1.0 Project overview

The Qingshan Yangzi River Bridge is a passage in Wuhan Fourth Ring Road to cross the Yangtze River. The main bridge design is a cable-stayed bridge with two towers and two cable surface hybrid beam (steel box and steel box combined beam). The south main pier of the bridge is located on the south bank slope, and the river bed elevation is +11. 4 ~ + 20. 5 m. The south main pier foundation uses 60 Φ2 — 5 m bored pile. The bearing cap is a dumbbell-shaped structure with a plane size of 98. 9 m * 39. 5 m, and the top elevation +12.0 m, 9 m thick. There are 18 m wide straining beam in the middle of the platform, and the thickness of the beam is the same as the cap. 

2.0 Design conditions

2.1 Geological conditions

The foundation of south main pier adopts the construction scheme of cofferdam construction after hydraulic reclamation. Before drilling pile construction, clearing the riverbed at the dock to the height of +6m is the first step, and then blowing fill the platform to +18m. After the completion of island-building, the overburden layers at the pier position from top to bottom are: 12m thick island filling sand, 6m thick fine sand and silty clay interlayer, and 18m thick fine sand.

2.2 Hydrological conditions

The worst flood water level at the bridge site in the last ten years is +25. After the three gorges project started to store water and generate electricity, the water level in the middle and lower reaches of the Yangtze river changed significantly. Through the statistical analysis of the water level at the pier in the recent ten years (2005-2015), the water level stayed between +11m and +16m from November to early April of the next year. It did not exceed +21.5m before June 15 of each year, except 2010, and the peak water level at the pier did not exceed +24 m.

3.0 Design of cofferdam structure 

The south main pier support platform is large in scale and deep in depth. The construction period of the support platform is during the flood season of the Yangtze River, and its support structure is under about 15 m earth pressure and 26 m water pressure. Also, the construction of the support structure should be completed before the water level of the Yangtze river rises. Large construction load and the short construction period are two major problems in the design of supporting structure. Because of these factors, south main pier piles caps adopts locking steel pipe pile cofferdam structure. 

In the construction of cofferdam, internal support assembly and locking steel pipe pile are in progress synchronously. Then, internal support will be transferred to a lower level after the foundation pit excavation. This method not only improves the pit depth of steel pipe pile cofferdam, and effectively shorten the construction period of the cofferdam.

The locking steel pipe pile cofferdam is the shape of regular polygon dumbbell, and it’s plane size is 101.7 m×41.3m. According to the water level during the construction, the cofferdam is constructed with a single wall steel cofferdam 2 m higher (the final set water level is +24 m). Steel pipe piles are supported by three layers of inner support. The elevation of the top, middle and bottom layers of inner support is +18, +13, +8 m respectively. The bottom sealing concrete is 5m thick, and the bottom of the foundation pit is excavated to -2m. The cap is constructed in three layers, each of which is 3m in height.

(1) Locking steel pipe pile. 

Φ 820mm× 18mm steel pipe with material Q345B is adopted for the main pipe of locking steel pipe pile. As there is a large concentrated force at the prop, it is designed to strengthen at the corresponding positions of middle inner support, bottom inner support, first-floor cap and bottom seal. The foreshaft of steel pipe pile is connected by two different sizes of pipe. 

(2) Internal support. 

Due to the large size of the cofferdam, the inner support is designed as two regular decapitated dumbbell structures on the plane to reduce the manufacturing difficulty and work as a beam structure in the later stage. Since the internal support and the locking steel pipe pile need to be constructed simultaneously, there is a gap of 160 mm between the outer edge contour line and the inner edge contour line of the locking steel pipe pile. After the internal support is installed, this gap will form support. The inner support is a 1.2m wide and 1.8m high box structure, which is made of Q345B. For this box structure, the top and middle inner support flange and web are 20 mm thick. The bottom inner support flange and web are 30 mm and 25 mm, respectively. In the design of internal support, 1,000 mm×16 mm steel pipes are set in these two regular decagons and the straining beams to increase the stability of the structure.

(3) Base sealing concrete and auxiliary piles.

C30 underwater concrete is used to seal the bottom, which is 5 m thick. Because there is no main pile within the scope of the cap beam system, 8 Φ 1. 8 m bored piles are set to assist construction and improve the force conditions of the concrete system. At the same time, to ensure that these auxiliary piles of the bridge do not affect the mode of stress, the auxiliary pile should be cut below the top of the base sealed concrete. Also, the top of back sealed concrete needs to be painted by isolated asphalt layer, meeting the requirement from the design organization. 

(4) Hoisting and lowering system. 

Except for the top of the inner support, the middle and the bottom of the inner support are assembled in the ground, and then lowered to the design position layer by layer by using the hanging system according to the soil extraction situation. There are eight suspension centres set for the whole cofferdam, and a total of 32,100 tonnes jacks arranged for lowering.

(5) Single-wall cofferdam. 

The water level during the construction is going to determine whether there is a need for high wall cofferdam. The total height of the single-wall cofferdam is 2. 76 m, and the top elevation is set to be +24. 5 m. Its main structure is 4 mm steel plate and ∠63 mm×6mm Angle steel.

4.0 Conclusion 

The locking steel pipe pile cofferdam is used for the south main pier of Qingshan Yangtze River Bridge. The cofferdam adopts the form of partial reinforcement of steel pipe pile to bear larger concentrated force. Instead of being in the shape of a circle, the structure of a regular polygon is used for the convenience of manufacturing and later reusing. 

By calculation, the depth of entry into the soil, the anti-floating of the cofferdam, the structural stress and deformation of steel pipe pile under various working conditions all meet the requirements of the code. Cofferdams are safe in bearing and reasonable in structure. In the construction of cofferdam, internal support assembly and locking steel pipe piles are in progress synchronously. This method not only improves the pit depth of steel pipe pile cofferdam, and effectively shorten the construction period of the cofferdam.

At present, the construction of the south main pier foundation of Qingshan Yangtze River Bridge has been completed, and now it has entered the bridge tower construction stage. The design, structure and construction method of this cofferdam are different from that of the traditional steel pipe pile cofferdam, which can provide a reference for the construction of other ultra-large deep foundation pits in the future.