Browsing by Author "Sharpe, Jacob"

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    Development of a Rock Strength Database
    (Minnesota Department of Transportation, 2018-06) Folta, Brian; Sharpe, Jacob; Hu, Chen; Labuz, Joseph
    Rock 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.
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    Failure of Rock at Low Mean Stress
    (2017-07) Sharpe, Jacob
    Conditions arise in many geoengineering applications where both tensile and compressive normal (effective) stresses act due to change in stress from excavation or pore pressure. However, testing of rock at these stress states associated with low mean stress, say p < C0/3, is rare because of experimental difficulties, where p = (σxx + σ¬yy + σzz)/3 and Co = uniaxial compressive strength. The objective of this research is to evaluate rock failure at low mean stress using dog-bone specimens of (dry) Dunnville sandstone. Results from these special triaxial extension tests were used in conjunction with conventional triaxial extension and compression experiments with right-circular cylinders to evaluate four failure criteria: (1) Mohr-Coulomb (MC) with a tension cut-off, (2) Paul-Mohr-Coulomb (PMC) with a tension cut-off, (3) Hoek-Brown (HB), and (4) Fairhurst (Fh). Results for the Dunnville sandstone show that the three failure criteria that either include a tension cut-off (MC and PMC) or have a “natural” tension cut-off (Fh) best capture failure in the low mean stress regime, -T/3 < p < C0/3, where T = uniaxial tensile strength. Of the four criteria considered, Fh provided the best overall fit because it is nonlinear and contains a tension cut-off. Fracture surfaces of the dog-bone specimens were evaluated for failure mode based on surface roughness and it was found that there is a transition of decreasing roughness from tensile failure to hybrid (opening and sliding) failure to shear failure.
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    Mechanical Response of a Composite Steel, Concrete-Filled Pile
    (Minnesota Department of Transportation, 2018-06) Hu, Chen; Sharpe, Jacob; Labuz, Joseph
    A steel pipe-pile section, filled with concrete, was instrumented and tested under axial load. Two types of strain gages, resistive and vibrating wire, were mounted to the steel-pipe pile and checked by determining the known Young’s modulus of steel E^s. The steel section was filled with concrete and a resistive embedment gage was placed in the concrete during the filling process to measure axial strain of the concrete. The axial load – axial strain responses of the steel (area A^s) and concrete (area A^c) were evaluated. The stiffening of concrete, related to curing, was also studied. Assuming the boundary condition of uniform axial displacement, i.e., equal axial strain in the steel and concrete, εz^s = εz^c = εz, the sum of the forces carried by the two materials, F^s + F^c, where F^s = εz * E^s * A^s and Fc = εz * E^c * A^c, provides a reasonable estimate – within 3% – of the pile force. For the particular specimen studied (12 in. ID, 0.25 in. wall thickness), the stiffness of the composite section of steel and concrete was about three times larger compared to the steel section without concrete. Further, the concrete carried about 70% of the load, but the axial stress in the concrete, at an applied force of 150,000 lb, was less than 20% of the compressive strength of the concrete.