Study On Freezing Reinforcement And Stability Of Shield Launching In Saturated Soft Soil

With the rapid development of underground engineering,the artificial freezing method has become a commonly used reinforcement technology in the initial project of metro shield tunnels because of its advantages of good water stopping performance,stable support,and flexible form.The origin of shield is one of the most important processes in the shield construction process,and it is also the accident-prone part of the subway tunnel construction process.If the strength of the horizontal freeze reinforcement is insufficient,the water inrush and the sand gushing phenomenon are likely to occur when chiseling the wall of the tunnel door which may cause severe engineering accidents.If the strength of reinforcement is too high,it will not only cause waste but also make it difficult to excavate the shield machine.Therefore,it is very important to study the formation and development of the temperature field and the stability of the soil in the initial freezing freeze of the shield.Based on the above mentioned problems,this paper combined with the freezing engineering of the origin of shield construction of Fuzhou metro line 2,has carried out a systematic study on the mechanism and stability of the artificial freezing and strengthening mechanism of saturated soft soil layer shield by theoretical analysis,numerical simulation and field measurement.The main research results are as follows:(1)The various factors affecting the physical and mechanical properties of the soil were analyzed,and the calculation methods for the thermophysical properties of the unfrozen soil and the frozen soil and the mechanical properties of the frozen soil were summarized.The soil in the frozen area of the Fuzhou subway was given according to the calculation formula and field tests.The thermophysical parameters and mechanical parameters,and the formation and development of frozen soil and freezing temperature field were studied,which laid the foundation for numerical simulation.(2)A freezing temperature field calculation model was established,and some of the measured results were compared with the numerical simulation.It is believed that the numerical simulation can better reflect the real situation of the temperature field,and the development of the temperature field using finite element analysis is reliable.The law of the development of the frozen wall over time is believed to be that the thickness of the horizontally frozen cup wall after active freezing for 30 days can reach 3.7m,the average temperature can reach-17.8℃,and the cup wall thickness can also meet the design requirements.(3)The use of plate strength design theory to push back the frozen wall with a safetyfactor of 3.157.A stability analysis model of the soil at the end of the shield was established.The initial stability coefficient and soil deformation law of the soil at the front end of the freezing were analyzed by the finite element strength reduction method.The stability coefficient of the soil at the front end of the freezing was considered to be 0.758.When the shear strength of the soil is high,a collapsed arch is formed in the upper part of the cavern and local damage occurs.As the shear strength decreases,the plastic damage zone extend to the ground.(4)The FEM analysis of the safety factor of the frozen soil at the rear end is 2.564,which is less than the theoretical calculation value of 3.157.It may be because the plate strength design theory assumes that the lateral water and earth pressure is uniformly distributed load,lead to stress distribution does not accord with the actual,used in practical engineering could be dangerous,and it may cause engineering accidents.(5)Increasing the thickness of the frozen wall cup has a positive effect on reducing the displacement field and stress field of the tip soil.The effect is most obvious when the thickness is 1.5m to 2.5m,and then gradually becomes gradual.The thickness of the frozen wall can meet the strength when the thickness of the cup bottom is 3m.With the deformation requirements,the wall thickness of the frozen wall has little influence on the displacement field and stress field of the formation.To increase the wall thickness of the frozen wall to improve the stability of the end soil is not obvious.(6)Through the information monitoring of frozen temperature field and the field measurement data,the change law of freezing temperature field is revealed in a comprehensive way.The actual measurement and analysis indicates that the development of the frozen wall meets the design requirements and cementation with the continuous wall is in good condition.The numerical simulation agrees well with the field measurement analysis results,can fully and accurately reflect the development of the frozen wall.