Study On Mechanical Properties Evolution And Damage Mechanism Of Artificially Frozen Saturated Sandstone

In the construction process of complex and difficult rock tunnel engineering,roadway engineering,underground military engineering,etc.,complex and diverse water-bearing strata are often encountered,and pore water pressure is one of the major challenges faced by underground engineering construction.Artificial freezing method is widely used to block high-pressure pore water when crossing complex strata rich in water.In this thesis,the freezing method of mine shaft construction crossing water-rich complex strata is taken as the research background,and the sandstone under the coupling action of in-situ stress and pore water pressure is taken as the research object.The triaxial compression test and microscopic test of frozen sandstone and sandstone after freezing and thawing under the coupling action of in-situ stress and pore water pressure are carried out by using the high-pressure geotechnical triaxial test system and the automatic low-temperature freeze-thaw cycle test machine,combined with Meso MR12-060H-I medium-size nuclear magnetic resonance analysis system and scanning electron microscope system.The mechanical properties evolution and damage mechanism of saturated sandstone under pore water pressure before freezing,during real-time freezing and after one freeze-thaw cycle were studied.The main research results are as follows:（1）Through the triaxial compression test of frozen sandstone under the real-time action of pore water,the triaxial compression test results of frozen sandstone without pore water pressure and normal temperature sandstone were compared and analyzed.The influence of pore water pressure on the mechanical properties of sandstone under freezing and normal temperature conditions was studied.The influence mechanism of pore water pressure and freezing on the macroscopic failure characteristics and microstructure of sandstone was analyzed by scanning electron microscopy.（2）The stress-strain curves,triaxial compressive strength and loss rate,elastic modulus and Poisson’s ratio of sandstone samples with three kinds of porosity（3.79%,11.61%and 16.21%）under different freezing temperatures（-5°C,-10°C,-15°C and-20°C）were obtained.The influence of freezing-thawing temperature and sandstone porosity on the deformation characteristics,triaxial compressive strength and elastic modulus of sandstone with pore water before and after freezing-thawing was analyzed.（3）The nuclear magnetic resonance（NMR）tests of sandstones with different porosities before freezing,during freezing and after thawing were carried out.The NMR porosity,T₂distribution curve and spectral area distribution histogram of sandstones at different stages were obtained.The change process and damage evolution law of pore structure of sandstones in different sizes during freezing and thawing were studied.（4）Based on the statistical damage theory and effective stress principle,considering the coupling effect of in-situ stress and pore water pressure on sandstone,the statistical damage constitutive model of sandstone considering in-situ stress,pore water pressure and freeze-thaw damage is established by using the elastic modulus of sandstone before and after freeze-thaw to characterize the freeze-thaw damage.There are 34 figures,16 tables and 99 references in this thesis.