Study On The Drilling Technique For Detecting Multilayer Cavities

Both of the underground karst caves or the goafs formed from mining activities pose a great threat to the normal production and life security of humans. Detecting of the underground cavities so as to learn their detailed and accurate positions and the shapes, as well as treatments base on the obtained data are important effective methods to avoid the secondary disasters caused by cavity collapse. To date, the physical prospecting method is widely use to detect the cavities, however, leading to a relative lower accuracy because of the multiple solutions of various method of physical prospecting especially in the case when there exists several layers of cavities in the direction of gravity. By the use of drilling method together with the 3D cavity auto scanning laser system, the positions and shapes of the cavities can be obtained with with high accuracy, which makes this method much popular in the detection of cavities in mines. The traditional positive circulation drilling method is much limited and is only suit to be use in the detection of single layer cavity, while as is currently the case, there are quite number of unexplored cavities underground that are multilayer in the mines, which is one of the key restrictive factors of safety production, especially for the safety production of integrated strip mining. The hollow-through DTH hammer drilling techniques with reverse circulation is an advanced drilling method which integrate the percussive rotary drilling and the reverse circulation discharging techniques together. This technology is widely used in oil and gas exploration, geology and mineral exploration and in hydrological well drilling subjects. Because of the unnecessary use of annular between borehole and the drilling pipes serving as a discharging passage, the use of this technology can conquer the hard discharging problem after drilling through the first cavity, which is much conmen for the normal drilling method. Besides, the hollow-through passage in the hollow-through DTH hammer drilling tool can serve as a safe passage for the 3D cavity auto scanning laser system going down and up. The use of hollow-through DTH hammer drilling techniques with reverse circulation together with the 3D cavity auto scanning laser system provides an ideal technical method to cope with the technical difficulties of detecting multilayer underground cavities in mines.In this paper, it was based on the subject of detection of multilayer cavity in the iron ore of Yuanjia village, Lan county, Taiyuan Iron and Steel Company(TISCO) that the drilling method and technical process study of detecting multilayer cavity was carried out by the use of the hollowthrough DTH hammer drilling techniques with reverse circulation together with the 3D cavity auto scanning laser system. Additionally, this study also aimed at providing solutions for the accurate exploration of unclear cavities especially for the open pit mines. This study was carried out in several aspects of research content as followed below.1. Focusing on the multilayer cavities distribution characteristics in the iron ore of Yuanjia village, as well as the working site rig and properties of the 3D cavity auto scanning laser system, the large diameter of center passage of hollow-through DTH hammer, the reverse circulation drill bit and the double barreled drilling pipes was designed and manufactured for allowing the 3D cavity auto scanning laser system going through and reaching the region of cavity. The center main spindle of the motor head of the rig was designed with large diameter so as to ensure the successful connection with the designed double barreled drilling pipes. The other accompanying drilling tools were also designed and manufactured. The field tests were conducted, by which the process techniques of the drilling method mentioned above were proposed and enacted.2. By the use of dynamic mesh technology of the computational fluid dynamic(CFD), the performance analysis of air driven DTH hammer was conducted. Good prediction of the working performance of the DTH hammer was obtained, providing a efficient way for the design and optimization of the DTH hammer.3. Because the debris cannot discharge from the pipe in time, causing the problems of bit burying and bit jamming happened in the drilling process of detecting multilayer cavity in the field tests of iron ore mine in Yuanjia village, the inner-jetting nozzles of the reverse circulation bit was designed with the structure of the laval nozzles, which replaced the normal nozzle with circular section, so as to improve the reverse circulation ability of the drill bit. The reverse circulation ability and the flow behavior of the drill bit with laval nozzles of inner-jetting nozzles was studied by the use of numerical simulation and laboratory experimental tests. Both of the simulation and experimental results showed that the reverse circulation ability of the new designed drill bit with the inner-jetting nozzles of laval nozzles structure was improved significantly.The innovation of this paper are as follows:(1) It is the first time that the technology of combining the hollowthrough DTH hammer drilling techniques with reverse circulation together with the 3D cavity auto scanning laser system, using to detect the multilayer cavities, was proposed. Field tests results showed that the this drilling method could provide accurate detection of the multilayer underground cavities.(2) It is the first time that the dynamic mesh techniques of CFD was used to analyze the flow behavior and working performance of the air driven DTH hammer. By using this method, The design efficiency of the air driven DTH hammer was improved significantly and the design cost decreases a lot, in the same time, providing a good solution method for the computer aided design of the DTH hammer.(3) The structure of laval nozzles was applied in the inner-jetting nozzles of the reverse circulation drill bit so as to increase the reverse circulation ability of the bit. Both of the numerical simulation and experimental results showed that the reverse circulation ability of the new designed drill bit with the inner-jetting nozzles of laval nozzles structure was improved significantly.To date, with the increased degree of integration of mining resource and the technical promotion of the advanced open-pit mining, the underground uncertain cavities in mines pose great threat to the security of production. Thus, it is important to strengthen to study of cavities in the mines especially for the multilayer cavities. In this paper, focusing on the detection technique of cavities by the use of drilling method, the technology of combining the hollowthrough DTH hammer drilling techniques with reverse circulation together with the 3D cavity auto scanning laser system was proposed. Engineering practice showed that this technique was efficiency and meaningful for solving the problem associated with the detection of multilayer cavities in mines, and was worth to be widespread.