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

Ran An,Chengsheng Li,Lingwei Kong

doi:10.3390/app10020544

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 (T2) spectrums and T2-weighted images of saturated samples after freezing at different temperatures. The pore volume distributions were determined from T2 spectrums based on the surface relaxation coefficient ( ρ 2 ) and the pore structures were visualized by T2-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 T2 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 μm and the secondary peaks with diameters of 20–500 μm. 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.

Highlights

Granite residual soils are extensively distributed in southeast coastal areas of China [1,2]
The main purpose of the present study is to assess the effect of freezing temperatures on evolution patterns of microstructure in granite residual soils based on Nuclear Magnetic Resonance Imaging (NMRI) measurements
Theand measuring of of spectrum curves and was cross-sectional images of pore freezing temperatures, theresults validity measurements discussed by the results of distributions were analyzed to study the pore structure variations of thawed soils under different freezing temperatures, and the validity of NMRI measurements was discussed by the results of

Summary

Introduction

Granite residual soils are extensively distributed in southeast coastal areas of China [1,2]. Under the hot and rainy climate in tropical and subtropical areas, the residual soils exhibit a special grain distribution from clay to gravel due to physical and chemical weathering processes [3]. The distributed areas of granite residual soils are generally encountered in the engineering problems of subway tunnel excavation [4]. To guarantee the safety of the tunnel construction in residual soil distributed areas, the artificial freezing method, which highlights a good sealing performance, high strength and other advantages, has been widely applied in in practical engineering. During the construction of metro tunnels with the artificial ground freezing method, the freezing process will affect the engineering properties of soils in subway wall. Lackner et al [6] found that the macroscopic viscoelastic behaviors of soils after artificial freezing treatments are related to the

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Conclusion