Every tree and blade of grass is pristine; a light boat travels on the Apeng River. The Apeng River integrates primitive gorges, hot springs, karst caves, and underground rivers, forming a unique valley landscape. This is the location of the Apeng River Grand Bridge on the Qianzhangchang Railway, constructed by the Third Harbor Engineering Bureau.
The Apeng River Grand Bridge is the first high-span passenger-freight railway cable-stayed bridge in China, with complex hydrogeological conditions. In particular, the deep foundation pit construction for main piers No. 14 and No. 15 on the Apeng River banks requires excavation to a maximum depth of 15 meters, creating foundation pit planes larger than a standard 5-a-side football field at the water-land interface on both sides of the river. Not only is the engineering volume large, but the geology is a typical 'karst' lava area, mainly composed of mudstone and conglomerate, which softens, collapses, and breaks easily when soaked in water, posing high excavation risks. There is also water pressure of 7 to 9 meters on the water-facing side. Ensuring the foundation pit does not deform has become a key control project in the construction.
The original design used steel sheet pile support for foundation pit excavation, but two trial pile tests failed. The 'culprit' was the hard rock layer. Although mudstone and conglomerate are prone to collapse and breakage after long-term water soaking, they are a 'hard bone' when not soaked.
To tame the hard rock layer and ensure foundation pit excavation, Deputy Chief Engineer Yuan Weimin continuously simulated schemes in his mind. One evening, a conversation with a classmate helped him find a new approach: combining secant piles and fish-belly beams, with secant piles for longitudinal support and fish-belly beam steel structures for lateral support, working together to prevent foundation pit deformation.
Constructing bored secant piles is like inserting chopsticks into the rock layer and then cutting off the protruding parts to form a seamless 'chopstick wall' that isolates water and soil. Laterally, two fish-belly beams are added on each side, functioning like 'crossbeams' of a house, applying lateral support force on top of the secant piles' surrounding pressure resistance, creating a 'double strength' to resist external pressure.
Yuan Weimin searched for cases and found that bored secant piles are a 'nemesis' for rock layers and suitable for deeper foundation pit excavation. Compared with traditional internal support methods, fish-belly beam support offers better overall stability and a lower probability of foundation pit deformation. It saves time and cost and provides open construction space.
Yuan Weimin quickly reported the idea of using this technology to the expert team. However, faced with the fact that the technology was still blank in railway applications, they rejected it and instead suggested using traditional cast-in-place piles and steel structure internal support systems. But Yuan Weimin did not give up.
To smoothly 'graft' the secant pile and fish-belly beam technologies, Yuan Weimin did five months of homework and finally persuaded the organization to hold an expert demonstration meeting to comprehensively diagnose the safety feasibility, economic rationality, and practical operability of the scheme, and approved the scheme for construction.
At the moment of commencement, Yuan Weimin was happy like a child, but soon replaced by seriousness. It was the first time this technology was applied in railway construction, and he was a bit afraid. The secant pile drilling construction encountered a problem right at the start—the drilling rig could hardly move on the site. He found the root cause was that most of the construction area was still relatively soft, and the rig's own weight far exceeded the bearing capacity. 'Find a way to harden the foundation to support the rig's weight.'
Yuan Weimin arranged for the construction team to level and compact the site, replace the muddy ground with block stones to thicken the 'base', and finally lay a layer of roadbed plates. With three measures, the foundation changed from soft to hard, and drilling construction proceeded vigorously.
After solving the soft foundation problem, ensuring each pile was at a 90-degree angle to the ground horizontal line became the key to the success of foundation pit support. Once deviated, the pile's secant degree would be difficult to meet requirements, and water stop would be in vain.
'Secant piles are one of the most effective water-stop structures today, provided that the secant degree reaches 30 centimeters,' Yuan Weimin introduced. 'Secanting is the process of achieving "you in me, me in you" by arc-cutting the overlapping concrete of plain concrete piles and reinforced concrete piles.'
To precisely control the verticality of each pile, Yuan Weimin and the technicians did not miss any details, discovering and solving problems early, often holding short meetings directly on site to overcome difficulties.
Thus, a 'straightening' scheme was introduced: using guide walls and total stations to accurately position piles, real-time control of verticality with the rig's built-in instruments during construction, and final verification of verticality with a 'plumb line' in the hole. 'It's no exaggeration to say that after implementation, the verticality deviation of each pile was controlled within 3‰,' Yuan Weimin said confidently.
After clearing the obstacles, he wanted to take a break, but encountered the problem of low efficiency. To facilitate cutting, the plain concrete piles used super-retarding concrete to delay setting time. However, construction revealed that the hardness of the plain concrete piles exceeded expectations, which greatly hindered the drilling of reinforced concrete piles and made cutting difficult. Yuan Weimin led the team to reconfigure the plain concrete pile concrete. Everyone worked together, using lower-strength concrete and adjusting the sand ratio to control the hardness of the plain concrete piles, successfully solving the problems of low efficiency and difficult cutting.
After the secant piles were completed, the water stop was 'leak-proof'. With the strong support of fish-belly beams, the maximum cumulative displacement of the foundation pit was only 2.3 millimeters... Now, the main piers, over 170 meters high, stand on both sides of the Apeng River, echoing each other and creating a magnificent sight.