| Due to the fast construction speed and good economic benefits,steel sheet pile cofferdam is widely used as a temporary retaining structure in the construction of bridge foundation caps under water.Because the construction of underwater cofferdam is greatly influenced by geological conditions,water flow and water depth,the complexity of construction environment puts forward higher requirements for the construction technology of cofferdam.At present,there are many domestic and foreign researches on the application of steel sheet piles to deep-water foundations or foundation construction with better geological conditions,while there are few research and available technical achievements on the construction technology of steel sheet pile cofferdam under ultra-thick sandy pebble layer.In this paper,taking the steel sheet pile cofferdam of No.12 pier of the main bridge of Ningxia Zhenluo Yellow River Bridge in bid A7 from Wuhai to Maqin as an example,combined with the actual situation of the project,this paper studies the design and construction of steel sheet pile cofferdam under the geology of ultra-thick sandy pebble bed in the Yellow River.The main research work and achievements are as follows:(1)On the premise of meeting the anti-floating stability and strength control conditions,this paper puts forward the calculation formula of the minimum safe thickness of bottom sealing concrete used in cofferdam construction in different seasons,which solves the problems of material waste and increased construction period caused by cofferdam construction in different seasons.(2)Combined with the whole construction process of steel sheet pile cofferdam,the key construction steps,such as installing internal supports,pumping excavation and pouring bottom concrete,are selected as the construction conditions,and the minimum depth of steel sheet pile in each condition is obtained based on the equivalent beam method.The deficiency of the formula for calculating the local scour depth of cofferdams is pointed out,and the correction ideas and formulas for improving the calculation of the local scour depth of cofferdams are put forward.The strength and stiffness of steel sheet pile and internal support structure are checked,and the optimal design scheme of internal support is put forward.The actual monitoring results show that the designed cofferdam structure meets the above requirements.(3)In view of the geological conditions of ultra-thick sandy cobble in which this project is located,the method of selecting m value of scale coefficient of horizontal reaction coefficient is revised by integrating existing research theories,and the method of selecting m value under geological conditions of ultra-thick sandy cobble layer is put forward.Based on the finite element software Midas civil,the soil is simplified into a series of soil springs related to m value to simulate the embedding effect of soil on cofferdam,and the finite element model of steel sheet pile cofferdam in the whole construction process is constructed,and the stress characteristics and deformation of steel sheet pile cofferdam in the whole construction process are obtained.(4)Pointed out the difficulties of steel sheet pile driving under the geological conditions of ultra-thick sandy pebble bed,found the limitations of traditional steel sheet pile driving construction methods under special geological conditions,and refined a set of overall construction scheme of steel sheet pile cofferdam under the geological conditions of ultra-thick sandy pebble bed.According to different difficulties,the corresponding solutions are given,and the construction scheme suitable for geological conditions is put forward through the on-site steel sheet pile insertion test,and the key technology of underwater blasting construction of steel sheet pile is put forward for the first time,so as to ensure the smooth construction of steel sheet pile cofferdam. |
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