# Influence of Different Freezing Modes on the Base Slab Displacement of an Upper Structure
## 2 Similarity Simulation Experiment
### Construction Scheme  
In a construction process by using AGF, the whole experimental stage was separated into three phases. The first phase is referred to as active freezing, which extracted heat from ground and a frozen wall was created with thickness and temperature set forth in the project. Frost heaving mainly occurs in this phase. The second phase is maintenance freezing, which adjusted brine flow to prevent further development of the frozen volume and avoid deterioration of strength during excavation. The third phase is related to tunnel excavation. To reduce the frost heave of ground, some measures have been proposed in a number of studies, e.g., by using different freezing modes, controlling the freezing time, and lay-outing pressure release holes [54; 55]. After weighing the feasibility against complexity of actual construction, two schemes were proposed with two different freezing modes. One is the simultaneous freezing mode, where freezing pipes were drilled from W\({}_{1}\) to W\({}_{4}\). The upline and downline were frozen by chilled brine simultaneously. The other is the sequential freezing mode, in which Sections I and II of the upline and downline were frozen at different times, respectively. To optimize the procedure between freezing and excavation processes, the interval time between two construction sections was set to be 30 days in the project (i.e., 70 min in experiments). The upline Sections I and II started freezing at 0 and 30 days, respectively, and the downline Sections I and II started freezing at 60 and 90 days, as shown in Figures 2b and 4. Excavation of each section was performed 45 days after the start of freezing to ensure a 2 m thickness of frozen soil. Based upon the time similarity ratios, these time points were converted and then applied in experiments. Six displacement sensors, located on the base slab, were used to measure heave and settlement of the station. Six temperature sensors were also embedded on the horizontal plane of the tunnel centerline with 80 mm (i.e., 2 m in the project) away from the outer excavation diameter. Data were collected by an automatic data collector, and the measurement interval was set to be 30 seconds. In terms of the two different schemes, three groups of experiments were carried out under an overlying force of 800 kN. The first group adopted a simultaneous freezing mode, where Sections I and II were simultaneously frozen in 104 min (i.e., 45 days in the project), and then the frost heave of base slab was measured. The freezing mode of the second group was sequential, which can be divided into four sections according to construction scheme of the engineering project. These four sections were frozen in sequence. Figure 4: Schematic diagram of the arrangement of sensors and freezing pipes in freezing experiments. A1\(-\)C2 are displacement sensors and T1\(-\)T6 are temperature sensors. Blue and red lines indicate the pipes with \(-\)28 \({}^{\circ}\)C chilled brine and return brine, respectively. When an active freezing process of each section was completed, the freezing mode was transferred to maintenance freezing without excavation. The experiment time was 312 min, which is equivalent to 135 days in the project. The freezing sequence of the third group is the same as the second one, but an excavation process was added. When an active freezing process was completed, excavation of the tunnel sections was conducted with a maintenance freezing mode. The simulation of excavation was done by a core drilling machine. Compared with the second group, the time increased by 15 days due to excavation of downline Section II. The testing time was 347 min, which is equivalent to 150 days in the project. To simulate the two freezing modes in experiments, two holes on each side of the soil chamber were reserved in advance. Two allocation tanks were used to provide the chilled brine for freezing pipes of each side, as illustrated in Figure 4. Then, brine was collected by a collection tank and returned to a refrigerator.