The Influence of lithophysal porosity on the in-situ stress-strain properties of Topopah Spring Tuff

摘要

Numerical analysis and a laboratory testing program were conducted in order to investigate the effect of lithophysal porosity on the elastic stress-strain properties of the lithophysae-rich tuff specimens and to find the locations of cavities in both analog and tuff specimens. In the first part of the study, a finite difference mesh containing circular holes was modeled for varying porosity ranges between 5 and 40% using commercially available software FLAG20u27 version 3.5. Elastic (Youngu27s) modulus and Poissonu27s ratio were calculated for each setup and normalized with respect to matrix elastic modulus and Poissonu27s ratio. The moduli calculated through numerical analysis were compared with those determined by the biaxial testing of urethane cubes containing circular holes extending through the cube that have same sizes and distribution of holes as those numerically analyzed. Correlation between moduli determined through testing and numerical analysis was very good. Secondly, ultrasonic testing was conducted on plaster of Paris specimens containing spherical STYROFOAM® inclusions and tuff specimens to determine the locations of the spherical STYROFOAM® inclusions and lithophysal cavities, respectively. The ultrasonic characterization technique was able to detect numerous inclusions within each plaster and some cavities in most of the tuff specimens. The ultrasound technique could not locate the exact positions and dimensions of STYROFOAM® inclusions in plaster of Paris specimens and cavities in tuff specimens but rather roughly detected such zones in specimens. Thirdly, lithophysal tuff and plaster of Paris specimens containing spherical STYROFOAM® inclusions were tested under uniaxial compression and moduli and compressive strengths were determined. Elastic moduli of plaster specimens were normalized with respect to the matrix modulus of a zero porosity plaster specimen. Similar decreasing trend in modulus with increasing porosity was observed and correlation between each data set was good in most of the porosity values. Fifteen tuff specimens, including five specimens from middle non-lithophysal units, were also tested under uniaxial compression. The compressive strength and elastic modulus values for tuff showed a decreasing trend with increasing porosity. The reason of variations in data is due to heterogeneities and discontinuities within the tuff.