Control of Borehole Accuracy of Rotary Drilling Rigs: Parameter Optimization And Practical Operation Key Points

    Parameter settings need to be dynamically adjusted in combination with construction geology and pile specifications, focusing on three key indicators: rotational speed, feed rate, and slurry parameters. The core is to balance efficiency and precision to avoid problems such as hole wall instability and hole diameter deviation caused by blind setting.

    Rotational speed settings need to be adapted to geological hardness, following the principle of 'slow rotation for soft soil and moderate speed for hard soil'. In soft soil and silty clay strata, the rotational speed should be controlled at 10-20 r/min to avoid hole wall disturbance and collapse caused by high-speed drilling, while also reducing hole diameter expansion. In hard soil and gravel strata, the rotational speed can be adjusted to 20-35 r/min, combined with high-quality drill bits to improve cutting efficiency, and avoid excessive rotational speed leading to drill rod vibration and affecting verticality. When encountering hard objects such as isolated stones, the speed should be reduced to 5-10 r/min, using low-speed probing drilling, and high-speed impact is strictly prohibited.

    The feed rate needs to match the rotational speed and geology, implementing 'stratified feeding and gradual progress'. In soft soil strata, the feed rate is controlled at 50-100 mm per time to prevent hole wall squeezing and deformation caused by too fast feeding. In hard soil strata, it can be appropriately increased to 100-150 mm per time to avoid increasing construction costs due to too slow feeding. In gravel and pebble strata, it needs to be controlled at 30-80 mm per time to reduce hole wall     scouring and facilitate timely discharge of slag. Throughout the process, real-time micro-adjustments need to be made according to the resistance feedback from the drill rod to avoid uniform feeding causing drill sticking or precision deviation.

    Slurry parameters are key to hole wall stability and directly affect the accuracy of hole formation. Slurry specific gravity needs to be adjusted according to the stratum: controlled at 1.05-1.10 for soft soil and silt strata, and increased to 1.10-1.20 for gravel and pebble strata to enhance wall protection; viscosity is controlled at 18-22s to ensure the slurry's ability to carry slag, while avoiding excessively high viscosity increasing drilling resistance and too low viscosity failing to form an effective mud skin; the sand content needs to be strictly controlled within 2%, regularly clean the slurry pool, replace aged slurry, and prevent excessive sand content from wearing drill tools and damaging the hole wall.

    Standardized operation is the core of parameter implementation and ensuring accuracy, which needs to be carried out throughout the entire process of drilling, hole cleaning, and lifting, with a focus on controlling equipment calibration, drilling posture, slag treatment, and other details.

Equipment calibration must be completed before construction to establish a solid foundation of accuracy. Adjust the level of the drill body in advance, and use a level gauge for two-way detection to ensure that the body is not tilted; Calibrate the verticality of the drill rod, check whether the drill rod joints and drill bits are intact, and avoid bending the drill rod or eccentricity of the drill bit that may cause hole diameter deviation or verticality exceeding the standard; Verify the coordinates of the pile position, use a total station for precise positioning, set positioning markers, and conduct regular checks during drilling to prevent pile displacement.

    Strictly control the posture during the drilling process and monitor the verticality throughout the entire process. The initial stage of drilling (the first 3-5 meters) is the key to verticality control, requiring low-speed and small feed rate drilling. The verticality of the drill rod should be rechecked every 1 meter of drilling, and the deviation should be controlled within 1%; During deep drilling, recheck every 3-5 meters. If any tilt is found, immediately stop the machine and correct it. Use the sweeping method to gradually adjust, and it is strictly prohibited to forcefully correct it. At the same time, maintain smooth drilling, avoid frequent start stop, rapid acceleration or deceleration, and reduce the impact of drill pipe vibration on the hole wall and accuracy.

    The treatment of soil slag should be timely and standardized to avoid affecting the aperture and depth of the hole. During the drilling process, the mud is circulated synchronously to ensure that the debris is fully suspended and discharged, avoiding the accumulation of debris at the bottom of the hole and causing deviation in hole depth measurement; Clean the drill bit debris in a timely manner every layer drilled to prevent residual debris from squeezing the hole wall and causing the hole diameter to expand; When lifting and lowering the drill, move slowly to avoid negative pressure inside the hole caused by high-speed lifting, suction of the hole wall, or impact on the bottom of the hole and damage to the drill bit caused by drilling too quickly.

    During the cleaning stage, it is necessary to balance accuracy and hole wall stability to avoid secondary pollution. After the final hole is drilled, the slurry replacement method is used to clean the hole, gradually replacing the mud with high sand content to ensure that the mud indicators meet the standards after hole cleaning; The depth of hole cleaning must meet the design requirements. Excessive hole cleaning can lead to thin sediment at the bottom of the hole and collapse of the hole wall, while insufficient hole cleaning can affect the bearing capacity of the pile foundation; After the hole cleaning is completed, promptly lower the steel cage to shorten the exposure time of the hole wall and reduce accuracy deviation.