Classification of piles

Development trends in pile foundation construction technology As we move into the 21st century, there are at least the following trends in the development of pile foundation construction technology that deserve attention: 1. Piles are becoming larger in size, both in diameter and length. Due to the load-bearing requirements of high-rise and super-high-rise buildings, pile diameters are increasing, and pile lengths are getting longer. In Europe, the United States, and Japan, the length of steel pipe piles has reached over 100 meters, with a diameter exceeding 2500 mm ； The pile tips of the steel pipe piles in the Shanghai Jinmao Tower reach a sand layer 80 meters below the ground surface, with a pile diameter of 914.4 mm ； In the Wenzhou area, the length of static-pressure reinforced concrete precast piles has reached over 70 meters, with a cross-sectional area of 600mm×600mm ； The reverse circulation drilling piles used in a project in Zhengzhou have a diameter of 1000–1100 mm and a length of 77.6 m ； The foundation piles driven by reverse circulation drilling for the main pylon footings of Nanjing’s Second Yangtze River Bridge have a diameter of 3000 mm and a depth of 150 m; the manually dug piles used in a project in Xiamen have a length of 73 m and a diameter of 1.8 m. 2　The dimensions of piles are trending toward being shorter and smaller. Driven pile technologies using small piles and anchor rods are becoming increasingly mature to meet the needs such as the renovation of old urban areas, the reinforcement of existing foundations, the correction of building alignment, the addition of floors, and the installation of supplementary piles. Small piles (also known as micro-piles or IM piles) are essentially small-diameter pressure grouting piles, with a pile diameter of 70–250 mm and a pile length of 8–20 m. The cross-section of the static press pile with anchor rods is 200mm×200～300mm×300mm ； The length of each pile section depends on the available construction headroom and the equipment used, ranging from 1.0 to 3.0 meters, while the depth to which the piles are driven into the ground ranges from 3 to 30 meters. 3　Moving forward in the direction of overcoming the challenges associated with hole drilling for penetration piles 　Taking Japan as an example, an association for rock drilling technologies was established, comprising 64 foundation companies, which has developed over 20 different methods for drilling large-diameter holes in rock. These include 3 methods using long spiral drilling, 5 methods using rotary drilling, 7 methods using impact drilling, and 9 methods using full-casing rotary excavation. In China as well, many organizations have successfully developed drilling methods for rock formations, as well as methods for drilling through layers of large gravel and boulders. 4 Development toward low-pollution pile construction methods Although cylindrical diesel hammer-driven precast reinforced concrete piles have advantages such as reliable pile quality, fast construction speed, and high bearing capacity, they suffer from drawbacks like high noise, significant vibration, and oil splashing during construction (collectively referred to as primary pollution), which imposes serious restrictions on their use in residential and commercial building constructions in urban areas. As a result, the static pressure-driven precast reinforced concrete pile construction technique has gained favor among clients in China, while abroad there is a trend toward using hydraulic pile drivers instead of cylindrical diesel hammers. 　　Although the mud wall drilling and percussion cast-in-place pile methods are widely used in soft soil areas with high groundwater levels, the use of mud leads to unsanitary conditions at the construction site, and the difficulty of removing this mud (known as secondary pollution) poses a problem for constructors. Therefore, drill bucket piles are used for cast-in-place pile construction; thanks to the dry soil extraction process and the use of stabilizing fluids stored in specialized tanks, the construction site remains orderly. In Japan’s construction industry, this type of pile has become the most common variant of slurry-supported cast-in-place piles, and its use is also increasing steadily in China. 　　Benot driven piles are widely used both at home and abroad due to their excellent environmental benefits – low noise, minimal vibration, and no mud contamination or discharge – as well as their contribution to a civilized construction site. In our country, these piles account for approximately 45% of the market share in relevant regions; more than 10 projects in Kunming, Wenzhou, and Beijing have successfully employed this type of pile. 5 Development in the direction of enlarged-diameter piles Test results on static load tests of ordinary-diameter drilled and enlarged-bottom poured piles in the Beijing area (pile diameter of 0.3–0.4 m, enlarged-bottom diameter of 0.8–1.2 m) show that, compared with straight-hole piles of the same diameter, the ultimate load of the former is 1.7–7.0 times that of the latter, while the ultimate load per unit volume of the former is 1.4–3.0 times that of the latter. Large-diameter drilled (excavated) and enlarged-base piles possess significant advantages such as high bearing capacity, less soil removal after hole formation, and a smaller cap area, and are widely used both domestically and internationally. There are over 20 types of bored and enlarged pile methods in our country, while Japan has nearly 30 types of large-diameter bored and enlarged pile techniques. 　　In addition to drilling and reaming, the forming processes for hole expansion include explosive expansion, ramming expansion, vibration expansion, hammer expansion, pressure expansion, punching expansion, injection expansion, extrusion expansion, and digging expansion. 6 Development in the direction of special-shaped piles To increase the bearing capacity of individual piles (side friction and end resistance), special-shaped piles have been widely developed both domestically and internationally. Broadly speaking, special-shaped piles include piles with abnormal transverse cross-sections and those with abnormal longitudinal cross-sections. 　　The expanded multi-branch load-bearing disc pile is a special-shaped pile with a heterogeneous cross-section in both the vertical and horizontal directions, consisting of components such as the pile shaft, branches, load-bearing discs, and pile root; its ultimate load per unit volume of pile material is more than twice that of a corresponding straight-hole pile. 7 Development towards embedded piles There are three methods for installing precast reinforced concrete piles and steel piles: driving, pressing (static compression), and embedding. As mentioned earlier, there is a type of pollution associated with the impact (driving) construction using barrel-type diesel hammers. The driving and pressing pile-driving methods generate soil displacement effects during construction, causing the foundation soil to bulge and shift horizontally, which has adverse effects on adjacent buildings and underground pipelines to varying degrees. 　　To eliminate one-time pollution and soil displacement effects, Japan has developed over 60 types of embedded pile construction methods in the past 20-odd years. The so-called embedded pile method is a general term for techniques that involve sinking precast piles into drilled holes and then using certain methods to enhance the load-bearing capacity of these piles. The pile-driving method used in the Beijing area involves first drilling holes using a long spiral drill to penetrate through hard interlayers or liquefiable layers, then placing pre-fabricated piles into these holes, and finally driving the piles down with hammers so that their ends reach the bearing stratum as specified in the design. 8 Development toward composite process piles Due to requirements regarding bearing capacity, environmental protection, as well as limitations imposed by engineering geological and hydrogeological conditions, pile types using a single processing method often fail to meet the requirements of projects; therefore, composite process piles are frequently used in practice. For example, bored and enlarged cast-in-place piles have two processes: drilling a straight hole and enlarging the hole ； Pile-end pressure grouting piles involve two processes: hole formation and pile construction, and grouting into the stratum at the pile end after pile construction ； Pre-drilled driven precast piles involve processes such as drilling, grouting, pile insertion, and light striking (or pushing). 9 Development toward high-strength piles As the requirements for driven precast piles increase, such as high bearing capacity, the ability to penetrate hard strata, resistance to high impact stresses, and rapid delivery, RC piles can no longer meet these demands; therefore, PC piles and PHC piles are being used more and more often. 10 Development towards various pile materials 　　Taking cast-in-place piles as an example, there is also a trend toward diversification in the types of pile materials, such as ordinary concrete, ultra-fluid concrete, sand-free concrete, and micro-expansive concrete. Driven piles also include composite material piles, such as synthetic piles with a steel tube shell and a concrete inner wall.