大型沉井基础初沉阶段受力特性及开裂控制

Mechanical Characteristics and Cracking Control of Large Caisson Foundation in Initial Sinking Stage

大型沉井基础在初沉阶段因平面面积巨大、高度低、整体刚度小、基底土体不均匀性等而导致受力安全问题十分突出。以长100.7 m、宽72.3 m的五峰山长江大桥北锚碇沉井基础为背景,运用仿真分析和现场试验,进行大型沉井初沉阶段的受力特性及开裂控制研究。结果表明:大型沉井在初沉阶段因面积大、高度低而受弯明显;采用传统大锅底下沉时内隔墙底部混凝土拉应力达到11.4 MPa,远超混凝土开裂限值2.2 MPa,开裂风险明显;采用中、小锅底下沉时,因中部隔墙底部支撑混凝土拉应力降至2.31和1.28 MPa;3种下沉方法下沉井底部的最大变形分别为−20.0,−1.6和−0.25 mm,变形均较小。工程实际采用小锅底下沉时,沉井顶部挠度、底部混凝土实测应力基本在有限元计算控制值的范围内;采用大锅底下沉时,在沉井中部隔墙及井墙底部布置预应力钢束,能有效降低混凝土的拉应力水平并提升沉井结构的抗裂性能。

In the initial sinking stage of large caisson foundation,the stress safety problem is very prominent due to the huge plane area,low height,small overall stiffness,the inhomogeneity of the foundation soil and so on.Based on the north anchorage caisson of Wufengshan Yangtze River Bridge with a length of 100.7 m and a width of 72.3 m,the mechanical characteristics and cracking control of large caisson in the initial sinking stage were studied by simulation analysis and field tests.Results show that the bending moment of large caisson is notable due to large area and low height in the initial sinking stage.The tensile stress of concrete at the bottom of the inner partition wall reaches 11.4 MPa when the traditional big pot bottom sinking method is adopted,which far exceeds the concrete cracking limit of 2.2 MPa,and the cracking risk is obvious.The tensile stress of concrete is reduced to 2.31 and 1.28 MPa because of the support at the bottom of the middle partition wall when the middle and small pot bottom sinking methods are adopted.The maximum deformations at the bottom of the caisson under three sinking methods are-20.0,-1.6 and-0.25 mm respectively,and the deformations are small.The measured deflection of the top of the caisson and the measured stress of bottom concrete are basically within the range of the control values calculated by the finite element method when the small pot bottom sinking method is adopted.The arrangement of prestressed steel tendons in the middle partition wall and the bottom of the caisson wall can effectively reduce the tensile stress of concrete and improve the anti-crack performance of the caisson structure when the big pot bottom sinking method is adopted.