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Pilch Rectanta[ion Project -Sediment Control Embankment November, 2002 <br />3.3 Piezometer installation <br />Based on the field observations and data reviewed, it was recommended that additional <br />piezometers be installed in the embankment core for collection of additional data and monitoring points <br />for the phreatic water levels in the embankment. In addition, it was recommended to survey the <br />embankment slope and crest for overall grade and the location and limits of features observed in the <br />field. The purposes for the new piezometers were to further characterize seepage through the <br />embankment, observe the core material intercepted in the boreholes and to further evaluate the cause of <br />elevated water surfaces recorded in P-3 and the wet areas observed on the downstream face of the <br />embankment.. <br />A total of four piezometers were drilled into the core of the embankment -one to the right of P- <br />3 (looking downstream) and three between P-3 and the left abutment. The two boreholes drilled <br />towards the left abutment intercepted the probable pathway of seepage [hat effected P-3 water levels <br />and the wet areas on the downstream face. Avery gravelly zone was intercepted in the two boreholes <br />at about 40-ft below the crest of the embankment. The split spoon borehole samples contained less <br />fines than in the core material above and below the gravelly zone. Perched water was also present in <br />this zone of material. The piezometers installed in the two holes (P-6 and P-7) recorded water levels a <br />few feet higher than in P-3. The two boreholes drilled on both sides of P-3, did not intercept such a <br />zone, but the piezometers installed in the boreholes (P-8 and P-9} eventually contained water levels <br />similar to P-3. (Note: The boreholes for P-8 and P-9 were dry at the time of installation; however the <br />cuttings from the bottom of the borehole were logged as moist to very moist. No `free' water was <br />observed in the borehole cuttings or samples. The water levels occurred over a 36-hour period. <br />The boreholes were drilled with 4-inch diameter solid-stem augers. Split spoon drive samples <br />were collected at selected intervals and at the bottom in each borehole. Blow counts required to drive <br />the sampler (referred to as standard penetration tests, SPT) were recorded at each of the sample <br />locations. The core material in the interval of the perched water in P-6 and P-7 were recorded as 10 <br />and 24 blows per foot, respectively. <br />The depth of boreholes were limited by the drilling equipment. However, review of the <br />. collected data found that additional boreholes to a greater depth were not necessary to complete the <br />The Homestake Mining Company -l- The Glasgow Engineering Group, lnc. <br />