| Artificial ground freezing method has been widely used in the construction of deep alluvium vertical shafts in Lianghuai mining area and in the construction of metros in coastal cities.Groundwater in the stratum has always been the decisive factor influencing the construction effect of the freezing method.When the groundwater velocity in the local stratum is large,it will not only extend the closure time of the frozen wall,but also reduce the bearing capacity of it,which seriously endangers the construction safety.Therefore,artificial ground freezing method in high-velocity infiltrated formations faces many theoretical and key technical problems,and corresponding application foundations and engineering technology research are urgently needed to meet engineering design and construction needs.For this reason,this paper takes the "water-heat coupling" problem as the main research line,and adopts a combination of laboratory tests,model tests,numerical calculations,and theoretical analysis.The research focuses on the heat transfer mechanism of freezing pipes,development law of artificial frozen wall,optimal arrangement method of artificial freezing pipes,theoretical solution of steady-state freezing temperature field,and mechanical properties of frozen wall in the strata with high-speed seepage of underground water.A circular sand with a particle size of 1 ± 0.15 mm was used to simulate the permeable strata.The change of the unfrozen water content of the sand sample with temperature was studied by nuclear magnetic resonance technology.Based on the test results,the applicability of the four commonly used mathematical models of unfrozen water content and the test results of saturated sand samples was analyzed,and the influence of the fitting parameters of models on the calculation results was studied.Through comparison,it was found that the Kozlowski model could well reflect the change law of the unfrozen water content of the sand sample.Based on the self-developed large-scale hydrothermal coupled physical model test system,the space-time evolution of freezing temperature fields under different flow conditions was experimentally studied.The test results showed that when the distance between adjacent freezing fronts was reduced to a critical value Lc,a"group-pipe-effect" would be generated,which would accelerate the expansion speed of the freezing fronts toward the middle area and shorten the closure time of the frozen wall.The convective heat transfer effect of water flow would partially offset"group-pipe-effect",so Lc would decrease with the increase of the seepage velocity.When the seepage velocity was less than 3m/d,the Lc was 400mm;and when the flow velocity was 6m/d and 9m/d,the Lc was reduced to 154mm and 130mm,respectively.Considering the ice-water phase transition process,a mathematical model of water-heat coupling was constructed,and the rationality of the mathematical model was verified based on the model test results.Based on the constructed mathematical model,the development laws of single-circle freezing temperature field and double-circle freezing temperature field under different flow conditions were predicted and analyzed by COMSOL Multiphysic numerical calculation software.The calculation results showed that groundwater with a flow velocity of less than 5m/d would not have a significant effect on the temperature field of artificial ground freezing.The layout of the single-circle freezing pipe was suitable for ground freezing with groundwater velocity of less than 10m/d,and the arrangement of the double-circle freezing pipe was suitable for ground freezing with groundwater flow rate less than 20m/d.For single-cycle freezing,after adopting the optimization method of partial encryption or adding auxiliary freezing pipes upstream of the water flow,the maximum flow velocity that the frozen wall could be closed was increased from 11 m/d to 12 m/d.For double-cycle freezing,while keeping the number of freezing pipes unchanged,the spacing of freezing pipes in different areas was optimized and adjusted according to the degree of influence of the water flow on the freezing temperature field.After optimization,the maximum flow velocity that the frozen wall can be closed was increased from 20 m/d to 26 m/d.To accurately describe the distribution law of the temperature field formed by a single freezing pipe under the action of a seepage field,the shape of the freezing front was simplified using a segmentation-equivalent method.The analytical solution of the steady-state temperature field was derived,and the accuracy was verified using a physical model test.Combined with the results of the model test and the calculation results of the analytical solution,the distribution law of the freezing temperature field formed by a single pipe under different seepage velocity was analyzed.It could be found that as the seepage velocity increased,the frozen area formed by a single pipe decreased while the proportion of the low-temperature zone increased and the average temperature of the temperature field decreased.According to the temperature distribution law of the "dangerous section" of the frozen wall under the groundwater with different flow rates,the position of the lowest temperature point was taken as the dividing point,and the temperature field was equivalent to two parabolic lines.According to the linear relationship of elastic modulus,cohesive force and freezing temperature field of frozen soil,the frozen wall was regarded as material whose mechanical parameters changed with a quadratic function of radius.Based on the Mohr-Coulomb criterion,the Drucker-Prager criterion,the generalized Tresca criterion,and Twin Shear Strength criterion,the elastoplastic stress calculation formulas of the frozen wall were derived.Based on the stress calculation formulas,the elastoplastic stress analysis of the artificial frozen wall under the groundwater with different flow rates was performed.Figure[83]table[33]reference[170]... |
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