| In deep alluvium freezing sinking project, in the formation of multi-ring frozen wall, frostheaving force appears to accumulate. It has an important impact on the safety of frozen pipes.Frozen wall is a non-homogeneous structure, the increase of the stress caused by the frostheaving is not uniform. In the wellbore tunneling process, there is a big difference in theunloading process of the frozen wall of each region. In the non-uniform loading and unloadingprocess, changes in stress and displacement fields beneath the excavation surface, impact onfrost heaving stress field of ground pressure, the outer tube radius, excavation radius, systematicstudy of these issues to reveal the frozen pipe fracture mechanism is very important.The variations of displacement field, stress field before and after unloading were analyzedby numerical simulation and model tests.The second part of the study is the frozen wall of double ring frozen pipes, throughsequential coupling method to calculate the stress and displacement fields of permafrost,calculate the displacement field, stress field after unloading, carry out study of the impact of soilpressure, the outer tube radius, the excavation radius on stress and displacement fields.The thirdpart of the contents is the model test, clay sample experienced high pressure consolidation-loadfreeze-axis unloading, explore the variation of the stress and displacement fields beneath theexcavation face.The main conclusions as follows:The numerical simulation results show that inner and outer tubes were overlap first, thenfrozen wall between inner and outer rings tube overlap. In the early freeze, shrinkage effect isdominant role, the dominant role is the frost heaving in the late. Frost heaving stress are negative,which means pressure. Stress values were first reduced and then increased until stable.Afterexperiencing unloading, inner and outer tube position appears stress concentration; vertical stressdistribution showing a distinct culture. At different stages of unloading, from the heart to theoutside of outer tube region, changes of radial stress, tangential stress is large. Vertical stress isthe pressure after unloading, the vertical stress value of non-unloading area is initial pressure.After unloading the various parts of the excavation face stress increases with the increase ofinitial pressure. Ground pressure has greater impact on center, inner tube, outer tube area, groundpressure then impact on the region between the inner and outer rings tubes, the area outside ofthe outer tube. Changing the radius of the outer tube rings, have an impact on the distribution ofunloading stress field, the stress extreme of the outer tube position is changed. Changeexcavation radius, have a significant impact on soil stress unloading part, the impact of stress onthe rest of soil is very small. Analysis of two test results, inner and outer tubes were overlap first, frozen wall betweeninner and outer rings tube overlap later. In the early freeze, shrinkage effect is dominant role, thedominant role is the frost heaving in the late. The final stress is greater than the initialconsolidation pressure.In successive unloading process, the stress decreases rapidly with time.After the completion of each unloading level, axial restraint remains unchanged. Whenunloading is completed, there occurs the phenomenon of stress recovery, the stress increases withtime. For the two model tests of unloading diameters were150mm,100mm respectively, theunloading stress of former were less than the stress of latter. With the increase of the unloadingradius in the center, the volume of the upward displacement soil increases. The verticaldisplacement of outside the center becomes larger, the larger of the degree of restraint weakened,leading to soil stress decreased. |
