Browsing by Author "Folta, Brian"
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Item Development of a Rock Strength Database(Minnesota Department of Transportation, 2018-06) Folta, Brian; Sharpe, Jacob; Hu, Chen; Labuz, JosephRock strength and elastic behavior are important for foundations such as spread footings resting on rock and drilled shafts socketed into rock. In addition to traditional rock quality information, stiffness and failure parameters are helpful for design. MnDOT has previously used a low-capacity load frame for routine rock testing but this apparatus does not generate sufficient force for testing hard rock. The report provides a comprehensive suite of results from 134 specimens tested under uniaxial compression and 33 specimens tested under triaxial compression on a wide variety of rock, including hard rock, which frequently is of interest for high-capacity foundation systems. Thus, an economic benefit is realized if the strength of the rock is measured, as opposed to correlated with an index parameter, due to the potential to reduce foundation size and construction time. Information from the testing was used to expand the MnDOT database of rock properties and allow for improved designs based on accurate measurements of Young’s modulus, uniaxial compressive strength, and friction angle.Item Strength testing under multi-axial stress states(2016-06) Folta, BrianA true triaxial apparatus was designed and fabricated to apply mechanical loading to achieve multi-axial stress states. The University of Minnesota plane strain apparatus was equipped with piston assemblies that generated intermediate principal stress σ_II up to 100 MPa. The minor principal stress σ_III was limited by the pressure vessel to 24 MPa and the major principal stress σ_I was applied by a closed-loop, servo-hydraulic load frame. Calibration tests were conducted on an isotropic, linear elastic material (aluminum 6061) to confirm the performance of the piston assemblies. Results from strength testing of Dunnville sandstone under multi-axial stress states were used to evaluate the Mohr-Coulomb (MC), Hoek-Brown (HB), and Paul-Mohr-Coulomb (PMC) failure criteria. A series of conventional triaxial compression (σ_II=σ_III) and extension (σ_I=σ_II) experiments were performed to evaluate the necessary material parameters for each failure criterion. The true triaxial apparatus was used to conduct experiments under various states of stress (σ_I≠σ_II≠σ_III) at constant mean stress (p=(σ_I+σ_II+σ_III)/3) of 28.3 and 56.0 MPa. The three criteria were compared in principal stress space on a plane normal to the hydrostatic axis called the pi-plane. Even though MC and HB are independent of σ_II, the stress states at failure were reasonably predicted at p = 28.3 MPa. This was not the case at p = 56.0 MPa, and a plane fitting method using a failure criterion such as PMC that includes σ_II may be needed.