Mechanical properties of seven fractures from drillholes NRG-4 and NRG-6 at Yucca Mountain, Nevada

The Yucca Mountain Site Characterization Project has been assigned the task of determining the suitability of the Yucca Mountain site. Among the concerns being investigated, the characterization of the mechanical properties of the fractures present in the host rock had direct relevance to repository design, and the pre- and post-closure performance assessment. Cores from drillholes NRG-4 and NRG-6 containing natural fractures were obtained from the Sample Management Facility at Yucca Mountain, Nevada. Seven selected fracture were sheared at constant normal stress, either 5 or 10 MPa, in the as-received condition (air-dry). Detailed profilometer data was collected from each fracture surface before testing. The tests yielded the normal closure as a function of normal stress, and the shear stress and dilation as a function of shear offset. The constitutive properties resulting from the measurements were: normal stiffness, shear stiffness, shear strength and coefficient of friction, and dilation. Peak friction ranged from 0.89 to 1.11; residual friction ranged from 0.76 to 1.00. The lowest initial dilation angel was found to be 5.29 degree and the highest was 11.28 degree. The roughness characteristics of the fracture surfaces agree qualitatively with the simple mathematical model of Brown (1984) derived from fracture data in many other rock types