Study Of Deformation And Strength Of Loess With Triaxial Test Based On Digital Image Measurement

Loess is widely distributed in China.Besides the engineering properties of general soil, loess has particular mechanics characteristics,such as collapsibility,structural property,and unsaturated.In order to meet requirements in the geotechnical engineering construction in loess area,study on engineering characteristics of loess is an important field of research on the geotechnical engineering and geotechnical mechanics.But accumulation of loess's test data is not enough abundant,because the limitation of triaxial test means,and the long test period,and much time consuming of the mechanics tests of loess.Digital image measurement method,which is based on sub-pixel comer detection,is a new technique of triaxial specimen deformation measurement.This technique measures the specimen deformation through the tracking of comer detection displacement,and the result precision can be up to sub-pixel level.This new technique can measure the triaxial specimen deformation of both saturated and unsaturated soil more exactly,and can deeply analyze the soil stress-strain property under triaxial stress state.As a typical unsaturated soil,it is difficult to accurately confirm the deformation of loess sample in order to study the characteristics of collapsible deformation and strength using conventional triaxial test method.A new deformation measurement method is provided in triaxial test of loess by using digital image measurement system.In this paper,deformation and strength characteristics of loess in the process of consolidation,collapse and shearing are analysed and inquired by means of triaxial staged consolidation test,triaxial collapsibility test, triaxial moistening test under different stress ratio and consolidation-undrained triaxial shear test of Yangling Q₃ loess in Shanxi by using of GDS triaxial testing apparatus along with digital image measurement system.Firstly,the relationship between strain and time of loess under different confining conditions is obtained by analyzing data of triaxial staged consolidation,the influence scope of end restraint is discussed,and the critical strength is determined through analysis of the relation between confining pressure and volumetric strain.The research shows that,to axial strain,radial strain and volumetric strain,each of which and time can be described with hyperbolic curve under the certain confining pressure.The final strain under each grade pressure can be obtained through coordinate transformation,and the final strain has an exponential relationship with the confining pressure by fitting.The formula found by fitting that can predict the consolidation deformation of soil under next grade load.The Soil specimen is divided into 11 segments and the radial strain is analyzed,what can find that the deformation difference of the middle five segments is uniform,and the deformation of other parts is great different.Therefore,in order to reduce the effect of end constraint on the deformation of specimen,using the data of middle five segments is proposed to analysis.The results based on the remolded loess triaxial collapsibility test can educe the time that every cross-section of specimen achieved required collapsible radial strain.The relationship between the accumulated collapsible volumetric strain rate of soil and the time is analyzed, and the factors of collapsible deformation for remolded loess are discussed.Study shows that collapsible strain with confining pressure,initial water content and initial dry density have reciprocal proportion relations.The collapsible strain can be gained by using the data of middle five segments of the specimen to process that significantly higher than the collapsible strain of overall specimen.However,due to the effect of end constraints is different on the soil under different initial conditions,so the curves of two methods are not exactly same.Based on the triaxial moistening test of remolded loess under different deviatoric stress,ε_vp^*,the collapsible volumetric strain in the unit spherical stress is defined in order to show the contribution rate of stress on the collapsible volumetric strain.Under the same water content condition,the greater confining pressure,the greater collapsible volumetric strain,but at the same time,the collapsibile volume strain under the unit spherical stress was smaller when the confining pressure was greater.The mathematical expression ofε_vp^* and water content w is made by the test results,which included two parameters those are the collapsible volumetric strain under the peak unit spherical stress and the characteristic water content w^*. Through normalization processing found that 4 kinds of k value and 4 kinds of confining pressure combined 16 kinds of circumstances,the test results are distributed in S-band.By an average of treatment,the average characteristic curve,the lower envelope and the upper envelope are determined by the average,maximum and minimum value of the characteristic water content.To parameterε_vpmax^*,the lower confining pressure,the greater the difference of eachε_vpmax^* under different k values conditions.The results reflect that the remolded loess can generate more collapsible volumetric strain in the unit spherical stress under the low confining pressure and the high deviatoric pressure consolidation,which is not conducive to security at actual projects,so it should try to avoid generating low confining pressure and high deviatoric pressure consolidation in the project.Finally,stress-strain curve types of remolded loess under different initial conditions are analyzed,and the influence factors such as confining pressure,initial water content,initial dry density are discussed,the relationships between shear strength and water content,dry density are studied.It is found that strain softening and hardening both appeared in the stress-strain curves of remolded loess under different initial water content and initial dry density. Compared the result of local deformation with overall deformation,confining pressure restricts the influence which end restraint to measurement result under higher confining pressure.The study also indicates that the advantages of data processing with the local deformation are more accurate measurement of strain softening curve,much clearer break point of volumetric strain.The relationships between the cohesive strength,the friction angle and water content of remolded loess are studied.And the research finds that the cohesive strengths and the friction angles confirmed with the local deformation and overall deformation both minish along with the accretion of water content,increase along with the increase of dry density,and the friction angle obtained from the local deformation is smaller than the overall deformation.Analysis indicates that the cohesive strength has an exponential relationship with the water content,the relationship between the friction angle and water content conforms to quadratic polynominal function.The influence on cohesive strength from water state variable plays main effect only when the level of water content is lower,the debilitation is not significant on the shear strength when the level of water content is higher. The influence on the friction angle from water state variable plays main effect only when the level of water content is higher,the debilitation is not sigcificant on the shear strength when the level of water content is lower.