The Research On The Stability And Bearing Capacity Of Ultra Large Open Caisson Foundation In Deep Silt Soft Soil

In recent years,with the vigorous development of bridge engineering,the foundations of many long-span bridges have reached a depth of more than 100 meters,and the open caisson foundation has become the preferred type of tower foundation and anchorage for river-crossing and sea-crossing bridges because of its good integrity,large buried depth and strong bearing capacity.As a result,caissons with larger size and buried depth have emerged.However,the existing theories and studies are mostly aimed at the foundation of small and medium caissons.At present,the mechanical characteristics and load-carrying performance of large open caissons during the sinking process are still unclear.The caisson foundation of the middle tower of Wenzhou Oujiang North Estuary Bridge is located in the deep soft soil area of the marine plain.The upper part of the site is a silt layer with the thickness of more than 40 meters,which has the characteristics of high water content,high porosity ratio,high compressibility,high sensitivity and thixotropy.Based on the construction process of the caisson foundation,this paper carries out the research on the mechanical characteristics,sinking stability and bearing capacity during the subsidence stage of large caisson in soft ground,through the methods of monitoring data analysis,theoretical formula calculation and numerical simulation.The main work and research results are as follows:1.Due to construction disturbance during sinking,the open caisson is easy to be unstable.Based on field monitoring data,the stress distribution and influencing factors of large open caisson foundation sinking in soft soil are analyzed,and the applicability of theoretical formula and the sinking stability of caisson are discussed through theoretical calculation.The results show that with the gradual deepening of caisson sinking,the influence of suction disturbance decreases,and the lateral earth pressure increases linearly with the depth of the caisson sinking.When calculating the sinking stability of the caisson,directly replacing the supporting force of the soil at the bottom of the caisson with the ultimate bearing capacity of the foundation will result in a small theoretical stability factor of sinking.2.At present,the mechanism of sudden sinking of caisson is not clear.Based on ABAQUS/Explicit dynamic finite element program and the measured data of sinking resistance of the caisson,the sudden and rapid sinking behavior of open caisson in soft foundation is systematically analyzed.The mechanism of the sudden sinking behavior of caisson is determined,which indicates that the essence of sudden sinking behavior in soft foundation is the instantaneous transformation of strain energy to damage dissipation energy caused by abrupt change of soil stress.By comparing the numerical calculation with the measured results,the methods of quantitative evaluation of the mechanical behaviors of soft soil around caisson and the corresponding sudden sinking behavior of caisson under load are proposed.3.In determining the bearing capacity,the methods of current standards are all for shallow foundation,but not applicable to the caisson foundation.A numerical model for calculating the bearing capacity of open caisson foundation under combined loads is established.The influence of vertical load and depth of penetration on the envelope of bearing capacity of soft foundation in H-M loading space is analyzed by using load-constant displacement detection method,and the movement mechanisms of caisson and soil under ultimate state are obtained.The ultimate bearing capacity of caisson foundation under various unidirectional loading modes is determined,and the mathematical expression of the envelope surfaces of bearing capacity of caisson under combined loading mode is proposed,and the corresponding failure envelope surfaces are drawn.Comparing with the envelope of bearing capacity of solid foundation under the condition of homogeneous ground,the overall bearing capacity of large cross-section open caisson sinking in soft soil is evaluated.