The Research On Subsidence Behavior And Stability Analysis Of Ultra Large Open Caisson In Deep Soft Soil Area

Due to its large section size and buried depth,strong stiffness and integrity,and the ability to bear large horizontal and vertical loads,ultra large open caisson has gradually become one of the most common foundation for long-span bridges.At present,there are few researches on the mechanical characteristics and the subsidence stability of the ultra large open caisson in soft soil area.In this paper,based on the first engineering practice of ultra large caisson foundation located in the deep silt layer of the strong tidal estuary and affected by typhoon in China,the researches on the resistance characteristics,the sudden sinking mechanism with different buried depth and the subsidence stability while initial positioning or final settlement via the methods of monitoring data analysis,theoretical formula calculation and numerical simulation have carried out:(1)On the basis of the field monitoring data,the resistance characteristics of the open caisson during subsiding and the applicability of the classical soil pressure theory has been analyzed and discussed in this paper.The results show that with the increase of the embedded depth,the base resistance increases in an oscillating way,while the lateral wall soil pressure increases in an approximately linear way.The earth pressure on caisson wall is roughly consistent with the static earth pressure,and the earth pressure near the blade feet is more inclined to the passive earth pressure.(2)On the basis of the change rule of the sinking resistance and the sinking coefficient during sudden sinking obtained via the calculation of monitoring data,the mechanical analysis is carried out respectively for the sudden sinking behavior with shallow or deep depth.The results show that the possibility of sudden sinking occurs with shallow depth is greater than deep depth.And their induced factors are different.The sudden sinking is caused by the uneven increase of the caisson's dead weight in the case of shallow depth,while it is caused by decrease of vertical resistance resulted from the disturbance of soil mass and the suction of mud in the case of deep depth.(3)Aimed at the problem of subsidence stability of the open caisson foundation,three-dimensional finite element numerical calculation method is adopted.By analyzing the changes of structural variables and safety factors of the open caisson,the anti-typhoon stability with different technical measures are evaluated.The results show that the appropriate thickness of gravel layer and riprap protective layer can effectively improve the stability of the open caisson,but excessive thickness will affect the bearing capacity of the earth and leading to instability.In this paper,the mechanism and rule of piping phenomenon affecting the settlement stability of the open caisson after final settlement in place has been revealed via the method of soil strength reduction.And the influence of different drainage conditions and pot bottom shapes on the piping phenomenon and the settlement stability has been evaluated via the simulation of the rebound amount of the basic soil.The results show that the complete drainage and the large pot bottom excavation are the most unfavorable excavation conditions which affect the settlement stability.