Experimental Investigation On Scour Development Around The Pile Foundation Of The Sea-crossing Bridge

In the marine environment,the presence of the bridges will change the flow pattern in its neighborhood and result in local scour.The local scour will decrease the bearing capacity of the bridges' pile foundations and is considered to be one of the main factors affecting the bridges' stability.The topic on the local scour around the bridges' pile foundations is a classic issue in coastal and offshore geotechnichal engineering,which involves both the hydrodynamics and soil mechanics.The mechanism and process of the local scour is complicated,and the corresponding experimental techniques and computational capabilities are also limited.Therefore,the issue on the scour characteristics around pile groups in complex configurations has remained unresolved and needs to be further explored.Therefore,a seires of experiments has been carried out to investigate the local scours around a single pile and pile groups.The following issues are addressed in this thesis:1.A series of flume experiments was conducted to study the scour development around a single pile with different water depths and flow velocities.In these experiments,a camera placed inside the model pile was used to monitor the change of scour depths at the periphery of the pile.It was found that the local scour was firstly appeared at the front lateral side of the pile and then developed around the periphery of the pile.The sediment at the back of the pile was deposited firstly and then washed away.With increasing of the water depth and flow velocity,the duration of the sediment deposition at the back of model pile was decreased,and the local scour around the pile was developed fast and needed a longer period to reach the equilibrium state.At the initial stage,the locations of the maximum and minimum scour depths were appeared at the front lateral side and back side of the pile,respectively.With the development of the scour process,the two locations would be changed subsequently.2.An experimental investigation on flow pattern and force characteristics past nine cylinders in a square configuration was carried out using the Particle Image Velocimetry(PIV)technique and load cell in a water channel at different spacing ratios(L/D,L is the center-to-center distance,D is the cylinder diameter)and Reynolds numbers(Re).The results showed that three distinct flow regimes were categorized with variations of the spacing ratios and Reynolds numbers,namely,shielding flow regime(at LID=1.5 and 2.0),transition flow regime(at L/D=2.5 and L/D=3.0&Re=1500)and vortex shedding flow regime(at L/D=3.0&Re=2500-5000).Moreover,an interesting feature of bistable flow pattern in the lee wake of the model was observed at L/D=1.5,consisting of one narrow wake with a higher vortex shedding frequency behind the central downstream cylinder and one wide wake with a lower vortex shedding frequency behind the lateral side of the centeral downstream cylinder.The analysis of force coefficients on each cylinder were also performed,the maximum values of mean lift coefficients were found to be on the upstream cylinders,and the minimum values were found to be on the downstream cylinders.However,both the minimum values of mean drag coefficients and root-mean-square forces were found to be on the upstream cylinders.3.A series of experiments has been conducted to investigate the characteristics of scour development around the dumbbell-shaped pile group.Cameras were positioned in some specific piles to monitor the scour development around the periphery of each specific pile.Moreover,the final bed topography was obtained by the laser scanner.It was found that in the unidirectional current test,the bed topography was observed to be spoon-shaped at flow velocity of 0.34 m/s.The maximum scour depth was 3.41D(laser scanning)or 3.57D(camera monitoring)located in the central part of the upstream pile group.However,the scour depth in the downstream pile group was relatively small,and a sediment deposition was observed in the lee wake of the whole model.In the tidal tests,the flow velocity and test duration were set to be the same as the unidirectional current test,and the maximum scour depths were about 70%of that in the unidirectional current test.Moreover,two different shallow scour pits were existed in the upstream and downstream pile group,respectively.With increasing of the tidal period,the number of tidal cycles showed a more significant effect on the scour depth,and this phenomenon would increase the disparities between the two scour pits,namely,the scour depth of the downstream scour pit was gradually deeper than that of the upstream scour pit.