Adrian R. Russell

Development of a new calibration chamber for conducting cone penetration tests in unsaturated soils

A calibration chamber has been developed to conduct laboratory-controlled cone penetration tests in unsaturated soils. The chamber allows independent application of lateral and vertical pressures to an unsaturated soil specimen. Horizontal pressure is applied by cell water pressure pushing on a rubber membrane enclosing the specimen, while vertical pressure is maintained by a hydraulic loading ram at the base of the specimen. Suction is controlled using the axis-translation technique. Air pressure is applied to the top of the chamber where it spreads uniformly across the top of the specimen. Pore-water pressure is applied through eight high air-entry value porous disks embedded in the bottom plate. A particularly original aspect of the chamber design is the specimen formation system comprising four moveable cylinder quarters, which enables the creation of specimens of repeatable properties from a variety of soil types. The results of typical cone penetration tests conducted on dry, saturated, and unsatura...

Particle Crushing in Granular Assemblies

Idealised assemblies of equally sized spherical particles are subjected to a range of macroscopic compressive principal stresses and contact forces are determined. Stress fields within individual particles are studied and a failure criterion for brittle materials imposed indicating that crushing occurs when the maximum contact force reaches a threshold particle strength value, irrespective of the presence and magnitude of other lesser contact forces acting on the particle. Failure surfaces are drawn in the three dimensional principal stress space.

Interaction between Retaining Walls and Unsaturated Soils in Experiments and Using Slip Line Theory

AbstractRetaining walls are widely used for shoring excavations and stabilizing earth slopes. They often support naturally formed soils above the groundwater table and compacted soils, which are in...

Cavity Expansion Theory and the Cone Penetration Test in Unsaturated Sands

Cavity expansion theory has long been used to interpret cone penetration test results in saturated soils. The pressure required to expand a spherical cavity approaches a limiting value at large strains, and this limiting value is thought to be related to cone penetration resistance. However, there is very little experience in interpreting the cone penetration test in unsaturated soils using cavity expansion theory. Practitioners often use correlations developed for saturated soils which may lead to non-conservative misrepresentations in estimated soil parameters. This paper gathers the results of cone penetration tests in two unsaturated quartz sands and highlights the similarities between spherical cavity limit pressure and cone penetration resistance. In some cases the effects of suction causes both the cone penetration resistance and cavity limit pressure to be doubled. The contribution of suction to effective stress is back-calculated using these results.

A computational geometry approach to pore network construction for granular packings

Pore network construction provides the ability to characterize and study the pore space of inhomogeneous and geometrically complex granular media in a range of scientific and engineering applications. Various approaches to the construction have been proposed, however subtle implementational details are frequently omitted, open access to source code is limited, and few studies compare multiple algorithms in the context of a specific application. This study presents, in detail, a new pore network construction algorithm, and provides a comprehensive comparison with two other, well-established Delaunay triangulation-based pore network construction methods. Source code is provided to encourage further development. The proposed algorithm avoids the expensive non-linear optimization procedure in existing Delaunay approaches, and is robust in the presence of polydispersity. Algorithms are compared in terms of structural, geometrical and advanced connectivity parameters, focusing on the application of fluid flow characteristics. Sensitivity of the various networks to permeability is assessed through network (Stokes) simulations and finite-element (Navier-Stokes) simulations. Results highlight strong dependencies of pore volume, pore connectivity, throat geometry and fluid conductance on the degree of tetrahedra merging and the specific characteristics of the throats targeted by the merging algorithm. The paper concludes with practical recommendations on the applicability of the three investigated algorithms.

Three-Dimensional DDA and DLSM Coupled Approach for Rock Cutting and Rock Penetration

AbstractRock cutting and rock penetration are typical problems in civil, mining, petroleum, and geothermal engineering disciplines. They involve dynamic fracturing and fragmentation of rock, high-speed movements of a cutter/impactor, and complex dynamic contacts between the cutter/impactor and the rock. In this study a new three-dimensional (3D) coupled approach is developed to address these problems. The distinct lattice spring model (DLSM) is used to simulate the dynamic fracturing process of the rock, and the discontinuous deformation analysis (DDA) is adopted to model the high-speed motion of the cutter/impactor. An explicit-implicit coupling scheme is developed to bridge DLSM and DDA. Moreover, to take account of interaction between DLSM and DDA, a 3D simplex sphere-to-block contact method is introduced. Finally, a number of numerical examples are conducted to verify the implementation of the coupled approach and its ability to model rock cutting and rock penetration problems.

Rigid Retaining Walls Interacting with Unsaturated Soils in Axial Symmetry

The slip line theory is applied to the problem of an axisymmetric retaining wall interacting with unsaturated soil. Active and passive failures of a rigid vertical wall are considered. The slip line governing equations assume the limiting equilibrium state of a Mohr-Coulomb soil. A linear variation of the contribution of suction to the effective stress with depth is assumed. A standard finite difference method is used to solve the governing equations. The finite difference procedures were validated with recently published literature on axisymmetric retaining walls retaining dry soils. It is shown that adopting the effective stress concept enables the influence of suction in unsaturated soils to be considered in a simple way. An example of analysis is presented to illustrate this. Moreover, the paper shows significant influence of the length scale and the magnitude of the circumferential stress on computed earth pressures.

Pore Characterisation in Monodisperse Granular Assemblies

The micro-scale geometric arrangement of pores was quantitatively characterised for monodisperse granular assemblies, particularly in relation to pore volume distribution and pore orientation characteristics. Using physical experiments and numerical simulations, the pore volume distribution was uniquely described by the analytical k-gamma distribution function [1-2]. A pore orientation tensor was defined to determine the preferred orientation of individual pores. This was subsequently used to define a global orientation tensor that revealed an isotropic pore network for the monodisperse granular assemblies considered in this study. The global orientation tensor was analytically linked to the parameters defining the pore volume distribution.

Microstructural pore changes in Gosford sandstone during compressive loading

X-ray CT scans were taken of Gosford sandstone under successively increasing pre-failure loads in unconfined compression to investigate the evolution of microstructural pore properties. It was found that the number of pores increased during early stages of loading, prior to a reduction of pore numbers at a point also corresponding to a slight stiffening of the stress-strain curve. Pore surface area, orientation and fractal dimension also showed subtle variations as loading progressed. Further, orientations of newly created pores tended to be near vertical and therefore perpendicular to the direction of maximum tensile strains. Energy dissipations associated with the creation of new surface and the redistribution of elastic stored energy through the newly fractured sandstone were back-calculated using the observed stress-strain response.A general increase in dissipation rate with the applied load was observed in the pre-failure region.

Cavity Expansion in Soil of Finite Radial Extent

The problem of drained cavity expansion in a soil of finite radial extent is investigated. Spherical cavities expanded from zero radius subjected to a constant stress condition at the finite boundary are considered. The new analytical solution procedure presented enables more advanced constitutive models to be implemented than possible than when using other solution procedures. Cavity expansion results generated for a Sydney quartz sand highlight substantial differences between cavity limit pressures for boundaries of finite and infinite radial extent.