand-held or conventional miner-mounted rig. The use of “through-the-chuck” drill rigs, which can feed the bolt into the hole, has overcome this problem. Once the quick set resin has cured the bolt has to be tensioned before the extra slow set resin hardens.

 

For post installation grouting of cables, sufficient hole clearance is required to insert grout or breather tubes, as is the case with the Jennmar SSB. In the case of plain strand cables, stepped hole diameters are required. These are 28mm for the point anchor, 45-55mm for the section with the grout/breather tube and 70mm at the collar to allow the hose to be inserted into the drill hole through a hole in the bearing plate. Difficulty can often be experienced in inserting the grout hose particularly if the drill hole passes through broken ground.

 

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