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CESARE,INC. <br /> indicated) within 20 degrees (+/-) of the current strike of the high wall. <br /> 2. There are areas of moderately fractured rock with semicontinuous fracture sets and <br /> repeating areas of highly fractured rocks. The highly fractured areas create rock blocks of <br /> 1 foot or less along their long axis and pose more of a rockfall hazard. Monitoring the <br /> change in rock mass character will reduce the rockfall risk as operations move along the <br /> high wall. <br /> 3. The black obsidian is mixed and overlain with a lighter/white rhyolite layer that is very <br /> poor rock overall but contains large spheres of rhyolite that pose a tumble down rock <br /> rolling hazard in the quarry area. <br /> 4. Overall, the rockfall and slope stability hazard along the high wall in the operational areas <br /> is being well managed. <br /> 1. SCOPE <br /> The purpose of the site visit was to address the high wall stability. The quarry was operational the <br /> morning of June 16, 2022. The CMTTS engineering geologist inspected the high wall cut face and <br /> surrounding area of the mine operations, as safety allowed. <br /> 2. SITE CONDITIONS <br /> The geology of the area has been mapped at the 1:250,000 scale on the"Geologic Map of the Pueblo <br /> 10 x 20 Quadrangle, South-Central Colorado", by Scott, et al., 1978. The area north of Silver Cliff is <br /> described as Miocene age rhyolite and glass (obsidian). The topography is low rolling hills. Vegetation <br /> on the surrounding hills has grass and low shrubs. <br /> The quarry is about 3.0 acres in size and has two large pits that have been excavated below the <br /> water table to create quarry ponds. One pit is on the western side and the other is on the far eastern <br /> side. Each has a high wall that is about 30 feet benched, with access roadways for equipment. <br /> The quarry consists of two types of rock, rhyolite and obsidian. The contact between the two is not <br /> well defined and there is significant mixing and distribution of each rock type. The obsidian in the <br /> quarry was highly fractured and internally friable black, glassy rock. The obsidian was mixed with <br /> and overlain by rhyolite deposits that were characterized by large spherical core stones in a friable <br /> matrix of lighter color(Exhibit 2). The obsidian which was exposed in the high wall of the quarry had <br /> several major joint sets and fracture patterns. The main joint set dipped 770 to 820 with a strike of <br /> 2400, the secondary joint set dipped 83° to 870 with a strike of 1501, and a third joint set dipped at <br /> 440 to 490 with a strike of 700 to 740. This intersecting joint pattern indicates a small degree of <br /> potential wedge failure of blocks (Exhibit 3). This fracture intersection creates 1.5 to 2 foot blocks. <br /> The obsidian unit becomes highly fractured in areas along the high wall (Exhibit 3). These fracture <br /> zones have four fracture sets that break the rock into blocks 1 foot or less on their long axis. The <br /> probability of rockfall in these zones is increased, although the overall hazard is from smaller blocks. <br /> Local areas of this highly fractured and jointed obsidian limit the long term stability of the overall <br /> rock mass. The strike of the joint sets and the strike of the high wall provide short term control of <br /> the high wall face. <br /> 22.6084 Obsidian Quarry Slope Consultation Letter 06.29.22 2 <br />