Investigation of the Mechanical Properties of Hanna Sandstone Using Newly Installed High Pressure and High Temperature Polyaxial Equipment

Abstract

ABSTRACT The design of civil infrastructure requires the assessment of the mechanical properties of rock layers supporting these structures. In this study, a newly installed high-pressure and high-temperature polyaxial equipment at the University of Wyoming is described and used in the mechanical testing. Uniaxial and triaxial compression tests are performed on dry and water-saturated cylindrical sandstone samples obtained from the Hanna Formation, Wyoming. The cylindrical samples are prepared and tested in two different sizes: 1) 25.4 mm diameter by 50.8 mm length, and 2) 76.2 mm diameter by 152.4 mm length. The sandstone samples are vacuum-saturated for 7 and 14 days. For the triaxial compression tests, the samples are subjected to three effective confining pressures of 5, 15 and 25 MPa and under two temperatures of 35 and 55 °C. The results show that the effective confining pressure has a clear positive effect on the compressive strength of all samples. Under the same effective confining pressure, larger samples show higher compressive strength and Young's modulus. For all three effective pressures, the dry samples exhibit higher compressive strengths, Young's modulus and more brittle behavior. Permeability decreases with the confining pressure. P and S wave velocities increase with the confining pressure. INTRODUCTION The design and performance of infrastructures can be highly affected by the strength and deformation of the rocks beneath or around them. Characteristics like rock type, sample size, porosity, water content, and temperature can change the strength and deformability of rocks. Studying the simultaneous effect of the aforementioned characteristics can improve our understanding of rock behavior, which leads to more dependable designs of civil infrastructures. The Hanna Basin is located in southeast Wyoming, bounded by Laramide-aged uplifts (Fig. 1). The formation was formed in the Tertiary period and extends from 0 to 7000 ft below the ground level (Bowen, 1918). A conglomeratic sandstone lays at the base of the formation, with conglomerate, sandstone, shale, and coal beds alternating to the top. In a study (Oliver and Youngberg, 1984) on a unit from the Hanna formation, 72.2% of the rock footage was comprised of Calcareous sandstone. The bulk mineralogy of the Calcareous sandstone was determined as follows: 54% quartz, 9.6% kaolinite, 9% K-Feldspar, 8.4% Dolomite, 6.1% Plagioclase, 5.4% Mica, 3.9% Calcite, 3.4% Siderite, and 0.3% Acmite.