PSI - Issue 17

O. Plekhov et al. / Procedia Structural Integrity 17 (2019) 602–609 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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phase transition front through water-saturated clays (Taber, 1930). Apart from this study, in 1920 Bouyoucos (Bouyoucos, 1920) published the paper, in which he showed that unfrozen water plays a key role in the formation of frozen soils. In 1924, the research on this topic was continued by Fisher (1924), who held an in-depth discussion of the problem of water freezing in capillary systems. In 1935, Schofield (Schofield, 1935) developed a method for calculating a decrease in the freezing temperature, taking into account the effect of negative pore water pressure. A large number of subsequent works have been devoted to the study of moisture transfer in freezing soils (Danielian, 1983; Feldman, 1988; Nakano, 1992; Black; 1995; Bittelli 2004; Konrad, 2005) and many other authors. In the Russia, theoretical and experimental studies in this field were systematized in the works of Ershov (1979, 2004), Grechischev (Grechischev, 1980). Modern studies show that the equilibrium content of unfrozen water depends on temperature and has a nonlinear character (for example, Istomin et al., 2009; Grigoriev, 2013; Bronfenbrener, 2013). Today, it is common knowledge that freezing of water-saturated soil with sufficient water supply is associated with a continuous water migration in the soil to the frozen zone, which increases the ice content in the frozen zone. The development of the first model for calculating the coupled water and heat flows for unsaturated frozen soils is generally credited to Harlan (Harlan, 1973). Later, the Harlan model was improved and extended in the works of Taylor (1978). The relevance of studying the process of formation of cryogenic flows is associated with both the necessity of analyzing the range of applicability of the two-phase models of phase transitions in porous media and the importance of predicting the factors that are responsible for abnormal operating modes of the monitoring equipment during the formation of ice walls. One of the traditional methods of monitoring the moment of closure of ice walls of the AGF system is the hydro observation wells. The artificial ground freezing (AGF) system is a set of freeze wells located circumferentially around the shaft construction site. The image of one of the existing AGF is given in Figure 1a. In the case of the Petrikov mine (Belorussia), the depth of the wells reaches 250 meters, the coolant temperature is - 27ºС. The arrangement of freeze and hydro-observation wells is shown in Figure 1b. The block-diagram of hydro-observation wells is shown in Figure 1d. The state of AGF system was monitored by two hydro-observation wells (GN1, GN2) of depth 82 m and 202 m located around the periphery of the mineshaft. From the analysis of the character of changes in the groundwater levels during freezing, we can judge the continuity of the AGF system. The continuity of the AGF system implies that there is no communication between inside and outside of zones of AGF system. The growth of the closed AGF system occurs at a rise of the water level in hydro-observation wells located inside the freezing zone. The difference in depths of hydro-observation wells is the criterion that makes it possible to control the growth of the ice wall in the vertical direction. At present, hydro-observation wells are also used for temperature monitoring. The results of the monitoring through the depth of the well GN2, which were obtained with the fiber-optic measuring system developed at the MI Ur O RAS (for details Panteleev, I. A., 2017), are presented in Fig. 1b. Monitoring of the level of groundwater during the formation of ice wall for the first shaft of the Petrikov mine (Belorussia) with the aid of hydro-observation wells were conducted four months. The results of water level measurements are shown in Figure 1d. Analysis of changes in the groundwater level in the wells indicates that in the interval of 40-80 meters freezing of the rocks is more intensive than in deeper interval of 140-200 meters. In contrast, the analysis of temperature variation with the depth of the well GN2 demonstrates a more intensive freezing in the lower layer (Fig. 1c). The resulting discrepancy between the results is evidently due to the processes occurring in a closed region during freezing of the soil with insufficient moisture content. To explain this situation we described the processes of phase transition front propagation in a porous medium using two AGF models, which variously consider the key effects observed during the propagation of the phase transition front in a saturated soil (the nonequilibrium moisture content in the soil at negative temperatures and kinetic peculiarities of water-ice transition in a porous medium). We analyzed in detail the formation of frozen ground in the region of a single freezing column, the closure of two growing frozen regions and the redistribution of moisture content inside the closed growing ice walls. It was shown that moisture content inside the closed AGF system could decrease due to liquid migration towards the phase transition front.