According to recent research into improving roadway development rates, self-drilling bolts are one of the technology developments that will overcome some key constraints to speeding up development rates in the short to medium term.

The research was funded by the Australian Coal Association Research Program (ACARP) to identify what was needed to improve roadway development rates to support a new generation of longwall mines.

Roof and rib support particularly were typically identified as the most significant constraint to roadway development, unless support requirements were minimal and difficult floor conditions were experienced.

In Australian coal mines roof bolts currently in use in roadway development vary between 1800mm and 2400mm in length, while roof bolting intensity typically varies from four 1800mm bolts/1.5m to eight 2400mm bolts/m, with six 2100mm bolts/m being the most consistently applied (41%).

Some 32% of mines reported using 4m or 6m tendons as part of the primary bolting phase. Rib bolts vary between 1200mm and 1800mm in length, while rib bolting intensity varies from spot bolting one 1200mm bolt/metre/rib, up to three 1200mm, two 1500mm, or two 1800mm rib bolts/m/rib.

As many as 67% were typically installing two or three bolts per cycle per rib, and in most instances full rib meshing as well.

Currently bolt installation is a two-step process involving drilling the hole, withdrawing the bit and then inserting the bolt. As self-drilling bolts are simply driven into the strata and left the second stage is eliminated, which can significantly speed up the bolting phase of development mining. This is where self drilling bolts come in.

Self drilling bolts

The five manufacturers currently working on these bolts include German company Hilti, Sandvik’s Australian office Voest Alpine Mining and Tunnelling (VAMT), rebar manufacturer OneSteel, research group Cutting Edge Technologies and bolting rig manufacturer Hydramatic.

Hilti has demonstrated its Hilti OneStep anchor – a self-drilling anchor with built-in adhesive cartridge – in various mines around the world. The OneStep anchor combines the drill bit head, drill steel, anchor and adhesive resin in a single unit. Researchers, however, said the anticipated price of the bolt may be a barrier to widespread adoption.

Voest Alpine Mining and Tunnelling and Sandvik's Mineral Ground Tools division have begun testing the Seedsman self drilling, tensionable, post groutable bolting system. Preliminary lab pull tests have been completed into concrete with the bolt standing up to 10 tonne tests. In situ trials are three to six months away, according to the company.

One Steel recently committed to commercialise production of a hollow bar suitable for a self drilling bolting system, as developed by consultant Peter Gray through an earlier ACARP project. The rolling of a 20 tonne sample was completed in October, and field trials are expected to take place in early 2006 after the bolts, drilling rig and resin injection system is assembled, and an underground test site is identified.

CSIRO's Queensland Centre of Advanced Technologies (QCAT) has developed another self drilling bolting system for incorporation into the Automated Coal Conveying/Bolting Module ACBM which CSIRO has been developing for the last four years. At the moment commercialisation of this bolting system is suspended because the design of the automated bolting system is not complete.

Cutting Edge Technologies have also developed a self drilling bolt, and it is expected that the bolt will be commercialised in January 2006 through a local bolt manufacturer. CET also have designs for an extensible conveyor system, although the status of that development is not known.