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.~ R. J. Irish Consulting Engineering Geologist, Inc. In the vicinity of Buena Vista the Sawatch Fault, a major element of the Rift, does not seem to evidence reactivation younger than about 2 million years ago. On the western side of the graben near Buena Vista, faults reportedly evidence reactivation from about 30,000 years ago to 4,000 years ago, and a recurrence level of 10,000 to 40,000 years. Conceivably, a Magnitude 6 to 7 earthquake could be generated along a fault within 5 miles of the quarry site, and that could subject the site to a peak particle velocity (PPV) of about 0.3 to 0.4 g.; but we consider the risk of that occurrence to be low during the next few hundred years. We suggest that a PPV of 0.1 g. would be a more realistic value. Importantly, the rock at the quarry site should dampen the seismic waves considerably. Assessment of Rock Quarry Cut-Slope Stability The back wall cut-slope of the planned quarry will face essentially due westward, whereas the side slopes, at right angles to the back slope, will face north and south. As currently planned, the cut-slopes are to be benched to promote cut-slope stability. Consequently, the joints of the 3 main joints sets that strike 5° to 15° either west or east of north are likely to appreciably influence, if not entirely govern, breakage along the back wall. Similarly, the near- horizontally dipping joints of 3 other joint sets should influence breakage across the side walls, with the joints of 1 set dipping out of each one of the 3 planned cut-slopes. They are likely to promote the placement of the risers because the rock will tend to break-out, after drilling and blasting, to those surfaces. Those joints typically are widely spaced, however, thus their influence should be relatively minor. Moreover, the surfaces of those planes are expected to be rough, thus the friction factors for those planes are expected to be high. Resistance to the sliding of blocks and slabs out of the cut-slopes along those planes should be high. Given the high degree of rock massivity, the relatively favorable geometric configuration of the 7 principal systems of steeply dipping joints relative to the planned back wall and side wall cut-slopes, and the superior intergranular strength characteristics of the granodiorite at this site, ~we conclude that these cut slopes should be benched with 10-foot wide steps (that wide mainly for operational necessity) spaced apart about 2D feet in elevation; and with the risers drill-and- blast-excavated to 1/4:1'S( 6°)2 slopes~h~t riser cut-slope angle would result in an equivalent cut-slope angle of abou~ 34:1 (about 56°)~hat proposed cut-slope angle is shallower than the angle of 1/2:1 (63°) for the theoretical joint commonly assumed to govern cut-slope stability during an empirical evaluation of potential cut-slope stability in granitic rock. Moreover, that 3/4:1 angle should allow the cut-slopes to respond satisfactorily to the most probable PPV for this site. as well. Some of the risers will break to high angle (80°+) to near-vertical joints that will dip out of those cut-slopes sections along strikes that closely sub-parallel the trends of the cut-slopes, at least for several tens of feet laterally along the slope. That, however, should not detract from cut-slope stability in general or riser cut-slope stability in particular because the resulting natural planed surfaces should be at least as stable as the 3/4:1 slopes cut through the rock. That ' Vertical o horizontal. sured from the horizontal. ' Risers cut steeper than about 1/4:1 tend to have unstable overhangs that need to be re-shot.