城市桥梁超大沉井关键施工技术

Key Construction Techniques for Very Huge Caisson of a City Bridge

武汉鹦鹉洲长江大桥为三塔四跨钢-混结合梁悬索桥,桥跨布置为(200+2×850+200)m。该桥北锚碇基础为"带孔圆环+十字撑"结构沉井,圆环内沿圆周均布16个直径8.7m的井孔。为降低沉井施工对周围房屋、长江大堤的安全影响,沉井施工前在其外围10m处设置地下连续墙结构进行防护。沉井共分8节,采取在底节上接高第二节后下沉9m,再接高3节下沉14m,最后接高3节下沉22m的"3次接高3次下沉"施工方案。为防止出现翻砂事故,采取沉井内侧环向均匀取土、中间缓吸反压的技术措施,采用5孔单孔直径1mm的空气幕气龛助沉。在沉井即将到达设计标高时,在沉井内侧沿沉井壁吸泥形成环形沟槽、开动空气幕实现沉井精确就位。采取长距离管道水力排渣施工方法,有效避免对城市环保和路面交通的影响。

The concrete composite main bridge of Yingwuzhou Changjiang River Bridge in Wuhan is a steel and girder suspension bridge with three towers, four spans and with span arrange- ment （200＋2×850＋200） m. The foundation for the north anchorage of the bridge is the caisson foundation of ＂holed ring＋crisscross bracing＂ structure. In the ring, 16 well holes of diameter 8.7 m were uniformly arranged along the periphery. To minimize the influences of the construction of the caisson on the safety of the surrounding buildings and the Changjiang river dyke, at the 10 m outside the caisson periphery, the diaphragm wall was built to serve as the protection facility before the caisson was constructed. The caisson was totally divided into 8 lifts, the construction scheme of 3 times caisson height extending and 3 times caisson sinking was applied, that was, the caisson was firstly sunk down 9 m after the second lift of the caisson was extended on the first lift. Then 3 lifts of the caisson were extended and the caisson was continuously sunk down 14 m and fi- nally another 3 lifts of the caisson were extended and the caisson was again sunk down 22 m. To prevent the accident of sand rushing, the technical measure of removing the soil cireularly and uniformly close to the inner side of the caisson and slowly dredging in the middle by negative pressure was taken and the air curtain air nozzles of 5 holes with diameter of each hole being 1 mm were also applied to aid the sinking of the caisson. Before the caisson was about to reach the designed elevation, the dredging along the caisson wall in the inner side was carried out to make a circular trench and at this stage, the air curtain started to work to realize the accurate setting of the caisson in place. The method of water power soil discharging through the long distance pipes was applied and the influences of the construction on the environment protection and road traffic in the city were therefore effectively avoided.