Experimental Investigation Of Scour Around Vertical Piles In Steady Currents

Vertical pile is an important component in offshore engineering.Local scour is one of the main hazards to the stability of this kind of structures built on erodible seabed.Hence,scour around vertical circular cylinders in steady currents was studied experimentally.This paper is organized as follows.In chapter 2,local scour around a single vertical pile in steady currents was studied.The effects of Reynolds number(Re = UD / ?)on the local scour around a single vertical pile are addressed.In the present study,the change in the Reynolds number is achieved by varying the diameter of the vertical pile while the incoming flow velocity remains constant.For each test with different Reynolds number,the propagating rate of local scour around the vertical pile was detected using Contact Image Sensor(CIS).The equilibrium profile was monitored by a threedimensional surface profiling camera system.An improved empirical formula was proposed to predict the time scale of local scour based on the measurements of the CIS.Based on new empirical formula,we can we can calculate the scour depth at any time.Then the relationship between the equilibrium depth S and the Reynolds number were identified.The experimental results suggest that the variation of the scour depth satisfy a law of power function.In order to ensure the on-bottom stability of a single pile,composite structure is generally used in the engineering applications to prevent the local scour around the single pile.In chapter 3,we focused on local scour protection considering a structure composed of a pile and spread foundation.The experiment adopts the method of spreading foundation around the pile to achieve scour protection.The spread foundations in experiment consist of two types.One is the deep buried spread foundation,mainly investigated the influence of the diameter and buried depth of the spread foundations on scour protection.Another is the shallow buried spread foundation,mainly investigated the influence of the diameter and height of the spread foundations on scour protection.The test results show that increasing the diameter of the spread foundation is the most direct and effective method for scour protection.The principle of scour protection is to weaken the scouring force of downward flow to the sand bed by top surface of the spread foundation.The upstream part of spread foundation can protect sand bed from scouring by the horseshoe,inducing the depth of scour pit in the upstream of cylinder.However,this method has the disadvantages of wasting space,high cost and difficult to construct.Therefore,the methods of burying the foundation and controlling the height of the foundation can be considered.Combined these two ways with the method of increasing the diameter of the foundation,we can find the most suitable method of scour protection in specific engineering environment.The conclusions of this experiment can provide suggestions for practical engineering.In chapter 4,scour around a group of piles in steady currents was studied.The influence of porosity p on the scour of group piles was investigated.The porosity p reflects the density of the pile groups.In the experiment,each test model consists of a number of unit piles(for experimental set-up details,please see 4.1.2).Various porosities p are obtained by changing the number of unit piles with diameter d = 2.5cm.The outer diameter(D)of the tested groups of piles keeps on a constant 15 cm.In this paper,we focus on the characteristics of scour with different porosity p.The test results show that the local scour around each unit pile is obvious when p > 0.75.While,the global scour around pile group is obvious when p < 0.75.Hence,when the porosity p is big enough,scour around pile group can be treated as a single pile with diameter D.The effects of porosity p on scour depth S and the radius of scour hole l were studied.With the decrease of porosity p,scour depth and the radius of scour hole all have the upward trends.