Problems Of Design And Risk Management Of Construction Of Bottomless Double Wall Steel Cofferdam In Deepwater Bare Rock Bed Conditions

The Three Gorges Reservoir area in the upper reaches of the Yangtze River is about175 meters high water state,and last for a long time in winter and spring with the success of the Three Gorges Reservoir,which greatly shorten the construction time at low water level.The water level dropped to flood level about 145 m in order to meet the needs of flood control in flood season in the summer,which make the water level in a year changed greatly.The steep terrain and the huge gap in mountainous rivers cause the fast flowing,thin sediment cover and even rock is exposed.Some of the bed rock joints and fissures are developed.The construction of deepwater foundation used the double wall steel cofferdam in the upper reaches of the Yangtze River face the prevention of cofferdam inflow of water,mud and cofferdam structure of anti floating problem.This article mainly aims at the the bottomless double wall steel cofferdam which is used in the exposed rock conditions to study.The main contents are as follows:1 Collect and analyze the relevant provisions of the design and construction of the temporary structure.Put forward the calculation content of allowable stress method and limit state method in the design of temporary structure.Then refine the design of double wall steel cofferdam.The following problems are found in the design and calculation of double wall steel cofferdam under the conditions of deepwater and bare rock bed:1)When the allowable stress method is adopted to design the steel cofferdam,the reasonable value of the safety factor K needs to be further determined.2)When the limit state method is used for the design and calculation of steel cofferdam,the partial coefficient and the combination coefficient of the function still need to be further studied.3)For environmental loads,wind loads and wave forces have been designed to give a calculation method for different return periods,but the calculation method of the water flow is not applicable to the different return periods.The influence of transverse flow force on steel cofferdam is larger,but the research is less,especially the problem of water flow force in the construction of cross sea bridge needs further research.4)The anti-floating design is an important content of design and calculation of double wall steel cofferdam under the conditions of deepwater and bare rock bed.Theanti floating capacity of steel cofferdam can be welded by welding steel bars at the periphery of the tough foot and the seepage pressure of the water pressure of the rock fissure water or the base of the river bed has a great influence on the buoyancy of steel cofferdam.5)The allowable stress force method is applied to calculate deep-water bare rock bed under the condition of no back cover double wall steel cofferdam of the local strength,stability and foundation bearing capacity.Using the limit state method to calculate the overall stability and the anti-floating stability of the steel cofferdam.2 Based on engineering examples,the local strength and stability of double wall steel cofferdam under combined load and working condition are checked by the allowable stress method.The integral stability of double wall steel cofferdam is checked by the method of probability limit state,including the anti overturning,anti sliding and anti floating stability of high water level.Calculation results show that: It is very necessary to test and calculate osmotic pressure which is between cofferdam foundation trench and the trench concrete when checking calculation of anti floating stability of steel cofferdam.It can greatly reduce the weight,reduce the construction cost of investment.The bonding force between the steel cofferdam and foundation trench concrete and foundation trench concrete and bedrock have a great impact on anti floating and anti slip of cofferdam.3 Using fuzzy comprehensive evaluation method to identificate and evaluate the risk of the construction of no bottom double wall steel cofferdam under the condition of deep water rock bed.Find the key risk factors affecting the construction of double wall steel cofferdam.The results show that the overall risk of the back cover of double wall steel cofferdam construction is at a low level.Water gushing in the cofferdam is the biggest factors in the construction,which is mainly affected by clearing foundation trench clean degree and the sealing effect of the curtain grouting.4 Give detailed sealing measures of the the main risk factors of water gushing.According to the key risks in design and installation of bottomless double wall steel cofferdam,give attention to matters to ensure the safety of the construction and to avoid the occurrence of safety accidents.