###### Abstract  
Artificial ground freezing method is widely applied in the construction of metro tunnel and significantly impact the microstructure of soils in artificial frozen-walls. To delve into the pore distribution characteristics of thawed residual soils, Nuclear Magnetic Resonance Imaging (NMRI) measurements were performed to investigate the relaxation time (\(T_{2}\)) spectrums and \(T_{2}\)-weighted images of saturated samples after freezing at different temperatures. The pore volume distributions were determined from \(T_{2}\) spectrums based on the surface relaxation coefficient (\(\rho_{2}\)) and the pore structures were visualized by \(T_{2}\)-weighted images. Subsequently, the pore size distribution curves from NMRI were compared and validated by mercury intrusion porosimetry (MIP) tests. According to the results, the peak areas of \(T_{2}\) spectrums were linearly related to freezing temperatures in a positive manner. Pore volume distribution curves of thawed soils have two peaks, which are the major peaks with diameters of 0.5-20 \(\upmu\)m and the secondary peaks with diameters of 20-500 \(\upmu\)m. As the freezing temperature drops, the volumes of pores with different diameters all increased. The damage degree of microstructure in thawed soils increases as the temperature drops, according to the visualized pore structure. Besides, NMRI measurements of saturated soils are more accurate to reflect the full diameter range of pores, compared to MIP method. artificial ground freezing method; residual soils; nuclear magnetic resonance imaging; freezing temperature; pore distribution +
Footnote †: journal: Medical  
microscopic properties such as soil particles, water and ice contents. [PERSON] et al. [7] experimentally found that the dynamic behavior of silty clay in Shanghai metro tunnel altered significantly after the artificial freezing process. As [PERSON] et al. [8] reported, the void ratio decreased while the mean pore diameter and hydraulic conductivity increased after freeze-thaw treatment of the soils in metro frozen-wall. It has been commonly recognized that the artificial freezing method significantly impacts the microstructures in soils, thereby causing its physical and mechanical properties to vary [9]. Nevertheless, the evolution patterns of pore distributions of soils under freezing-thawing effects remain unclear. Accordingly, the study on the pore distributions of thawed soil after freezing process is of great implication to gaining comprehensive insights into its damage mechanism of soils under freezing effects [10]. The pore size distribution (POSD), as a vital intrinsic property of soils, is associated with the microstructure and seriously impacts the physical, hydraulic and mechanical properties of soils, namely permeability, soil-water characteristic curves and shear strength [11; 12; 13]. Various testing methods, including mercury injection porosimeter (MIP) tests, X-ray computed tomography (CT) and Nuclear magnetic resonance scans (NMR), have been employed to analyze the microstructures of soils [14; 15; 16; 17]. The MIP test, a most commonly used method, requires freezing treatment that can to some extent cause damage to the microstructure of the sample [18]. X-ray CT tests consist of a tedious procedure for sample preparation, and the result of pore measurement in a micron level exhibits relatively low precision [19]. The low magnetic field nuclear magnetic resonance imaging (NMRI) techniques have been extensively employed in microstructure detections of porous media for their rapidity, efficiency and non-destructiveness [20]. In recent years, the NMRI technique have been frequently adopted to study geotechnical engineering to ascertain the porosity, the damage degree caused by freezing-thawing cycles, as well as the free water content in a frozen soil at different saturations [21; 22; 23; 24]. However, reports on the application of NMRI method to investigate the pore structure of frozen-thawed soils in subway tunnel excavations are exceedingly rare.