Study On Stratum Deformation Characteristics Of Large Underground Space Excavation Under Different Unloading Levels

With the increasing emphasis on the development and construction of ground space and the development and utilization of underground space in recent years,the importance of foundation pit engineering as an important part of underground space development is self-evident.Foundation pit engineering has a strong space-time effect in the construction process,and its construction safety issues need more attention.With the increase of excavation depth,the unloading level of foundation pit soil is also changing.Therefore,based on the foundation pit project of Xiong ’an intercity station hub,this paper mainly uses numerical simulation analysis method to study and analyze the deformation characteristics of large underground space excavation stratum under different unloading levels,and considers the influence of time and space effect on the deformation of foundation pit in the process of foundation pit excavation,and obtains the research conclusion with practical value for practical engineering.The specific research contents and conclusions are as follows :(1)Through the numerical modeling and analysis of the foundation pit from zone Ⅰ to zone Ⅴ,it is found that the deformation distribution curve of foundation pit excavation under different unloading levels has obvious spatial effect.The closer to the position of the pit angle,the smaller the deformation;the distribution characteristic curve of surface settlement deformation is ’ flute type ’,and the deformation distribution characteristic curve of retaining pile is mostly ’ parabolic type ’.The distribution characteristic curve of bottom heave deformation is obviously affected by the earth pressure on both sides of the foundation pit and the excavation width of the foundation pit.(2)Taking the foundation pit in zone Ⅰ as an example,the excavation model of foundation pit considering time effect is established,and the creep constitutive model and consolidation theory are introduced.It is found that the creep amount of foundation pit increases with time,and the creep rate decreases gradually.The creep deformation occurs rapidly within 30 days after construction.The soil creep after construction has the greatest influence on the horizontal displacement deformation of the retaining pile,and has little influence on the surrounding surface settlement and the uplift deformation of the pit bottom.In the calculation of post-construction consolidation,the deformation of foundation pit increases with the increase of consolidation time,but the consolidation deformation rate decreases gradually.9 days before post-construction consolidation,the deformation of foundation pit is the fastest,and 187.5 days after construction,the consolidation deformation of foundation pit tends to be stable.The excess pore water pressure at the bottom of the foundation pit gradually dissipates with time.At 185 days,the dissipation is basically completed.(3)Taking the foundation pit in zone Ⅳ as an example,a partition stepped excavation model is established.The results show that the partition step excavation has obvious advantages in controlling the deformation of foundation pit compared with the integral excavation.When the step-by-step excavation is adopted,the closer to the corner of the pit,the more effectively the deformation of the retaining pile can be reduced.The advantage of partition step excavation is obvious in reducing the vertical uplift deformation of foundation pit bottom.The research results can provide reference for practical construction.(4)Taking the foundation pit in zone Ⅳ and zone Ⅴ as an example,the influence of different geological conditions on the deformation characteristics of foundation pit is compared and analyzed.It is found that when the geological conditions change,the deformation of foundation pit will be different,but the deformation distribution characteristic curve of foundation pit will not change.