Research On Reverse Circulation Down-the-hole Hammer Drilling With Casing Technology

With the development of constructions in our country, the quantity of drilling engineering in some complex stratums has already been getting more and more than before. Drilling in these parts underground totally breaks the stability and balance of these stratums, and wall of hole slumping,clumps falling,leakage loss,water gushing,diameter curtailment,diameter augmentation can be found at every turn. So, absolutely, to drill in complex conditions really becomes a difficulty. However, Down-the-hole Drilling with casing technology can successfully deals with this problem. This technique is researched to protect the wall of hole, thus it makes itself a good method against the complex stratums of cracking,leakage loss,quicksand. Nowadays, drilling with casing technology, in our country, is mostly used for small diameter drill holes with its hole-diameter of casing less than 146mm.It uses single eccentric drill bit, conventional air-powered down-the-hole hammer, single wall drill pipe and normal circulation to expel rock clippings and usually be taken to rock anchoring, dam foundation examination hole drilling, etc. Moreover, drilling with casing technology can not be applied in grouting pile hole and water well with large diameter at the present time. In other countries, many researches about the structure of casing bit have already been carried out. They all use normal circulation to transfer rock cuttings, and the specification of bits is serialized. Large diameter casing bit can drilling with the casing with outer diameter of 34 inches (863.6mm) and inner diameter of 33 inches (838.2mm). When the casing bit opens, its diameter is 34.882 inches (886.0mm) and when it contracts, 31.850 inches (809.0mm).In 1980s, Center Sampling Recovery technology was born in the USA and Canada, which is also called the CSR. And in the middle of 1980s, the CSR technology was introduced into our country. Hollow-through down-the-hole hammer drilling with reverse circulation is a high and new technology in the field of drilling with air. It totally contains three techniques. First, the rock is drilled with high energy impact of down-the-hole hammer. Second, multiple drilling fluids (air, liquid and solid)with reverse circulation are used. Third, cores or samples are recovered continuously without the stopping of drilling process. Therefore, Hollow-through down-the-hole hammer drilling with reverse circulation technology has high drilling efficiency, high core recovery percent, strong adaptability in drilling complexities and many other finer effectiveness.There are some defects when use down-the-hole hammer drilling with normal circulation.1. The clearance between wall of hole and the casing is too small, and the rock cuttings can not be moved out easily if they are not tiny enough, in result, the cuttings are broken up again and again, and this makes the drilling efficiency get low and the wastage of energy become much, especially in large diameter drilling with casing.2. The disturbance often makes the wall of hole out of balance when rock clippings, air and water run out to surface from the clearance between wall of hole and the casing. And during this process, a moiety of clippings block the clearance, consequently, to expel becomes much more difficult without evidence. 3. There is a big friction between the wall of hole and casing. Certainly, to make the casing down the hole, more energy of impact will be needed, and the life of casing shoes and casings will be largely cut down.Aiming at solving those problems above, the technical scheme for reverse circulation down-the-hole hammer drilling with casing technology is put forward, that is, it equips with casing bit which is suitable for reverse circulation, hollow-through down-the-hole hammer and double-passage drill pipe to form a reverse circulation down-the-hole hammer drilling tool.To realize the goals above, first for most, a lot of studies on the mechanism of reverse circulation down-the-hole hammer drilling have been carried out, and then reverse circulation with inner jet orifice experimental facility was developed, also based on it, a large number of experiments were performed sequentially. In succession, researches on flow field of reverse circulation drill bit were done by the way of CFD. Analysis with CFD has four steps, including model building,fore treatment,sovling,post treatment. Inventor, Solidworks and other softwares are used to modeling the fluid pathway. Gambit software is used to fore treatment which involves mesh generation and boundary conditions setting, and Fluent, CFX and other specialized CFD software are utilized to solve problem and post treatment. Second, large numbers of researches on simulation and dynamic process in down-the-hole hammer were put into effect to develop a series of hollow-through reverse circulation down-the-hole hammer, and simultaneously, a naissance of double wall drilling tool was come to reality ground on great effort of researches. Finally, according to the essential requirment of drilling with casing that the bit diameter should expand to make itself a little larger than the casing when drilling so that the casing can easily goes down with the bit, and the bit diameter can dwindle to make itself a little smaller than the casing when draw back the drilling tool so that the casing can be left in the hole with a ease, and also according to the specialty of large diameter drilling with casing which has a mass of rock clippings to expel when working in a hole, lots of works were done to work over the structure of down-the-hole hammer drilling with casing bit, and successfully, several kinds of the bit with novel structure were developed, particularly, QFGZ type bit has ever won the patent of invention of China.Experimentation of micro-pile hole drilling with reverse circulation hollow-through down-the-hole hammer technology was taken and GQ320 type hollow-through air-powered down-the-hole hammer was developed. After its manufacture by Shanghai Jintai Engineering Co. Ltd., the drilling experiment was carried out by Guangzhou Geo-engineering Co.The results of this drilling experimentation show that GQ320 type hollow-through down-the-hole hammer can be easily start-up, its performance is steady, its impact is powerful, and furthermore, each part has enough strength intensity.