Study On Underwater Excavation Method And Numerical Simulation Of Deep Foundation Pit

The upper stratum of the riverside city is the cohesive soil with low mechanical strength and poor permeability,and the lower part is composed of sand,gravel and pebbles that are rich in water and water permeability.If traditional dewatering and water stopping methods are used in deep foundation pit projects,it is likely that precipitation or water-proof effect is not reached,or the construction cost is sharply increased.In this kind of stratigraphic environment,The new method of underwater excavation can effectively solve the problems that are difficult to solve in the traditional precipitation and water stop construction.However,due to the scarcity of deep foundation pit projects with underwater excavation,there is a lack of relevant literature and engineering experience,the technical indexes,construction methods,foundation pit deformation characteristics and influencing factors of the new method with underwater excavation are not yet clear,and further research is needed.In this paper,we take the foundation pit of Changde Yuanjiang tunnel as the engineering background,study on underwater excavation method and numerical simulation of deep foundation pit.The main work and results of this article are as follows:(1)The effects of groundwater seepage on foundation pits,the impact of confined water on the inrush of foundation pits,and the research status of deep pit excavation and water stoppage are summed up.The basic theory of seepage and the failure mode and failure mechanism of foundation pits under the action of confined water are introduced.Based on these ones,a new method for underwater excavation in deep foundation pits was introduced.The applicable conditions for the new method were clarified,and the construction steps and technical points of underwater excavation in deep foundation pits were summed up.(2)Using the FLAC3D numerical simulation software for modeling and analysis,the simulation of water in the pit and the simulation of the confined water are solved in detail.Study on the numerical simulation method of the deep foundation pit with underwater excavation,and compared with the traditional methods of precipitation excavation.The results show that under specific geological conditions,the underwater excavation method of deep foundation pit has obvious superiority compared with the traditional method of precipitation excavation,which effectively constrains the settlement of the wall,the uplift of the bottom of the pit and the horizontal displacement of the diaphragm wall.In the background engineering,the new engineering method is particularly significant to the bottom of the pit,and the height of the bottom of the pit with underwater excavation method is 1/8 of the amount of the uplift of the precipitation excavation method.(3)The influence rule of the water level in the pit,the pressure water head,and the buried depth of the retaining structure(the underground continuous wall)on the construction effect of deep foundation pit with underwater excavation method is analyzed.The results show:?The water level in the pit has a constraint on the deformation of the foundation pit,but the selection of the water level in the pit is limited.It should be synthetically determined according to the coordination relationship between the deformation of the foundation pit and the rationality of the construction.? The excessive water head of the confined water has adverse effects on the deformation of foundation pit,but not too low.It is very important to correctly handle the balance between the water level in the pit and the water head of the confined water.?The burial depth of underground diaphragm wall has certain restraining effect on foundation pit uplift,settlement behind wall and horizontal displacement of diaphragm wall.But the depth is too deep the deformation restraint effect is not obvious.?During the underwater excavation of foundation pit,the influence of each factor on the deformation of the foundation pit isn't single.In the actual project,the design parameters of the water level in the pit,the pressure water head and the buried depth of the continuous wall should be combined with the construction method,the deformation of the foundation pit,the construction cost,the construction efficiency and the relations among the factors.