Breakage Mechanism And Binary-Medium Constitutive Model For Freeze-Thaw Tailings Subjected To Cyclic Triaxial Loading

The mineral composition of the tailings is complex,it contains a certain amount of crystal water,unstable compounds,and cementing substances,etc.,which leads to a higher cohesive force of the tailings.The above substances are unevenly distributed within the tailings,hence causing an apparently difference in geotechnical properties comparing with the naturally occurring soils.In engineering practice,the dynamic constitutive model is the basis of the safety and stability analysis for tailings dams.At present,the influence of freeze-thaw cycles on the physical and mechanical properties of geotechnical materials has been one of the most difficult and hot topics in geotechnical engineering.Based on the abundant outcomes that researchers have achieved previously,the static and cyclic mechanical properties of freeze-thaw tailings have been in-depth investigated in this paper.The study was undertaken by performing a series of laboratory tests,involving the freeze-thaw test,the conventional and cyclic triaxial shear tests,as well as the characterization analysis.Based on the test results,a so called cyclically binary-medium constitutive model of freeze-thaw tailings was established.The model was proposed within the framework of Breakage Mechanics theory,combined with the micromechanics principle of composite materials.The main work and achievements of the research are listed as follows:(1)Freeze-thaw tests are carried out at an open system.And a series of conventional triaxial shear tests were later performed on the freeze-thaw tailings at both undrained and drained conditions,in which the stress-strain relationship,pore pressure,shear strength as well as the elastic modulus are investigated.The experimental results indicate that for the undrained tests,the tailings samples exposed to just a few cycles of freeze-thaw(NFT=0,1)show a strain-softening behavior under different confining pressures,and their pore water pressures have a decreasing trend after reaching the peak value.While for the drained test,the specimens show an obvious stain-softening and bulk dilation features only under the condition of a few cycles of freeze-thaw(NFT=0,1)and of a relatively low confining pressure(σ3=50kPa,100kPa).When reaches failure,a shear band was observed extending through the diagonal line of the sample that presents strain-softening behavior.As the number of freeze-thaw cycles and the confining pressure increase,the stress-strain relationship of the specimen changes to a strain-hardening form,accordingly,the failure pattern changes to bulging failure.Besides,with the increasing number of freeze-thaw cycles and the confining pressure,the decreasing trend of pore water pressure gradually disappeared in the undrained test,while for the drained test,the dilation characteristic gradually disappeared,and the specimen trends to present a continuously shrink during the whole loading process.In addition,the shear strength and elastic modulus of the tailings decrease with the increase of cycles of freeze-thaw,but increase with the increasing confining pressure.(2)The impact of freeze-thaw cycles on the microstructure of tailings is analyzed through a combination experimental investigation,involving the conventional triaxial tests and the chemical and structure characterization of the tailings specimens.The results show that the chemical composition of the tailings is not affected by the freezethaw cycles,by contrast,the physical and mechanical changes of freeze-thaw tailings are contributed the adjustment of its microstructure and to the failure of the cement structures,which were induced by the phase change of in-pore water.After freezethaw process,the strength of tailings decreases.When under only a few cycles of freeze-thaw(NFT=0,1),the deterioration of the mechanical properties is primarily due to the local damage of cement structures.With the increased cycles of freeze-thaw(NFT=5,15),the deterioration is dominated by the reorganization of the pore structure.(3)Tailings specimens were remolded after the conventional triaxial drainage test(CD),and the newly obtained specimens are denoted by frictional samples,which were also subjected to CD test.Subsequently,the stress-strain relationship,the deformation behavior and the damage mechanism are compared between the above two types of materials.The results show that the strength of the frictional sample is significantly lower than that of the tailings sample.Firstly,the growth rate of the deviatoric stress of the former is always smaller than that of the latter.As the loading continues,the difference between their deviatoric stresses gradually decreases,and their residual strengths are very close to each other when the specimens fail.Concerning the shape of the volumetric curve,the frictional specimen and the tailings specimen appear to be very similar,despite the volume contraction of the former is slightly larger than that of the latter.Tailings sample containing a certain amount of crystal water,unstable compounds and cement,etc.The cement aggregates the former and the tailings particles with each other forming a cemented structure(or larger particle aggregates).Consequently,it brings a high cohesive force to the tailings specimen,which is considered the primary factor that leads to a distinctly difference in the mechanical behavior between the two types of specimen.During the initial loading process,the external loading of the tailings sample is mainly borne by the cohesive force.As the loading continues,the cemented structures are partially destroyed within the areas that reach their elastic limit,resulting in a decline of the cohesive force.Meanwhile,the external loading was gradually carried on by the frictional force.After the cemented structure begins to fail,the growth rate of the deviatoric stress of the tailings sample gradually slows down,or even reverses to a negative value.(4)A series of cyclic triaxial shear tests were performed for the freeze-thaw tailings specimens under undrained conditions.Three sets of both experimental confining pressure(σ3)and cyclic shear stress ratio(CSR)were applied in the cyclic triaxial test,where the dynamical mechanical properties,involving cyclic shear resistance,dynamic shear strain,dynamic pore pressure and the number of vibrations to reach failure,were investigated.As it has been done in the conventional triaxial test,the cyclic triaxial shear test was also performed on the frictional specimens for the purposed of comparison.From a general point of view,the experimental results indicate that the increase in the cycles of freeze-thaw leads to the deterioration of the cyclic shear resistance,which accelerates the accumulation rate of residual shear strain and pore water pressure.Besides,the increase of cyclic shear stress ratio and confining pressure will also cause an increase in the accumulation rate of the residual shear strain.On the other hand,the pore pressure curve shows a good normalization with regard to cyclic shear stress ratio.Under the same condition of freeze-thaw cycles and confining pressure,an empirical formula similar to the Seed model can be used to roughly predict the pore pressure curve.Finally,owing to the complex composition,the tailings sample presents a high cohesive force.Its cemented structure is almost completely destroyed after cyclic loading and remolding.Hence,when compares with the tailings specimen,the friction sample presented a significantly lower cyclic shear resistance under the cyclic loading,which is coincident with a higher accumulation rate of its residual shear strain and its dynamic pore water pressure.(5)In the framework of Breakage Mechanics of geotechnical materials,a binarymedium structure,composted of bonded elements and frictional elements,was abstracted from tailings as its main configuration.Based on the binary-medium conception,the failure mechanism of tailings was analyzed from both macro and micro points of view.By introducing micromechanics of composite materials,the local and total strain concentration tensors have been derived in order to describe the relationship between the strain of bonded elements and fraction elements,as well as the macro strain of the representative volume element(RVE).Meanwhile,the evolution law of breakage ratio was assumed through the breakage analysis.And the generalized Hooke’s law and a double hardening constitutive model were applied to predict the mechanical behaviors of the bonded elements and friction elements,respectively,thereby establishing a binary-medium constitutive model of tailings in a concise way.Subsequently,the results of the CD tests under different freeze-thaw cycles and confining pressure are predicted by the model proposed.The research results show that the proposed model can capture the main mechanical properties of bonded blocks and weakened bands of the geotechnical materials to some extent.Depending on the different parameters selected,the model can duplicate the strainsoftening and bulk dilation behaviors of tailings at the condition of a very few number freeze-thaw cycles and of lower confining pressure.In addition,it can also predict the strain hardening and bulk contraction properties as the increase of the number of freeze-thaw cycles and the confining pressure.(6)According to the dynamic elastoplastic theory,the cyclic loading was divided into three stages,involving the initial loading stage,the unloading stage and the reloading stage.Based on the segmentation,the constitutive relationships were established for each stage of the cyclic loading,respectively.However,they were finally written in a uniform expression by considering that the breakage parameters do not change within the proceeding loading or unloading process.The proposed model can interpret the failure mechanism of the freeze-thaw tailings from both macro and micro points of view,and it also is able to duplicate the development of the dynamic shear strain and the dynamic pore pressure to some extent.Depending on the different parameters selected,the model could predict the accumulation effect of the residual shear strain and the pore pressure under the different conditions of freezethaw cycles(NFT)and cyclic shear stress ratio(CSR).Thus,it initially verified the applicability of the model proposed.