## 1 Introduction  
Artificial ground freezing (AGF) is widely used in urban rail transit construction. However, there are many problems in the use of AGF, such as uneven ground settlement, tunnel segment leakage, and so on. Sand clay combination strata are common in China's coastal areas, and the seepage in the sand layer aggravates the uncertainty of the problems above. [PERSON] [1] found that the existence of seepage delays the closure of freezing curtain. [PERSON] et al. [2] obtained that the symmetry of frozen walls decreased, and the formation time of the frozen wall increased under the condition of seepage from model test. [PERSON] et al. [3] also reached a similar conclusion through model test. [PERSON] et al. [4] used COMSOL to simulate the formation process of freezing wall under different seepage velocities and found that the freezing temperature field inhomogeneity increases as the initial stratum seepage velocity increases. The maximum thickness of the downstream frozen wall initially increases and then decreases. However, the average thickness of the frozen wall linearly decreases, and when the seepage velocity is greater than 7 m/d, the frozen wall can hardly be formed. Some scholars put forward the concept of critical seepage velocity; that is, when the seepage velocity is greater than this threshold, the frozen curtain does not close. [PERSON] et al. [5] proposed that when the seepage velocity is greater than 2 m/d, the stratum is difficult to be frozen according to their coupled thermo-hydraulic model. [PERSON] et al. [6] draw the conclusion that the frozen curtain does not close with seepage velocity up to 2.5 m/d by model test and Bakholdin's analytic solution of temperature field. Seepage also affects the formed frozen curtain. In sudden seepage, [PERSON] et al. [7] found that when the seepage velocity was 0-30 m/d, it caused obvious damage to the upper reaches of the frozen curtain, while the wall was strengthened in the middle and lower reaches to a small extent, but when the flow velocity was more than 30 m/d, the frozen curtain was completely damaged. Reasonable arrangement of freezing pipes increases the critical seepage velocity to adapt to more complex situations. After encrypting the freezing pipes in the range of 120 degrees upstream of the outer ring of the double-loop pipes, [PERSON] et al. [8] shortened the freezing time and increased the thickness of frozen curtain. [PERSON] et al. [9] found that when the freezing pipe layout coincides with the seepage direction, reducing the freezing pipe spacing cannot shorten the freezing time. Coolant properties are also important influencing factors. When the cooling medium is brine, the critical seepage velocity is 1-2 m/d [10]. AGF using liquid nitrogen as refrigeration circulating liquid can be applied when the seepage velocity is 20 m/d and 50 m/d [11]. As a non-destructive testing technology, computed tomography (CT) has been widely used in geotechnical field in recent years [12]. The change of micro-structure in different parts of soil can be measured by the statistical characteristics of CT number [13]. [PERSON] studied the ice thickness and spacing of different soil in different freezing time through X-ray test [14]. The ice thickness and spacing in frozen soil increase with the decrease of temperature, and the ice thickness and spacing of different types of soil are different. [PERSON] [15] reflected the damage of rock through CT image and CT number, studied the relationship between strength change and damage expansion of mudstone and water supply and loading and unloading process, and put forward the concept of damage rate. CT scanning can reduce the extraction of pore data by artificial intervention. Based on genetic algorithm, [PERSON] et al. [16] determined the gray-level threshold of CT image segmentation and achieved the recognition of water, ice, and rock, which assured the digital expression of water and ice content. [PERSON] et al.