Research On Sinking Resistance And Drag Reduction Technology Of Double-wall Steel Cofferdam In Seditious Soil Bed

Depend on its excellent structural performance,double-wall steel cofferdam has been widely used in the construction of river and sea crossing bridges.At present,the research on double-wall steel cofferdam is mainly about structural performance analysis,while the research on sinking resistance mostly focused on the caisson foundation.Therefore,it is necessary to study the sinking resistance of double-wall steel cofferdam.This paper studied the numerical value and distribution of the sinking resistance of the double-wall steel cofferdam under multiple working conditions through a variety of research methods,and makes a parametric analysis on the ultimate soil resistance beneath cutting curb,then discusses the drag reduction measures applicable to the 72# pier double-wall steel cofferdam.The main works and results are as follows:(1)Throughing geological drilling and field sampling,the actual geological data were obtained.The standard method was used to calculate the ultimate tip resistance and sidewall friction of the cofferdam under typical working conditions.The results showed that the sinking resistance is largest when the cutting curb of the cofferdam is embedded into the silty soil,hard plastic clay and silty clay,and the proportion of sidewall friction increases with the increase of sinking depth.(2)Abaqus was used to establish a three-dimensional numerical analysis model to determine the distribution characteristics of the tip resistance and sidewall friction of steel cofferdam.The results show that the unit tip resistance and sidewall friction of cutting curb increase with the process of digging and sucking mud.and the sidewall friction presents a nonlinear distribution of "located at static earth pressure and active earth pressure,small at the top and large at the bottom,sudden change at the soil boundary and decrease near the cutting curb".(3)With the help of the finite element limit analysis software Optum G2,the influence of multiple parameters on the ultimate soil resistance beneath cutting curb and the failure mode of foundation soil was explored.The results show that the ultimate soil resistance beneath cutting curb and the failure mode of foundation soil are significantly correlated with the friction angle of contact surface,and positively correlated with the buried depth of external soil,the height of soil beneath cutting curb,the tread width of cutting curb and the internal friction angle of soil.The influence law of multiple parameters provides a theoretical basis for drag reduction of cofferdam,which is helpful for rational selection of drag reduction measures in the process of sinking.(4)Based on the existing measures of internal excavation and mud suction of 72# pier double-wall steel cofferdam,combined with the results of the previous study and the practical problems in the field,a new type of drag reduction equipment was developed and the targeted measures of "adding sand bags at the top of cofferdam,mechanical inserts outside the cofferdam,mechanical inserting and water jet dredging with reaming blade inside the cofferdam" are put forward.The field subsidence monitoring confirmed the feasibility of drag reduction measures.