Properties of joints in Anvil Points oil shale

Properties of joints in two oil shales from Anvil Points, Colorado, containing 20 and 40 gal/ton of kerogen were determined by direct and triaxial shear. Twenty gal/ton oil shale was found to be about 2 to 5 times stiffer than 40 gal/ton oil shale, in terms of both normal and shear stiffness. The 20 gal/ton shale was also found to be stronger against sliding, by a factor of about 2, and the joint properties were found to be somewhat more reproducible for the lean shale than for the rich shale. The shear data was found to correlate well when the shear stress was normalized by the normal stress acting across the joint. The resulting shear stress/normal stress ratio vs shear displacement plots were essentially parabolic in form, with considerable nonlinearity, even at very low shear stress levels. The lean shale slid at a constant shear stress, while the rich shale continued to harden during sliding. A few direct shear tests were conducted at a strain rate two orders of magnitude higher than the rest, with no systematic variation in joint properties resulting. Also, the effect of staging, or retesting a joint which had been previously sheared at a different normalmore » stress level, was found to be negligible.« less