Research On Mechanical Properties And Failure Criterion Of Frozen Sandy Soil Under True Triaxial Stress State

Artificial freezing method is widely used in mine shaft engineering with deep alluvium and urban underground engineering due to its good sealing effect and green environmental protection.Under the combined action of the interaction force between frozen soil and structure and in-situ stress,deep-frozen soil is in complex stress state.The deep shaft excavation process will cause the redistribution of internal stress of the frozen wall,which is a typical loading and unloading process.The mechanical parameters of frozen soil obtained by conventional triaxial tests cannot reflect the influence of complex stress state and stress path on the strength and deformation characteristics,and energy dissipation of deep-frozen soil.The true triaxial tests were carried out on artificial frozen sandy soil of mine freezing engineering by using the true triaxial test system of frozen soil.The unidirectional loading test under different intermediate principal stress states,the loading and unloading test for various true triaxial stress state,true triaxial tests with constant minor principal stress（σ₃）and constant intermediate principal stress ratio（b）for different small principal stress,negative temperature and moisture content are carried out.The effects of intermediate principal stress,loading and unloading rates and intermediate principal stress coefficient on strength,deformation characteristics and energy dissipation characteristics of frozen sandy soil were analyzed,and the generalized nonlinear strength criterion of frozen sandy soil in full stress space is constructed.The main conclusions are as follows:The unidirectional loading test of frozen sandy soil shown that intermediate principal stress（σ₂）has dual effect of strengthening and weakening the mechanical properties of frozen sandy soil,and there exists a critical intermediate principal stress(σ_2c),andσ₃has an obvious effect onσ_2c.Whenσ₂<σ_2c,the strengthening effect is dominant;whenσ₂>σ_2c,the weakening effect is dominant.With the increase ofσ₂,σ_ciincreases first and then tends to be stable,andσ_cdincreases first and then decreases.Initial tangent modulus and deformation modulus decrease with the increase ofσ₂.With the increase ofσ₂,the deformation difference betweenσ₂direction andσ₃direction increases gradually,and that betweenσ₂direction andσ₁decreases.The deformation anisotropy coefficient of frozen sandy soil increases significantly with the increase ofσ₂.Compared withσ₂,the effect ofσ₃on energy dissipation characteristics of frozen sand sandy soil is more obvious.Under the loading and unloading stress path,stress-strain curve characteristics are affected by the loading rate,unloading rate,and initial stress state.Whenσ₃is 2.5 MPa and 4.0 MPa and the loading rate is 0.75 mm/min,(σ₁-σ₁₀)-ε₁curve shows strain hardening characteristics.The increase ofσ₃and loading rate and the decrease of unloading rate are helpful to improve the failure strain of(σ₁-σ₁₀)-ε₁curve.The amplitude and variation rate ofε₂andε₃decrease with the increase in loading rate and the decrease in unloading rate in the sameε₁.Power function can better reflect the relationship between loading and unloading rate and the strength of frozen sandy soil.Loading rate has the greatest influence on the energy dissipation characteristics of frozen sandy soil,followed by unloading rate and initial stress state.The intermediate principal stress coefficient（b）can significantly improve the failure stress and deformation modulus of frozen sandy soil.The stress-strain curve can be mainly divided into three stages,with evidence of strain hardening characteristics in the different test conditions.The failure strength and stress-strain curve slope of frozen sandy soil under true triaxial stress state（b>0）is greater than that under conventional triaxial stress state（b=0）.The failure strength and friction angle increased with the increase in b from 0 to 0.6,but decreased when increasing b from 0.6 to 1.0,whereas the cohesion varied little with the variation in b.With the increase of b,the deformation in the direction ofσ₂changed from dilative to compressive,and that in the direction ofσ₃remained dilative throughout.The deformation difference betweenε₂andε₃increases gradually,and that betweenε₂andε₁decreases.The deformation anisotropy coefficient of frozen sandy soil increases significantly with the increase of b.Based on the meridian function in the form of power function and the extended spatial mobilized plane（SMP）criterion deviatoric plane function,a generalized three-dimensional nonlinear strength criterion is proposed.It comprehensively considers the compression-tensile strength ratio,hydrostatic pressure effect,and intermediate principal stress effect of friction materials.The meridian function reflects the cohesion effect,friction effect and hydrostatic pressure effect of frozen soil.By modifying the parameters of the deviatoric plane function,it can be converted into a variety of classical three-dimensional strength criteria.Comparisons between the failure criterion and experimental results were presented for frozen sandy soil,which reveals that the proposed failure criterion captures the experimental trend quite well.Figures[171]Tables[27]References[223]...