During the construction of cast-in-place piles with rotary drilling rigs, hole collapse has become the biggest obstacle in the boring process. Through technical research and engineering practice, the author adopts manual pouring to form a protective wall and concrete backfilling to form a protective wall to suppress the occurrence of hole collapse.
　　Keywords: rotary drilling rig; hole collapse; suppression
　Introduction
　　With the rapid development of China's infrastructure construction, especially urban construction, pile foundations for high-rise and super high-rise buildings are widely used. Rotary drilling rig bored cast-in-place piles are favored for their safety, environmental friendliness, speed, and efficiency. However, the promotion of new equipment and new technologies is accompanied by a process of improvement and perfection. After construction units introduced and put rotary drilling rigs into use, hole collapse became the biggest obstacle in the boring process of rotary drilling rigs. Through technical research and engineering practice, manual pouring to form a protective wall and concrete backfilling to form a protective wall have gradually become the preferred methods to suppress hole collapse.

　　1 Scope of Application
　　Through application examples of methods for suppressing hole collapse in rotary drilling rig boring, it is proved that they are suitable for treating severe hole collapse caused by pebbles, quicksand, and water inflow during rotary drilling rig boring, and are more targeted and feasible than treatment schemes such as mud slurry wall protection, backfilling cohesive soil, and pre-grouting.
　　2 Process Principle
　　The process principle of this method is to address severe hole collapse caused by pebbles, quicksand, and water inflow during rotary drilling rig boring by using concrete backfilling to form a protective wall and manual pouring to form a protective wall to suppress hole collapse. If the collapse location is shallow, manual excavation and concrete pouring can be used to form a protective wall first, and then the rotary drilling rig can be used for boring; if the collapse location is deep, the rotary drilling rig can quickly clean the slight collapse sediment, promptly pour low-grade concrete to fill, and after 48 hours, use the rotary drilling rig to continue boring.

　　2.1 Manual Pouring to Form a Protective Wall Method
　　Based on the "Geological Survey Report" and the geological conditions of already constructed pile holes, analyze the location of hole collapse caused by pebbles, quicksand, and water inflow. If the collapse location is in the shallow part of the pile hole, first use manual excavation, dig to the bottom of the collapse location, and pour concrete protective wall as the manual excavation proceeds. After the concrete protective wall reaches strength, it supports the soil at the collapse location, and then the rotary drilling rig is used to drill to the design depth.

　　2.2 Concrete Backfilling to Form a Protective Wall Method
　　When rotary drilling rig boring encounters hole collapse caused by pebbles, quicksand, and water inflow, and the collapse location is in the middle or bottom of the pile hole, first use the rotary drilling rig to drill to the collapse location, quickly remove the slightly collapsed sediment, and promptly use underwater pouring of C15 concrete, backfill to the top of the collapse, wait 48 hours for the rig to continue operation. At this time, the concrete has filled the collapsed part, the rotary drilling rig removes the middle part, and the remaining part will form a concrete protective wall to prevent further collapse. If the collapse is severe, manifested by incomplete cleaning of collapsed sediment and rebound of pile hole progress, layered C15 concrete pouring and drilling are required until the bottom of the collapse is treated.

　　3 Construction Process Flow and Operation Points
　　3.1 Construction Process Flow
　　3.1.1 Manual Pouring to Form a Protective Wall Method
　　Set out and determine pile position and elevation — excavate the first section of pile hole earthwork — install formwork and pour the first section of concrete protective wall — secondary measurement of elevation and pile position cross axis on the protective wall — install lifting frame, ventilation, lighting facilities, etc. — excavate the second section of pile shaft — clean the four walls of the pile hole, check the verticality and diameter of the pile hole — remove the upper section formwork, install the second section formwork, pour the second section of concrete protective wall — repeat the manual excavation process to the bottom of the collapse location — remove the protective wall formwork, lifting frame, ventilation, lighting facilities — position the rotary drilling rig — drill with the rotary drilling rig to the design hole depth.

　　3.1.2 Concrete Backfilling to Form a Protective Wall Method
　　Position the rotary drilling rig — align the drill bit with the center of the pile position — drill — install steel casing — drill with the rotary drilling rig to the bottom of the easily collapsing location — install tremie pipe and hopper — underwater pour C15 concrete — continue drilling with the rotary drilling rig to the design hole depth.

　　3.2 Operation Points
　　3.2.1 Manual Pouring to Form a Protective Wall Method
　　Based on the "Geological Survey Report" and the geological conditions of already constructed pile holes, analyze the location of hole collapse caused by pebbles, quicksand, and water inflow.
　　Excavated soil and stone must be transported away from the hole mouth by more than 2m, and motor vehicles must pass at least 3m away from the hole mouth, and must not affect the safety of the well wall.
　　When transporting steel bars, formwork, concrete, and other tools into the hole, they must be securely and reliably tied, and coordinate with personnel inside the hole in advance.
　　Lighting inside the hole must use a transformer to reduce 220V to a safe voltage below 36V. Electrical equipment must be strictly grounded, connected to neutral, and use leakage protectors. Each hole must have a separate electrical switch; multiple uses of one switch are strictly prohibited. Cables above the hole must be elevated more than 2m.
　　The hole mouth is equipped with a cover made of multi-layer boards, and a dedicated person is assigned to watch. After lifting out earthwork and during construction intervals, the cover must tightly cover the hole mouth.

　　3.2.2 Concrete Backfilling to Form a Protective Wall Method
　　When positioning the drilling rig, it must be kept stable without tilting or displacement. To accurately control the drilling depth, a control scale must be made on the frame or pipe for observation and recording during construction.
　　The casing position must be buried correctly and stably. The space between the casing and the hole wall must be filled with clay. The deviation between the casing center and the pile hole centerline must not exceed 50mm.
　　The drilling speed must be determined based on soil conditions, hole depth, rig load, and hole quality. Especially when drilling below the water level, "light pressure, light drilling, slow up and down" must be adopted to prevent violent actions from causing water flow that impacts pebbles, quicksand, and other soils, resulting in severe hole collapse. Measure the elevation of the collapse bottom in advance, calculate the required tremie pipe length based on the pipe bottom being ≤600mm from the hole bottom, and immediately lower the tremie pipe after the excavator moves, connect the hopper, and pour C15 concrete.
　　C15 concrete is suitable for pumping by concrete pump truck, with a suitable slump of 160-190mm. Pouring must be completed in one go, with the height controlled by visually observing the upper edge of the collapse; if the collapse location is below the water surface, the height should be controlled at the original water surface position.

　　4 Quality Control
　　4.1 Ensure Age and Improve Protective Wall Quality
　　Both the concrete backfilling to form a protective wall method and the manual pouring to form a protective wall method use concrete pouring to form a protective wall. After pouring, the concrete must reach a certain strength before formwork removal or proceeding to the next process. If removed early or forcibly, the concrete protective wall will collapse due to softness and low strength, inevitably causing rework. Therefore, according to the strength requirements of the protective wall concrete, in the manual pouring method, sulfoaluminate rapid-hardening cement is used, and formwork removal is allowed after curing for at least 12 hours. In the concrete backfilling method, C15 concrete must be naturally cured for more than 48 hours before rotary drilling rig drilling.

　　4.2 Prevent Hole Deviation and Ensure Pile Hole Verticality
　　The causes of hole deviation are: during drilling in loose strata, encountering large smooth boulders or protruding stones pushes the drilling tool away from the hole axis; when the drilling tool enters hard strata from soft strata or sand-gravel layers with large particle size differences, the resistance on the drill bit is uneven, causing the drilling tool to tilt and resulting in hole deviation. Or the drilling rig position shifts, or the base partially sinks, causing the drill hole to tilt. Preventive measures are: slow down the drilling speed, reinforce the rig base, improve ground bearing capacity, and use rotary drilling rigs with good guiding performance and strong mast rigidity for construction.

　　4.3 Quality Inspection Methods
　　① Pile position deviation is measured with a straight ruler.
　　② Pile hole depth is measured with a measuring rope.
　　③ Pile verticality is measured with a hole detector.
　　④ Mud specific gravity is measured with a hydrometer, mud consistency with a funnel viscometer, sand content with a sand content meter, and sediment thickness is relatively measured with a measuring rope.
　　⑤ Pile integrity and pile foundation concrete quality are tested using detector ultrasonic testing.

　　5 Safety Testing
　　5.1 Strengthen safety production education and safety briefings. Entering the site requires wearing a safety helmet, work clothes, anti-slip shoes, and protective gloves.
　　5.2 When the drilling rig is drilling, personnel at the hole mouth should concentrate. When drilling tools need to be suspended or placed, they must be firmly supported, and the operating position must not be left unattended.
　　5.3 Set up conspicuous warning signs next to equipment that endangers personal safety. Personnel are strictly prohibited from approaching or crossing.
　　5.4 Non-electricians are not allowed to disassemble or repair electrical equipment at will. Cables crossing roads must be buried deep or elevated.
　　5.5 Moving drill pipes or lifting tremie pipes must be uniformly directed by a dedicated person.
　　5.6 After hole formation, the hole mouth must be covered. After pile formation, negative holes must be backfilled in time to prevent people and objects from falling into the hole.

　　6 Environmental Protection Principle
　　Because the rotary drilling rig is a low-noise, vibration-free, environmentally friendly drilling rig, the noise during drilling is within 65 decibels. Targeted treatment and concrete protective wall construction avoid mud preparation and discharge pollution. If the excavated original soil has too high water content or is in a fluid-plastic state, and if equipped with an on-site fluid-plastic soil water-reducing solidification treatment device, the excavated fluid-plastic soil is treated by water reduction and solidification to become dry soil, and then transported by dump truck, meeting urban environmental protection requirements.

　　7 Conclusion
　　The method for suppressing hole collapse in rotary drilling rig boring has the characteristics of high construction efficiency, reliable quality, fast hole formation speed, high hole formation rate, strong adaptability, less waste slurry, low noise, and low pollution. It overcomes the shortcomings of hole collapse, mud discharge, and long construction period during rotary drilling rig boring, greatly improving construction quality.