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Memo to Steve Renner <br />Fruita Highwall Stability <br />page 6 <br />Ultimately, after the analyses have been completed and accepted, it <br />is the performance of the highwall which demonstrates whether or <br />not it is stable. The Fruita highwall shows no immediate evidence <br />of instability. <br />Aerial Photographic Reconnaissance <br />As discussed above under "geomorphology" the portal bench and <br />portal highwall of the Fruits mine occupies one of the steep <br />gradient linear valleys characteristically incised into the Book <br />Cliffs. This valley, in. which the bench fill was placed, is <br />flanked by a prominent spine of bedrock on either side. The <br />regular spacing of these alternating valleys and apinea are <br />believed to be controlled by regional alignment of jointing. The <br />valleys occupy the more easily eroded zones with closer joint <br />spacing, while the apinea represent the more resistant bedrock <br />zones with greater joint spacing. Examination of the stereographic <br />aerial photography of the Fruits mine site and surrounding area <br />clearly shows the influence of a regularly spaced structural joint <br />set trending approximately north-south with a regular spacing of <br />between 1,250 and 1,500 feet. <br />Aa also discussed previously, the stability of rock highwalla is <br />commonly controlled by macro structural elements, such as faults, <br />joints or bedding. For the experienced practitioner, aerial <br />photography often provides telltale evidence of the development of <br />slope stability problems, even before it may be apparent to the <br />observer on the ground. The subtle changes in ground moisture and <br />ground.aurface texture_ characteristic of a rotational.or block <br />glide landslide ale often detectable on aerial photography before <br />the accelerated failure occurs. For this reason I consider aerial <br />photographic analysis to be a desirable component of any rock <br />highwall stability analysis. <br />The Division possesses periodic aerial photography of the Fruita <br />mine site and surrounding area. Traditional stereo pairs were <br />obtained in black and white in October, 1980, and in natural color <br />in September, 1982. In addition I examined natural color <br />stereographic oblique photographic pairs obtained in November of <br />1990 and November of 1993, before and after the resculpturing of <br />the portal bench and partial backfilling of the highwall. <br />My examination of the traditional 1980 and 1982 vertical <br />stereographic pairs detected no indication of joint set opening or <br />relaxation of the elopes above the highwall. I could detect no <br />increase in .soil moisture, no subtle vegetative change, no <br />surficial evidence of elope instability, and no evidence of <br />increased debris calving. In short I detected no evidence of <br />