1. Current Status of Pile Foundation Construction Technology

　　According to construction methods, piles can be divided into three major categories: non-displacement piles, partial displacement piles, and displacement piles. Further subdivided, there are more than 300 pile construction methods. The changes, improvements, and updates of construction methods can be said to be advancing rapidly. Figure 1 lists some construction types of piles.

　　Taking embedded piles as an example, only three major categories are listed in the figure; in fact, there can be more than 80 types when further subdivided. The so-called mid-excavation construction method pile involves inserting a long spiral drill smaller than the pile diameter by 30–40 mm, or a spiral drill with mixing blades installed at the end of the drill pipe, or a drilling bucket drill, into the hollow part of the pile, continuously drilling into the stratum near the drill bit so that soil rises along the hollow part, and sinking the pile while discharging soil from the pile top. During construction, compressed air and water are usually injected at the pile end to promote drilling and also make pile sinking smooth. In order to enable the pile to obtain greater bearing capacity, after the pile is embedded in the hole, the final driving method, pile-end reinforcement method, or enlarged-head reinforcement method can be adopted respectively. According to the mid-excavation embedding process, drilling rig, bearing capacity mobilization method, and type of precast pile used, mid-excavation construction method piles can be further subdivided into more than 40 pile types. Pre-bored embedded piles can also be subdivided into more than 40 types.

　　Taking the hole-forming methods of bored expanded-base cast-in-place piles with slurry wall protection as an example, there are also more than 40 methods. Expansion methods can be divided into reverse circulation expansion, drilling bucket expansion, direct circulation expansion, and submersible drilling expansion, etc. Among them, the reverse circulation expansion method is further divided into upward-opening reamer, downward-opening reamer, sliding reamer, and push-out reamer, etc.; the drilling bucket expansion method is further divided into horizontal push-out, sliding, and combined downward-opening and horizontal push-out, etc.

　　Taking pile-end pressure grouting as an example, grouting processes can be divided into two major categories: closed grouting and open grouting. The core component of the pile-end pressure grouting construction process—the pile-end pressure grouting device—can be divided into five major categories: reserved pressure grouting chamber, reserved pressure-bearing bag, reserved grouting cavity, reserved grouting channel, and reserved special grouting device. Combined, there are currently more than 20 pile-end pressure grouting pile construction methods, including 18 in China.

　　2. Selection of Common Pile Installation Techniques

　　The selection of pile type should consider the following principles:

　　(1) "Adapt to load," that is, the magnitude of the load transmitted from the superstructure to the foundation is the main factor controlling the bearing capacity requirements of a single pile.

　　(2) "Adapt to soil stratum," that is, according to the engineering geological conditions of the building site, groundwater level conditions, and depth of the pile-end bearing stratum, select the pile type by comparing the bearing capacity and technical-economic indicators of pile structures under various schemes.

　　(3) "Adapt to machinery," that is, consider the existing pile construction machinery and equipment of pile foundation construction units in the region; if pile construction machinery really needs to be introduced from other regions, its economic rationality must be considered.

　　(4) "Adapt to environment," that is, consider the impact on the environment during pile installation. For example, in the case of driven precast piles and driven cast-in-place piles, the impact of vibration, noise, and oil pollution on the surrounding environment must be considered; for slurry wall-protected bored piles and embedded piles, the treatment of mud water and mud soil must be considered, otherwise adverse environmental impacts will result.

　　(5) "Adapt to cost," that is, the cost of the pile type adopted should be relatively low.

　　(6) "Adapt to construction period," that is, when the construction period is tight and the environment permits, driven precast piles can be used because their construction speed is fast; similarly, if construction conditions are suitable, manually excavated piles can also be used because this pile type can increase the number of working faces and the construction progress is also relatively fast.

　　In short, when selecting pile type and technique, comprehensive research and analysis should be conducted on the characteristics of the building (type of building structure, nature of loads, function of piles, safety grade of the building, etc.), topography, engineering geological conditions (soil layers penetrated, geotechnical characteristics of the pile-end bearing stratum), hydrogeological conditions (type of groundwater, groundwater level elevation), construction machinery and equipment, construction environment, construction experience, characteristics of various pile construction methods, supply conditions of pile-making materials, cost, and construction period, etc., and technical-economic analysis and comparison should be carried out, finally selecting an economical, reasonable, safe, and applicable pile type and pile-forming technique.

　　Table 1 is a reference table for the selection of common pile installation techniques. This table was compiled after integrating domestic and foreign construction practices and consulting a large amount of literature and materials, and can be used as a reference for type selection.

　　China has a vast territory, and its engineering geological and hydrogeological conditions are complex and variable. Economic development in the eastern and central-western regions is uneven, and the requirements of various projects differ. A large amount of construction practice shows that the various commonly used pile types in China generally have the following characteristics: coexistence of large-diameter piles and ordinary-diameter piles; coexistence of precast piles and cast-in-place piles; coexistence of non-displacement piles, partial displacement piles, and displacement piles; among non-displacement cast-in-place piles, coexistence of drilling, percussion-grabbing hole formation, and manual excavation methods; among displacement piles, coexistence of hammering, vibration, and static pressing methods; among partial displacement cast-in-place pile grouting processes, coexistence of pre-grouting piles and post-grouting piles; coexistence of advanced and modern process equipment with traditional and relatively outdated process equipment, etc. It can thus be seen that various pile types all have suitable strata, soil conditions, environments, and demands in China, and also have conditions for development, improvement, and innovation.

　　It should be noted that no pile type is omnipotent; each has its applicable scope. The key lies in finding the entry point and maximizing strengths while avoiding weaknesses. No matter how good a pile type is, if quality is neglected during construction or its applicable scope is exceeded, quality problems will occur and may even cause major accidents and economic losses.

　　3. Development Trends of Pile Foundation Construction Technology

　　As we enter the 21st century, at least the following trends in the development of pile foundation construction technology deserve attention.

　　3.1 Pile dimensions are developing toward longer and larger sizes.

　　Based on the bearing requirements of high-rise and super-high-rise buildings and large bridge main tower foundations, pile diameters are becoming larger and pile lengths longer. The length of steel pipe piles in Europe, America, and Japan has reached more than 100 m, with pile diameters exceeding 2,500 mm; the steel pipe piles of the Shanghai Jin Mao Tower penetrate into the sand layer 80 m below the ground surface, with a pile diameter of 914.4 mm; the length of statically pressed reinforced concrete precast piles in the Wenzhou area has reached more than 70 m, with a pile section of 600×600 mm2; a reverse circulation drilled bored pile in a project in Zhengzhou has a diameter of 1,000–1,100 mm and a pile length of 77.6 m; a reverse circulation drilled bored pile in a building in Xiamen has reached a depth of 103 m; the reverse circulation drilled bored piles for the main tower pier foundation of the Nanjing Yangtze River Second Bridge have a diameter of 3 m and a depth of 150 m.

　　3.2 Pile dimensions are developing toward shorter and smaller sizes.