Khalid A. Alshibli

Assessment of Localized Deformations in Sand Using X-Ray Computed Tomography

The internal fabric and localized deformation patterns of triaxial sand specimens were investigated using computed tomography (CT). Three displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses (0.05, 0.52, and 1.30 kPa) in a microgravity environment aboard the Space Shuttle during the NASA STS-79 mission. CT scanning was performed on these flight specimens, as well as on an uncompressed specimen and a specimen tested in a terrestrial laboratory at 1.30 kPa effective confining stress. CT demonstrated high accuracy in detecting specimen inhomogeneity and localization patterns. Formation of deformation patters is dependent on the effective confining stress and gravity. Multiple symmetrical radial shear bands were observed in the specimens tested in a microgravity environment. In the axial direction, two major conical surfaces were developed. Nonsymmetrical spatial deformation was observed in the 1-G specimen. Analysis tools were developed to quantify the spatial density change. Void ratio variation within and outside the shear bands is calculated and discussed.

Influence of pore water chemistry on the swelling behavior of compacted clays

The influences of the exchange complex and pH of the solution used for cation saturation on Atterberg limits, compaction, and swelling potential of a compacted clay were investigated. The study involved transforming the exchange complex from a heterogeneous to a homogeneous one so that a frame of reference can be set for the clay behavior under such an ideal condition. The employed method for altering the exchange complex successfully yielded homo-ionic clay. The introduction of different species of cations gave rise to different particles associations. When introduced to the tested clay, potassium cations bond its particles with a rather strong bond (K-linkage), causing a drastic decrease in the specific area of the clay (about one-fourth of its untreated specific area), a decrease in the CEC, as well as a drastic decrease in the swell potential. For example, the swell pressure decreased from 1.87 kg/cm2 for the untreated samples to 0.4 kg/cm2 for the K-treated samples (under the same conditions). Also, the swell potential vs. time relationships can be modeled accurately using a rectangular hyperbola.

Strength Properties of JSC-1A Lunar Regolith Simulant

The paper presents a description of physical properties and a characterization of shear strength behavior of JSC-1A lunar regolith simulant. Specific gravity of solids, particle size analysis, and minimum and maximum index densities were measured according to ASTM standards. A series of axisymmetric triaxial experiments were conducted on JSC-1A specimens at two densities and a wide range of confining pressures. The stress-strain and volume change behavior of JSC-1A is presented and discussed. The results show that the peak friction and dilatancy angles are highly influenced by specimen density and confining pressure. Two models were developed to predict the peak friction and dilatancy angle, as a function of relative density and mean effective stress at the critical state. It was found that JSC-1A closely resembles the morphology and specific gravity of solids of some of Apollo regolith samples reported in the literature. Shear strength properties are presented within the framework of Mohr-Coulomb and friction-dilatancy theories. The effect of reduced moon gravity on strength parameters is also presented and discussed.

THE LOUISIANA PLANE STRAIN APPARATUS FOR SOIL TESTING

The mechanical description of a new plane strain (biaxial) apparatus for soil testing is presented. The design took into consideration flexibility in accomodating different specimen sizes, easy assembly procedure, and well-controlled boundary conditions. The apparatus is well instrumented with load, displacement, and pressure sensors and has the capabilities to capture localization and shear band development. A comparison between two experiments that were conducted on F-75 Ottawa sand is presented and the effect of inhibiting lateral movement of bottom end platen is presented and discussed. It has been found that restraining lateral movement of bottom end platen has resulted in higher peak and residual load values.

Combined TDR and P-Wave Velocity Measurements for the Determination of In Situ Soil Density--Experimental Study

This paper summarizes a new nondestructive approach for the evaluation of soil density and water content. This new measurement methodology involves evaluating the dielectric permittivity and the P-wave velocity in soils as the water content is increased. These values are then related to the volumetric water content, porosity, and skeleton shear stiffness, which are needed to back-calculate the density and water content of the tested soil specimens. Experimental laboratory results are briefly summarized. These test results show a potential for developing a new device. Electronic equipment and sensors for the proposed device include a TDR system, miniature piezoelectric accelerometers, signal conditioner system, and oscilloscope for data acquisition.

Shear Band Characterization of Triaxial Sand Specimens Using Computed Tomography

A thorough quantitative analysis of the internal density distribution and strain localization of axisymmetric triaxial sand specimens is presented. Computed tomography technique was used to acquire detailed three-dimensional images of a series of Ottawa sand specimens subjected to Conventional Triaxial Compression (CTC) conditions at very low effective stresses in microgravity and terrestrial laboratories. Analysis tools were developed to quantify the distribution of local void ratio, track the onset, propagation, thickness, and inclination angle of shear bands, and calculate the variation of void ratio within and outside shear bands. It has been found that shear bands initiate in the post-peak strength regime in CTC specimens, where a rather complex pattern of shear bands develops such that behavior is highly influenced by large-scale kinematics of the specimen. Four main deformation patterns were identified and their contribution to the overall volume change of the specimens was quantified.

Evaluating the Light Falling Weight Deflectometer Device for In Situ Measurement of Elastic Modulus of Pavement Layers

Field and laboratory testing programs were conducted to evaluate the potential use of the light falling weight deflectometer (LFWD) device for measuring the in situ elastic modulus of pavement layers and subgrades. The field tests were conducted on several highway sections selected from different projects in Louisiana. In addition, nine test sections were constructed and tested at the Pavement Research Facility site of Louisiana Transportation Research Center. All sections were tested using the Prima 100 model-LFWD in conjunction with the falling weight deflectometer (FWD), plate load test (PLT), and dynamic cone penetrometer (DCP) tests that were used as reference measurements. Linear regression analyses were carried out on the collected test data to develop models that could directly relate the LFWD stiffness modulus with moduli obtained from FWD and PLT and the DCP penetration rate. In addition, multiple nonlinear regression analyses were conducted to develop models that could predict FWD and PLT moduli on the basis of the LFWD elastic moduli and selected soil properties (moisture content and void ratio) of the tested materials. The results showed that the FWD, PLT moduli, and DCP-penetration rate could be predicted directly with LFWD at a significant confidence level. However, the prediction models were improved when the soil properties were included as variables. Laboratory tests also were conducted to determine the influence depth of the LFWD, and the results of these tests showed that the LFWD influence depth ranged from 270 to 280 mm.

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...

ESTIMATING VOLUME CHANGE OF TRIAXIAL SOIL SPECIMENS FROM PLANAR IMAGES

A series of conventional triaxial experiments were conducted on sand specimens aboard the Space Shuttle during the NASA STS-79 mission. A 360° video coverage to monitor the specimen deformations was recorded during the experiments. Post-flight analyses included retrieving the video images for further analysis. Analysis tools were developed to correct distortion in specimen images due to using wide-angle lenses employing quadratic polynomial functions implemented in a computer code. Planar projections of specimen video images were used along with the x-ray computed tomography images to reconstruct three-dimensional (3D) renderings of triaxial sand specimens at different axial strain levels. Volumetric change of the specimens was accurately calculated and compared to measurements recorded during the Shuttle flight. 3D renderings that show the progress of specimen deformations were also developed and displayed. Such results are valuable for documenting true 3D recordings of soils instability phenomena tested using standard triaxial compression procedures and for applications related to educational demonstrations of conventional testing of soils.

EVALUATING THE POTENTIAL USE OF A PORTABLE LFWD FOR CHARACTERIZING PAVEMENT LAYERS AND SUBGRADES

An evaluation of the Light Falling Weight Deflectometer (LFWD) device to reliably measure the in-situ elastic modulus of pavement layers and subgrades is presented in this paper. For this purpose, field tests were conducted on selected highway sections from different periods within Louisiana. In addition, six test sections were constructed and tested at the Louisiana Transportation Research Center (LTRC) Pavement Research Facility (PRF) site. All sections were tested using the Prima 100 model - LFWD in comparison with other standard tests including the Falling Weight Deflectometer (FWD) and the Plate Load Test (PLT) that were used as reference measurements. Regression analyses were conducted to determine the best correlations between the elastic modulus obtained from LFWD and those obtained from FWD and PLT tests. Good correlations were obtained, which demonstrated that the LFWD can be a promising device for in-situ characterizing of highway layers and subgrades.