Study On The True Triaxial Tests And Strength Characteristics Of The Q₃Loess

Considering the deficiencies of the conventional triaxial shear test and the the directshear test, a new true triaxial apparatus was used to test the strength behaviors of the Q3undisturbed and remolded loess under the three-dimensional principle stress condition. Themost significant feature of this new true triaxial apparatus is that it can independently load theprincipal stress to reflect the actual stress conditions in the soil. In this paper, a series of truetriaxial tests were carried out to research the effects of the water content, dry density,confining pressure, and the principal stress ratio b on the strength behaviors of the Q3undisturbed loess and remolded loess. Based on the test results, the properties of stress-strainand principal strains in different conditions were analyzed. Furthermore, the test results werecompared with the Mohr-Coulomb, Lade-Duncan and twin shear strength criterions in p-qplanes and π planes, respectively. The main conclusions of this study are outlined below.1）Focusing on the properties of stress-strain and principal strains in different conditions,it can be found that the deviatoric stress decreases with the increase of water content, andincreases with the increase of confining pressure and principal stress ratio. In the respect oflateral deformation, the greater confining pressure and the smaller principal stress ratio are,the smaller swell increment in small principal stress direction and the greater compression inintermediate principal stress direction will be. Paying attention to the volumetric strain, undercondition of b=0, the greater confining pressure is, the greater volumetric strain will beneeded to reach the same major principal stress; under the condition of b>0, however, thegreater confining pressure is, the smaller volumetric strain will be needed to reach the samemajor principal stress. Similarly, the greater dry density is, the smaller volumetric strain willbe needed to reach the same major principal stress. Besides, the greater dry density anddeviatoric stress are, the greater absolute value of small and intermediate principal stress willbe.2）In this study, the failures of loess are the lateral expansion failure, the trapezoidalfailure, and the conical damage. 3）Focusing on the strength envelope in the π planes, under the same water content, thefailure surfaces of both undisturbed and remolded loess are extended outward with theincrease of hydrostatic stress, which are a series of similar curves in the π planes. On thecontrary, in the same hydrostatic stress, the failure surfaces of both undisturbed and remoldedloess are converge inward with the increase of water content. Besides, under the same watercontent, the failure surfaces of the undisturbed loess are greater than that of the remoldedloess.4）Comparing the test results with the Mohr-Coulomb, Lade-Duncan and twin shearstrength criterions in p-q planes and π planes, it can be found when the Mohr-Coulombcriterion is adopted to describe the undisturbed and remolded loess, the strength of the loess isoverall smaller than that of the true triaxial test results. When the water content is20%, theconsistency between the Mohr-Coulomb criterion and the true triaxial test results is satisfying.Similarly, when the twin shear strength criterion is adopted, the strength of the loess is alsooverall greater than that of the true triaxial test results. It’s worth noting that, however, thedifference between the twin shear strength criterion and the true triaxial test results willincrease with the increase of the value of b.