Pore Distribution Characteristics of Thawed Residual Soils in Artificial Frozen-Wall Using NMRI and MIP Measurements

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 mu m and the secondary peaks with diameters of 20-500 mu 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.