Prestressing of Strands to Improve Cable Performance

High load capacity cable bolts and Flexibolts have the potential to be pretensioned to relatively high loads of 30 tonnes while still allowing further roof deformation and bolt loads to increase before failure loads are reached. The benefits of pretensioning cables have been assessed in this project by an initial laboratory based testing program followed by a series of field evaluations. Results from laboratory testing of shear and opening behaviour of a single joint have shown that:

• the resistance to joint opening increases with cable pre-tension load,
• joint shear resistance increases by approximately the pretension load at small shear displacement but once shearing increases, the shear resistance is less dependent on the level of pretension,
• pretension loads will be maintained if the cable anchorage length is adequate and the end fittings and plates remain relatively stiff, and
• the stiffness of the reinforced joint to both opening and sliding is increased significantly by a cable pretensioned to 15 tonnes.

The most efficient method of pretensioning cables is to stretch the free length between an anchored portion and the cable end fitting at the hole collar using a hydraulic jack. The stretch is then taken up within the end fitting so that releasing the jack allows the majority of the jack load to be retained as pretension. A tension locker end fitting for both strand cables and Flexibolts was developed during the project and used for all field based trials.

Six field trials were completed at three collieries; West Wallsend, Newstan and Wyee. These mainly involved pretensioning long point anchored Flexibolts but at Wyee one trial was attempted with two stage fully encapsulated Flexibolts which were pretensioned after the point anchor had developed but before the secondary grout had cured. Results from the fieldwork showed that:

• pretensioning Flexibolts to 25 tonnes can be reliably achieved provided prior roof deformation is small and within the first 2 to 3m of the roof,
• there is no advantage in pretensioning cables if the roof has deformed to the point where the height of softening is within or above the cable anchorage horizon,
• pretensioned Flexibolts can reduce and in some cases eliminate the time dependent creep deformation in the roof that commonly occurs between development and longwall induced stress changes, and
• at West Wallsend, the onset of longwall induced roof deformation was delayed until the longwall was within about 8m of the trial site; this contrasts with the performance in adjacent areas reinforced with non pretensioned Flexibolts where the longwall started to affect roof movements when it was 50 - 60m away.

Experience with various Flexibolt plates and end fittings during the field trials has shown that a butterfly plate should be used with a domed plate to reduce the potential to overload point anchored Flexibolts and minimise plate punching in weak immediate roof.