Browsing by Subject "Triaxial testing"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Characterization of Gulf of Mexico Clay Using Automated Triaxial Testing(2012-02-14) Murali, MadhuriWith increasing development in the oil and gas industry, exploration and production is continuously moving deeper off the continental shelf and onto the continental slopes. This increases the risk of submarine slope failures leading to damage of offshore structures. Thus there is a need to study and understand properties of offshore marine clays on slopes. This study was undertaken in order to understand better the characteristics of a sub-marine clay deposit taken from the Gulf of Mexico. This thesis presents the results of SHANSEP triaxial testing performed on undisturbed samples of Gulf of Mexico clay. Background information is given about the clay, the sampling program and the laboratory testing program. The GEOTAC Truepath automated stress path triaxial apparatus implemented for this research and the laboratory procedures used are described in detail. Data is summarized from the various types of tests run on the clay (CKoU compression and extension, CIU compression and extension tests, consolidations tests) and the stress history of the deposit is evaluated. The SHANSEP reconsolidation technique was used for a comprehensive program of Koconsolidated-undrained (CKoU) triaxial compression and extension tests at overconsolidation ratios (OCR) ranging from one to eight. Eighteen tests were run on jumbo piston core samples from one particular core. The consolidation phase of these SHANSEP tests provided most of the preconsolidation pressure values used to establish the stress history at the two test sites. These tests were used to estimate the in situ Ko and how it varies with OCR. The undrained shear phase of the tests provides detailed information on the values of S and m for use in the SHANSEP undrained strength equation, Su= 0vo = S(OCR)m, effective stress failure envelopes, etc.Item Laboratory quantification and detection of pre-existing fractures and stress-induced microfracturing through combined ultrasonic and triaxial-stress testing(2016-08) Ramos, Matthew John; Espinoza, David N.; Torres-Verdin, CarlosSimultaneous triaxial stress testing and ultrasonic wave propagation were utilized to quantify natural fractures and microfracturing in Berea Sandstone and Silurian Dolomite. Experimental results indicate that the presence of fractures distinctly decreases wave velocities, with calculated dynamic elastic moduli decreasing by up to 7.5% in artificially fractured sandstone. Wave analysis of intact and artificially fractured Berea Sandstone reveal the nonlinear mechanical and geophysical response or fractured rocks subject to isotropic and deviatoric stress loading paths. Specifically, fractures increase hysteretic stress-strain behavior, and tend to amplify the stress dependence of wave attenuation and the filtering of high-frequency wave components. Additional deviatoric loading tests of Berea and Silurian samples provide evidence for the onset of stress-induced microfracturing, detected at a threshold of 1% shear wave anisotropy called the “shear wave crossover” (SWX). The SWX and subsequent increases in shear wave anisotropy evidence microstructural damage development well before quasi-static indicators such as the volumetric strain positive point of dilatancy (PPD) and yield/failure in all samples. Specifically, Berea and Silurian samples exhibit up to 5% and 7% shear wave anisotropy at the PPD, respectively. Additionally, stresses at the SWX and PPD were compared to peak axial stress to understand linkages between damage at several scales and ultimate rock strength. The SWX occurs at an average of 27% lower axial stresses, and 5% less shear wave anisotropy than the PPD, indicating that samples undergo irreversible microstructural changes earlier than previously thought. The SWX and PPD both provide meaningful estimates of failure stress, however samples must be subjected to higher stresses and strains to reach the PPD, making it less favorable for sample preservation. Furthermore, correlation between the SWX and peak stress under several different radial stresses, present a viable technique for using dynamic measurements to predict static rock failure properties, while also preserving sample competence for future tests. Linking the dynamically measured SWX to static rock failure properties provides an additional avenue for developing accurate transforms for several rock types. Therefore, the SWX can add value across industries for predicting rock behavior and maximizing the value of expensive samples and rock testing.Item Undrained, monotonic shear strength of loose, saturated sand treated with a thixotropic bentonite suspension for soil improvement(2010-08) Rugg, Dennis A.; El Mohtar, Chadi Said; Rathje, Ellen M.Liquefaction is a phenomenon that occurs in loose saturated sand deposits that are subjected to earthquake loading. This phenomenon can cause massive displacements and significant destruction. Many methods for mitigating liquefaction have been proposed and investigated including compaction, drainage, and grouting. One such liquefaction mitigation technique involves the addition of bentonite fines to the pore spaces of a loose, saturated sand via permeation of an engineered clay suspension. This method of soil improvement has provided the basis and motivation for this research. Also, the effect of plastic and non-plastic fines on the static and cyclic response of sands is somewhat contradictory throughout the literature. Thus, the primary objective of this study was to characterize the affect of an engineered bentonite pore fluid on the undrained monotonic response of loose, saturated Ottawa sand in order to determine its feasibility for use as an effective method for liquefaction mitigation. The permeation of engineered bentonite suspensions is proposed as a passive site remediation technique. Thus, the suspensions were delivered to loose Ottawa sand specimens in the laboratory by permeation in a newly designed three-way split mold. This split mold was used to create easily tested specimens that would have an initial soil fabric similar to that expected after permeation in the field. The bentonite suspensions were treated with sodium pyrophosphate to reduce the initial yield stress and viscosity in order to allow for permeation. Three different bentonite suspensions were utilized throughout this study each having different properties and delivering slightly different amounts of bentonite to the loose, saturated sand. The affect of this engineered pore fluid on the undrained shear response of loose, saturated Ottawa sand was compared to the undrained shear response of clean sand and dry-mixed sand and bentonite. The specimen preparation method (dry-mixed or permeated) was shown to have a significant effect on the response of the sand specimens. While the dry-mixed specimens produced larger and more sustained positive pore water pressures than the clean sand (resulting in an increased tendency to flow), the permeated specimens showed a marked decrease in the generation of excess pore water pressures, displayed a more dilative response, and thus resulted in a soil structure that was less likely to flow. Finally, the results of tests on specimens permeated with engineered bentonite suspensions show that there is little to no change in the effective friction angle at critical state. A method for effectively testing permeated soil specimens was developed in this study. This method has laid the framework for further investigations into the use of engineered bentonite suspensions for liquefaction mitigation by permeation grouting.