## 2 Experimental Program
### Test Content and Scheme
#### 2.2.2 Freezing Test Scheme  
There is no good condition to simulate the tunnel excavation; therefore, only the freezing scheme and control method were taken into account in the test. With the comparison of different freezing modes, the freezing scheme and process optimization of freezing method and the dynamic control method of freezing scheme are studied in the test. The freeze process is a two-phase procedure: active freezing phase and maintained freezing phase. In the active freezing period, using master freezing tubes and enhancing freezing tubes is main method to form frozen soil curtain. According to the arrangements of freezing tubes in the curtain, the test in this paper studied the following four active freezing schemes: single freezing mode of the solid pipe, synergy freezing mode of the solid pipe and hollow pipe, principal freezing of the solid pipe with hollow pipe as complement and single freezing mode of the hollow pipe. The single freezing mode of the solid pipe is to freeze the master freezing tubes in the solid pipe, and to verify the freezing effect under the condition that the master freezing tube works alone. The synergy freezing mode of the solid pipe and hollow pipe is to use master freezing tubes and enhancing freezing tubes at the same time to achieve the maximum freezing strength, so as to ensure the rapid formation of the frozen soil curtain as soon as possible. During the principal freezing of the solid pipe with the hollow pipe as a complement, it is necessary to open the enhancing freezing tubes in a timely fashion to improve the freezing strength with the master freezing tubes open. As a kind of optimized freezing mode, the effect of freezing and time control are worth exploring. Finally, the freezing mode of the hollow pipe is used to examine the freezing effect when the enhancing freezing tubes are opened only in the hollow pipe. In this phase, the brine temperature of the master freezing tubes and enhancing freezing tubes was maintained at \(-24\)\({}^{\circ}\)C.  
After active freezing, the phase of maintaining the freezing phase began. The design concept emphasized the importance to dynamically control the thickness of the frozen soil curtain during excavation. Therefore, maintaining the freezing phase is complex and important. There are two methods to control the freezing area, namely heat control and cold control. The difference between the two methods is controlling the freezing tubes or turning on limiting freezing tubes. The heat control method recycled brine with relatively high (\(-2\)\({}^{\circ}\)C-\(8\)\({}^{\circ}\)C) temperature in limiting freezing tubes and then carried cold away, with the purpose of controlling the development of the curtain is thickness outward from the tunnel. In the heat control phase, the brine temperature of limiting freezing tubes was maintained at \(2\)\({}^{\circ}\)C. The cold control of frozen soil is realized by raising the brine temperature from \(-24\)\({}^{\circ}\)C to \(-20\)\({}^{\circ}\)C in the freezing system or changing the freezing mode of the master freezing tubes to reduce the cold produced by the cold source, which is named the intermittent freezing method. In summary, the realization of heat control is based on providing an external heat source in the freezing area, while that of the cold one depends on the reduction of cold provided in unit time. In addition, the effect of soil improvement and insulation measure is also involved in this test. To set a reasonable combination of active freezing phase and maintaining freezing phase, the whole test was divided into three main modes: non-limiting freezing, heat control limiting freezing and cold control limiting freezing. The controlling freeze schemes in the maintaining freezing phase are taken into consideration. The difference among the three above modes is the measure taken to control the development of the frozen soil. The non-limiting freezing mode allows the infinite expansion of the frozen soil.