Richard A. Coffman

Consideration of Internal Board Camera Optics for Triaxial Testing Applications

The application of small board cameras, located within a triaxial cell to determine radial and axial strain, was investigated. Specifically, charge-coupled device (CCD) sensors were utilized in conjunction with precision pinhole apertures to capture images from within the triaxial cell. The cameras were fully immersed in electronics-grade silicone oil and were able to withstand cell pressures that are common to triaxial testing (up to 1034 kPa (150 psi)). The small size of the cameras allowed for implementation within the triaxial cell, thereby avoiding: (1) the cumbersome corrections that are required to account for refraction at the confining fluid–cell wall–air interfaces and magnification due to cell wall curvature, and (2) the amount of space required for outside-of-the-cell monitoring systems that utilize cameras. The final design of the cameras was based on an iterative testing process in which various types of small board cameras, lenses, and finally pinhole apertures were investigated. The advantages of the lensless pinhole aperture camera design included: (1) lack of optical aberrations, such as those encountered in traditional lensed camera systems, (2) practically infinite depth of field, allowing for sharp, close-up images, and (3) wide-angle field of view without the distortions that are associated with the use of wide-angle lenses. As discussed herein, the pinhole cameras were optimized for optical resolution and light entry to minimize the effect of diffraction patterns that are commonly associated with pinhole apertures. The resolution of the cameras was determined to be sufficient for the potential application of the cameras (volume measurements). The instrumentation presented herein provides a novel alternative to the state-of-the-art outside-of-the-cell photogrammetric instrumentation that is currently employed to monitor soil specimens during triaxial tests.

Development of an Internal Camera–Based Volume Determination System for Triaxial Testing

A triaxial testing cell was instrumented with an internal camera monitoring system. By placing the camera monitoring system inside of the triaxial cell, optical distortions due to refraction at the confining fluid–cell wall and cell wall–atmosphere interfaces and the curvature of the cell wall were eliminated. The components of the system are presented. Furthermore, the photogrammetric techniques that were utilized to analyze the photographs that were captured from within the triaxial cell are discussed. The proposed methods for acquiring and analyzing the photographs are presented and the potential for the inclusion of an internal camera–monitoring system for triaxial testing applications are discussed.

Design and Fabrication of End Platens for Acquisition of Small-Strain Piezoelectric Measurements During Large-Strain Triaxial Extension and Triaxial Compression Testing

A triaxial testing device was integrated with piezoelectric transducers to measure small-strain (<10−3%) dynamic soil properties during large-strain (15 %) triaxial testing. To incorporate the technology into existing equipment, end platens were designed and fabricated to facilitate direct contact between the transducers and the soil specimen. The platens protected the sensitive electronics while providing a seal between the confining fluid and the pore fluid. Two types of transducers were incorporated into the apparatus, bender elements and bender disks, used to measure shear wave and compression wave velocities, respectively. The 3.81-cm (1.5-in.) diameter acrylic end platens were designed to house the transducers, a porous stone, and openings to facilitate pore fluid drainage and wiring for the transducers. The top platen included a vacuum attachment and piston mount that enabled triaxial compression and triaxial extension testing. Removable stainless steel inserts were designed and fabricated to house and secure the transducers. These stainless steel inserts were used to ground the apparatus and allowed for maintenance and, if necessary, replacement of individual transducers. To ensure that the transducers were not damaged when subjected to the pore fluid, the transducers were waterproofed. Accurate readings of shear wave and compression wave velocities were obtained via proper design, fabrication, calibration, and implementation of the integrated small-strain components. Accurate readings of axial deformation, shear stress, and confining stress were also obtained via proper design, fabrication, and implementation of the vacuum connection components. Calibration results, as obtained from tests on specimens of medium-dense, dry, Ottawa sand, are presented and discussed. The system time delay was determined to be 5.67 × 10−5 seconds for the bender elements and 3.50 × 10−5 seconds for the bender disks. Measured shear wave velocity values ranged between 178 and 251 m/s and the corresponding compression wave velocity values ranged between 291 and 451 m/s.

Machine Learning Based Predictive Modeling of Debris Flow Probability Following Wildfire in the Intermountain Western United States

It has been recognized that wildfire, followed by large precipitation events, triggers both flooding and debris flows in mountainous regions. The ability to predict and mitigate these hazards is crucial in protecting public safety and infrastructure. A need for advanced modeling techniques was highlighted by re-evaluating existing prediction models from the literature. Data from 15 individual burn basins in the intermountain western United States, which contained 388 instances and 26 variables, were obtained from the United States Geological Survey (USGS). After randomly selecting a subset of the data to serve as a validation set, advanced predictive modeling techniques, using machine learning, were implemented using the remaining training data. Tenfold cross-validation was applied to the training data to ensure nearly unbiased error estimation and also to avoid model over-fitting. Linear, nonlinear, and rule-based predictive models including naïve Bayes, mixture discriminant analysis, classification trees, and logistic regression models were developed and tested on the validation dataset. Results for the new non-linear approaches were nearly twice as successful as those for the linear models, previously published in debris flow prediction literature. The new prediction models advance the current state-of-the-art of debris flow prediction and improve the ability to accurately predict debris flow events in wildfire-prone intermountain western United States.

Slope Stability Monitoring Using Remote Sensing Techniques

A slope failure across Interstate I-540 near Chester, Arkansas is being monitored using traditional surveying techniques (total station) and advanced remote sensing techniques (Light Detection And Ranging [LiDAR]). The initial movement of the slope was observed by Arkansas Highway and Transportation Department personnel in May 2010. The department performed a geotechnical site investigation; the results of which were used to perform a slope stability back-analysis. The results of the slope stability back-analysis and the movements observed using the total station and LiDAR are discussed and compared.

Discussion of “Implementation of LRFD of Drilled Shafts in Louisiana” by Xinbao Yu, Murad Y. Abu-Farsakh, Sungmin Yoon, Ching Tsai, and Zhongjie Zhang

Morgan Race, S.M.ASCE; Sarah Bey, S.M.ASCE; and Richard Coffman, M.ASCE Graduate Research Assistant, Dept. of Civil Engineering, Univ. of Arkansas, Fayetteville, AR 72701. Graduate Research Assistant, Dept. of Civil Engineering, Univ. of Arkansas, Fayetteville, AR 72701. Assistant Professor, Dept. of Civil Engineering, Univ. of Arkansas, Fayetteville, AR 72701 (corresponding author). E-mail: rick@uark.edu

Response of a drilled shaft foundation constructed in a redrilled shaft excavation following collapse

Two drilled shaft foundations (DSFs) of equal size (1.2 m diameter) were installed at the Turrell Arkansas Test Site (TATS). The soil stratigraphy at the TATS consisted of 6.1 m of clay underlain by 3.0 m of silt underlain by sand. After drilling the excavation for the North 1.2 m DSF, the silty soil collapsed from the sidewall of the excavation into the bottom of the excavation. Following the collapse, the excavation was redrilled and the construction of the DSF was completed. The measured capacity, unit side resistance, and end bearing resistance of the South 1.2 m diameter DSF were predicted by using software programs and mean values of soil data. The measured response of the North 1.2 m diameter DSF was backward modeled to determine the appropriate strength and stress reduction. Based on the measured data for this site, a 10 percent reduction in unit weight within the silt layer and a modification of the soil properties within the top 3.0 m of the sand layer produced predicted responses that matched the measured responses.

Discussion of “Measurement of Stiffness Anisotropy in Kaolinite Using Bender Element Tests in a Floating Wall Consolidometer” by X. Kang, G.-C. Kang, and B. Bate

The procedures utilized and results obtained from a newly designed floating wall consolidation device are compared with those obtained from a modified triaxial insert, traditional fixed wall, back-pressure saturated, constant-rate-of-strain consolidation device that incorporated bender elements (BP–CRS–BE). Specifically, the need for additional measurements within the floating wall consolidation device including machine deflection and tip-to-tip measurements are highlighted and discussed. The procedures that were utilized to collect and reduce the measured shear wave and compression wave data, as collected using the newly designed floating wall consolidation device, are also questioned.

Characterization of Landslides Using Advanced Remote Sensing Techniques, Standard Monitoring Techniques, and Laboratory Testing

Two slope failures that affecting two interstates in the state of Arkansas were monitored using traditional surveying techniques (total station) and advanced remote sensing techniques (Light Detection And Ranging [LiDAR] and RAdio Detection and Ranging [RADAR]). One failure was located near Chester, AR (calibration site) and the other slope failure was located near Malvern, AR (validation site). The results of monitoring program completed at both sites (calibration and validation) and geotechnical explorations/laboratory testing (validation site) are discussed and compared in this case study. INTRODUCTION While individual slope failures are not as spectacular or costly as other natural disasters such as earthquakes, major floods, and tornadoes; slope failures are more widespread. In aggregate, the total financial loss due to slope failures is probably greater than that for any other single geologic hazard (Griffiths et al., 1999). The ability to precisely identify the extents of a landslide and to monitor and preemptively mitigate potential landslide disasters can help save money and ensure slope remediation is properly performed. During the past six years the Arkansas State Highway and Transportation Department (AHTD) has spent over nine million dollars repairing slope failures that have occurred in the state of Arkansas. Therefore, a necessity exists to quantitatively identify the surface extents, movement rates, and direction of movements of a given landslide using advanced remote sensing techniques. Specifically, the Gamma Portable RADAR Interferometer (GPRI-II) and a Leica 289 Geo-Congress 2013

Back-Pressure Saturated Constant-Rate-of-Strain Consolidation Device With Bender Elements: Verification of System Compliance

A back-pressure saturated, constant-rate-of-strain (BP-CRS) consolidation device was modified to incorporate bender elements (BE). A series of laboratory tests were conducted on a dummy brass sample and on kaolinite soil samples, using the BP-CRS and BP-CRS-BE devices, to determine the amount of system compliance for the BP-CRS-BE device, as compared to the BP-CRS device. The amount of machine deflection was determined for both the BP-CRS-BE device and BP-CRS device. Two approaches (quick and slow) were evaluated for determining the machine deflection. The machine deflection results, as obtained from both methods, were comparable; therefore, the use of the quick method is recommended. The respective average amount of maximum machine deflection (brass sample) and maximum corrected vertical deformation (kaolinite sample) for the BP-CRS and BP-CRS-BE devices were 0.78 mm and 0.88 mm (machine deflection) and 3.15 mm and 3.43 mm (soil deformation), respectively. Consolidation parameters were determined by subtracting the amount of respective machine deflection from the amount of vertical deformation that was measured during the tests that were performed on kaolinite soil samples. The consolidation parameters, as obtained from both devices, were also comparable. The average values of recompression index (Cr), compression index (Cc), swell index (Cs), and coefficient of consolidation (Cv) for the kaolinite samples that were tested in the BP-CRS and BP-CRS-BE devices were 0.07 and 0.08, 0.19 and 0.21, 0.08 and 0.07, and 9.3E-8 m2/s and 9.6E-7 m2/s, respectively. Because similar values were obtained for the consolidation parameters, as obtained by using wither the BP-CRS device or the BP-CRS-BE device, the use of the newly designed BP-CRS-BE device is advocated because the BP-CRS-BE device also enabled collection of shear wave velocity measurements while the sample was being subjected to various stress levels.