What Is a Rotary Drill?

Rotary drilling rig is used for drilling in rock formations with hardness coefficient f6 in open-pit mines. It can also drill hard interlayer rocks with f = 7-11. Rotating torque and shaft pressure are applied during drilling to make the drill bit rotate and feed to cut the rock.

A bridge spans a place

1 Drill and drill bit selection

According to the geological data and initial drilling, the soil quality of the stratum is mainly sandy soil. The quality of the mud wall protection during the hole formation process is the key to the success of the pile foundation. Therefore, a positive circulation drilling machine with better wall protection effect is selected for hole formation. The length of the pile foundation of the approach bridge pier on land is less than 51m, and the diameter of the pile is 1.2m and 1.5m. GPS215 rotary drilling rig can be selected; the length of the pile foundation of the main bridge pier in the irrigation canal is 55m and the diameter of the pile is 1.5m. The GPS220 rotary drilling rig was selected. This kind of drilling rig has the advantages of large torque and good air tightness of the circulation system. It is suitable for hole formation in deep-water pile foundations in water. Because the geology of the bridge area of the project is clay and sandy soil, the three-wing drills were selected for the hole-forming drills.

2 Protective tube buried

According to the pile diameter and technical requirements, the protective tube is made of steel, the diameter of the 1.2m bored pile protective tube is 1.5m, the wall thickness is = 5mm, the length of a single protective tube is 2.0m, and the depth below the original ground is 1.0m. ; 1.5m bored pile protection tube diameter is 1.8m, wall thickness = 6mm, the length of a single protection tube is 2.5m, and the depth below the original ground is 1.5m. Before burying, the vertical and horizontal positioning piles are used to lead the stake out point out of the excavation operation range.After the foundation pit is dug to the designed depth, the protective cylinder is placed in the pit. After correcting the position, the layers are compacted around the protective cylinder. Fill the clay to the top of the protective cylinder, and at the same time restore the pile site to the protective cylinder. Use the red paint on the protective cylinder wall to make obvious marks.

4 Construction process and instructions

The main procedures of hole drilling are: drilling rig in place, mud circulation, hole drilling, final hole and inspection.

(1) Drilling machine in place: This process includes hole setting out, drill bit in place and drilling machine in place.

Hole setting out: The vertical and horizontal axes of the pile position have been released when the steel protective tube is buried, and it can be used only after restoration and verification. Set up pile guards at appropriate positions on the longitudinal and transverse axes of the pile position, and pay attention to protection during drilling to prepare for checking the pile position.

Drill bit in place: The drill bit must be placed in the hole before the drilling rig is in place. The drill bit in the protective casing of the main channel needs to be suspended on the drilling platform with a wire rope. According to the water depth in the hole, several drill rods are prepared in advance.

Drill rig in place: Use a car crane or floating crane to suspend the drill rig in place, adjust the position of the rig so that the vertical line of the center of the turntable and the axis of the pile match, and the deviation is not greater than 1cm; use the vertical ball to adjust the verticality of the drill pipe to The inclination is not greater than 1%; the level of the turntable is checked with a spirit level, and the deviation of the bubbles must not be more than half a division. After all the indicators of the rig have passed the inspection, the rig is fixed by the guide chain.

(2) Mud circulation: This work includes lifting the drill bit, connecting the suction and discharge pipeline, connecting the grouting pipeline in the hole (reverse circulation), starting the pumps to circulate the mud, and running the main engine.

Lift the drill bit suspended from the protective cylinder into the hole in advance with a crane, use a clamp plate to clamp the turntable, pad a sealing rubber pad, and fasten the square drill rod and drill bit flange with bolts. Do not leak air.

When connecting the mud pipeline, pay attention to the tightness of the interface and no air leakage.

During reverse circulation mud circulation, first start the mud pump, fill the protective cylinder with mud, and then start the air suction pump of the drilling rig. When the circulation is normal and lasts about 5 minutes, drilling can be started.

(3) Drilling: This process includes drilling, discarding drilling slag, testing mud, supplementing or adjusting mud.

Number the drill pipe before drilling, measure the length of each drill pipe and make a record. In order to ensure the plumbness of the hole during the drilling process, decompression and slow drilling are adopted, and the drilling rate and mud are adjusted according to the different soil conditions of the formation, and slag sampling is performed in stages in time, which is consistent with the drilling data of geological survey. In contrast, when there are major differences in geological conditions, they should be reported to the supervision engineer in time, and corresponding measures should be taken to ensure the quality of pore formation. Make the original drilling record during the drilling process. In addition, during the drilling process, a special person is arranged to check the mud performance at any time, and the mud index (specific gravity, viscosity, colloid ratio, sand content, etc.) is adjusted in time according to different formations to ensure the quality of pore formation.

(4) Final hole and inspection: This process includes slurry replacement, final hole inspection, rig displacement, and drill repair.

Change the slurry and clear the hole: When the cumulative length of the effective length of the drill pipe and drill bit reaches the planned drilling depth, after the supervisor's consent, immediately change the slurry and clear the hole, lift the bit 20cm away from the bottom of the hole, and turn the idler to detect the specific gravity of the slurry mouth , Viscosity, pH value, sand content, until the above indicators of slurry in and out are completely consistent, hole clearing time must not be less than 30min, after re-examination of the hole depth meets the design requirements, you can unload the drill, move the drill and repair the drill bit.

Final hole detection: After the rig is removed, the final hole detection can be performed. The main detection is hole depth, hole diameter, verticality, hole axis, thickness of the sediment layer and the state of the hole wall. The thickness of the sediment layer at the bottom of the hole is measured with a sedimentation tube and determined after the reinforcement cage is lowered to ensure that the thickness of the sediment is as small as possible; the maximum deviation of the verticality of the pile body should not be greater than 1/100; other indicators are reinforced after the hole is cleared Cage inspection, the inspection results also provide orientation guidance for the steel skeleton into the hole.

(1) Concrete strength (MPa): Use 4 sets of test pieces to check whether it is within the qualified standard range.

(2) Pile position (shelving pile): Use theodolite to check whether the vertical and horizontal directions are within the allowable deviation of 50mm.

(3) Drilling inclination: According to the results of the control inspection before the perfusion, the allowable deviation is 1%.

(4) Thickness of sedimentary layer (support pile): The record before pouring is not greater than the relevant regulations.

(5) Elevation of the bottom surface of the steel skeleton: according to the records before pouring, the allowable deviation is ± 50mm ^[2]

Practice has proved that in the construction of large-diameter cast-in-situ piles such as high-rise buildings, ports, dams, and bridge foundation projects, the use of rotary drilling rigs for cast-in-situ pile construction is easy to operate, safe and reliable, highly adaptable, low in use cost, and high in hole formation efficiency. The hole wall has the advantages of good stability, etc., and reasonable construction organization and management during construction can play a better role in promoting the construction of cast-in-situ piles ^[3] .