Study On Bearing Capacity Of Open Caisson Foundation In Water Under Local Scouring Effect

Taizhou Yangtze River Highway Bridge is a suspension bridge with two span and three towers. The middle tower foundation is the important part of the structure of multi-span suspension bridge. In the area of deep overburden layer, the deformation of the tower foundation is large under vertical load. There are deep water, thick overburden layer, easy scouring and the ship collision force etc in the caisson foundation of the middle tower. The combined method of theoretical analysis and numerical calculation was used in this paper. Bearing capacity of foundation of tower caisson in water was researched under the influence of local scour. The theoretical basis for the evaluation of carrying capacity of open caisson foundation in water was provided. The main research contents and results are as follows.(1)As the layering summation method of subsidence displacement computation was established to the large caisson foundation, using the Mindlin solution of the half space body subjected to a concentrated load as the fundamental solution to gain the accurate additional stress in foundation. The analytic solutions of additional stress in foundation were given under rectangular uniform load of the basement. The loading area was divided into several regions under the rectangular non-uniform distributed load of the basement, a semi-analytical method was proposed for solving the additional stress of foundation. As a result, the subsidence displacement of the caisson foundation was5.18cm.(2) Using numerical analysis software FLAC3D, the Morh-Coulomb elasto-plastic model was used to soil. The numerical simulation calculation of the caisson foundation of the middle tower was completed considering only the vertical design loads and included in the level of ship collision force. Based on displacement and opposite force of the basement as well as the regularities of distribution of plastic zone of caisson foundation, the influences of bearing properties of open caisson foundation in water were researched under ship collision force. The numerical calculation results(4.5cm only for vertical design loads and6.0cm for both vertical design loads and ship collision force) and field test of foundation subsidence displacement(5.1cm-6.7cm) were compared. The feasibility of numerical simulation method was verified.(3) Based on the flood scour pit pattern obtained by model test in20years and300 years, FLAC3D numerical model was established. The simulation process of soil scour is same as the soil excavation process, and the equivalent node load incremental form was given in the scour boundary due to soil initial stress caused by the release. The effects of local scour on bearing capacity of open caisson foundation in water were investigated by numerical calculation method, which got the foundation subsidence displacement6.14cm for20years flood scour and5.74cm for300years flood scour.(4) In open caisson of the middle tower of Taizhou Bridge, the surcharge analytic method was used without considering the scour and consider the scour in two cases. Based on the results of numerical simulation, combined with the requisition of open caisson foundation deformation control, the deformation of open caisson foundation varies with different surcharge coefficients of vertical load were researched. The surcharge safety was obtained for open caisson foundation. The surcharge coefficient without considering the scour is2.56. The surcharge coefficient considering20years flood scour is2.10, and the surcharge coefficient considering300years flood scour is2.04. And then the effects of local scour on the surcharge safety of open caisson foundation in water were discussed.