Detailed explanation of the use of rotary drilling rigs

The construction technology of the rotary drilling rig is an advanced pile foundation construction technology. It was invented by the Italians after World War II. Later, it was used on a large scale due to the full development of construction in Europe and Japan, making it more complete and more functional. It has become the main pile foundation hole-forming tool in the construction of railways, highways, water conservancy, workers and people all over the world.

Rotary drilling rig construction site

1. Basic introduction of rotary drilling rig and technology

The rotary drilling rig has the characteristics of large power, large output torque, large axial pressure, flexible maneuverability, high construction efficiency and multiple functions. It can adapt to the soil and geological conditions in most areas of our country. At present, rotary drilling rigs have been widely used in various drills. Hole cast-in-place pile project. Rotary drills are mainly used to make holes for foundation piles. The drill bits come in various forms: such as revolving buckets, short spirals, core drill bits, etc. According to different geological conditions, different drill bits can be replaced to achieve high speed and high quality. The hole formation requirements. It is mainly composed of walking device, main engine, connecting frame, mast, auxiliary winch, main winch, power head and drill pipe.

The basic working principle of the rotary drilling rig is: relying on the power head to provide torque to act on the drill pipe and the drill bit, so that the drill bit can rotate and cut the soil, and at the same time, the pressurized oil cylinder transmits pressure to the drill pipe and the drill bit through the power head to realize the pressurization of the drill bit. Drill in. When the muck enters the drill bit, the main hoist lifts the drill pipe drill bit out of the hole, and unloads the soil with the help of the special mechanism of the drill bit. Close the bucket door, lower the drill rod, put the drill bit in the manhole again, continue drilling, repeat the above process to form a pile hole [1].

Rotary-dug bored pile technology is known as the "green construction technology". It is characterized by high work efficiency, good construction quality, less dust and mud pollution, high work efficiency, good construction quality, strong adaptability and low construction noise. The advantages of less mud and no pollution to the environment make it suitable for use in the construction of urban viaducts. Although rotary drilling technology has many advantages, there are also some restrictive factors in the promotion and application of this advanced technology:

(1) The equipment is expensive.

(2) Higher operating costs.

(3) Restricted by engineering quantity.

(4) Constrained by the construction ground [2].

2. Case analysis

A certain viaduct construction in a city has been taken as an example to illustrate the application of rotary drilling rigs in the construction of urban viaduct pile foundations. The viaduct is located in the center of the city, with dense population and heavy traffic along the line. The terrain of the site is plain, with flat terrain and high groundwater level, which requires high construction. The main requirements are as follows:

1. The structure is simple, atmospheric, and easy to maintain and repair, and must meet the requirements of vehicle operation safety, passenger comfort, urban planning, urban landscape, economy, waterproof, fireproof, and environmental protection.

2. It should meet the durability requirements, and meet the requirements of strength, rigidity, stability, wind resistance, earthquake resistance, crack control, etc., and meet the requirements of the construction process.

3. Meet the requirements of building boundaries, driving across railways, highways, rivers, and flood discharge, and the impact of structural deformation, foundation settlement and other unfavorable factors should be fully considered.

4. Comprehensively consider the interface settings of equipment and other auxiliary facilities, and reserve a reliable connection with the overhead main structure to ensure the normal installation and use of related equipment.

3. Rotary drilling rig construction technology and operating procedures

The construction process of the rotary drilling rig for this project.

The main process steps are described in detail as follows:

1. The protective tube is buried. The burying of the protective tube is the beginning of the construction of the rotary drilling machine. Whether the plane position and verticality of the protective tube are accurate, and whether the surrounding of the protective tube and the bottom of the protective tube are tight and impervious to the rotary drilling machine. The quality of the pile has a great influence. According to the geological conditions, the length of the buried protective tube can reach 6.0m. In this project, the length of the protective tube is 4-6m, the inner diameter of the protective tube is 1.05m, and the outer diameter is 1.11m. When burying the protective tube, use a level to check the verticality. The top of the protective tube is generally 0.3m higher than the original ground to facilitate the positioning of the drill bit and the protection of the pile hole.

2. Mud modulation. This viaduct has the characteristics of high ground water level due to the fine sand, silt and coarse sand layer in the rock and soil of the foundation, and the various performance indexes of the mud are particularly important. The mud material is bentonite or clay, polyacrylamide and caustic soda. Adding a proper amount of caustic soda can increase the viscosity of the mud. Relevant experiments have shown that the slurry clay particles are uniformly suspended, less sedimentation, and stable in performance, which meets the needs of drilling.

3. Drilling construction. When the drill bit drops to a predetermined depth, rotate the drill bucket and apply pressure to squeeze the soil into the drill bucket, close the bottom of the drill bucket, and lift the drill bucket to unload the soil at the stacking site. Pay attention to the groundwater level in the hole and add water in time to prevent collapse. During the construction of the drilling rig, ensure that the mud surface is never lower than the bottom of the protective cylinder to ensure the stability of the hole wall.

Reasonably control the drilling and lifting speed of the drill bucket. The drilling speed of the rotary drilling rig should be strictly controlled during the drilling process, and the lifting speed of the drilling bucket should be maintained at 0.70～0.85m/s. If the speed of the drilling rig is too fast when the drilling bucket is raised and lowered, the mud between the outer wall of the drilling bucket and the hole wall will wash the hole wall , Coupled with the large negative pressure generated at the bottom of the drill bucket, causing the hole wall to shrink and collapse. When the bucket is in silt or sub-sand layer, its lifting speed should be slower. In order to keep the drill pipe vertical when drilling or passing through the junction of soft and hard layers, it is advisable to advance slowly. During the drilling process, the accumulated soil in the orifice and the scattered soil on the ground should be removed at any time.

4. Clear the hole. Hole cleaning is an important part of the construction of bored piles to ensure the quality of the pile. The quality requirements of the pile hole, the thickness of the sediment at the bottom of the hole, the amount of drilling slag in the circulating fluid, and the mud on the hole wall are ensured by the hole cleaning. The hole cleaning method of the positive circulation rotary drilling technology is: after the pile hole is finished, the drilling tool is increased by 20-50m, and a large pump is used to pump in new mud with performance indicators that meet the requirements, and the positive circulation is maintained for more than 30 minutes. If the mud coming out of the hole is basically free of scumming by hand, and the error between the measured hole depth and the actual drilling depth is within 30cm, the hole cleaning is completed at this time.

5. Fabrication and installation of steel cages. The steel cage production requires that the cut steel support frame is placed on the same horizontal plane and aligned with the center line at an interval of 2 to 3m, and then the fixed-length main reinforcement is placed straight on the welding support frame. Sleeve the stirrup into the main bar according to the design requirements and keep it perpendicular to the main bar for spot welding or binding. After the stirrups are welded to the main reinforcement or tied up, wind the hoops on them at a specified interval, tie them with lashing wires and fix them by spot welding at intervals. The hoisting of the steel cage should be hoisted at three points to keep the cage axes coincident. When entering the hole, the steel cage should always be kept vertical, align with the hole position and put it gently to maintain stability and avoid collision with the hole wall. Once it encounters obstacles, find out the cause immediately, and prohibit shaking and forced impact.

6. Underwater concrete pouring. Underwater concrete pouring is the last critical process. The construction quality will seriously affect the quality of the cast-in-place pile. Therefore, the following points must be paid attention to during the construction:

(1) The catheter must be tight and moderate in length, and ensure that the bottom end is 32 to 50 cm away from the bottom of the hole.

(2) The mixing of concrete must be uniform, and the slump should be controlled at 18～23cm.

(3) Concrete pouring must be operated continuously, and it is strictly forbidden to interrupt pouring [3].

Fourth, the conclusion

The viaduct pile foundation adopts a rotary drilling rig construction method, which has the advantages of reliable construction quality, fast speed, high efficiency, strong adaptability and environmental protection. It can be seen from this case that despite the high investment cost of rotary drilling rigs, under suitable geological conditions, the final comprehensive economic benefit index is still better than that of ordinary rotary drilling rigs.

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