## 1 Introduction  
Artificial freezing technique is an efficient method in soft clay reinforcement, it can both reduce the cost and shorten the construction period. In tunneling, especially in soft clay, artificial ground freezing (AGF) has been widely used [1; 2; 3]. However, the freezing-thawing cycles change the microstructure of soft clay and result in greater compressibility [4; 5]. Information of the microstructural change in the soft clay is helpful for clarifying the deformation mechanism of the ground. To explore the microstructural change, pore size distribution (PSD) is selected as the index to quantize the soft clay microstructure [6; 7]. There are five methods to measure the PSD of soft clay: mercury intrusion porosimetry, scanning electron microscopy, time domain reflectometry, computer tomography, and gas adsorption [8]. The first two methods are most commonly used. The mercury intrusion porosimetry methods obtain the PSD of soil by pressing mercury into the soil structure. [PERSON] determined the PSD of clay samples by mercury intrusion porosimetry, indicating that the range of equivalent pore diameter covers almost five orders of magnitude [9]. Mercury intrusion porosimetry tests on kaolin soil showed that there were test result errors associated with certain pore-size diameters [10]. Liquid water and ice coexist in frozen soft clay, and water retention properties are best obtained at high matrix potentials from draining and at low matrix potentials from freezing [11]. [PERSON] and [PERSON] determined the surface fractal dimensions for porous media by mercury intrusion porosimetry [12]. Scanning electron microscopy uses the pictures of soil to observe the microstructure of soil [13; 14; 15]. Imprecise PSD of soil specimens can be obtained from scanning electron microscopy analysis [16; 17; 18]. Some scientists combine multiple methods to study the microstructure of soil. [PERSON] and [PERSON] compared the porosity of frozen-thawed soil tested by scanning electron microscopy and mercury intrusion porosimetry. They concluded that freeze-thaw progress produced more connected and slender pores in the clay [19]. [PERSON] et al. used mercury intrusion porosimetry and scanning electron microscopy to describe the microstructure and morphological character of clay [20; 21]. Studies have shown that each method has its limitations, so combining multiple methods can get more accurate PSD. In a break from traditional methods, low-field NMR (LF-NMR) technology offers a quick, lossless and repeatable way of precisely measuring water in samples [22; 23]. Currently, NMR technology has been widely used in various industries, including chemistry, fuel, and medicine [24; 25; 26]. Geotechnical engineering has also benefited from NMR technology in recent years [27]. Low field NMR can obtain the signal from water in soft clay much more precisely, intelligently, and quickly [28; 29]. Comparing NMR with MIP, [PERSON] and [PERSON] found that Low field NMR is an efficient tool for quantifying the PSD in coal non-destructively [29]. Additionally, [PERSON] et al. optimized the scheme of NMR measurement [30]. Progress has been made through technological developments concerning soft clay microstructure behavior when subjected to freeze-thaw cycling. The effect of freeze-thaw cycles on soil microstructure was tested by NMR and implied that pore crystallization could alter the soft clay structure and lead to severe frost heave, deterioration of soil engineering properties [31]. However, the current microstructure research on frozen-thawed soft clay mostly focuses on seasonal freezing-thawing. Unlike soft clay subjected to seasonal freeze-thaw cycles, soft clay subjected to artificial ground freezing (AGF), which completes the freeze-thaw cycle in a short time, experiences severe uneven settlement on subway foundations [18].