Andrew Druckrey

Quantifying Morphology of Sands Using 3D Imaging

AbstractParticle morphology plays a significant role in influencing engineering behavior of granular materials. Surface texture, roundness, and sphericity represent distinct multiscale measures needed to fully describe particle morphology. Most studies reported in the literature rely on two-dimensional (2D) projected images of particles with a few three-dimensional (3D) images that mostly focused on relatively large-size aggregate samples. In this paper, 3D synchrotron microcomputed tomography (SMT) was used to acquire high-resolution images of glass beads, F-35 Ottawa sand, #1 dry glass sand, GS#40 Columbia sand, Toyoura sand, and Hostun RF sand. New roundness and sphericity indexes are proposed and calculated for the samples based on 3D measurements of surface area, volume, and three orthogonal diameters of particles. In addition, the surface texture of particles were measured using optical interferometry technique. The measurements reported in this paper can serve as a good source for other researchers...

Shear bands as bottlenecks in force transmission

The formation of shear bands is a key attribute of degradation and failure in soil, rocks, and many other forms of amorphous and crystalline materials. Previous studies of dense sand under triaxial compression and two-dimensional analogues from simulations have shown that the ultimate shear band pattern may be detected in the nascent stages of loading, well before the bands known nucleation point (i.e., around peak stress ratio), as reported in the published literature. Here we construct a network flow model of force transmission to identify the bottlenecks in the contact networks of dense granular media: triaxial compression of Caicos ooid and Ottawa sand and a discrete element simulation of simple shear. The bottlenecks localise in the nascent stages of loading —in the location where the persistent shear band ultimately forms. This corroborates recent findings on vortices that suggest localised failure is a progressive process of degradation, initiating early in the loading history at sites spanning the full extent, yet confined to a subregion, of the sample. Bottlenecks are governed by the local and global properties of the sample fabric and the grain kinematics. Grains with large rotations and/or contacts having minimal load-bearing capacities per se do not identify the bottlenecks early in the loading history.

3D Behavior of Sand Particles Using X-ray Synchrotron Micro-Tomography

Granular materials are composed of discrete particles in contact with each other. At the micro scale, each particle is in contact with neighboring particles where individual particle properties and contact mechanics govern particle interaction. At the meso scale, multiparticle assemblies interact through contact networks at particle contact. In this paper, synchrotron X-ray micro-tomography (SMT) is used to study particle kinematic behavior during in situ axisymmetric triaxial testing. High-resolution SMT images are used to discretize and track individual particles at multiple SMT-imaging stages of axisymmetric triaxial experiment. A particle-tracking algorithm, similar to digital image correlation, was developed by the authors to track individual particles throughout the entire test, and particle kinematics are quantified at each imaging stage. The algorithm allows tracking of individual particles and quantifies the relative (between images) and absolute (total) translation and rotation of particles as the test progresses.

ONR MURI project on soil blast modeling and simulation

Current computational modeling methods for simulating blast and ejecta in soils resulting from the detonation of buried explosives rely heavily on continuum approaches such as Arbitrary Lagrangian-Eulerian (ALE) and pure Eulerian shock-physics techniques. These methods approximate the soil as a Lagrangian solid continuum when deforming (but not flowing) or an Eulerian non-Newtonian fluid continuum when deforming and flowing at high strain rates. These two extremes do not properly account for the transition from solid to fluid-like behavior and vice versa in soil, nor properly address advection of internal state variables and fabric tensors in the Eulerian approaches. To address these deficiencies on the modeling side, we are developing a multiscale multiphase hybrid Lagrangian particle-continuum computational approach, in conjunction with coordinated laboratory experiments for parameter calibration and model validation. This paper provides an overview of the research approach and current progress for this Office of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI) project.

Influence of Loading Rate on Fracture Strength of Individual Sand Particles

Dynamic loading on granular materials, such as impact, blast, or projectile penetration, can impose large inter-particle forces to cause significant particle fracture within individual particles. Extensive research has been conducted at different strain rates on granular media mass, but very little has been published to investigate the influence of strain or loading rate on individual particles. Therefore, a gap in the knowledge base is present since comprehensive multi-scale modeling of granular material begins at the micro (particle) scale. In this paper, individual natural sand particles are compressed to fracture at loading rates of 0.2 mm/min, 2.25 m/s, and 14.5 m/s using quasi-static unconfined compression and unconfined mini-Kolsky bar techniques. Fracture loads are compared for various “types” of particles within the natural sand, and compared to conventional quasi-static failure definitions for particles. Particles exhibited loading rate dependence when comparing Weibull characteristic tensile strength with loading rate.

Evaluation of Compacted Aggregate Base Course Layers

Field and laboratory tests were conducted on 10 projects during base course layer construction to evaluate the quality of the constructed base layers. Base aggregates were also collected from these sites for laboratory testing. The field testing program consisted of the in place density by the sand cone method, the dynamic cone penetration (DCP) test, the light weight deflectometer (LWD) test, and the GeoGauge test. Laboratory tests conducted are the particle size analysis, the standard compaction test (AASHTO T 99), and the repeated load triaxial test (AASHTO T 307) for determining the resilient modulus. Analyses were conducted on field and laboratory test results. High spatial variability in field density and moisture content exists in base course layers under construction, as demonstrated by the relative compaction test results. High variability exists along the depth of base course layers, as demonstrated by the dynamic cone penetrometer test results and the estimated profile of California Bearing Ratio (CBR) along the depth of the investigated base layers. Spatial variability and non-uniformity were also demonstrated by the results of the light weight deflectometer and GeoGauge, in which the layer modulus varies within a large range of values.

Construction of poly-ellipsoidal grain shapes from SMT imaging on sand, and the development of a new DEM contact detection algorithm

This paper aims to construct smooth poly-ellipsoid shapes from synchrotron microcomputed tomography (SMT) images on sand and to develop a new discrete element method (DEM) contact detection algorithm.,Voxelated images generated by SMT on Colorado Mason sand are processed to construct smooth poly-ellipsoidal particle approximations. For DEM contact detection, cuboidal shape approximations to the poly-ellipsoids are used to speed up contact detection.,The poly-ellipsoid particle shape approximation to Colorado Mason sand grains is better than a simpler ellipsoidal approximation. The new DEM contact algorithm leads to significant speedup and accuracy is maintained.,The paper limits particle shape approximation to smooth poly-ellipsoids.,Poly-ellipsoids provide asymmetry of particle shapes as compared to ellipsoids, thus allowing closer representation of real sand grain shapes that may be angular and unsymmetric. When incorporated in a DEM for computation, the poly-ellipsoids allow better representation of particle rolling, sliding and interlocking phenomena.,Method to construct poly-ellipsoid particle shapes from SMT data on real sands and computationally efficient DEM contact detection algorithm for poly-ellipsoids.

3D Particle-Scale Displacement Gradient to Uncover the Onset of Shear Bands in Sand

In situ synchrotron micro-computed tomography (SMT) scans of a specimen composed of uniform silica sand were acquired at multiple strains during a triaxial compression test. Individual sand particles were identified and tracked through multiple strain increments. The paper presents the concept of displacement gradient to expose the inception of shear bands (strain localization) in sheared sand. Each particle’s neighboring particles were identified and translation fields for all particles were calculated. The second order norm between the particle translation vector and neighboring particles translation vectors were averaged, resulting in a relative displacement value for each particle. The relative displacement concept is effective to uncover the onset of localized shearing within sheared granular materials.

Field Monitoring of Concrete Piles of an Integral Abutment Bridge

Integral abutment bridges (IABs) are a cost-effective design method for bridges. Recently, Louisiana Department of Transportation and Development (LA-DOTD) constructed their first two IABs; one was supported by HP steel piles driven in clayey soil while the other was supported by pre-stressed precast concrete (PPC) piles driven into fine sand deposit. The use of PPC piles has long been recognized as a good option for Louisiana bridges. However, there are concerns that the rigidity of the piles driven in sandy soils might cause excessive stresses in the bridge superstructure. This paper presents the instrumentation plan of two 36 in (914.4 mm) square PPC piles and the bent-soil interaction. Sisterbar strain gauges were attached to the pre-stressing strands in the piles along with nine pressure cells that were attached to the face of the bent supporting the piles. The bridge deformations were mainly controlled by the piles’ rigidity, soil resistances surrounding the piles, and connection behaviors between the pile-bent. Based on the observed temperature effects, the design of the piles of the Caminada Bay Bridge is very conservative. The piles experienced very low bending moments and very small amount of pressure on the backfill soil.