Laboratory water jet assisted drag tool rock cutting studies at high traverse speeds
Water jet assistance has shown many promising benefits to drag tool rock cutting. However, the basic failure mechanism of hybrid cutting is not well understood. In addition, most previous laboratory investigations have been carried out with cutting speeds of less than 0.25 m/s, whilst typical tool speeds for a production boom-type tunnelling machine cutting hard rock is over 1.0 m/s. Potentially erroneous conclusions may be obtained unless laboratory cutting speeds are comparable with those used in the practical situation. Based on the research work carried out under a three-year contract sponsored jointly by the Science and Engineering Research Council and the European Coal and Steel Community, this thesis examines the cutting mechanisms when a water jet and a drag tool are acting together. Over one thousand cuts have been carried out in five rock types which cover a wide range of strength and abrasivity. A linear cutting rig was modified to enable cutting speeds up to 1.10 m/s to be obtained. Jet pressures up to 70 MPa were provided by a 75-kW water pump. Based on the cutting mechanisms of the drag tool and the effect of the water jet action, a hybrid cutting model is proposed. To obtain significant tool force reductions, the jet power must be greater than either the threshold jet power for slotting, or a critical jet power for hydraulic fracturing. Depending on the jet power, rocks can be separated into two groups, one with significant jet penetration and the other without. For rocks with significant jet penetration, the force reductions with water jet assistance can be estimated from the jet penetration characteristic. An optimum jet penetration was found to exist which provided the maximum force reductions. For rocks without jet penetration, the force reduction is marginal except when the jet power exceeds the critical jet power for hydraulic fracturing. An expression is given which characterises the functional relationship between force reduction and jet penetration. When the jet penetration for the rock is insignificant, an equation is proposed to estimate the critical jet power required.