The Development of a Novel 3D Stadium Shear Device for Perpetually Deforming Granular Soils

Abstract

Soils behave completely differently under different circumstances. For years, we have placed a particular effort into investigating how soils deform under load by conducting intensive laboratory tests. We have designed a variety of experimental apparatuses with which we tried to replicate the true behaviour of soils as they deform in the field. Among those testing configurations, the direct shear box and triaxial test devices are the most widely used conventional tools we use to determine the shear strength of soils. However, samples in those tests can only be sheared under very limited strains as their shape change continuously, thus losing the initial geometrical assumptions used to interpret the data. The shear strength at steady state conditions, known as critical state, is of importance to many geotechnical problems. Since this strength characterises the conditions for the onset of shear-driven natural hazards such as landslides, devices are needed that can shear soil samples to their true critical state. The ring shear device is one of the only few available large deformation devices that has been developed over the years. However, one of its major disadvantages is the imposition of radially dependent stress field on the sample due to its circular geometry. In this Thesis, we propose a new device which we call the three-dimensional “Stadium Shear Device” (3D SSD). Our idea for developing this device is devoted to the study of the shear behaviour of soils as they truly enter and remain in their critical state. It is designed in a manner that will be shown to impose uniform stress conditions. The device can shear granular systems indefinitely, while simultaneously allowing for measurements of shear and normal stresses, and the vertical displacement during the tests. We have particularly investigated the performance of this device under large shear deformations and low-stress levels, to address many applications involving such conditions, including shallow landslides and pipeline movements on seabeds. The stress conditions developed in this configuration are simulated with a Discrete Element Method (DEM) model, with which we get stress responses that are cross-validated with the experimental results obtained by the physical 3D SSD. Most importantly, the DEM simulations also confirm that the stress uniformity in SSD samples. Using this understanding, we complete a Mohr’s circle stress analysis for SSD tests. The performance of the 3D SSD device is first tested with glass beads and Jasmine rice systems, from which the effectiveness of this apparatus in capturing the mechanical properties of granular media is demonstrated. Furthermore, the device also shows its capability for measuring stress responses under different confining pressures. We thereafter conducted more robust experiments with a wider range of natural soils with various properties, including particle shapes, particle gradations and initial densities. The results are compared against results from previous studies, and in general, similar trends are observed. In addition, we characterise the shear strength and dilatancy properties of those soils under low stresses as these were barely addressed in the past. The velocity profiles developed in the 3D SSD are further studied by employing dynamic X-ray radiography, where a thin shear band is observed to develop close to the shear boundaries for systems with relatively small size particles. Hence, a second design is next proposed and constructed. The major change from the original design to the modified one is the slenderness of the modified SSD samples thanks to the belt configuration. The velocity profile in the modified design is then compared to additional X-ray studies with the ring shear device. We find a relatively linear velocity profile within the modified 3D SSD, whereas half of the sample in depth in the ring shear is hardly sheared under comparable shear velocities. Besides, preliminary results related to the particle segregation are also analysed, which show a noticeable effect during shear. Given those new results, further investigation into the roles of the strain uniformity and segregation are recommended, as this will help to determine more accurately the appropriate shear strength properties of granular soils using large deformation shear devices.