1. The principle of air DTH hammer drilling

　　The principle of air down-the-hole hammer drilling is to apply pressure and torque to the bottom of the hole through the drill pipe on the ground, and use compressed air to drive the down-the-hole hammer to impact and break the rock to achieve impact rotary drilling while using compressed air to cool the drill bit, and expel cuttings from the bottom of the hole to the surface.

　　2. Classification of complex formations

　　(1) During the drilling process, various complex situations often occur in the hole, such as water gushing, rock breaking, hole wall collapse, drilling diameter expansion or reduction, etc., making the drilling work unable to proceed normally, and serious accidents in the hole will occur, was forced to stop drilling, and even caused the drilling to be scrapped. The formation prone to the above situation is called complex formation.

　　(2) In engineering geology and hydrogeological drilling, the complex formations often encountered mainly include: clay formations, broken and collapsed formations, and pebble formations.

　　3. Drilling in clay formations

　　(1) Clay and clay shale formations are highly water-sensitive formations. The rocks are soft and swell after absorbing water, and the borehole necks down. Drilling into this type of formation at present mostly uses the method of rotary drilling. When the DTH hammer encounters such formations, it is very easy to get muddled drills, forming mud pockets or mud rings, resulting in stuck and buried drill accidents. Usually, core drilling is used to deal with such formations by adding various flushing fluid additives, such as water glass, PAM, PHP low-solid phase mud, etc., aiming to use high-quality mud wall protection to reduce water loss.

　　(2) Practice shows that when the air DTH hammer drills into the clay layer, the footage is still very fast, but to avoid the difficulty of lifting the drill caused by necking, decompression drilling should be used, and the drill should be drilled repeatedly up and down after a certain depth Tool, carry out hole sweeping, and wait until the clay stress is released and the necking is no longer present. If it is difficult to lift the drill due to necking, the following measures should be taken: first, lift the drill, use the power head to lift the drill to the highest point; second, drill and grind, let the power head drive the drill to rotate continuously; The third is to move the drilling tool up and down in series. After rotating in situ for about 10 minutes, use the power head to move up and down in series for about 1 minute. To effectively solve or avoid mud bags and mud rings and prevent accidents, the foam should be added to the holes during the period, because of its small specific gravity, low water content, and good lubrication, it can effectively reduce the clamping effect of clay on drilling tools.

　　4. Drilling in broken and collapsed formations

　　(1) Usually when core drilling encounters broken and collapsed formations, in addition to decompression drilling and appropriately reducing the speed, we should focus on the retaining wall material and try to keep the hole wall stable.

　　(2) Attention should be paid to the drilling of air DTH hammers in such formations: first, the design of the drilling structure should not be too complicated, because the number of variable diameters will inevitably affect the upward return wind speed; second, the drilling pressure and rotation speed should be strictly controlled to make them lower than the normal value, that is, light pressure and slow rotation, so as not to accelerate the formation collapse and cause drill stuck and broken drill accidents.

　　(3) If the collapse is so serious that conventional methods cannot be drilled, it may be considered to raise the drilling tool and pour in cement slurry (generally, cement above C40 is used to expect higher early strength). After the cement slurry penetrates the cracks to cement and solidifies the broken rocks, it is then drilled with a down-the-hole hammer.

　　(4) When the down-the-hole hammer drills into the broken formation, the high-speed airflow carries large cuttings back up, and repeatedly scours the rough hole wall, causing the fragments of the hole wall to fall off and cause the block to fall. If the falling pieces are large and the air volume is insufficient, it will be difficult to carry them out of the drill hole, and fall to the DTH hammer and the diameter change of the drill pipe, resulting in drill sticking or wind, making it difficult for the lower cuttings to go back up, and at the same time the drill pipe When the torque increases, the drill pipe is easily broken. Practice shows that in the event of a stuck drill accident, the drilling tool can stop rotating, the DTH hammer continues to impact, and at the same time lift the drill tool, and use the vibration of the DTH hammer to release the stuck drill as much as possible. When lifting the drilling tool, first try to rotate at the lowest speed, and continue drilling after the rotation is normal. When the drill sticking is serious, before other measures are taken, the air supply should be kept uninterrupted to prevent the drill from being buried.

　　5. Drilling in riverbed pebble formations

　　(1) Drilling in pebble and gravel formations At present, there are many drilling technologies, such as traditional follow-up pipe, impact hole forming, ordinary rotary method, etc., more advanced pre-splitting blasting in holes, reverse circulation continuous coring, ODEX, Double-rotation follow-up methods, etc. But there are certain disadvantages. The traditional drilling method is slow, low in efficiency, and poor in quality. The more advanced method has a high cost and is limited by specific conditions to varying degrees.

　　(2) At present, there are better construction methods in China for drilling in the pebble and gravel strata of the river bed. For example, Chongqing Nanjiang Geotechnical Survey and Design Institute adopted the eccentric tube air down-the-hole hammer technology in the survey and construction of the Jialing River embankment and achieved success in the drilling of river bed pebbles and gravel layers. Because the rock-breaking method of the eccentric down-the-hole hammer has changed the traditional rock-breaking mechanism (cutting, grinding, fracturing), the volume of coarse particles (2-3 cm) will be broken when the impactor works, and the casing follow-up and sealing will isolate The leakage of air in the pebble layer generates high-speed airflow (>15cm/s) in the gap between the casing and the drilling tool, and quickly discharges the cuttings out of the hole, avoiding repeated crushing. In addition, for rocks that are difficult to break in time, under the action of rotary drilling pressure, the drilling tool can squeeze it into the hole wall, which is beneficial to continue to break down.