Nick Hudyma

Porosity dependence of the elastic modulus of lithophysae-rich tuff: Numerical and experimental investigations

Abstract The influence of porosity on the mechanical properties of rock has received much research attention. Portions of the Yucca Mountain high-level nuclear waste repository may be placed in tuff units containing lithophysal cavities, which are large, generally noninterconnected cavities that can be considered a form of macroscopic porosity. This paper presents the results of numerical modeling and uniaxial compression testing of analog models and tuff rock, in order to assess the relationships between elastic modulus and porosity. The first part of the paper presents numerical simulation of uniaxial compression testing to calculate the elastic modulus of two-dimensional models containing randomly distributed circular holes in plane strain. The range of porosities investigated is approximately 5–40%. In the second part, the elastic modulus determined from the uniaxial compression testing of analog models and tuff specimens is presented. The analog specimens were made of plaster of Paris containing varying amounts of spherical shaped Styrofoam® inclusions to simulate a cavity structure similar to tuff. The results from the numerical analysis and analog material testing show an exponential decrease in elastic modulus with increasing porosity, whereas the elastic moduli of tuff show a linear decrease. The difference in the two behaviors can be attributed to the nonuniform cavity shapes in the tuff specimens.

Field Measurements of Shear Strength of an Underconsolidated Marine Clay

Abstract This paper presents the observations of cone penetration testing (CPT), in situ vane shear testing and undrained triaxial testing of underconsolidated marine clay in the Craney Island Dredged Material Management Area (CIDMMA), Norfolk, VA. Comparison of vane shear measurements, taken during 1981–1983, to CPT measurements, taken during 1993, indicates a change in undrained shear strength. Both the vane shear and cone penetration resistances are lowest at the mid-heights of the clay layers and the excess pore water pressures are highest at the mid-height of the clay layers, indicating that the clay layer is underconsolidated.

Ground Proving Three Seismic Refraction Tomography Programs

During a recent ground-proving exercise at the shared University of North Florida and University of Florida karstic limestone geophysical and ground-proving test site in central Florida, the limestone bedrock surface was mapped along several survey lines with both intrusive and geophysical techniques. Analyses of the site investigation data revealed a highly erratic limestone bedrock surface, which is common in karst terrane. Analysis of seismic refraction data demonstrated that three commercially available refraction tomography software systems can reveal the undulating bedrock surface. However, the tomography data revealed marked differences in the compression wave velocities at the top of the bedrock surface at various locations along one of the survey lines. Compression wave velocities were highest within slots or valleys and lowest at the tops of blocks or pinnacles. This variability is attributed to the geologic history of the limestone, which includes how the limestone was formed and how the limestone weathered. Ground proving via cone penetration tests and geotechnical borings appears to corroborate this finding and demonstrates the importance of measuring multiple material parameters during site characterization activities in complex terranes.

Changes in Swell Behavior of Expansive Clay Soils from Dilution with Sand

Volume changes associated with expansive soils cause considerable distress to lightly loaded engineered structures, especially slab-on-grade residential structures, roadways, and masonry block walls. There are numerous techniques, such as physicochemical stabilization, mechanical stabilization, and preventative practices, that are recommended for mitigating the effects of expansive soils. In this paper, two expansive soils from southern Nevada were mixed with varying percentages of fine silica sand, and a series of laboratory tests were conducted on the soil mixtures to assess changes in soil classification, plasticity index, swelling behavior through surcharge expansion tests, and potential volume change (PVC) tests. Additionally, the clay mineralogy was determined by x-ray diffraction. For both soils, an increase in the percentage of mixed silica sand by 0 to 75 percent decreased the plasticity index such that the soils were categorized as non-expansive or having very low swell potential. The relationship between plasticity index and percentage of added sand was described using a simple dilution model. The swell characteristics of the soils, based on surcharge expansion tests and potential volume change tests, decreased with increasing percentages of mixed silica sand, and non-soil-specific empirical models were developed to predict the changes. These experiments show that soil mixing is a promising technique that could be implemented to mitigate the effects of expansive soils.

Quantifying Surface Roughness of Weathered Rock - Examples from Granite and Limestone

It is well established within the geotechnical community that weathering affects rock in a variety of ways. Weathering not only degrades engineering properties but also changes the surface appearance. Weathering classifications are partially based on the qualitative assessment of the surface appearance of rock. This study focuses on quantifying surface roughness of weathered rock. Surface textures of both granite and limestone specimens were collected using a commercially available laboratory laser scanning system. Point clouds were analyzed using two different techniques to assess surface roughness. Granite surfaces were assessed using triangulated point clouds and surface normal vectors. Limestone surfaces were assessed using statistical methods to describe deviations from a cylindrical shape. The most highly weathered granite had the largest range of surface normal vector orientation. Less weathered granites had smaller ranges of surface normal vector orientation. Results from weathered limestones were more ambiguous. The least weathered specimens had very small deviations from a perfect cylinder whereas the most weathered specimens had the greatest deviation from a perfect cylinder. However, no clear distinction could be made between deviations from a perfect cylinder for intermediate stages of weathering for limestone specimens. Close range laser scanning was able to capture surface textures from both granite and limestone specimens, however, relationships between weathering grade and surface texture were only statistically significant for granite specimens.

Segmentation of cracks in X-ray CT images of tested macroporous plaster specimens

Precise segmentation of cracks is essential to characterize the structural properties of a rock specimen under compressive force. A two-dimensional internal cross-sectional image of rock can be created using X-ray computed tomography (CT scanning). Cracks in rocks usually have very poor local contrast which makes it difficult to detect and segment cracks from the background using existing popular edge detection algorithms. In this paper, we propose a two-dimensional matched filtering technique followed by local entropy based thresholding, morphological operators and length filtering to detect and segment cracks from the cross-sectional images of rock. The proposed algorithm is tested on several macroporous plaster specimens. Experimental results demonstrate the effectiveness and robustness of the algorithm compared to hand-labeled ground truth segmentations.

Characterization of a Sinkhole Prone Retention Pond Using Multiple Geophysical Surveys and Closely Spaced Borings

Sinkholes are one of the well known geological hazards in the state of Florida. In Alachua County Florida, sinkholes have become problematic in highway retention ponds, especially along State Road 26 west of I-75. As part of a Florida Department of Transportation effort to minimize the effects of sinkholes in retention ponds, multiple geophysical surveys and closely spaced boring were used to characterize a retention pond that had experienced previous sinkhole activity. Within the approximately 0.6 hectare retention pond, a survey grid consisting of lines and stations, both with 1.52 meter spacing was established. One of the survey lines was aligned with 12 geotechnical borings that were previously conducted to establish the site stratigraphy. The borings were placed approximately 3 to 7.6 meters apart. The depth of the borings ranged between approximately 9 to 12 meters. Five different geophysical techniques, ground penetrating radar (GPR), electrical resistivity (ER), gravity survey, multielectrode resistivity (MER), and capacitively coupled resistivity (CCR) were used to characterize the subsurface stratigraphy in the vicinity of the borings. As expected in a karst region, the stratigraphy determined from the closely spaced borings was very erratic; the limestone bedrock surface varied between depths ranging from the ground surface to greater than 9 meters, within a lateral distance of 15 meters. Open voids within the limestone, one over 6 meters in length, were also detected in the borings. The results of the geophysical surveys compared favorably with the stratigraphy determined through borings, and multiple geophysical techniques were able to detect anomalies in the same vicinity at the same depths. Based on the work conducted at this site, both traditional intrusive exploration techniques and geophysical surveys were required to fully characterize the subsurface. Two recommendations for retention pond site characterization were derived from this work: geotechnical borings must be conducted to determine depth of bedrock, and at least two geophysical methods measuring different subsurface properties and have approximately the same depth of investigation with the depth of investigation being at minimum the soil/bedrock interface.

Qualitative assessment of surface roughness of limestone specimens from the orientation of unwrapped triangulated point clouds

Weathering reduces both the strength and stiffness of rock. Since weathering is a surface phenomenon it can be assessed using surface imaging techniques. The surface roughness of twelve cylindrical limestone specimens from Florida were assessed using analysis of close-range photogrammetry. Dense point clouds produced from the specimens were unwrapped and triangulated using Delaunay triangulation. Outward normal facing vectors were computed for each triangle and the γ-values were assessed. Smooth surfaces had lower average γ-values and lower standard deviations than rough surfaces. Surface roughness could easily be distinguished using lognormal probability density functions based on average and standard deviation of the γ-values. For smooth specimens, histograms of γ-values were similar to the probability density functions. However, for rough specimens the histograms of γ-values were not similar to the probability density functions. The histograms for the roughest specimens showed a bimodal distribution of values with peaks at both the low and high ends of the histograms.

Development and assessment of a photogrammetry system for rock specimen surface characterization

The use of field and laboratory three-dimensional imaging techniques are commonly used to assess the roughness of planar rock joint surfaces for shear strength estimations. A photogrammetry system for capturing the surface features of cylindrical rock specimens is presented. They system consists of a DSLR camera, photo-turntable, scale block, photogrammetry software and point cloud processing software. The process to develop a cylinder shaped point cloud, unwrap the point cloud, and triangulate the unwrapped point cloud is presented. The photogrammetry system was assessed using a smooth limestone specimen as a benchmark. Increasing the number of digital images used to generate the point cloud increased the number of points in the point cloud. However the increase in the number of point significantly decreased when using more than eight digital images. A minimum of six digital images are required to fully capture the surface of the specimen. The computing time ranged between ten and twenty-four minutes to generate a point cloud consisting of approximately twenty million data points using twenty-four digital images.

Simulated rock profiles for surface weathering estimation

The estimation of rock surface weathering, much like the estimation of shear strength of rock joints, is facilitated by the use of statistical methods on linear profiles. Unfortunately, a number of factors ranging from the size of the sampling interval used to the mathematics of the roughness index chosen can misrepresent the true roughness of a surface. A method is proposed to simulate rock profiles using a combination of generated signals and a Markov process for the purpose of studying and anticipating the effects of many variables on roughness estimates. The large-scale properties of rock are represented as sine waves, sawtooth waves, and a random Markov process and the small-scale properties are simulated with red noise. The generative parameters are varied to produce a corpus of simulated profiles, where sampling and measurement error is also simulated by lightly deflecting measurements with Gaussian-distributed noise. Using the simulated profiles, five different measures for roughness are compared and the results indicate that no one measure alone exhibits sensitivity to all generative parameters. The Fourier transform is applied to compare the frequency content of these simulated profiles to that of real limestone rock, providing some validation that the conclusions drawn from our simulated profiles also extend to profiles from real rock samples.