| In recent years,China has vigorously developed urban infrastructure,with rapid development in the field of underground space development.While people enjoy the convenience of infrastructure,we cannot ignore the potential safety hazards that exist.In many underground projects,such as subway shield tunnels and underground pipelines,water seepage defects may occur due to uneven overlap between shield tunnel linings or aging of underground pipelines.These defects will cause stress concentration,which will change the originally balanced stress and seepage fields.Since sand has characteristics such as no cohesion,poor stability,and strong permeability,when the groundwater level exceeds the position of the defect,the groundwater as an infinite body will have extremely high water pressure and will carry a large amount of particles to rush out of the defect,causing water and sand leakage phenomenon.As a result,the roadbed loses its soil support,leading to road surface collapse,which poses a huge threat to people’s lives and property safety.However,research on the water and sand leakage caused by double waterproof defects in underground engineering is still weak.The study on the evolution process and seepage mechanism of double-hole water and sand leakage is very limited.Therefore,this paper takes the sand in the rich water sand layer of Nanchang city as the research object,designs a visualization indoor experimental device for double-hole water and sand leakage,and studies the influence of different double-hole aperture,particle grading,and hole spacing on the evolution process of water and sand leakage.At the same time,a suitable numerical simulation model is established using the PFC-CFD coupling method to analyze the micro mechanism in the seepage process.The study found that:(1)Through indoor test equipment,it can be found that the higher the fine particle content and the closer the distance between the double holes,the sand is more prone to liquefaction.Both type 1(with 80% fine particle content)and 5cm type(with hole spacing of 5cm)sands experience liquefaction,with all the sand being lost.The process can be roughly divided into three stages: surface settlement and disturbance stage,water flow penetration stage,and nearly complete loss stage.Other types of sand form soil arches,which can be divided into two stages: surface settlement and disturbance stage,and soil arch formation stage.(2)By analyzing the results of indoor double-hole water and sand leakage tests,it can be concluded that under the condition of double-hole seepage,taking the left hole as an example,the earlier the left hole arches,the smaller the settlement scale,as the left hole diameter decreases from 12 cm to 10 cm,the arching time of the left hole shortens from 60 s to 24 s,the cumulative sand leakage decreases from 1.93 kg to 0.38 kg,and the maximum settlement depth decreases from 13.65 cm to 1.61 cm.As the fine particle content decreases from 60% to 40%,the arching time of the left hole shortens from 60 s to 28 s,the cumulative sand leakage decreases from 1.93 kg to 0.80 kg,and the maximum settlement depth decreases from 13.65 cm to 7.73 cm.As the right hole diameter decreases from 12 cm to 10 cm,the arching time of the left hole shortens from60 s to 50 s,the cumulative sand leakage decreases from 1.93 kg to 1.52 kg,and the maximum settlement depth decreases from 13.65 cm to 9.27 cm.As the distance between holes increases from 10 cm to 15 cm,the arching time of the left hole shortens from 60 s to 34 s,the cumulative sand leakage decreases from 1.93 kg to 0.87 kg,and the maximum settlement depth decreases from 13.65 cm to 7.29 cm.(3)Through the exploration of the indoor dual-hole water and sand leakage test,it can be found that under the condition of dual-hole leakage,the right hole will deprive part of the water flow on the right side of the left hole,thereby affecting the leakage situation of the left hole.This leads to the displacement field of the sand moving towards the outside of the hole,and as the distance between the holes becomes closer,the order of sand loss between the holes changes from top to bottom to bottom to top,and the surface subsidence trough changes from W-shaped to V-shaped.(4)The suitable theoretical model for the surface curve of dual-pore leakage and sand loss was established by extending the Peck curve to a hyperbolic threedimensional model.The final surface curve of dual-pore leakage and sand loss was preliminarily calculated using this model,and the theoretical volume loss rate of the calculated curve was within 6% of the measured value,indicating that the applicability of this theory is strong.(5)The stress-strain curves of soil samples under different confining pressures were obtained through indoor conventional triaxial tests,and a numerical simulation of conventional triaxial tests was established using PFC3 D to calibrate the microscopic parameters of soil samples by comparing them with the stress-strain curves obtained from indoor tests.The PFC-CFD coupling method was used to simulate the doublepore leakage and sand loss disaster test,and the simulation process was consistent with the development of soil settlement deformation in the test process,and the sand loss curve was in good agreement.Therefore,the PFC-CFD coupling method can be used to simulate and solve the problem of double-pore leakage and sand loss effectively.(6)Through PFC-CFD simulation of double-pore leakage and sand loss test,it can be found that for sand soils with sand collapse,the particle velocity field and water flow velocity field exhibit an expanding trend throughout the leakage and sand loss process.For sand soils that form stable arches,the particle velocity field and water flow velocity field expand continuously in the early stage,and then start to decrease after the particle aggregation reaches a certain level.Finally,the particle velocity field returns to 0,and the water flow velocity field remains unchanged.(7)By using the post-processing function of PFC,the microscopic flow field of water and the microscopic force chain of particles can be obtained.From the velocity field,it can be intuitively seen that the larger the diameter of the right pore and the smaller the distance between the pores,the more significant the right pore’s stripping effect on the water flow from the left pore.From the contact force chain,it can be found that the force chain at the stable arch is strong and dense,indicating that the particle formation of the stable mechanical structure of the arch is stable.For sand soils with sand collapse,the force chain at the pore appears to form an arch,but it is destroyed,indicating that the process of forming an arch is a continuous process of formation and destruction. |
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