Study On The Influence Of Loading And Unloading Rate On The Mechanical Properties Of Frozen Sand Under True Triaxial Stress

As a foundation or structure,the strength and deformation characteristics of frozen soil are important basis for the design and construction of frozen soil engineering.In engineering construction,frozen soil is usually in three-dimensional unequal stress state.The results of uniaxial and conventional triaxial tests cannot fully reflect the mechanical properties of frozen soil under complex stress state.In this paper,the artificial frozen sand is taken as the research object,and the uniaxial compression tests with water content of 10%,13%,16%,20%and 24%are carried out by using the true triaxial frozen soil testing machine.The loading test was carried out with temperature（-5°C,-10°C,-15°C）,minor principal stress（1.0 MPa,2.0 MPa,3.0MPa）and loading rate（0.15 mm/min,0.3 mm/min,0.45 mm/min,0.6 mm/min,0.75mm/min）as variables.The unloading test was carried out at-10°C with small principal stress（1.0 MPa,2.0 MPa,3.0 MPa）and unloading rate（5 N/s,10 N/s,15N/s,20 N/s,25 N/s）as variables to study the effect of loading and unloading rate on the strength and deformation characteristics of frozen sand.Based on Weibull distribution and D-P strength criterion,a two-parameter damage constitutive model was established.The main conclusions are as follows:（1）The stress-strain curve of frozen sand in uniaxial compression test is strain softening.The peak stress,peak strain and elastic modulus of frozen sand increased first and then decreased with the increase of water content.According to the Weibull distribution theory,the constitutive model of frozen sand under unidirectional load is derived,which can better reflect the whole process of frozen soil failure.（2）In true triaxial loading and unloading tests,the stress-strain curves of frozen sand show strain hardening characteristics under different loading rates.Under different unloading rate conditions,the stress-strain curves show strain hardening characteristics when the minor principal stress is 1.0 MPa and 2.0 MPa,and the stress-strain curves show strain softening characteristics when the minor principal stress is3.0 MPa.The strength of frozen sand increases with the increase of loading rate and decreases with the increase of unloading rate.The strengthening effect of loading rate on frozen sand and the weakening effect of unloading rate on frozen sand decrease with the increase of loading rate,and the power function can reflect the relationship between them.（3）In the true triaxial loading and unloading test,compression deformation and expansion deformation occurred in the middle principal stress direction and the small principal stress direction respectively,and the expansion deformation in the small principal stress direction was significantly larger than that in the middle principal stress direction.With the increase of loading rate,the medium principal strain and small principal strain of frozen sand gradually decrease;With the increase of unloading rate,the medium principal strain of frozen sand first increases and then decreases,while the small principal strain gradually increases,and the bulk strain shows the trend of shear contraction and then dilatancy.The strength,failure stress ratio and elastic modulus of frozen sand are positively correlated with the loading rate,and the failure stress ratio is linearly correlated with the loading rate.The strength and elastic modulus of frozen sand are negatively correlated with unloading rate,and the deterioration rate of strength and elastic modulus increases with the increase of unloading rate.The effects of loading rate and unloading rate on small principal strain,large principal strain,bulk strain and medium principal strain are weakened in turn.Large principal strain and small principal strain jointly control the change trend of bulk strain,and small principal strain is more sensitive to rate change.（4）According to the Weibull distribution and Drucker-Prager strength criterion of the micro-unit strength of frozen soil,combined with the equivalent strain hypothesis,a two-parameter constitutive model of frozen sand under true triaxial loading and unloading is established.The parameters F₀and m reflect the strength characteristics and ductility brittleness of frozen sand respectively.The relationship among loading rate,unloading rate and parameters F₀and m is established.The parameters F₀and m can be calculated directly according to the equation,and the predicted stress-strain curve can be obtained,which provides a basic basis for frozen soil engineering construction.Figure[33]table[19]references[102]...