During the hole-forming process of cast-in-place pile construction using a full-casing full-rotation drilling rig, a steel casing is used to protect the hole wall. It features good pile quality, no mud pollution, environmental friendliness, and reduced concrete filling coefficient. It can effectively solve the problems of hole collapse, necking, and high filling coefficient that occur when conventional methods are used for cast-in-place pile construction in urban high-fill areas and karst landforms. It is currently applied in construction fields such as bridges, buildings, and petrochemicals.

I. Mechanical configuration for full-rotation cast-in-place pile construction

Main equipment and components:

1. Full-casing full-rotation drilling rig: hole forming

2. Power station: provides power for the full-rotation main machine

3. Casing: hole wall protection

4. Reaction fork: provides reaction force to prevent displacement of the main machine during full-rotation turning

5. Operation cabin: operating platform and personnel working area

Auxiliary equipment:

1. Rotary drilling rig or grab: soil removal, rock penetration, hole cleaning

2. Pipe jacking machine: casing extraction, enabling continuous operation of the full-rotation process

3. Crawler crane: lifting and transporting the main machine, power station, reaction fork, etc.; providing support for the reaction fork; lifting and lowering reinforcement cages and concrete tremies; performing grab-type soil removal, etc.

4. Excavator: leveling the site, clearing and transporting spoil, etc.

II. Construction process flow for full-rotation steel casing cast-in-place piles

1. Construction preparation

The main work of construction preparation is site leveling. Since the drilling rig is large and has many associated auxiliary items, there are certain requirements for access routes and the working platform. Therefore, construction preparation must consider the construction access routes and working planes necessary for operations such as fabrication of pile foundation reinforcement cages, spoil transfer, lifting and installation of reinforcement cages, and pile foundation concrete pouring.

2. Surveying and setting out

First, carefully review the coordinates, elevations, and other relevant data provided in the design drawings. After confirmation, use a total station to set out the pile positions. After the pile center is set out, stretch cross lines along the pile center to beyond 1.5 m and mark the protection stakes.

3. Positioning of the full-rotation main machine

After the point is set out, lift and place the full-rotation base. The center of the base must coincide with the pile center point. Then lift and place the main machine, install it on the base, and finally install the reaction fork.

4. Lifting and placing the steel casing

After the main machine is in position, lift and install the steel casing.

5. Measuring and adjusting verticality

After the rotary drilling main machine is in position, perform rotary drilling. The casing is driven in rotation while being pressed down simultaneously, enabling rapid drilling of the casing into the stratum. During steel casing drilling, use plumb bobs in the X and Y directions to adjust the verticality of the casing.

6. Casing drilling and soil removal

While the casing is drilled into the stratum, use a crane to lower the grab bucket along the inner wall of the casing to the bottom of the hole for grab-type soil removal, or use a rotary drilling rig for soil removal.

7. Fabrication and installation of reinforcement cage

After the hole is formed to the design elevation, clean the hole. After passing acceptance by the geological survey unit, supervisor, and client, install the reinforcement cage.

8. Concrete pouring, casing extraction, and pile formation by pouring

After the reinforcement cage is installed, pour concrete. After the concrete is poured to a certain height, extract the casing. The casing can be extracted using a pipe jacking machine or the full-rotation main machine.

III. Advantages of full-rotation construction:

1. It can solve pile construction in special sites, special working conditions, and complex strata, with no noise, no vibration, and high safety performance;

2. No mud is used, the working surface is clean, and the possibility of mud entering the concrete can be avoided, which helps improve the bond strength of concrete to reinforcement; it prevents soil backflow, does not scrape the hole wall when lifting the drill or lowering the reinforcement cage, and produces less drilling spoil;

3. When operating the drilling rig, the characteristics of the strata and rock can be judged very intuitively;

4. Drilling speed is fast; for general soil layers, it can reach about 14 m/h;

5. Drilling depth is large; depending on soil conditions, it can reach a maximum of about 80 m;

6. Hole verticality is easy to control, and the verticality can be accurate to 1/1000;

7. Hole collapse is unlikely to occur, hole-forming quality is high, bottom cleaning is thorough, speed is fast, and sediment can be cleaned to about 30 mm;

8. The hole-forming diameter is standard and the filling coefficient is small. Compared with other hole-forming methods, it can save a large amount of concrete. Rotary drilling hole formation is severely affected by hole collapse because the backfill layer is too thick and contains large block stones.

Hole-forming effect of the casing:

The full-rotation drilling rig is not only used for pile foundation construction in various complex strata such as quicksand, karst landforms, and ultra-high backfill, but can also perform secant pile construction, subway steel column installation, and pile extraction and obstacle clearing construction.