Investigation of stress-dilatancy response at low stress levels with emphasis on simple shearing conditions

Abstract

In the present work, the assessment of dilatancy angle of cohesionless soils under low confining pressures and its contribution to peak friction angle are investigated. For this purpose, 3 different types of tests are conducted to obtain peak angle of dilation, peak friction angle and critical state friction angle on Kilyos sand. Alternative methods are used because of triaxial test being a time-consuming procedure and the apparatus being relatively expensive. Besides, triaxial tests being not practical to conduct ay very low stress levels is another reason to use alternative methods. An equipment which would allow the computation of peak dilatancy in a very short period of time is designed together with its electronic components and the required Arduino code. The main philosophy behind the equipment is to impose uniform shearing strain and to measure the associated change in the volume of a sample. Later the correlation between shear and volumetric strains is considered to calculate rate of dilation. With the devised mechanism, peak dilation angles of sand specimens with relative densities ranging from 14% to 79% are determined under the pressure resulted solely from their own weights. The equipment has transparent sidewalls that allow capturing images. Images captured during the tests are used for particle image velocimetry analyses. With the results of PIV analyses, shear planes, slip surfaces and the associated strain fields are determined. The number and intensity of shear planes are inspected and it is noted that their numbers increase with increasing relative density. Besides, the angle that active shear band makes with the horizontal axis is observed to be increasing as the relative density increases. Furthermore, sample uniformity during shearing is studied by plotting volumetric strain – shear strain relationship for different positions within samples. This aided the investigation of shear localization response. Examining the graphs obtained from both the tests and the associated PIV analyses, it is realized that volumetric strain versus shear strain relationship can be represented very well with a 3rd order polynomial. Moreover, for the same level of stress and density combinations, peak friction angles of the soil are determined using a simple test that is available in literature. Lastly, critical state friction angle is obtained using a simple method. Having experimentally determining the magnitudes of ψ_p, φ_p^ and φ_cs^, stress – dilatancy parameters r, Q and R are determined.