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<br />' 111. EXISTING SITE CONDITIONS <br />3-Ol. GEOLOGIC CONDITIONS <br />Subsurface conditions at the Morrison Quarry generally consist of a relatively thin (less than 20 ft <br />thick) veneer of overburden soils overlying Precambrian biotite gneiss, granitic gneiss, schist, <br />quartzite, and minor pegmatite. The central pit area is near an unconformity between these <br />' Precambrian metamorphic rocks and the younger Fountain Formation sandstones and siltstones to the <br />east. <br />Faulting is prevalent along the Colorado Front Range and several faults and/or fault systems have <br />been mapped or tentatively mapped within the quarry during its operation. These fault systems or <br />faults include east-west trending to northwest-southeast trending features. Faulting in the mine area is <br />typically characterized by fault gouge development. A fault has been mapped which extends through <br />or near the area of instability at the southeastern end of the central quarry area. <br />' During our site visits, Naley & Aldrich has observed rock mass characteristics immediately north and <br />south of and within the zone of instability. The rock mass within the failure zone was comprised of <br />weathered rock blocks generally between six inches and one foot on a side with sufficient weathering <br />' to suggest a potential for clay material coating or as infilling on discontinuity surfaces. <br />Considerable hydrothermal alteration has occurred as a result of granitic intrusions beneath the eastern <br />portion of the mine area and quartz veining is typically associated with this alteration mechanism. As <br />a result, quartz "float" in the overburden soils can be used as a possible indicator of the degree of <br />underlying alteration, <br />As part of an investigation for a proposed water storage project two core holes were drilled along the <br />eastern part of the active mine area north of the subject slope failure area. These holes encountered <br />between 2 and 12 ft of overburden soils overlying highly altered biotite gneiss and weathered and <br />' altered gneiss. Below a depth of about 60 ft a transitional contact occurred into relatively non-altered <br />Ibiotite gneiss underlain by granitic gneiss and quartzite. It is assumed that similar conditions with <br />depth prevail to the south within the subject area and that improved rock quality can generally be <br />' ;tnticipated below a depth from original ground surface of about 60 ft. <br />lluring preliminary mapping performed for this work effort, observations were made of bedrock type <br />' and structure exposed in outcrops for use in performing stability analyses. The majority of the rock <br />observed in outcrops consisted of gneiss with minor schist interbeds. In the southeast corner of the <br />central pi[, there is also an altered rock zone that appears to consist of a low-grade metamorphic rock <br />assemhlage with a frequent occurrence of talc. This may represent a localized metamorphic zone <br />derived from a mafic igneous parent rock. The majority of this latter rock type occurs at the south <br />end, or to the south of the slope failure area that is the subject of this report. <br />' Typical rock exposure within and to the north of the failure area revealed three general rock types; <br />Ij hard to very hard, moderately weathered to fresh, gray, white, reddish-brown and pink, fine to <br />' medium grained gneiss and granitic gneiss, with smooth to rough, open joints on 0.5 to 3 ft spacings, <br />and with rust colored coatings up to I/8-in. thick on some joints; 2) very soft, very severely to <br />completely weathered, dark gray, coarse to medium grained biotite schist, with very close rough, <br />' 3 <br />' ~~ ~~ <br />