Common Problems and Solutions During the Drilling Phase of Rotary Drilled Cast-in-Place Pile Construction

Ⅰ. Borehole wall collapse

1. Cause analysis

Borehole wall collapse is the most common risk in the drilling stage, which is mainly affected by geological conditions, construction technology and management factors:

- Unstable geological conditions: loose strata such as sand layers, silt, backfill soil, etc. are prone to instability due to insufficient cohesion between particles; groundwater seepage or pressurized water will scour the hole wall, resulting in failure of mud wall protection.

- Insufficient mud wall protection performance: When the mud density, viscosity or colloid rate does not meet the standard, it cannot effectively balance the formation pressure, especially in the highly permeable gravel layer, it is easy to cause leakage and aggravate the collapse of the hole.

- Improper construction operation: Too fast drilling speed (especially in the quicksand layer), the drill bit hits the hole wall, the hole cleaning time is too long or the concrete is not poured in time, which will weaken the stability of the hole wall.

- Defects in casing burial: Insufficient casing burial depth (it is recommended to bury at least 1m in the clay layer), loose backfill around or leaking joints, resulting in the collapse of the hole mouth and downward expansion.

2. Performance characteristics

- Initial signs: the mud level in the hole drops suddenly, fine bubbles appear at the hole mouth, the amount of slag discharge increases abnormally but the footage stagnates, and the load of the drilling rig increases significantly.

- Severe hole collapse: the lowering of the kelly bar is blocked, the hole depth suddenly becomes shallower, and even the ground sinks, and the steel cage cannot be installed normally.

3. Preventive measures

- Optimize mud performance: adjust the mud parameters according to the formation. The mud density in the sand layer is recommended to be 1.2~1.4g/cm³, the viscosity is 25~30s, and the colloid rate is ≥95%; add bentonite, CMC thickener or polymer stabilizer when necessary.

- Control drilling parameters: use low speed and slow footage for loose formations (such as sand layer footage ≤0.5m/min) to avoid excessive swing of the drill bit; clay blocks or flakes can be added to compact the hole wall when encountering flowing sand layers.

- Standard construction of casing: 50cm thick clay is rammed at the bottom of the casing, and the surrounding area is symmetrically backfilled and compacted; during the flood season or in the confined water area, a heightened casing or siphon is used to maintain the head stability.

- Process connection management: Concrete pouring is completed within 24 hours after the hole is formed to shorten the exposure time of the hole wall; continuous grouting is performed after the hole is cleaned to keep the liquid level 1.5~2m above the groundwater level.

4. Emergency treatment

- Minor hole collapse: Immediately inject high-viscosity mud and reduce the drilling speed, and at the same time throw clay balls or cement mortar to seal the cracks.

- Severe hole collapse: Backfill the mixture of clay and gravel to 1~2m above the collapsed hole section, and re-drill after the sediment is dense; if the collapsed hole continues to expand, the entire hole needs to be backfilled and 5%~8% cement is added to enhance consolidation, and re-drill after 7 days.

- Casing failure: Remove the casing and rebury it in a stable stratum, and use double-layer casing or steel casing follow-up technology to enhance local support.

Ⅱ. Underground obstacles

1. Obstacle types and risks

- Geological obstacles: Unexplored boulders, inclined rock faces, hard interlayers, etc., which can easily cause drill bit deviation, wear or jamming, causing drilling deviation or pile breakage.

- Artificial remains: old concrete foundations, abandoned pipelines, wooden piles, etc., which may damage drilling tools or cause safety accidents (such as digging up gas pipelines).

2. Cause analysis

- Insufficient exploration: The spacing between conventional geological exploration points is too large (such as >10m), making it difficult to identify local obstacles; geophysical exploration methods (geological radar, cross-hole CT) are not fully applied.

- Construction preparation omissions: Insufficient site historical investigation, and no manual excavation verification of existing underground facilities.

3. Preventive measures

- Refined geological survey: Increase the number of exploration points to within 10m, use in-hole video or three-dimensional seismic wave detection in key areas; conduct pipeline disclosure and manually excavate exploration trenches before construction.

- Drill tool adaptation and process optimization:

When encountering boulders, use a core barrel or rock-embedded drill bit (such as a roller bit) and use "low stroke and high frequency" impact crushing;

In hard rock formations, use a graded hole expansion process, first drill a small hole and then gradually expand it to the designed aperture.

- Real-time monitoring and adjustment: Install the torque and speed sensors of the drilling rig, and warn of obstacles with abnormal data; suspend drilling when a sudden increase in resistance is found, and determine the type of obstacle through rock chip analysis.

4. Emergency treatment

- Boulders treatment: Small boulders can be crushed by using alloy drill teeth with higher hardness; large boulders need to be loosened and removed by underwater blasting (charge ≤1kg).

- Pipeline protection: Stop drilling immediately when the pipeline is accidentally touched, close the site and notify the property owner; continue construction after using casing isolation or relocating the pipeline.

- Drilling jam treatment: Avoid forced drilling to cause the kelly bar to break, and inject lubricant (waste oil + bentonite), or use a vibrating pile puller with reverse circulation slag removal to remove the jam.

Ⅲ. Borehole deflection

1. Cause analysis

- Equipment installation defects: uneven drilling rig base, vertical deviation of guide frame>1/100, or bending of kelly bar leading to initial deflection.

- Uneven formation: soft and hard boundary rock surface (such as inclined bedrock) and pebble layer with large particle size difference make the drill bit unevenly stressed and deviate to the soft layer side.

- Improper operation: excessive pressure in hard rock layer forces the drill bit to deflect, or insufficient number of hole sweeping fails to correct the hole shape.

2. Performance and hazards

- Detection indicators: vertical deviation>1/100 pile length or design allowable value (such as inclined pile ±2.5%), hole diameter is "plum blossom" or "cross hole".

- Structural impact: reduce the vertical bearing capacity of pile foundation, induce eccentric load, and in severe cases lead to tilt of superstructure.

3. Preventive measures

- Precise equipment leveling: Use a total station to calibrate the level of the drill rig turntable and base to ensure that the lifting pulley, the center of the kelly bar and the pile position are in line; re-measure the verticality every 4~5m of drilling.

- Drill tool optimization: Regularly check the straightness of the kelly bar (bending ≤0.1%), and repair the worn drill bit in time; use drill tools with stabilizers to reduce swing in hard rock formations.

- Process control: Hang the kelly bar at the junction of soft and hard areas and drill at a low speed (speed ≤20rpm), backfill the probe stone and flush it flat before drilling; use the "backfill and re-punch method" to correct the hole wall during impact drilling.

4. Correction technology

- Sweep hole correction: Hang the drill bit in the deflected section and sweep the hole up and down, and adjust the drilling pressure and speed in real time with ultrasonic detection.

- Backfill and re-drill: When the deviation is serious, backfill the mixture of hard gravel and clay to 0.5m above the top of the deflected section, and use a small diameter drill bit to correct the deflection after compaction, and then expand the hole in stages.

- Mechanical assistance: Install a hydraulic corrector or guide frame to force the drill bit to return to its original position, which is suitable for deep hole correction with a depth of more than 30m.