Gazing at the bustling scene of steel girder hoisting for the superstructure on the south bank of the Rongshan Yangtze River Bridge project in Luzhou, Sichuan, Wang Peng, project manager of the Wuhan-based central enterprise CCCC Second Harbor Engineering Co., Ltd., still vividly recalls the numerous difficulties in constructing the No. 2 main pier on the south bank.

With a total length of 1,505 meters and a main span of 530 meters, the Rongshan Yangtze River Bridge is a cable-stayed bridge with unequal-height towers, double cable planes, and composite girders. It is a key transportation project for public benefit that Sichuan has built to completely eliminate hidden safety hazards in ferry transport. Once completed, the local government will abolish the Rongshan and Lianshitan ferries, providing convenience for residents on both banks and promoting Luzhou's integration into the Chengdu-Chongqing metropolitan area.

The geology at the pile cap of the No. 2 main pier of the bridge is characterized by hard exposed bedrock, scattered gullies, and uneven distribution of cobble layers, which experts call "karst-like topography under cobble-filled conditions." The bridge's pile cap is designed as an embedded cap, and there is no precedent at home or abroad for constructing a pile cap in such an environment.

When the project team first entered the site, Wang Peng led the team in a survey and found that embedded pile caps generally use cofferdams as temporary water-retaining structures. However, common cofferdam construction methods posed a series of problems in this project, such as difficulty in ensuring the smooth lowering of cofferdam steel sheet piles, easy collapse of the cobble layer, and difficult water sealing. In addition, the pile cap is in a deep-water environment and is highly susceptible to the Yangtze River flood season.

Faced with the dilemma of having no experience to draw on, Wang Peng and his team stayed on site continuously for a month, invited experts more than 10 times for on-site surveys, and proposed multiple schemes. The core issue was how to prevent cobble layer collapse during cofferdam construction and poor water-sealing performance of the cofferdam. After repeated surveys and analyses, the builders for the first time proposed a complete set of construction technologies: "casing rotary drilling for pilot holes + steel sheet pile rock-socketing + sand-mud mixture filling for water sealing."

Technicians used the rotary drilling rig for pile foundation construction to drill pilot holes first, opening a path for the subsequent insertion and driving of steel sheet piles. However, using ordinary drill pipes would damage the underwater cobble layer and cause collapse, so improvements had to be made to the drill pipes. The project team adopted a specially processed protective casing device. After drilling the pilot hole and before removing the casing, sand was filled into the hole to effectively prevent cobble collapse. This protective casing acted like "safety armor" protecting the surrounding rock strata.

Just as construction of the No. 2 main pier pile cap was in full swing, Wang Peng heard complaints over the walkie-talkie from Liu Longkui, the on-site person in charge of the construction team: "I've been doing pile cap construction for more than 20 years, and I've never felt so frustrated." It turned out that the rock at the pile cap location was too hard, and coupled with limited space, the excavator's hydraulic breaker had power but nowhere to apply it.

"The key is to rely on ingenuity," Wang Peng said. "We need to work on the details." After discussion and analysis, the project team decided to apply targeted measures according to the rock hardness and geological conditions: hydraulic splitting for hard rock without cracks that is difficult to drill, mechanical breaking for areas with cracks that are easy to drill, and water drill coring for pile foundation locations. The rational use of this combination of methods rapidly advanced the excavation work.

On March 5, 2022, the steel sheet pile cofferdam for the No. 2 main pier pile cap completed bottom sealing. On March 26, after 36 hours of continuous effort, the pile cap concrete pouring was completed, ensuring construction of the No. 2 main tower column. After 240 days of hard work, the project department finally "cracked" this hard nut on schedule.

Today, all main towers of the bridge have been topped out, and superstructure steel girder construction is progressing steadily. The bridge is expected to open to traffic in 2024.