Underground » Mining Technology and Production
Drilling and installation of secondary support is considered to be a critical yet low-productivity aspect of underground coal mining. It presents a major obstacle to the vision of rapid roadway development. Rock bolt drills are considered to be high-risk equipment, with key operational risks associated with rock falls, manual rod handling injuries, and incorrect operation of equipment. The industry clearly recognises the need for an improved technique for cable bolt drilling, to enhance safety and productivity.
This project leveraged CRCMining's extensive water jet drilling expertise, investigating a novel technique for continuously drilling cable bolt holes using a water jet drilling tool. This was recognised by the ACARP Roadway Development Task Group as having the potential to minimise manual handling and man-machine interactions (and therefore operator exposure), as well as providing step-change improvements to cable bolt installation productivity. A continuous water jet drilling system solution was conceived and the requirements established in a prior project phase. The main objective of this current project was to assess the drilling performance of such a system (drilling speed, diameter control, hole quality) in actual mine conditions.
Two water jet drilling tools were trialled in the roof strata of a portal entry at Wambo Underground Mine:
1. A CRCMining-developed tool with a flexible shaft remotely powering the rotation of the cutting head [drilled diameter 45mm];
2. An advanced self-rotating compact water jet tool deployed on a flexible conduit [drilled diameter 28mm]. Functionality and robustness of the drilling tools were verified in drill tests at CRCMining's water jet laboratory on a variety of sandstone samples, before being tested at the Wambo trial site.
The field testing program highlighted the significant variability in the penetration rate of water jet drilling systems with changes in rock type and strata conditions. When drilling through roof strata of the Wambo seam, a drilling rate of
1.7 m/min was achieved in sandstone at 6.5m depth (from the roof), whereas the rates in siltstones and mudstones (which collectively make up around 30% of the rock encountered in an 8m support hole at the trial site) were relatively low at ~0.1 m/min. Penetration rates actually dropped to zero in two specific zones:
1. Hard siltstone/sandstone bands at about 0.6m to 0.9m depth;
2. A sandstone band at about 2m depth. The drilling tool was also trialled in the rib (coal), where it achieved more consistent and higher penetration rates of up to 4 m/min.
Field testing indicated that to ensure that a water jet-based continuous drilling system is absolutely capable of productively drilling through all encountered rock and strata conditions, a likely requirement is the move from high-pressure (~80 MPa) to ultra-high-pressure (+300 MPa). This would necessitate redesign and upgrades to the drilling tool and water reticulation systems, and whilst potentially feasible, has prompted parallel investigation and reassessment of alternative system solutions.
Despite the inconsistent field drilling performance of the water jet tools used in this project, a significant breakthrough resulted from this work: development of an improved flexible drilling conduit able to withstand a 150mm bend radius and transmit significant compression forces (translating to high weighton- bit). CRCMining is continuing development of this outcome via integration in a radial drilling system technology, and is exploring opportunities for continued development for coal strata support that builds on the outcomes of this project to leverage the novel flexible conduit technology and explore coupling of this with more traditional or perhaps even standard mechanical tools for drilling.