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hfarch 1998 Slump lnvest+gatian and Mitrgat+an Report ~ Page J <br />2.0 SCOPE OF WORK <br />2.1 CAUSE OF SLUMP <br />As part of the investigation to determine the cause of the slump, MWA excavated a trench <br />along the axis of the slump as shown in Exhibit I-I to observe and describe the stratigraphy of <br />the site and any subsurface slump related features. Upon completion, the trench was backfilled <br />with the excavated material. <br />MWA reviewed all readily available project files that described the development, reclamation <br />and geologic setting of the mine. During the review, MWA looked for information regarding <br />the location of the former high-wall relative to the slump, the depth of mining, the location and <br />type of spoils replaced in the mined areas, whether the slump was observed prior to or during <br />mine development and the locations of any former seeps or springs in the vicinity of the slump. <br />2.2 GROUNDWATER EVALUATION <br />In order to evaluate groundwater conditions (occurrence, depth and flow direction), MWA <br />installed six piezometers in the vicinity of the slump, as shown in Exhibit I-I. Two of the <br />piezometers were installed within the slump mass (PZ-3 and PZ-4) and two beyond the lateral <br />edges of the slump (PZ-5 and PZ-6). The two piezometers inside the slump were distributed <br />with one near the head of the slump and one near the toe. The two piezometers outside the <br />lateral edges of the slump were located perpendicular to the axis of the slump and <br />approximately 50 feet from the apparent limits of the slump. piezometers PZ-1 and PZ-2 are <br />discussed in Section 2.4. <br />The boreholes were drilled vertically with attack-mounted drilling rig fitted with 6-inch <br />outside diameter hollow-stem augers. Daring drilling, soil samples were collected every three to <br />five Eeet and logged by an MWA geologist. The piezometers were constructed of 2-inch <br />diameter, schedule 40 PVC and slotted over the lower 8 to 10 feet. A sand filter pack was <br />installed from the base of the piezometer to a minimum of eight inches above the top of the <br />slotted section. A one- to three-foot layer of hydrated bentonite chips was placed above the <br />sand in each piezometer. Acement-bentonite grout mixture was then placed up to [he ground <br />surface. The piezometers were completed with steel standpipes with a 3-foot stick-up. The <br />vertical and lateral coordinates of the piezometers were then surveyed by a licensed surveyor, <br />using the Hayden Gulch Coal Mine coordinate system. <br />2.3 INSTALLATION OF MOVEMENT MARKERS <br />The dimensions of the slump were determined in the field and surveyed. Then, eight <br />temporary movement markers consisting of rebar stakes were installed in the vicinity of the <br />slump, as shown in Exhibit I-I.. Of those placed outside the slump, two were located on the <br />ridge-top above the slump to ensure that at least two of the markers were placed on stable <br />ground. The other three markers outside the slump were placed approximately 150 feet away <br />from the edges of the slump, two along the eastern edge of the slump, and the third below the <br />toe of the slump. The movement markers were surveyed for vertical and laceral location using <br />the Hayden Gulch Coal Mine coordinate system. <br />Montgomery Watson Americas ' PO. Box 774018 * Steamboat Springs, Colorado " 80477 (970f 879-6260 <br />¢!\pJlSl.mp Mie~uio.~6l.mp Rgoa <br />+/H/?9ila <br />