Experimental Research On Structural Internal Force Of The Ultra-deep And Large Caisson During Construction

With the development of bridge construction in China towards wide waters and offshore areas,as well as the complication of geological conditions and the increase of water depth,the open caisson foundation has been widely used in bridge engineering because of its advantages such as good integrity,strong bearing capacity,and large rigidity.Changes in structural internal force during caisson construction may lead to the destruction of the caisson structure and affect the safety of the caisson.So far,there have been few studies on the structural internal force during construction of ultra-deep large caisson.Therefore,the study of structural internal force of ultra-deep and large open caisson is an important issue during the construction of ultra-deep large caisson.This paper is based on the research and development project of China National Railway Corporation “Research on Key Technologies for Design and Construction of Ultra-deep and Large Section Caisson”.The ultra-large caisson foundation of No.29 main bridge pier of Hutong Yangtze River Bridge is taken as the engineering background and research object,and the internal force of the structure and the sidewall earth pressure during the open caisson construction are analyzed by means of field monitoring test data analysis and numerical simulation analysis.The main conclusions obtained are as follows:(1)The caisson will produce a pressure relaxation region at the foot blade during the caisson sinking.When the caisson is in the upright state,the distribution of lateral earth pressure is like scoop,with the upper part being small,the largest at the step,and the second at the foot blade.(2)In the situation when the caisson is excavated at the bottom like the cauldron,the structure stress of the caisson is similar to the long span beam.On the side wall of open caisson is compressive stress and the middle partition wall is tensile stress,and the tensile stress of the middle partition wall will gradually decrease as the caisson continues to increase.And when the height is reaches 72 meters for the NO.29 caisson,the tensile stress of the middle partition wall will gradually change from the tensioned state to the pressed state.(3)When excavating to form a large pan bottom,as the distance between the section and the cutting edge is further and longer,the position where the tensile stress occurs is further away from the side wall,which is related to the bottom state of the caisson and the transfer of stress.Tensile stress at a section 7 which is 5 meters above the cutting edge appeared at a distance of 15 meters from the side wall.And tensile stress at a section 6 which is 19 m above the cutting edge appeared at a distance of 24 meters from the side wall.According to the numerical simulation,the dangerous position on the cross section can be determined and the monitoring can be strengthened so as to ensure the safety of the caisson during the construction process.(4)The tensile stress of the partition wall in caisson gradually disappears with the vertical distance from the blade.For NO.29 caisson,at 51 meters away from the cutting edge,the tensile stress disappears,and the full section of the caisson is compressive.(5)Sudden sinking can cause data sudden changing of steel strain gauge during the caisson suction sinking.And the change can be used as a basis for early warning.