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-'• 3.0 ENGINEERIIVG GEOLOGY <br />~ 3.1 Gencral <br />Field observations were made along the base of the 'fwentymile Sandstone cliff and <br />stereo pairs of aerial photographs were reviewed to obtain engineering geologic data. The <br />field observations focused on the engineering geologic characteristics of: (1) the cliff face, <br />(2) joint orientations, (3) joint patterns, (4) joint faces, (5) intersection of joints, (6) relative <br />frequency of spacings between joints, (7) strikes and dips of bedding planes, (8) lithologic <br />variations.in the sandstone, (9) faults and fault zones, and (10) other engineering geologic <br />features such as past rock failures and their types of failure, debris flows and erosion. <br />1 Integration of these observations defined the engineering geologic hazards with respect to the <br />l cliff face and to County Road 27 and led to a description of Hazard Zones (Figure 3). <br />1 <br />The presence of water in the rock decreases stability. Our investigation did not <br />]• measure the presence of the water in the Twentymile Sandstone nor did we determine its <br />hydraulic gradient at the site; such an analysis was beyond the scope of work. However, we <br />did consider the general influence of water on the hazards for this study. We assumed the <br />gradient in the sandstone is northwesterly. Evidence of water seeps was observed in several <br />areas where evaporate salts are present in outcrop. Such salts are deposited as the pore <br />water seeps through the free surface of the rock. Water stains were also observed. <br />3.2 EnaineerinE Geologic Hazard Zones <br />Zones of engineering geologic hazards were defined using the criteria outlined <br />below. Each zone is typified by apparent internally consistent engineering geologic <br />characteristics that are different from other zones. The criteria are: <br />(1) orientations of joint planes and intersection of joints with respect to the <br />1 face of the cliff, <br />(2) locations of existing rack failures along the face of the cliff, <br />~• 4 <br />