Rotary drilled cast-in-place piles have been developed internationally for decades and are known as “green construction technology”, characterized by high work efficiency, good construction quality, less dust and mud pollution, wide application range and high mechanization. In recent years, they have been gradually recognized and applied in China. The Yufeng Liyuan project adopted rotary drilled cast-in-place piles according to actual conditions and achieved good economic and social benefits.

     1 Brief Introduction to the Working Principle of Rotary Drilled Piles
      Rotary drilling construction utilizes the rotation of the drill pipe and drilling bucket, with the bucket's own weight plus hydraulic pressure as the drilling pressure. Through the rotation of the drilling bucket, soil is excavated, filling the bucket with soil debris, which is then lifted and discharged. During construction, the rotary drill first rotates a bucket-type drill bit with a bottom flap to break the rock and soil, and directly loads the rock and soil into the drilling bucket. Then, the drilling rig's lifting device and telescopic drill pipe lift the bucket out of the hole to discharge the soil. This cycle of cutting, extracting, and discharging soil continues until the design depth is reached. 

      2 Project Overview
      The Yufeng Liyuan residential community project is located on Daxue East Road, Nanning City. It consists of two 32-story high-rise residential buildings, with two floors of shops and thirty floors of residences above ground, and two basement levels. The foundation uses rotary drilled cast-in-place piles. The site's rock and soil layers are: artificial fill, clay, silty clay, silty sand, round gravel, and mudstone. According to the geological survey report, from the perspective of site geology and construction conditions, manual excavated piles are not suitable due to the long pile length; static pressure precast piles are difficult to penetrate due to the presence of silty sand and round gravel layers; the project is in the city center with schools and residential areas nearby, so if drilled (percussion) cast-in-place piles were used, the site civilization would be poor and the impact on the surroundings would be significant. After comprehensive consideration, the project adopted rotary drilled cast-in-place piles, with the bearing layer being moderately weathered mudstone. Pile diameters are 800mm, 1000mm, and 1200mm, and pile lengths range from 28m to 41m. Construction uses rotary drilling with mud slurry wall protection, then installation of reinforcement cages, and underwater concrete pouring. A total of 184 rotary drilled piles were installed, with a construction period of about two months.

      3 Rotary Drilled Piles and Other Supporting Construction Machinery
     The project uses commercial concrete, delivered by tanker from the concrete company and poured by pumping, so no corresponding machinery is needed on site. However, on-site processing of reinforcement, preparation of wall-protecting mud slurry, and lifting of reinforcement cages still require corresponding supporting machinery. The required construction machinery is shown in Table 1 below.

      4 Analysis of Rotary Drilled Pile Cost Composition
      The project used three pile diameters: 800mm, 1000mm, and 1200mm, without enlarged heads, and pile length taken as 35m. Various cost components were calculated according to the construction drawings, where longitudinal bars and stirrups are both Grade III steel; half of the longitudinal bars are full length and half are 2/3 length; the stirrup densification zone is 5d (d is pile diameter), and the stiffening hoop spacing is 2m. The calculation is based on the raw material prices during construction: steel at 4500 yuan/ton, concrete at 300 yuan/m3, and construction cost at 330 yuan/m3 (including labor, machinery mobilization, etc.). Below is an example for 800mm pile diameter.
    (1) Pile diameter 800mm:
    (a) Concrete:
      Concrete volume: 0.503×35=17.6m3;
      Concrete cost: 17.6×300=5280 yuan
    (b) Reinforcement (longitudinal bars 10 22, stirrups 8@200):
     Longitudinal bar weight: (5×35+5×35×2/3)×2.98=869 kg
     Stirrup weight: (13.6×31+13.6×2×4)×0.395 =210 kg
     Stiffening hoop: 17×2.5×1.58=67 kg
     Total reinforcement weight: 869+210+67=1146 kg
     Total reinforcement cost: 1146 ×4500/1000=5156 yuan
   (c) Construction cost: 17.6×330=5808 yuan
    Total cost: 5280+5156+5808=16244 yuan
       5 Conclusion
      Based on comparison and analysis, because the pile end bearing capacity of this project is relatively low, with pile end bearing capacity accounting for only about half of the total bearing capacity, according to the unit bearing capacity cost in the table above, the 1200mm and 1000mm pile diameters are 16% and 33% higher than the 800mm pile diameter, respectively. In this case, using smaller diameter piles results in lower unit cost than larger diameter piles. Therefore, when the construction period allows and the pile end bearing capacity is low, using smaller diameter piles is more economical.
　　
      Compared with manual excavated piles or static pressure precast piles, the rotary drilled piles used in this project have a slightly higher cost. The reasons are as follows: the pile end bearing capacity in this project is low, and the standard value of ultimate end resistance of moderately weathered mudstone in the geological survey report qpk is only 4000 kPa, which affects the economy of rotary drilled piles; due to construction technology and schedule control, enlarged heads could not be used. If enlarged heads could be used to improve pile end bearing capacity, costs could also be reduced; besides the concrete and reinforcement used in the pile body, a large part of the cost composition is the construction cost of rotary drilled piles. With the popularization and continuous development of this technology, costs are expected to be further reduced.
　　
      Rotary drilling technology has high drilling efficiency during construction, produces less waste slurry and pollution on site, has low vibration and noise, can reduce disturbance to residents and environmental pollution, improves the civilization level of the construction site, and has little impact on the surrounding environment. It is a relatively ideal green construction technology with considerable economic and social benefits.