Study On Mechanical Characteristics And Sudden Sinking Mechanism Of Large Caisson Foundation During The Whole Sinking Process

With the continuous development of long-span bridges,the open caisson foundation is also increasing.During the sinking construction of large caissons,the large plane size and deep height,the main structure of caissons are self-important,the foundation soil covered by caissons is not uniform,so it is difficult to adjust the state of caissons in the construction process.During the sinking process,the safety of caissons structure and settlement control are faced with significant difficulties.Once the control is careless.It is easy to cause caisson cracking and caisson geometric posture exceeding the standard,which will cause irreversible economic losses.In this thesis,based on the "China Railway Corporation Science and technology research and development plan major topic" large caisson foundation construction process stress and deformation characteristics research ",Wufengshan Yangtze River Bridge north anchorage caisson foundation as the research object.It studies the mechanical characteristics of the whole process of caisson sinking from numerical simulation,studies the friction characteristics of the foundation of caisson combining with the field measured data,and studies the mechanism of sluggishness and sudden sinking in the middle and later stages of caisson sinking.The theoretical basis and control measures for safe and smooth subsidence construction of wells is putting forward.The main research contents and achievements are as follows:(1)The FEM of the caisson foundation is established by Ansys.The different excavation methods are simulated.The main methods are full compartment excavation,uniform excavation of angular point support,excavation in four districts,excavation in eight districts,reserved middle soil excavation.The results show that in the initial sinking stage,the larger the support area at the bottom of the caisson,the more favorable the force of the structure.The traditional full compartment excavation will result in the risk of structural cracking.The second and third sinking can adopt a larger excavation area to reduce the resistance at the bottom of the caisson to improve the sinking speed.(2)The theoretical formulas for calculating static and dynamic friction coefficients are put forward based on the theoretical study of the force composition of end resistance,deep friction of cutting edge and side wall friction during sinking of caisson foundation.(3)The side wall friction accounts for more than 70% of the total resistance in the middle and later stages of the sinking,and is the main factor of the stagnant settlement.(4)Combining the field measured data with the theoretical calculation results,the resistance characteristics and the mechanism of sudden settlement in the middle and late stages of subsidence are studied.Based on the analysis of four different sudden sinking positions during the sinking,it is concluded that the dynamic friction coefficient is 0.77,0.62,0.56 and0.57 times of the static friction coefficient at the same depth,and the unbalanced sinking forces of 294.817 MN,494.132 MN,585.754 MN and 626.754 MN are generated respectively.The unbalanced sinking force is the key factor to cause sudden sinking.(5)After sinking to 45 m,the sinking coefficient can be increased by about 24% by air curtain,and by 1.10 ～ 1.13 by composite measures in the later stage,which is about 47%higher than that in the absence of sinking measures,and by about 28% higher than that of single measures.The sinking measures can effectively improve the sinking coefficient in the later period of sinking,reduce the risk of sinking legs,and reduce the risk of sudden sinking.