Research On Mechanical Properties Of Carbonate Soil In Deep Seasonal Frozen Area

At present,more and more engineering constructions are building in cold regions,during the process of interaction of construction environment and permafrost environment,there has been seen a series of deterioration trend of permafrost environment,such as permafrost temperature and carrying capacity decline,combined with the background of global warming,permafrost engineering encounters many engineering problems during construction and operation.Many scholars have to participate in frozen soil research to provide new ideas and methods for engineering construction of cold areas.In order to study the influencing factors of the strength of frozen saline-alkali soil,triaxial shear tests of frozen soil samples under three negative temperatures(-6?,-10?and-14?),four confining pressures(0,0.5 MPa,1.0 MPa and 1.5 MPa)and four loading rates(0.4%/min,1%/min,1.6%/min and 2%/min)were carried out.The stress-strain curves of frozen soil samples under different conditions were obtained and the analysis shows that the stress-strain of frozen soil samples under low confining pressure presents a strain-softening type,and high confining pressure presents a strain-hardening type.The yield strain of soil samples is significantly sensitive to chamber temperature and increases linearly with the decrease of chamber temperature.The failure strength and initial elastic modulus can be expressed by the linear relationship between temperature and loading rate or temperature and confining pressure.In order to investigate the stress-strain evolution law of saline-alkali soil under freezing-thawing cycles in cold regions and guide the engineering construction of saline-alkali soil area.In this paper,GDS triaxial was used to conduct UU tests on saline-alkali soil taken from the place in Heilongjiang Province under different confining pressures and different freeze-thaw cycles.The relationship between soil sample strength index c and,model parameters K and n,break ratio R_f and the number of freezing-thawing cycles N were obtained by curve fitting.The tangent modulus E_i was modified according to the error comparison between model prediction results and the real value,so the modified Duncan-Zhang model was established considering the number of freezing-thawing cycles N of saline-alkali soil.The results show that the undrained stress-strain curve of saline-alkali soil is strain hardening after freeze-thaw,and the strength of saline-alkali soil decreases the most after three freeze-thaw cycles,accounting for more than 71%of the whole strength deterioration process.After freezing and thawing,the internal friction angle,cohesion c and parameter K decrease gradually,and the break ratio R_f decreases firstly and then increases,while parameter n increases firstly and then decreases.The modified Dun-can-Zhang model considering the number of freezing-thawing cycles was established in this paper,and the predicted results are in good agreement with the experimental results.In order to investigate the long-term settlement of foundations in saline-alkali soil in deep frozen region,a series of cyclic loading tests were carried out by using GDS dynamic and static triaxial test system,and the cumulative plastic deformation curves of remolded soil samples under different vibration frequencies f,compacting degree k and cyclic stress?_d were obtained.It is found that with the increase of loading frequency,cumulative plastic deformation increases firstly and then decrease,and the growth rate of cumulative plastic strain has the same trend.The relationship between cumulative plastic deformation and compactness is not always negative.The cyclic stress?_d has a significant effect on the cumulative plastic deformation,and the greater the cyclic stress?_d the greater the cumulative plastic deformation is.Monismith index model can well predict the cumulative plastic deformation of saline-alkali soil under freeze-thaw.