The main tower pile foundation of Longyan Bridge has poured 13 piles since the first pile foundation was poured on April 17, 2016. The main tower pile foundation of Longyan Bridge consists of 21 piles, with a pile diameter of 2.5m, a maximum pile length of 86m, and a maximum drilling depth of 97m. The long construction time and great construction difficulty of its pile foundation are rare in China. 　　The middle of the above figure is the location of the main tower cap, with a total of five rows and 21 piles (pile diameter 2.5m). The north side of the cap has partially intruded into the Longjin River, the east side is adjacent to the Longxia High-speed Railway line, the south side encroaches on part of Luolong Road, and the west side is close to shops. The construction space is extremely narrow, and railway safety must be ensured (the closest point to the railway is only 15m), the traffic on the south road must not be affected, and it also bears the environmental protection responsibility for the Longjin River. That is to say, apart from the main structure space, there is very little space around for construction. 　　The circles in the figure indicate the locations and sizes of karst caves. As can be seen from the above figure, karst caves of various sizes are almost everywhere in the entire underground space. The pile with the most karst caves is No. 21 pile, with a total of nine karst caves. The largest karst cave is the second karst cave of No. 10 pile, nearly 30m high. According to their vertical positions, these karst caves are called first-layer karst caves within 30m from the ground, second-layer karst caves within the range of 30m-60m, and third-layer karst caves within the range of 60m-100m.
　　The depth of the railway piles on the east side is about 40m, located between the first-layer karst caves and the second-layer karst caves. That is to say, the weight of the railway is entirely pressed on the roof of the second-layer karst caves. The piles of Longyan Bridge must all penetrate the second-layer karst caves to reach the top or bottom of the third-layer karst caves, and the roof of the second-layer karst caves must also be protected. The construction difficulty is extremely great.
　　The karst caves at the pile foundation locations are extremely developed, not only numerous but also with large cavities, and they are connected with the karst caves at the bottom of the adjacent Longjin River, forming an underground river. This situation is rare in China. 　　The extracted rock samples are mainly limestone and granite, either crescent-shaped in the karst cave area (extremely prone to collapse underground) or slightly weathered rock (extremely hard, with hardness reaching 115 MPa). Basically, 6 to 10 alloy roller cone drill bits are used per meter. The rock-socketed depth of large bridges in China is generally 4 to 5 times the design pile foundation. For a 2.5m pile foundation, a rock-socketed depth of only about 10m is required to meet the requirements. Because it needs to penetrate karst caves, the rock-socketed depth of this bridge's pile foundation reaches about 70m, which is 7 to 8 times the normal rock-socketed depth and is unique in China. 　　Then, a medium-sized drill bit was used to drill to the bottom of the second-layer karst cave, and a 2.86m or 3.05m casing was lowered to seal the second-layer karst cave.
　　Next, a drill bit not smaller than the design pile diameter was used to drill to the bottom of the third-layer karst cave, and a 2.65m casing was lowered.
　　Finally, a 2.5m drill bit was used to drill to the design elevation to form the hole. Considering that there are still some fissures at the bottom of the third-layer karst cave, the project department also prepared a 2.42m casing, which was套 on the reinforcement cage and lowered to the specified position. At this point, the pile can be poured with confidence.