Due to the large quantity and wide distribution of industrial and civil buildings using manually excavated pile foundations, and in accordance with the construction requirement that “the free height of concrete falling from a height should not exceed 2 meters,” it is generally felt that using traditional construction methods to cast deep hole pile core concrete—with tremie feeding and layered vibration—will be very difficult. In particular, the safety of operators working deep in the shaft will face serious threats. This is because the pile diameter only allows one person in the shaft; the operator must hold the steel ladder with one hand and operate the vibrator with the other. Although safety protection is provided, the hydration heat of freshly cast concrete rushes toward the worker, and the steel ladder and tremie must be dismantled as the concrete pouring surface rises. The worker's physical exertion is enormous, the operating conditions and construction environment are very dangerous, and the efficiency is low and speed is slow, lacking operability in actual concrete pouring. To overcome the various unfavorable factors of traditional construction methods, selecting the vertical high-drop non-vibratory concrete casting technology for deep hole pile cores will be a safe, fast, and reliable construction method.

　　The vertical high-drop non-vibratory concrete casting method for deep hole pile cores is an important innovation over traditional construction methods. The main key to construction is to prepare a concrete mixture with good workability such as fluidity, filling ability, and segregation resistance, giving full play to its excellent uniformity, cohesiveness, fluidity, and filling ability. At the same time, the impact vibration-compaction effect of the kinetic energy from the high drop of the concrete mixture at a certain height is utilized to achieve the self-compacting effect of high-drop vibration-free casting. Its construction key points are the main control points for ensuring the quality of pile core concrete casting.

　　I. Optimize the concrete mix proportion to achieve good fluidity, filling ability, segregation resistance, and homogeneity

　　The essence of concrete mix proportion optimization design is the careful selection of raw materials and the adjustment and trial mixing of the mix proportion, so that the concrete has sufficient plastic viscosity (i.e., cohesive viscosity) to prevent segregation and bleeding, and also has good fluidity and filling performance to form a uniform and dense structure. In the performance trial mixing of concrete, the following should be followed:

　　1. According to the influence patterns of water content, admixtures, sand ratio, and supplementary cementitious material properties on concrete workability, adjust the mix proportion to control multiple workability indicators within appropriate ranges.

　　2. Reduce the water-binder ratio through high-efficiency water reducers, retarders and plasticizers, and appropriate air entrainment to ensure concrete workability and strength grade.

　　3. When the concrete workability is suitable but the strength value differs greatly from the trial mix value, the strength grade can be met by increasing the cementitious material paste volume and reducing the water-binder ratio.

　　4. If segregation occurs in the concrete mixture, it can be solved by increasing the sand ratio or reducing the fineness modulus of fine aggregate, as well as increasing supplementary cementitious materials and reducing water content.

　　5. If the prepared concrete mixture is “sticky,” appropriately increase the fineness modulus of fine aggregate, control the fine powder content; adjust the type or amount of supplementary cementitious materials; or change the cement type to ensure good compatibility between cement and admixtures.

　　II. Control concrete slump and slump flow

　　The main defect in manually excavated pile concrete is that after segregation, local paste enrichment and local accumulation of coarse aggregate occur, directly affecting the internal quality of concrete. Controlling the concrete slump and slump flow segregation resistance will be an important key to controlling the workability of high-drop vibration-free concrete.

　　The slump flow index of concrete mixture quantifies the flow state of concrete under its own weight after overcoming yield stress, viscosity, and friction. The closer the slump flow is to a circle, the better the homogeneity and deformability; the center-edge difference reflects the suspended flow ability of gravel in mortar, segregation resistance, and stability, so the smaller the center-edge difference, the better. At the same time, it is required that there be no accumulation of coarse aggregate in the center, and no mortar exudation or bleeding at the edge of the concrete mixture flow.

　　III. Concrete workability testing

　　For the testing of prepared concrete mixtures, in addition to general workability tests, the testing of concrete fluidity and slump flow is particularly important.

　　For concrete fluidity testing, the traditional method uses the slump test, which is simple and easy to perform, but it cannot accurately quantify the yield shear stress and plastic viscosity that affect fluidity, and even with a large slump, the mixture may still be viscous and thick, failing to meet fluidity requirements. Therefore, a comprehensive evaluation of fluidity should be conducted from both time and space perspectives. The L-type flow meter test method overcomes the shortcomings of the slump test and is less affected by human factors, making it a relatively ideal method for measuring fluidity indicators.

　　For the testing of concrete slump flow workability, an inverted slump cone with a bottom cover is fixed on a support, with the bottom 50 cm above the ground. Fill the cone with concrete and level it, quickly slide open the bottom cover, and use a stopwatch to measure the flow-out time. Combine this with the measured concrete flow diameter (slump flow) and the height difference between the center and edge (center-edge difference) obtained during the slump test. Generally, the control should be: slump around 20 cm, slump flow greater than 60 cm, flow time 8–10 s, and center-edge difference less than or equal to 30 mm, indicating excellent workability.

　　IV. Concrete drop height and funnel setup

　　Another key to vertical high-drop non-vibratory construction technology is that the concrete must have a certain drop height. The drop height of concrete pouring must satisfy the condition that the clear distance between the concrete discharge outlet and the top surface of the pouring layer is greater than 6 meters, making full use of the kinetic energy generated by the concrete falling as a free body to achieve the effect of vibrating and compacting the concrete. During pouring, the concrete supply must be continuous to ensure pouring homogeneity. To prevent segregation caused by collision with the pile hole wall and reinforcement cage during falling, a discharge funnel is placed at the wellhead, with the funnel discharge pipe centered, so that the concrete falls vertically in the cavity.

　　V. Precautions for vertical high-drop concrete pouring

　　1. Construction of the enlarged bottom part of manually excavated piles. The enlarged head at the bottom of the excavated pile is an important part of the pile foundation bearing force. Since the impact kinetic energy of high-drop concrete is limited by amplitude and cannot be uniformly transmitted, the enlarged head part must be mechanically vibrated and compacted.

　　2. Within 6 m of the pile top, due to insufficient concrete drop energy, mechanical layered vibration and compaction must be used.

　　3. A slump cone must be set up at the pile core concrete pouring point to control concrete quality at any time.

　　4. In combination with actual conditions, formulate a strict construction plan, focus on key control points, and implement careful technical disclosure to operators.

　　5. For pile core concrete pouring of manually excavated piles with large diameter and shallow depth, if the high-drop construction process is adopted, the main technical measures must be determined through theoretical calculations and simulation tests.

　　The increasingly perfected vertical high-drop non-vibratory concrete casting technology for deep hole pile cores has been widely recognized and promoted with the widespread use of manually excavated pile foundation types. The use of high-drop casting will greatly improve the working conditions, labor intensity, and operating environment of operators, accelerate construction progress, and fully leverage its technical advantages of high efficiency, high quality, energy saving, noise reduction, and safety and reliability, gradually replacing the dangerous and heavy traditional construction methods.