Abstract:

The drilling tasks in hydraulic engineering were usually under severe conditions. In the narrow closed cavity, the normal DTH hammer drilling technique was generally applied to drill boreholes, but dust pollution will seriously affect the working environment and workers’ health. To solve the dust pollution in the cavity, reverse circulation drilling technique with DTH air hammer was firstly applied to drill the drain holes in the cavity of a hydropower station, and the upward and downward holes were successfully drilled. The reverse drilling technique with DTH air hammer can control and reduce the dust pollution in the cavity. Furthermore, this technique can maintain good working environments. Two types of drill bits with different structures were designed for test, which were named as type A and B, respectively. The main difference between type A and B drill bits were the positions of suction nozzles. The suction nozzles of type A drill bit were evenly distributed between any two flat keys of the spline. While the suction nozzles of type B were distributed on every flat key of the spline. The CFD numerical simulation results had shown that the air suction rate in annular space of type A drill bit was 0.199 987 04 kg/s, which is slightly larger than that of type B drill bit with a air suction rate of 0.177 524 64 kg/s in annular space between drilling tool and borehole wall . In other words, type A drill bit can suck more air from the annulus space. The test results on drilling upward and downward holes indicated that the structure of drill bit which has double row with 12 suction holes on the spline had the stronger suction capacity and better reverse circulation effect of the slag discharge. Therefore, there was less dust in the cavity and more rock cuttings discharged from the central channel while using type A drill bit. In hence, the reverse circulation discharging effect of type A drill bit is superior to that of type B drill bit.

Key words: reverse circulation with DTH hammer, reverse circulation drill bit, upward borehole, dust pollution, environment protection