Experimental investigation of stress and strain characteristics of frozen sandy soils - A case study of Tabriz subway

Temporary soil improvement techniques are considered among the most important geotechnical topics. Artificial ground freezing is considered to be eco-friendly, economic, safe and applicable for all types of homogeneous, loose and soft soils. By lowering the soil temperature below zero, the pore water of the soil freezes which leads to improved mechanical properties of soil; it increases the shear and compressive strength of soil, without entering any chemicals into the soil environment. In practice, artificial ground freezing consists of two parts; (i) formation of frozen body before construction or test (active step) and (ii) maintenance of the frozen body during the construction and test procedure (passive step). There are two methods for using artificial ground freezing; (i) open method and (ii) closed method. In the open method, liquid nitrogen is used for cooling. With the evaporation of the liquid nitrogen, the soil freezes and the nitrogen gas is released to the atmosphere. In the closed method, a brine is used which is connected to a refrigeration plant. The brine is circulated until the formation of frozen body. The brine used can be ethylene glycol or calcium chloride. One of the difficulties of studying frozen soils is the absence of triaxial compression apparatus for frozen soils in the national and international markets, because it is an unconventional test apparatus. In this research, a triaxial compresion apparatus was developed for testing of frozen soils in the geotechnical laboratory of the University of Tabriz. This apparatus was placed in a cold and insulated room with minimum heat transfer. The temperature of the room was monitored continuously. This triaxial apparatus for frozen soils was developed using a closed system that is connected to a refrigeration plant.
In this study, void ratio, ice saturation and texture of soil were considered as constant parameters and temperature, confining pressure and strain rate were considered as variable parameters. The soil samples were remolded in the laboratory to represent the in situ soil in the line 2, station H of Tabriz Subway. The in situ soil was poorly graded sand and fully saturated. A special type of mold was designed for this research, that was composed of aluminum material with high heat transfer. The mold was insulated from top and bottom and it was rigid in radial direction. A series of unconsolidated undrained triaxial tests were conducted on samples of the frozen soil using the developed triaxial compression apparatus. The results show that, at constant confining pressure and strain rate, decreasing temperature leads to increase in the shear strength and modulus of elasticity of the soil. Decreasing temperature also results in the yield point of frozen soil occurring at higher strains. Also, at constant temperature and strain rate, increasing the confining pressure increases the shear strength and modulus of elasticity of the soil. At constant temperature and confining pressure, increasing the strain rate leads to a moderate increase in the modulus of elasticity and a significant increase in shear strength of frozen sandy soil. Furthermore, the results show that the poorly graded frozen sandy soil samples tested in this research program show softening behavior. All the stress-strain curves show a peak and a residual state.