The rotary drilling rig was invented by Italians after World War II. Due to its high efficiency and reliable quality in use, it brought substantial profits to owners and construction companies. Later, Europeans and Japanese, along with the comprehensive resumption of construction in various countries, used it on a large scale, making it more refined and multifunctional. It has now become the main tool for pile foundation hole formation in railway, highway, water conservancy, and civil and industrial construction worldwide. Rotary drilling is mainly used for hole formation in foundation pile foundations. Its drill bits come in various forms, such as rotary buckets, short augers, and core drill bits. According to different geological conditions, different drill bits are replaced to achieve high-speed and high-quality hole formation. The rotary drilling rig is a construction machine suitable for hole formation operations in building foundation engineering. It is widely used in foundation construction projects such as municipal construction, highway bridges, and high-rise buildings. With different drilling tools, it is suitable for dry (short auger), wet (rotary bucket), and rock formation (core drill) hole formation operations. The rotary drilling rig features large installed power, large output torque, large axial pressure, mobility and flexibility, high construction efficiency, and multiple functions. The rotary drilling rig is suitable for the soil and geological conditions of most regions in China and has a wide range of applications, basically meeting the needs of bridge construction, high-rise building foundations, and other projects. At present, rotary drilling rigs have been widely promoted in various bored pile projects.

　　Because of its advantages such as fast construction speed, good hole formation quality, low environmental pollution, flexible and convenient operation, high safety performance, and strong applicability, the rotary drilling rig has become the main hole formation equipment for bored pile construction. To ensure project progress and quality, the owners of many key projects have designated it as the specified construction equipment, thereby replacing traditional percussion and rotary drilling rigs. At present, most rotary drilling rigs in China's engineering sector are products from Germany and Italy and have already occupied a dominant position.

　　Types of Rotary Drilling Rigs

　　Small Machines

　　Drilling rig torque 100 kNm, engine power 170 kW, drilling diameter 500–1000 mm, depth about 40 m, total machine weight about 40 t. The application market positioning of small machines is as follows: ① slope protection piles for various buildings; ② load-bearing structural piles for parts of buildings; ③ various piles with a diameter of less than 1 m in municipal projects of urban renewal; ④ piles for other purposes. The market coverage rate of small machines exceeds 30%.

　　Medium Machines

　　Drilling rig torque 180 kN?m, engine power 200 kW, drilling diameter 800–1800 mm, depth about 60 m, total machine weight about 65 t. The application market positioning of medium machines is as follows: ① bridge piles for transportation facility bridges such as various expressways and railways; ② load-bearing structural piles for large buildings and port wharves; ③ viaduct piles in cities; ④ other applicable piles. The market coverage rate of medium machines exceeds 90%.

　　Large Machines

　　Drilling rig torque 240 kN?m, engine power 300 kW, drilling diameter 1000–2500 mm, depth about 80 m, total machine weight more than 100 t. The application market positioning of large machines is as follows: ① extra-large bridge piles for various expressways and railway bridges; ② special structural load-bearing foundation piles for other large buildings. The market coverage rate of large machines exceeds 10%.

　　Application of Rotary Drilling Rigs in Municipal Engineering: At present, Guangzhou has many projects constructed with rotary drilling rigs, such as the pile foundation of Guangzhou Xinguang Expressway Section 6, the pile foundation of Guangzhou Dongfang Heavy Machinery, Xiaobei Station and Wuyangcun Station of the subway, etc. The following introduces an application example of the enclosure structure construction of a certain subway station in Guangzhou. The project is located under Huanshi Middle Road, adjacent to the intersection of Huanshi Middle Road, Lujing Road, and Xiaobei Road on the west. Directly above Huanshi Middle Road are an elevated bridge and a pedestrian bridge. The northwest side of the station is close to the Guangzhou-Kowloon Railway and the Inner Ring Road Viaduct, and the surrounding area is densely built with high-rise buildings.

　　Construction Steps of Rotary Drilling Rig: Position the rotary drilling rig → install the casing → after the drill bit lightly touches the ground, rotate to start drilling! When the drill bit is full of soil and sand, lift it out of the hole → swing the rotary drilling rig back and dump the soil and sand inside onto a dump truck or the ground → close the movable door of the drill bit, swing the rotary drilling rig back to its original position, and lock the rotary body of the drilling rig → lower the drill bit → after drilling is completed, clean the hole and measure the depth → place the reinforcement cage and tremie pipe → perform concrete pouring → pull out the casing and clean the sediment at the pile head, backfill, and complete the pile.

　　Quality Control Measures: (1) Pile Position Setting-out

　　After site leveling, organize qualified surveying and setting-out personnel to set out all pile positions, drive cross protection piles, conduct measurement review, and record setting-out data for filing.

　　(2) Casing Installation

　　Check pile position deviation by the string line method, and it should meet specification requirements.

　　(3) Hole Formation

　　Use drill bits for drilling, and select different rotary table speeds and penetration rates for control according to soil structures at different layers. When drilling in sand layers and after entering the strongly weathered layer, because the soil layer is too hard, it will cause the drill cone to jump and deviate and increase the swing of the drill pipe. Therefore, low gear and slow speed should be selected, with the rotary table speed parameter set at 13–40 r/min, and the hole depth controlled according to design requirements.

　　(4) Fabrication and Installation of Reinforcement Cage

　　After the rotary drilling rig has excavated soil and cleaned slag to meet design requirements, begin lowering the reinforcement cage. Its fabrication and installation must be strictly carried out according to the design drawings. The main reinforcement and stirrups should be welded evenly, and the lap length and welds should meet specification requirements. During installation, protective blocks should be installed to ensure alignment and the thickness of the protective layer. The fabrication deviation of the reinforcement cage should meet design and specification requirements.

　　(5) Concrete Pouring

　　After the tremie pipe connection is completed, suspend the concrete water-stop plug near the mud surface. The distance from the bottom end of the tremie pipe to the bottom of the hole should be controlled at about 0.4 m so that the concrete water-stop plug can be discharged smoothly. Before starting pouring, the amount of concrete stored in the hopper should be no less than 1.0 m3, so that after the concrete water-stop plug is squeezed out of the tremie pipe, the bottom end of the tremie pipe can be embedded at one time more than 0.8 m into the underwater concrete. Raise or dismantle the tremie pipe in a timely manner to ensure that the bottom end of the tremie pipe is embedded 1–4 m below the concrete surface. Pouring should be continuous. Once accidents such as equipment failure, power or water outage, or tremie pipe blockage or water ingress occur, effective measures should be taken for handling so as to resume concrete pouring as soon as possible, and records should be kept.

　　(6) Preventive Measures for In-Hole Accidents

　　① Select experienced and highly responsible construction teams to ensure the quality of operators; ② strengthen inspection of drilling tools, and strictly prohibit the use of poorly processed drilling tools; ③ frequently observe and test the in-hole water head height and the relative density and viscosity of the mud, and solve problems promptly when found. In particular, during drilling slag discharge, drill cone lifting for soil removal, or suspension of drilling for any reason, the specified water level in the hole and specified mud performance indicators should be maintained to prevent hole collapse; ④ drilling operations should be carried out continuously in shifts. At locations where soil layers change, take slag samples to identify the soil layer and check them against geological data. Take corresponding measures according to soil layer conditions to ensure construction quality; ⑤ lifting and lowering the drill cone must be stable. When the drill cone is lifted out of the wellhead, collision with the casing or hole wall should be prevented, and hooking the bottom of the casing should be prevented. The assembly and disassembly of the drill pipe should be rapid.

　　Issues to Note in Rotary Drilling Hole Formation Construction:

　　(1) The verticality of hole formation in the soil layer and the installation of the steel casing has a huge impact on the verticality of the pile body;

　　(2) The reasonable matching use of the concrete casing for positioning and the steel casing can effectively shorten the positioning time of the drilling rig and improve construction efficiency;

　　(3) In soft rock and soil layers, use the rotary bucket to complete hole formation in one pass; in hard rock, first use a spiral drill bit to form a small hole, and then use the rotary bucket for reaming to complete hole formation.

　　The rotary drilling rig has obvious advantages and strong applicability in bored pile construction for subway projects; high degree of automation, low labor intensity; high drilling efficiency; good pile quality; and low environmental pollution. It can significantly improve construction efficiency and project quality. When construction methods and processes are reasonably selected, it can maximize savings in material, financial, and human resources. It has win-win significance for enterprise benefits and social safety.