Influence Of The Dilatancy Of The Dense Sand On The Thrust Force Of The Shield

With the development of urbanization,shields are gradually becoming the most efficient method for subway construction.When the shield advances,especially in the dense sand formations,it is easy to see the phenomenon of “armpits” due to neglecting the shield shell.This is because the dilatancy force generated by shearing the sand will affect the shield thrust.However,the existing experiments and theories on the microscopic analysis of sand dilatancy are insufficient,especially with little related research.In this paper,through theoretical derivation,experimental research and numerical simulation,systematic research is conducted.Firstly,the dilatancy model is proposed on the microscopic level and the influencing factors of the dilatancy of the particles are analyzed.Then,a series of experiments of reshaping the dense sand with different gradations are performed.The verification was carried out and the particle flow software PFC was used for verification.From the microscopic point of view,the influence of dilatant deformation on shield thrust was explored and the following conclusions were obtained:1)Based on the four-particle pore cell clusters,a particle radius coefficient m was introduced for analysis.When the particle radius changes at the same time,the change of m obtained has no effect on the principal stress ratio and the void ratio,ie,the radius coefficient m has no effect on dilatancy.When the particle size is different,the radius changes,and the limit state of the principal stress ratio of the particles changes.The pore ratio of the particles first increases and then decreases with the increase of the particle radius coefficient.The larger the particle radius gap,the smaller the particle porosity ratio.The greater the dilatancy effect.2)Through a large number of triaxial consolidation drainage experiments with different particle-graded sandy soils and direct shear consolidation fast shear experiments,the dilatancy of sand is enhanced as the particle size distribution increases.In the gravel sand,the shear strength of the soil is the highest,and the dilatancy is also the strongest.The shear strength of the fine sand is the lowest,and the dilatancy is the weakest.3)Through the tri-axial and direct shear PFC numerical simulations of different granular grading sands,the relationship between the mesoscopic parameters of the particulate stress chain and particle displacement in the PFC and the dilatancy deformation is analyzed.In the three-axis and direct shear numerical simulations,it can be seen that with the widening of the particle size distribution,the particle stress chains will be strengthened,which is reflected in the fact that the shear strength of sand increases with the increase of particle size distribution.The increase in particle displacement is reflected in the fact that the dilatancy of the sand increases with the widening of the particle size distribution.4)Through the triaxial consolidation drainage test in the laboratory,the size of the dilatancy force of the standard size sample soil is obtained,and the basic factors affecting the dilatancy force are analyzed.The composition of the thrust of the shield is analyzed,and the shearing force will change the frictional resistance between the soil and the shield shell.At the same time,the range of the shear disturbance zone during the sand shield advancement is corrected,and the factors influencing its scope are analyzed.Through the range of shear dilatancy force and shear disturbance area,a shield thrust formula considering the effect of dilatancy force was established.