Research On The Technology Of DTH Hammer Drilling With Casing For Complex Stratum

The drilling and coring for complex stratum has always been the main and difficult problem in the geological and mining exploration, the engineering of exploration, and the geotechnical engineering. Since the complex formation of sedimentary layers generally contain clay, flow sand, egg stones, gravel, big stones, it posed difficulties and problems to the drilling, hole-wall-protecting, and coring of the stratum, which can not be simply solved by any usual drilling technology. The hydraulic and hydroelectric engineering at home has undergone two phases, the down-the-hole hammer core-drilling with casing and the diamond core drilling (the two significant breakthroughs in technology). In particular, the development and broad application of SM-gel diamond core drilling for the gravel layer has fundamentally transformed the drilling technology in this field, for it has solved to a certain degree some of the drilling and coring problems. However, till now, the SM gel diamond core drilling can not be applied to the complex stratum. As a result, the quality and efficiency of drilling remain at a very low level.Through the analysis and research on the drilling applicability into the complex stratum, the drilling techniques, the drilling fluid, the coring tools, etc., this paper has proposed the correspondingly new drilling methods and technology for different stratum, aiming to promote the drilling technology as such. Therefore, some major findings and conclusions are made as follows:(1) Heavily drawing on the stress wave theory, the paper describes and illustrates the down-the-hole hammer drilling process, analyzing especially the drilling process of rock-breaking with DTH hammer and the relative factors. With the eccentric drilling as the case in study, the paper has also generated a theoretical model for the analysis of pipe drilling pressure, case weight, percussive capacity, pipe drilling speed and depth, reaming gauge, the value of pipe drilling pressure in the process, and made the following findings:â‘ the pipe drilling speed is determined by the percussive power of DTH hammer, rock-breaking power per volume, and the diameter of aperture; â‘¡in terms of different drilling conditions, the paper has proposed, respectively for simple operating mode and complicated operating mode, the formula to calculate the maximum casing depth, namely l₀ and l_max, of both vertical and horizontal holes;â‘¢based on the analysis of the relation between pipe drilling pressure and drilling speed, the paper has concluded that the most appropriate pipe drilling pressure is 0.5～0.9kN per centimeter of the casing bit diameter.(2) The pipe drilling with air-powered DTH hammer is with limitation applied to the shallow, loose and even-grained formation with few aerial strata, which is of medium drilling characters. Though with high efficiency, it is simply a full-section drilling with casing, and not capable of coring. With the integration of air-powered DTH hammer drilling and core drilling, this research has developed a new drilling and coring method—the technology of air-powered DTH hammer and core drilling with casing, by combining the high drilling speed and efficient hole-wall protection of the former with the core gathering capacity of the latter. The drilling equipment is mainly characterized by the following features:â‘ the drill tool is structured in the way of drilling with simultaneous and concentric case; the equipment follows the mechanism of stage drilling and employs the pre-casing bit in the form of centre bit (profile); to meet the requirements of coring in either gravel or loose strata, the drilling tool is in double-piped making and tri-piped making, respectively;â‘¡the outer and core barrels are in rich supply and available in market, since they are general equipment for geological and oil drilling;â‘¢the core extracted by air-powered DTH hammer and core drilling can illustrate the factual state and characteristics of the formation (its position, its composition, etc.);â‘£owing to the use of highly-efficient DTH hammer drilling, coring and casing are completed simultaneously to achieve the high drilling efficiency;Besides, the coring bit in pipe drilling and casing bit have also been developed in the research; and practical and feasible regulations have been made for the air-powered DTH hammer and core drilling with casing. As general apparatus for geological exploration at home, such as air compressor, percussion tool, drill, the drilling equipment employed in the technology are easily available. Moreover, this technology is better applied to the drilling into shallow complex formation within 50m, especially to the core drilling into the II type complex stratum.(3) Based on pipe drilling with GJ reamer, the research has innovated the technology of percussive diamond core drilling with casing, which has the following characteristics:â‘ the expansive-contracted reamer is cone-like and symmetrically structured; â‘¡the drilling apparatus are able to meet the demand of conventional drilling on the bearing of pressure and twist;â‘¢the drilling bit and coring bit, in concentric pattern, are combined to function well in both orientation and adjustment in direction, and can avoid the hole deviation;â‘£the design of drainage outlet contributes to the cooling and flushing of reamer without reducing the necessary cooling liquid at hole bottom.⑤the adjustable rings in the hanging cavity can not only help the contraction of reamer, but also guarantee the load-bearing capacity of hanging apparatus by increasing or decreasing the rings, so as to eliminate the abnormal stress on the claw.â‘¥the sand flushing system can prevent grainy stuff from sticking to the drilling tool, so as to guarantee the expansion and contraction of core bit.The technology of percussive diamond core drilling with casing has been well applied to the drilling into complex stratum to achieve remarkably positive effects in technology and economics. And this method can reduce the drilling accident and the consumption of drilling material so as to enhance the efficiency and quality of drilling; it also simplify the hole structure and makes it possible to solve the backfire accident.(4) At home and abroad, there still remain four problems in the technology of air-powered DTH hammer drilling with casing: first, the gauge of drilling tool is not uniformed and the series of drilling products are not complete; second, there are obvious defaults in the design of drilling tool; third, the manufacturing technique of drilling tool, such as the selection of raw material and the heat treatment process, can not live up to the requirement; four, the production of complete set of drilling equipment as well as the operating regulation for the equipment need improving urgently.Due to the characters of the complex stratum in Xiao Wan hydropower station, the high-slope anchoring and reinforcement projects in the high slope bank have encountered the drilling difficulty mainly in the collapsed formation deposits and the foundation bed broken under the impact of geological formation and aeolian erosion. To solve the technological problems in the complex formation, this research has innovated two types of air-powered DTH hammer drilling equipment, The two types of drilling equipment have the following properties:â‘ the use of both centre bit and eccentric reaming bit forms the two-stage rock breaking; the design of slag flushing system ensures sufficient percussion impact of the percussive drilling tool;â‘¡the eccentric drilling with casing that employs the centre bit can better prevent the hole deviation;â‘¢the drilling equipment has high endurance, due to the design of key connection to deliver the torque, the selection of high strength material and the special heat treatment in manufacturing the drilling tool, as well as the proper techniques of fixing the gear tooth engagement;â‘£the keys are interlocking devices that strongly and reliably attach the centre bit to the adjuster to avoid the risks of bit losing.At the same time, in accordance with the drilling characters in Xiao Wan, the selection of the drilling tool, the air compressor, and the DTH hammer has followed the principle of mutual adaptation and matching, and the innovation of drilling tool has also been made. Moreover, the operation specifications and regulations for DTH hammer drilling with casing have also been proposed.(5) Since the pulling speed has direct effects on the anchoring efficiency, the research has developed a series of hydraulic pull casing that has the pulling force of 65t. The technical features are as follows:â‘ the series of pull casing are simple, small, and light, easy to be assembled, dissembled, and operated;â‘¡the separation of operating board from the hydraulic pump unit is better applicable to the high slope drilling projects suffering from difficult delivering and shifting;â‘¢for the sake of pull casing in high slope, the foundation of operating board and the petrol cylinder have been joined on hinges;â‘£each device can pull out a series of casing pipes of different gauges.(6) The technology of DTH hammer drilling with casing has been successfully applied to the drilling and coring into the complex stratum in more than ten projects, such as the project of slope rectification and anchoring in the Sichuan-Tibet Highway at Longdan River of Mt. Erlang, the cover curtain grouting test of the foundation in Gold Ping Power Station, the project of pre-stressing anchoring for the cable platform on the slope of the left bank of the Guandi Power Station of Yalong River. The application of the innovated eccentric drilling tool with casing, the hydraulic pull casing device, and the operation specifics and regulations has been proved efficient in both drilling and coring. The new drilling technology has been able to solve the drilling problems to a certain extent in the complex stratum with deep covering, cobble and pebble layers, and formation deposits.