Abstract
The Artificial Ground Freezing (AGF) method, which is widely used in tunnel excavations, significantly affects the properties of geotechnical materials in frozen walls under extremely low temperatures. In order to simulate the AGF process, the freezing treatment with a temperature of −30°C and thawing treatment temperature of 25°C were performed on natural specimens of granite residual soil (GRS). Subsequently, triaxial (TRX) tests were conducted to evaluate mechanical properties and Nuclear Magnetic Resonance Image (NMRI) tests were applied to detect pore distributions of GRS. To clarify variations of microstructure after freezing-thawing, the relaxation time (T2) distribution curves and T2-weighted images from NMRI results were thoroughly analyzed from the perspective of quantization and visualization. Results show that the shear strength as well as the cohesion of GRS are reduced sharply by the AGF process, while the internal friction angle decreases gently. The pore size distribution (PSD) converted from the T2 curve is constituted of two different peaks, corresponding to micro-pores with diameters from 0.1 to 10 µm and macro-pores with diameters from 10 to 1,000 µm. Under the AGF impact, the expansion in macro-pores and shrinkage in micro-pores simultaneously exist in the specimen, which was verified from a visualized perspective by T2-weighted images. The frost heaving damage on shear strength is attributed to the microstructural disturbance caused by the presence of large-scale pores and uneven deformations in GRS, which is subjected to the AGF impact under an extremely low temperature.
Highlights
A growing number of metro engineering projects have been under construction in large and medium-sized cities in China, as a result of rapid developments of urbanizations (Fang et al, 2011; Chen et al, 2015; Liu et al, 2021)
The soil columns exhibit more brittle behavior at different confining pressures and great reductions in peak stress after freezingthawing. This phenomenon is attributed to the pressure caused by ice lens formation increasing the friability of Granite residual soil (GRS), leading to a reduction of peak stress when large deformation occurs to the specimen (He et al, 2020)
The shear strength decreases synchronously with the pore enlargement of GRS during the Artificial Ground Freezing (AGF) process. Based on this experimental study, the following conclusions are drawn: 1) The process of freezing-thawing on the shear strength of GRS under an extremely low temperature was investigated by performing triaxial loading tests
Summary
A growing number of metro engineering projects have been under construction in large and medium-sized cities in China, as a result of rapid developments of urbanizations (Fang et al, 2011; Chen et al, 2015; Liu et al, 2021). Urban environments and geotechnical properties have been inevitably influenced by subway tunnel excavations at a varying degree during construction and operation periods (Fu et al, 2014). The AGF method has great advantages, the frost heaving damage and AGF Impact on Soils thaw-induced deformations of geotechnical materials during construction adversely affects the stability of underground structures (Vitel et al, 2015). Due to inferior properties, such as high permeability, high compressibility, low strength, and -disturbed characteristics of GRS, the AGF method serves as an important constructing technique of metro tunnels and cross passages in the formations (Fonseca et al, 2006). The structural damage of GRS caused by F-T process induces potential dangers to the safe operation of subway tunnels, increasing the probability of losing stability control during excavations
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