1. Project Overview
Project name: Bishan County Sunbao Public Rental Housing Construction Project (Sections I and II) Buildings 1#-6#, kindergarten, commercial buildings, underground garage; Project address: No.1 She, Hufeng Village, Biquan Street, Bishan County; Total construction area: 180,866.9 square meters; Total cost: 130.04078738 million yuan; Structure type: frame structure; Building height: 33 floors/99.3 meters; Commencement date: November 31, 2011, required construction period: 540 days. The three connections and one leveling are all complete. According to the design requirements, the foundation part adopts manual excavation. After the construction unit entered the site, it organized manual excavation pile construction.
Geological survey report groundwater effect evaluation: During the survey period, the water level of Binan River was 268.30 meters, and the historical highest water level was 274.10 meters. Because the building area is mostly backfill soil and about 100 meters away from Binan River, foundation excavation is affected to some extent by the river water; the silty clay in this site has a heavy sand content, with sandy particles when pinched by hand. It is easy to collapse under the action of water, and finally the cohesive components dissolve in water, leaving quicksand-like material.
2. After the construction unit entered the site
A construction plan was prepared based on manual excavation. According to the construction sequence, manual trial excavation was first carried out for the tower foundation. At about 2 meters down, quicksand was found to be particularly serious. After the construction unit reported to the supervisor, owner, and design for on-site inspection, the construction unit was required to use all methods to excavate the quicksand under the condition of ensuring absolute safety... However, being too close to the river, the goal could not be achieved. To avoid unnecessary safety accidents, the construction personnel first used steel bars to probe the depth of the quicksand and found that the deepest was about 8 meters; some wells contained thin mud, and during manual work, the mud surged upward, making manual construction impossible. Based on the on-site inspection by all participating parties, it was considered that manual operation posed serious safety hazards. The owner's leadership determined that due to the limited construction period, manual operation was impossible, and only mechanical rotary drilling could be used; however, the owner believed that mechanical rotary drilling technology was not mature for high-rise buildings. For example:
(1) Can the mechanical centering accuracy during excavation meet the requirements of current design specifications?
(2) Does the bottom hole reaming and slag cleaning meet the design and specification requirements?
(3) How to control the depth for taking pile core samples for pressure testing? How to determine the secondary detailed survey? What acceptance specifications to use? These questions can only be concluded after practice. Therefore, in order to achieve the scheduled construction period and complete the pile foundation construction task before the flood season, this is the reason we need to discuss.

2.1 Let us take the Zoomlion model ZR280A-1 rotary drilling rig as an example and refer to the construction process of mechanical rotary drilling pile foundation with water injection into the well wall: construction preparation → pile position setting out → embed pile casing → mechanical positioning → centering and drilling → final hole quality inspection → hole cleaning and slag removal → lower reinforcement cage → assemble conduit and check → check concrete elevation quality and pour concrete → remove conduit → chisel pile head and check quality → proceed to the next construction process. As the object of discussion: after construction preparation and setting out and positioning are completed, when the drilling rig is positioned, the machine must be initially leveled, and the rotary drill pipe and drill bit must be aligned with the center point of the pre-embedded steel casing (the mark made, i.e., the design pile center coordinate point), until the video shows X and Y axes display 0, then uniform rotation can begin. During the drilling process, the equipment has a mast vertical adjustment function, so that the rotation angle and memory angle automatically rotate within 1f, ensuring that the drill bit always maintains vertical and uniform rotation. After hole formation and pile pouring, the measured center deviation value of 30mm meets the design requirements. During construction, to prevent groundwater and quicksand from exerting pressure on the inner wall and causing hole collapse and economic losses, the general construction method adopts the water injection rotary drilling method. Water is injected to make the water level in the well higher than the groundwater level, so that the pressures offset each other to achieve wall protection. If quicksand is encountered during construction and wall protection is not possible, chemical mud wall protection can be used. The preparation method is: water + sodium bentonite + flake sodium hydroxide + carboxymethyl cellulose, with a particle size requirement of ≥300 mesh, mixed in a ratio of 75:25:2:1. The mixing effect of the drill bit on the soil during drilling is used to naturally create slurry. The specific gravity of the mud can be adjusted according to actual needs. During construction, the mud specific gravity is generally controlled between 1.2 and 1.3. During circulation, the mud forms a mud cake on the hole wall surface, which, together with the self-weight of the mud, protects the hole wall and prevents collapse. Through hole formation construction, the mud wall protection effect is relatively good. By adjusting the mud concentration, the wall protection requirements can be fully met (by adjusting the mud specific gravity and supporting groundwater pressure through slag removal with a slag scoop and adding clean water to the hole). According to actual construction needs, the mud specific gravity is generally controlled above 1.3, which is beneficial for drilling and hole wall stability.

2.2 When the drill bit enters the moderately weathered layer, the pressure displayed on the equipment screen is 200KM and the penetration speed significantly slows down. Based on the operator's experience and comparison with the geological survey report (operator's experience value), it indicates that the moderately weathered layer has been entered. When the design depth is reached, the reaming drill bit can be replaced to adjust the size for reaming. The expansion drill pipe descends until the enlarged head is fully opened. After the drill bit is fully opened, rotate rapidly in place without pressure for about 10 minutes, stop rotating, and lift the drill bit uniformly. Then lift the expansion drill bit to the ground, replace with a sand scoop for slag cleaning (the sand scoop is best to have a flat bottom and basically the same as the bottom of the expansion drill bit), stir uniformly counterclockwise repeatedly. After the sediment cleaning is completed, gently place the expansion drill bit at the bottom of the hole without applying pressure. If the expansion drill bit can be easily fully opened (reaching a stroke of 40cm), it can be judged that the bottom expansion is completed. Otherwise, repeat the steps of bottom expansion and slag removal until the conditions for completion of bottom expansion are met. Strictly detect the pile bottom elevation after bottom cleaning. If it is lower than the previously measured pile bottom elevation after bottom cleaning, the rotary drilling rig display data must be reset to zero and re-read. Until the rotary drilling rig penetration meets the distance for full opening of the expansion drill bit, and the design dimensions and pile bottom elevation no longer change. During drilling, drilling records must be kept and compared with the design. If there is a serious discrepancy, a change must be requested. When entering the moderately weathered layer, the core sampling method is: using the friction between the steel wall of the drill bit and the inner and outer walls of the rock layer to generate heat, the mud becomes thicker, the rotation speed gradually slows down, and the friction resistance between the mud and the inner wall of the drill bit and the inner rock core increases, so that it is slowly sucked out, and the sample is quickly taken for inspection. Strictly speaking, during the advanced secondary detailed survey, we have already taken pile core samples for inspection, and we basically know whether they are qualified. If hole collapse is really serious, use 5-8mm steel casing for wall protection. After reaching the design depth, cleaning the slag at the bottom of the well is particularly important. Re-lower the casing to check for collapse. If there is, readjust the mud concentration and clean the slag until qualified. When lowering the pre-fabricated and accepted reinforcement cage, use cross lines to adjust the centering accuracy. If qualified, proceed to the next construction process. When lowering the conduit, use threaded connection conduits with an inner diameter of 250-280mm, bottom pipe length of 4m, and middle sections generally 2.5m each. The length of the conduit can be adjusted according to the depth of the well to facilitate pipe removal without affecting pile quality and for faster operation. Before using the conduit, an appearance inspection and butt joint inspection must be passed before proceeding to the next process.

2.2.1 Appearance inspection:
Check whether the conduit is deformed, dented, bent, damaged, or cracked, and check whether its inner wall is smooth. For new conduits, check whether the inner wall is smooth and free of welding slag. For old conduits, check whether there is concrete adhesion and consolidation on the inner wall.

2.2.2 Butt joint inspection:
The conduit joint threads should be kept in good condition. After connection, it should be straight and have good concentricity. Only after passing the above inspections can it be put into use. Unqualified conduits are strictly prohibited. The length of the conduit should be equipped according to the hole depth to meet the needs of hole cleaning and underwater concrete pouring, i.e., it can reach the bottom of the hole during hole cleaning; during underwater pouring, the bottom end of the conduit should be about 500mm from the bottom of the hole, and the concrete should be able to smoothly pour from the conduit to the bottom of the hole. When lowering the conduit, the connection at the hole mouth should be firm, with a sealing ring. When lifting and lowering, the position should be centered, the axis straight, and it should be lowered steadily to avoid catching the reinforcement cage and scraping the hole wall.

2.2.3 Concrete requirements:
(1) Concrete is mixed and transported by truck. The concrete slump is controlled at 18-22cm. The concrete from the mixer truck is directly poured into the hopper at the hole mouth for storage and can meet the concrete volume for at least one pile to ensure continuity of concrete construction.
(2) Underwater concrete pouring: Before pouring, calculate the initial pouring volume of concrete for pile holes of different diameters and depths. During construction, ensure that the initial pouring volume makes the first impact squeeze the mud at the base upward, achieving the purpose of secondary bottom cleaning and ensuring that the mud thickness is less than 50mm to meet design requirements. During pouring, the conduit embedment depth is controlled at 2-6m. Before removing the pipe, a dedicated person measures the concrete surface in the hole and keeps records. When the poured concrete approaches the pile top elevation, the final pouring volume should be controlled to ensure that the pile top elevation meets the design requirements after slurry discharge.
(3) Test block preparation: During the pile pouring process, randomly take 1-2 batches of concrete to make test blocks. Each pile should have one set of test blocks. The prepared test blocks should be demolded after 12 hours and placed in a standard curing box for curing. After concrete pouring is completed, immediately pull out the casing, clean the pouring equipment and tools, and stack them neatly.

2.3 Regarding the secondary detailed survey, the rotary drilling rig has almost no construction gap during the construction process. Only advance detailed survey or post-pile core probing can be done, and the thickness of the sediment at the pile bottom can be re-obtained. Re-sampling from the pile core drilling and on-site measurement of sediment thickness fully meet the design requirements; it is also necessary to conduct static load tests to determine the reliability of the technology. Using diameter 1000CM, steel bar diameter 14MM grade II steel, pile length 14-15M, pressurize 1900Ra (KN), after 24H, the measured settlement value is 0.0195MM, meeting the design requirements. What needs special mention here is: during the drilling process of the rotary drilling rig, especially when the quicksand is seriously over-thick and causes hole collapse; during concrete pouring, if the construction gap is too long, the poured concrete may produce broken piles, pipe blockage, etc., causing quality accidents and increased investment losses. Who is responsible? Therefore, the author hopes that the subcontracting mode of mechanical rotary drilling piles should be subcontracted to units with labor subcontracting qualifications, as they have the ability to bear risks. The supervision engineer should strictly review the special construction plan of the safety and quality assurance system while requiring the preparation of emergency plans, clarifying handling methods and responsibility division; at the same time, require professional subcontracting teams with high integrity and economic capacity to undertake the task, to avoid unnecessary economic disputes.

At present, science and technology have developed to a new era, and new mechanical equipment is constantly emerging. Recently, the Chongqing Construction Field Notice on Restricting and Prohibiting the Use of Outdated Technologies (No. 4) will be implemented from July 1, 2012. Our use of mechanical equipment construction will move towards a new high era.

Through the author's experience and understanding of mechanical rotary drilling, it is indeed advisable from the perspectives of safety, construction quality, progress, and precision, and is suitable for modern buildings. Construction quality is mainly related to construction technology, processes, and personnel operation skills. If we effectively adopt pre-control, implement a professional technical disclosure system, and strengthen on-site supervision during construction, qualification is very easy; I hope that manufacturers of rotary drilling rigs will continue to innovate and reform to provide more high-precision mechanical equipment for modern construction.