| Underground rail transit is a strategic element in the development of urban agglomerations in coastal areas of China.Due to the presence of the water-rich and soft soil layer,as well as the underground integrated pipeline corridor and various adjacent structures,the subway project faces great construction difficulties.It is therefore imperative to seek an economic and safe measure for the construction of subway projects.The artificial freezing method utilizes the circulation of refrigerant(e,g.,calcium chloride solution)in the freezing pipe to form frozen walls in the ground with good bearing and sealing performance and no pollutants enter the ground during construction,and the ground is less disturbed after the thawing of frozen walls.It is now popular in subway construction in coastal cities,such as the Tianjin Metro Line 1 and Shanghai Metro Yangpu Line,where this method is applied to some of the connecting passages.However,when a highly permeable stratum with large groundwater flow velocity is involved,serious hazards of safety will arise,e.g.,construction accidents due to freezing failure in the Shenzhen Metro Phase Ⅰ project and Shanghai Metro Line 4.In addition,the freezing pipe fracture followed by brine leakage during the active freezing period likewise deserves attention.Even though no immediate threat to engineering safety is posed,safety hazards such as the thawing of frozen ground and strength loss of frozen soil will arise because of brine leakage.Therefore,the risks from two uncertainties,groundwater seepage and soil salinity,shall be addressed in the design of the artificial freezing project.In this study,the influence of groundwater ion type and content on the freezing point of saline saturated sand was analyzed by the cooling curve method with the typical sand in Tianjin Binhai New Area.The freezing point prediction model for offshore saline sand applicable to multiple solutes was established based on the phase-change theory of salt solution.Soil freezing characteristic curve(SFCC)of offshore saline sand was obtained by using the STM volumetric water content and temperature sensor,and the applicability of 14 common models was evaluated,and a modified model was established by introducing the relationship between salt concentration and freezing point based on the optimal model.The COMSOL Multiphysics was used to simulate the water-heat-salt coupled migration in the freezing construction process of offshore saline sand layer,and to analyze the influence of groundwater pressure head on the development of frozen wall.For the problem of brine leakage after the freezing pipe fracture during the active freezing period,the above method was used to simulate the frozen wall development under different brine leakage conditions.The concept of the area ratio of structural defects of the frozen wall is proposed to further investigate the range of brine leakage and groundwater pressure head on this characteristic index.It is suggested that in addition to temperature monitoring to observe the frozen wall development during the active freezing period,additional unfrozen water monitoring should be provided to reasonably deal with the adverse effects of original salinity or brine leakage.The main conclusions obtained from the above studies are as follows:(1)The freezing point tests on saturated sand samples with different salt types and contents were carried out.The results show that the freezing point of offshore saline sand is significantly influenced by salt species and content,as shown by the depression of freezing point with increasing salt content.The influence of cations on freezing point can be roughly ranked as Mg2+>Ca2+>Na+>K+for a given chloride ion concentration;HCO3-has a significant influence on freezing point for a given sodium ion concentration,while the effects of Cl-and SO42-are weaker.(2)The freezing point of offshore saline sand with single and multiple solutes was calculated based on the Extended UNIQUAC model.By comparing with the measured freezing points,it was found that the predicted results of the model deviated greatly and the osmotic suction measured was greater than that calculated by the model,which was found to be probably related to the osmotic suction and the inability of the model itself to reflect the colloidal double electric layer.Based on the Extended UNIQUAC model,a correction model for the freezing point of offshore saline sand was established by introducing an empirical relationship related to the salt content.It is found that the model can better predict the freezing point of offshore saline sand with multicomponent solutions.(3)From the SFCC tests on offshore saline sand samples,different curve patterns were observed.The unfrozen water content declines sharply when soil temperature was lower than the freezing point,but the decrement continued to decline.Based on the discussion on the residual water content in unsaturated soil mechanics and frozen soil physics,the strongly bound water content is taken as the residual water content of frozen soil.By comparing 14 common empirical models for soil freezing characteristic curve,the Michalowski model shows optimal prediction accuracy with comprehensive and rational parameters.(4)A parametric coupled model based on COMSOL Multiphysics was established for the freezing of offshore sand layer and the applicability of the model was verified by the experimental results of model tests with three freezing pipes.The results show that the numerical simulation of water-heat-salt coupling in this study could describe the development of soil temperature during the freezing process of the model test,which tends to stabilize over time from the initial nonlinear change of water and salt profiles.The numerical simulation and the model test result both show that the freezing time required to reach the steady state of the soil temperature increases with the increase of the seepage velocity.The freezing fronts predicted by the water-heat-salt coupled theoretical model are consistent with the experimental results at different freezing durations.(5)Numerical simulation of the freezing construction process of the connecting passage in the offshore saline sand layers in Tianjin was carried out based on COMSOL Multiphysics finite element software.It was found that a large pressure head difference and salt content exist in this typical sand layer.The temperature field on the upstream side under the influence of pressure head develops slowly;the higher the salt content under the same pressure head difference,the slower the temperature field development,which is consistent with the practical engineering observation.According to the influence of groundwater seepage and brine leakage on the structural defects of frozen wall,the concept of area ratio of structural defects of frozen wall was defined,and the adverse influence of brine leakage range and pressure head on the development of frozen wall was analyzed accordingly.It is suggested that in addition to temperature monitoring,additional unfrozen water content monitoring devices should be installed during freezing construction to detect and deal with engineering problems caused by brine leakage in a timely manner. |
PDF Full Text Request