Microscopic mechanisms of microwave irradiation thawing frozen soil and potential application in excavation of frozen ground
Introduction
Engineering activities are becoming increasingly frequent in seasonally frozen and permafrost regions (Pei et al., 2017; Jia et al., 2020). Because the hardness and strength of frozen soil are far greater than those of thawed soil (Tang et al., 2019) excavation in frozen ground is rather difficult and of high cost. Artificially thawing frozen soil before excavation may significantly increase the efficiency of construction and reduce cost (Lindroth et al., 1995). Traditional methods of artificial thawing (basically conduction heating), such as asphalt laying, film covering, hydro acupuncture and electric power thawing (Qiu, 2007) generally have problems of low efficiency and high cost. Microwave irradiation is expected to be highly efficient in both heating and energy utilisation; furthermore, it has strong penetrability and the energy can be well focused (Hamid, 1982). Therefore, it should be a competitive way to thaw frozen soil both efficiently and economically.
Microwave heating technology has been applied in such fields as rock breaking, mineral separation, coal treatment, oil shale exploitation and de-icing of pavement (Liu et al., 2019). Microwave irradiation can significantly reduce the strength and hardness of hard rock (containing microwave-absorbing minerals) (Kingmana et al., 2004), thus improving rock breaking efficiency and reducing excavation difficulty. Studies show that the strength of hard rock decreases with increasing microwave irradiation time (Hassani et al., 2016). Iron-bearing minerals have a strong ability to absorb microwaves (Lu et al., 2017), so they can be separated from other minerals with microwave irradiation. For example, Li et al. (2013) studied microwave pre-treatment of braided iron ore and found that thermal stress causes a large number of intergranular fissures under suitable microwave power and time, which increases the release of iron ore by about 20% to 30%. Omran et al. (2015) compared the amounts of iron ore released from oolitic iron ore pretreated by conventional heating and microwave heating. They found that the grindability of ore pretreated by microwave heating was better than that of ore treated with conventional heat, and much less energy was consumed. Microwave heating is also used to desulfurize coal (Chelgani and Jorjani, 2011) and increase its grindability (Sahoo et al., 2011; Lester et al., 2005). In addition, pretreating oil shale with microwave heating can significantly improve its pyrolysis efficiency and increase the amount of oil produced (Chanaa et al., 1994; Liu et al., 2010; Wang et al., 2009). Applications of microwave heating in the above fields indicate the unique advantages of this method.
Pilot studies of microwave irradiation thawing frozen soil by Hamid (1982) and Lindroth et al. (1995) have confirmed the feasibility of microwave irradiation as an artificial thawing method. Rattanadecho (2004) conducted theoretical and experimental researches on microwave irradiation thawing frozen soil, and found that variation of layered configurations and unfrozen layer thickness changes the degree of temperature level within layered sample as well as the thawing rate. However, this method has not yet been widely used in the field. One important reason is that the mechanisms of thawing and softening of frozen soil by microwave irradiation are not fully understood, therefore, the practicability and efficiency of this method cannot be well evaluated.
Aiming to revealing the mechanisms of microwave irradiation thawing and softening frozen soil and evaluate the practicability and efficiency of this method, in this study, microwave irradiation experiments on frozen soils with five moisture contents were conducted. The efficiency of this method in thawing frozen soil was estimated by comparing the melting rates of frozen soil undergoing microwave irradiation and conduction heating respectively. Variation of the unfrozen water content and the unconfined compressive strength (UCS) of frozen soil during microwave irradiation thawing were measured by the nuclear magnetic resonance (NMR) method and the UCS test apparatus respectively. Accordingly, the microscopic mechanisms of microwave irradiation thawing and softening frozen soil were interpreted. Based on the above experiments results, key issues regarding the application of this method in excavation of frozen ground were discussed.
Section snippets
Background theories
In this study, microwave irradiation experiments on frozen soils were conducted, in the process of which variation in the phase composition of pore water (ice) was measured via the NMR technology. A short introduction on the basic theories of microwave irradiation and nuclear magnetic resonance is given below to indicate how the test results were achieved and thus gain a better understanding of the results.
Methodology
Frozen soil samples with different moisture content were prepared for the microwave irradiation tests. To gain a thorough understanding of the thawing process of frozen soil, surface and internal temperature, T2 spectrum of the samples were measured during the tests. As strength is one of the most important parameters for design and execution of excavation works in the field, we also tested the UCS of frozen soil samples at different stages of microwave irradiation. Besides, the UCS of samples
Surface temperature changes during microwave irradiation
Samples with a temperature of −14.4 °C were irradiated for 85 s. The surface temperature of the samples uniformly increased to about 40 °C and the frozen soil melted rapidly under microwave irradiation (Fig. 2).
During microwave irradiation, the surface temperatures of samples with various moisture contents varied with irradiation time in three stages (Fig. 3a). During Stage I, changes of surface temperature were almost the same and increased continuously, whereas the duration of Stage II
Thawing mechanism of frozen soil subjected to microwave irradiation
At room temperatures, soil consists of soil particles, pore water and air. The pore water can be divided into free water, capillary water and adsorbed water (Coates et al., 1998). The three categories of pore water have different freezing points. The freezing point of free water is generally 0 °C. That of capillary water is determined primarily by the capillary force it suffers, larger capillary force leads to lower freezing point. The freezing point of capillary water in silt is between −4 °C
Conclusions
In the present study, variations in temperature, unfrozen water content and strength of frozen soil samples during microwave irradiation were investigated experimentally. The mechanisms of melting and softening of frozen soil under microwave irradiation were discussed. The main conclusions are as follows:
- (1)
Microwave irradiation can be used as an efficient thawing method. After microwave irradiation (with a power of 100 W) for 60 s, a frozen soil sample with an initial temperature of −14.4 °C was
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 41702334, 41702339) and the Shaanxi Post Doctoral Science Fund Project (2018BSHEDZZ18). Special thanks go to the editors and anonymous referees for their constructive comments.
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