Laboratory Study of Mechanical and Transport Properties of Bishop Tuff

The Bishop Tuff is a welded tuff that formed from ashfall and a rhyolitic pyroclastic flow around 764,800 years ago in Owens Valley, California (Andersen et al., 2017). Because of this unit's proximity to the Owens Valley fault zone, we seek to understand this rock's mechanical properties. Samples previously defined as Unit C within the Bishop Tuff by Evans and Bradbury (2004) were collected along the Sad Boulders trail in Bishop, California. Through the application of a range of stress conditions, we aim to document the failure mode of the Bishop Tuff and create a general characterization of its mechanical properties, including bulk and shear moduli, permeability, porosity, and stress-strain curves at varying conditions. We tested five cylindrical samples of 38 mm in diameter and 75 mm in length using a NER Autolab 3000 apparatus to deliver a holistic picture of the mechanical responses of the Bishop Tuff. Preliminary results show that Bishop Tuff has an average porosity of 28.9% and is easily compressible with a Young's modulus of 7.88 GPa. The Argon permeability of the intact sample is 4.64x10-15 m2. After failure, the permeability decreases to 2.29x10-15 m2 suggesting that the rock compacts. During uniaxial loading tests, we saw unconfined samples fail catastrophically with distinct surface level fractures at 19-22 MPa, whereas when samples were confined, yield strength at 10 MPa confining pressure is 26 MPa and decreases to 22 MPa at a confining pressure of 30 MPa with pore pressure compaction. By documenting the responses to deformation shown by this rock unit, our data contributes to a general understanding of how this tuff will behave as gradual compaction continues. This in turn will affect the safety of future infrastructure in the fault zone. This project was hosted by the NSF funded RORD REU.