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Geotechnical Stability Exhibit (6.5(4)) <br />This report is an assessment of the potential offsite impacts resulting from the proposed <br />excavation of the access roadway. It is based on field observations, geotechnical reseazch <br />and blasting analysis. <br />Site Description: The site is situated on the north side of the Navajo River Valley and is <br />comprised primarily of Mesa Verde Sandstone underlain with Mancos Shale. From south <br />to north, the site increases in elevation approximately 400 feet. The sandstone formation <br />across which the access roadway is planned is described generally as homogenous, <br />exhibiting thick laminated stratification. <br />Scope of Project: Construction of the proposed roadway will begin at a point where the <br />existing entrance road ends. The alignment of the roadway will take it across the base of <br />the sandstone valley wall and upwazd to the rim of the wall. Construction of the roadway <br />will involve a side hill cut for the entire length. No fill is anticipated. The cross-sectional <br />dimensions of the road cut will vary from top of cut to toe depending on the depth of the <br />cut. The road surface width is not expected to exceed 24 feet. The final cut slope ratio <br />will be no steeper than .25:1 (V:H). <br />The primazy rock size desired from the excavation of the roadway is 30" and lazger for <br />use as riprap and landscape structures. <br />Blastin¢: Drilling and blasting will be required for most of the excavation. Side hill cuts <br />are the proposed method of extraction. Drilling and blasting will done only by licensed <br />and certified professionals. <br />Based on the desired rock size, the drill hole patterns and hole diameter will be conducive <br />to minimize fragmentation. The depth of the drill holes will vary from approximately 6 to <br />28 feet, bore hole diameter is anticipated to range from 2" to 31/2" depending on depth, <br />spacing and burden. Primary spacing of drill holes is anticipated to be 9 x 9'. <br />Sequential MS delay, non electric blasting methods will be utilized to maximize rock size <br />and to minimize vibration. Stemming will be employed to minimize potential fly rock <br />and air blast. The dimensions of the road cut will dictate that the azea to be blasted is <br />fairly small and thus the amount of explosives used for any given shot will be minimal. <br />It is recommended that smaller trial blasts be performed at the outset of excavation. <br />Seismic vibration, air shock, and resulting rock breakage will be monitored to better <br />evaluate the blasting methods used. <br />~-- `~ .~~ <br />o.,; F ' ~ ~ 2001 <br />` •..~rro and Saf afion. <br />ety <br />