Abstract: Cofferdams are often used in the construction of deep water piers of bridges. The hydrodynamic load acting on cofferdams is a key factor to control the structural design of cofferdams and to insure the safety and stability of their subsidence process. The existing design specifications consider the hydrodynamic forces on cofferdams roughly to some degree. In order to understand the characteristics of hydrodynamic load during the construction of a special shaped steel cofferdam with high-low blade for bridge pier, the flow field around cofferdam is simulated by computational fluid dynamics (CFD) method. A threedimensional fluid domain is constructed based on the cross-sectional terrain data of the river channel. The subsidence process of steel cofferdam is simulated by overlapping grid technique efficiently. Results show that there is a partial acceleration of flow on both sides of the cofferdam, and the distribution of this acceleration effect is asymmetrical. The flow velocity of the deep water side of the cofferdam is relatively greater, and below the cofferdam the flow velocity of the downstream side decreases gradually with the increasing river depth. The external surface pressure on cofferdams are mostly negative, compared to the positive pressure at the upstream face. With the increase in subsidence depth, the negative pressure in each area increases gradually. In the initial stage, the water flow is influenced by the cofferdam and the protection tube. As the cofferdam gradually sinks to the riverbed, the influence of the protection tube gradually disappeared. Along with the increasing subsidence depth, the turbulence intensity in each region of the flow field shows an overall trend of increasing at first and then decreasing. Finally the maximum turbulence intensity area stabilizes at the wake of the cofferdam. The coefficients of drag and lateral force do not change with the incoming flow speed, which depends on its dimensionless property. The drag and lateral force increase with the increasing of incoming flow velocity, as well as the increasing underwater penetration. However, under the influence of the three-dimensional flow effect, the lateral force shows a local minimum when the sunk depth is about 8 m.

Key words: abnormal shape steel cofferdam, numerical simulation, drag force, lateral force, overlapping grid