Generalised sand behaviour
The response of sands under generalised loading conditions has been studied in the Soil Mechanics laboratory with the aid of the Hollow Cylinder apparatus for the past decade. Although these studies concur on the significance of principal stress rotation on the response of sands, quantifying the effects of anisotropy has always remained a challenge. Given that the load carrying structure re-adjusts to a specific combination of applied stress ratio and rotation when sheared under free principal stress rotation (Georgiannou and Konstadinou, 2014), current research is concentrated on imposing continuous stress rotation to assess its effect on sand response; a condition invariably encountered in practical geotechnical engineering problems.
To study sand’s anisotropy under generalised loading, the testing program includes rotation of the stress principal axes with either monotonically increasing, constant or cyclically changing deviatoric stress.
Preliminary observations show that stress-dilatancy theories based on monotonic experiments under various fixed principal stress directions dramatically fail to predict the anisotropic soil response when principal stress axes begin to rotate. It can be seen that contraction of the sand is initiated, under constant mean effective stress and stress ratio, solely by principal stress rotation. Under these conditions the amount of contraction and/or dilation can be associated with fabric changes.
The dilatancy and non-coaxiality of sand were investigated under generalised loading conditions with either fixed or rotating stress principal axes. Having observed that under loading at fixed stress principal axes coaxiality between the principal directions of fabric and stress is achieved ultimately, fabric changes following the rotation of the stress principal axes will be examined by changing the number of cycles, alternating rotations, applying bouts of rotations followed by increases in stress ratio with fixed or rotating stress principal axes.
An effort is made to correlate the anisotropy in sands’ behaviour with the anisotropy in fabric characteristics and present the techniques used to quantify these characteristics; the magnitude of fabric anisotropy defined as the difference or the ratio between the major and minor principal values of the fabric tensor.
funding: (European Social Fund and Greek national funds, Project code MIS:3049187). 2019-2021