Prevention of common accidents in rotary drilling and cast-in-place pile construction

Precautions for drilling holes

- Regardless of whether there is groundwater in the hole and what the surface soil is like, a surface casing must be set up, and the casing must be at least 30cm above the ground. Before the surface casing is inserted to the predetermined depth, the drill bit reamer must be used.

- To prevent the soil and sand in the drill bucket from falling into the hole and causing the properties of the stabilizing fluid to deteriorate or settle to the bottom of the hole, the valve at the bottom of the bucket should always remain closed during the drilling process.

- The lifting speed of the drill bucket in the hole must be controlled. Because if the drill bucket is moved up and down quickly, the water will flow through the gap between the outside of the drill bucket and the hole wall at a faster speed, causing the hole wall to be eroded; sometimes, when the drill bucket is lifted, negative pressure will be generated below it, causing the hole wall to collapse and shrink.

- According to the size of the drilling resistance, consider the necessary torque to determine the appropriate number of revolutions of the drill bucket.

- When drilling in the bearing layer at the pile end, it is necessary to consider that the bearing layer at the pile end is loosened due to the suction phenomenon of the drill bucket, so the drill bucket should be lifted slowly. If the bearing layer at the pile end is inclined, in order to prevent the drill bucket from tilting, drilling should be slightly pressured.

- To prevent the hole wall from collapsing, the water level in the hole should be ensured to be more than 2m above the groundwater level (example of dewatering well construction). - During drilling or lifting the drill bit, the same liquid level should always be maintained, especially there should be no lack of slurry, otherwise there will be a risk of borehole collapse. The rotary drilling rig slurry replenishment is generally pumped.

- Hole cleaning. The blade of the drill bucket is inclined, which will disturb the soil layer at the bottom of the hole; when drilling, the drill slag and the fallen soil will settle at the bottom of the hole to some extent, so the slag cleaning work is very important.

- The first bottom hole treatment: It is carried out before the steel cage is placed, and the slag drill bucket is generally used to remove the sediment; if the sedimentation time is long, a water pump should be used for circulation treatment.

- The second bottom hole treatment: It is carried out before concrete pouring, usually by pump suction or air lift.

Accidents and prevention in bored pile construction

Common accidents in bored pile construction: hole collapse, drill suction, hole tilt, diameter reduction, the pile hole is over large, drill stuck, failure to drill, broken kelly bar

Hole collapse

- Mud hole collapse:

1) The mud density (viscosity) is too low, and the hole wall support pressure is insufficient, resulting in hole collapse;

2) The mud density is too high, the diffusion speed is fast, and the stratum loosens and peels off when it encounters water, resulting in hole collapse;

3) The mud pH value is greater than 11, the mud colloid becomes a suspension, and the stratification does not play the role of arm guard, resulting in hole collapse;

4) The mud position is too low, resulting in hole collapse;

5) The mud material is not right, resulting in hole collapse;

6) The soil around the arm is loose and leaks, resulting in hole collapse;

7) Underground caves cause hole collapse due to leakage;

8) The bottom layer leaks seriously and the hole collapses;

9) Mud is not replenished in time and the hole collapses.

- Water gushing hole collapse:

1) The pressure of the pressurized water is released, resulting in water gushing hole collapse;

2) The rising tide causes water gushing hole collapse.

- Collapse due to support damage:

- Collapse of the hole wall caused by underground river

- Collapse caused by vibration of large machinery on vibrating and fluid strata

- Collapse due to operation:

1) Collapse due to excessively fast drilling speed;

2) Collapse due to excessively fast drilling speed;

3) Collapse due to excessively high drilling speed;

4) Collapse due to mud disturbance;

5) Collapse due to drilling out of control;

6) Collapse due to suction piston formation;

7) Collapse due to misalignment of the hole center.

Prevention measures for hole collapse

- Correctly select mud density

- Correctly select mud pH value

- Predict and ensure mud level

- Predict and observe mud leakage, and replenish mud in time

- Select wall protection methods of different layer depths in a targeted manner

- Predict pressurized water and high tide, and adopt reasonable drilling technology

- Predict rotten wood and boulders, and adopt corresponding drilling tools and drilling speeds

- Choose a good drilling technology when there is an underground river

- Avoid working with large machinery at the same time, and large machinery is not allowed to pass within 5 meters of the pile hole within 8 hours after drilling and pouring- Choose a reasonable operation method according to the soil quality

Suction drill

Types:

1) Suction drill in fluidized clay;

2) Suction drill in expansive soil;

3) Suction drill in drilling hole;

4) Suction drill when the drill bucket is stationary in clay for too long;

5) Suction drill when the drill tool structure is unreasonable.

Preventive measures:

1) Select or modify the drill tool;

2) Sweep the hole;

3) Do not allow the drill to be spurted out or stay in the clay for too long;

4) When there is fluidized clay and expansive soil, the drilling speed should not be too fast.

Borehole tilt

Causes:

1) The verticality of the kelly bar is not enough;

2) The selection of drilling tools for soft and hard interlayers is unreasonable;

3) The underground caves are not predicted and processed;

4) The probe stones on the hole wall are not removed

Preventive measures:

1) Observe the level frequently to correct the inclination of the kelly bar;

2) Select flat-head drilling tools and double-entry drilling buckets in soft and hard interlayers to avoid the inclination of the drilling guide;

3) When there are underground caves, measures such as filling the caves or adding casings should be taken to prevent the inclination of the borehole;

4) If there are probe stones, a two-step drilling method should be adopted, that is, loosening first and then taking soil.

Diameter reduction

Causes:

1) No corresponding measures are taken when drilling in the fluidized clay layer

2) No corresponding measures are taken when drilling in the expansive soil layer

3) The drill bit is lifted too quickly, generating negative pressure

Preventive measures:

1) Sweep the hole

2) Do not lift the drill too quickly

3) Prevent the drill from sucking the drill

4) Ensure that the drilling tool flows smoothly

The pile hole is over large

Causes:

1) Serious shaking of the main engine;

2) Unreasonable selection of drilling tools;

3) Mud ratio does not match the soil quality, causing the mud skin on the hole wall to fall off;

4) The mud weight is too high, causing partial hole collapse;

5) Operational reasons:

① The lifting and lowering speed of the drill is too fast, and the water flow washes away the mud skin;

② The drilling speed is too fast, destroying the combination of the mud skin and the hole wall soil and causing it to fall off;

③ No measures are taken to reduce the disturbance of the mud on the hole wall, causing the mud skin to fall off. The reasons that can cause the pile hole over large can cause hole collapse.

Preventive measures:

1) Choose a stable main engine;

2) In the case of sandy soil, increase the strength and viscosity of the mud skin;

3) The operation method varies according to the soil quality.

Drill stuck

Causes:

1) Stone or sand stuck between rock and drill tube

2) Drill teeth embedded in rock cracks

Preventive measures:

1) Choose a reasonable drill tool structure

2) Take corresponding measures when drilling through sandy soil layers into rock layers, especially when drilling through casing or spinning casing

3) When drilling into muddy cemented and strongly weathered hard rock, do not use long drill teeth. In order to avoid blocking the rear angle, a guide plate can be added between the teeth and the bottom of the drill

Cannot drill

Type:

1) Clay cannot be drilled;

2) Clay cannot be drilled due to slippage;

3) Loose soil cannot be drilled due to slippage;

4) Various rock formations cannot be drilled

Causes:

1) Insufficient drilling pressure;

2) Unreasonable selection of drilling tools;

3) Slippage;

4) Excessive drilling pressure;

5) Unreasonable selection of center drill

Preventive measures:

1) Reasonable selection of excavation angle;

2) Adjust the coverage rate of the cutter head;

3) Select drill teeth according to the soil quality;

4) Select drilling tools according to the soil particle size

5) When drilling clay, the method of destroying the viscosity of the regenerated clay is used to deal with slippage. When drilling into rock formations, slippage should be dealt with by selecting drill teeth and changing the drilling pressure.

6) In soft soil layers, excessive drilling pressure will cause the drill bucket to bottom out and fail to drill.

7) The center drill with poor drilling ability will top up to the formation where it cannot drill, which will cause failure to drill.

Kelly bar breakage

Causes:

1) The torque of the kelly bar and the main machine do not match;

2) Improper drilling methods in loose boulder and blocky layers;

3) Improper soil unloading method;

4) Too high speed when using a small torque kelly bar

Preventive measures:

1) Pay attention to the matching torque of the kelly bar and the main engine;

2) Pay attention to the matching speed of the kelly bar and the main engine;

3) Drilling into loose boulders and rock layers should be done in two steps, first use the spiral drill to loosen the boulders and rocks, and then use the drill bucket to take the soil;

4) When throwing soil, first open the bottom to throw the soil, then close the bottom to throw the soil, and then open the bottom to throw the soil;

5) The drilling speed should not be too high when using a small torque kelly bar