宜昌香溪河大桥4号桥塔墩钢围堰封底技术

Techniques of Base Sealing for Steel Cofferdam of Pylon Pier No.4 of Yichang Xiangxi River Bridge

宜昌香溪河大桥主桥为(2×48+78+470+78+2×48)m的双塔双索面斜拉桥,其4号桥塔墩采用18根3.0m钻孔桩基础,基础采用先钻孔平台、后圆形双壁钢套箱围堰(内径30.2m,外径33.2m)的方案施工。封底施工是钢围堰施工的关键工序,为了保证抽水后围堰能够承受水头差产生的巨大压力,必须从封底混凝土厚度设计及灌注施工方面保证封底质量。采用MIDAS Civil软件建立钢围堰结构整体有限元模型,通过封底混凝土受力计算,确定采用厚度为5.0m的C30水下混凝土封底。在4号桥塔墩钢围堰吸泥下沉至设计高程后,运用水下找平技术将围堰内河床标高找平至+139.0m,采用水下导管法、按照"由外向内、由中心向四周"的顺序灌注混凝土。围堰抽水期间的应力及变形监测结果表明,封底结构安全且无渗水现象。

The main bridge of Yichang Xiangxi River Bridge is a cable-stayed bridge with double pylons,double cable planes and with span arrangement(2X48+78+470+78+2X48)m.The foundation for the pylon pier No.4 of the bridge is the one comprised of 18 nos.of the^3.0 m bored piles and in the construction of the foundation,using the circular double-wall steel boxed cofferdam(the internal diameter of the cofferdam being 30.2 m and the external diameter being 33.2 m),the scheme of building up the boring platform before constructing the cofferdam was adopted.The base sealing for the cofferdam was the critical working procedure in the construction and to ensure that the cofferdam could bear the tremendous pressure caused by the water head difference after the water in the cofferdam was pumped out,the quality of the base sealing must be guaranteed in the two aspects of the thickness design and casting of the base sealing concrete.The software MIDAS Civil was used to establish the finite element model for the overall cofferdam structure and by means of calculating the forces of the concrete,it was determined that the C30tremie concrete of 5.0 m thickness should be used.After the cofferdam was sunk to the designed elevation by the air lifting,the underwater leveling technique was then used to level the riverbed part in the cofferdam to the elevation of+139.0 m and the concrete was cast in the sequences from the outside to the inside and from the center to the circumference of the cofferdam,using the tremie ducts.The monitoring results of the stresses and deformation of the cofferdam during the pumping-out of the water in the cofferdam showed that the base sealing of the structure was safe and there was no problem of the water seepage.