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<br />5.0 Uncertainties To Be Resolved <br /> <br />A number of uncertainties should be resolved in final design. <br />They are discussed below. <br /> <br />. <br /> <br />5.1 The upper surface of the Pierre has not been sufficiently <br />defined to insure that there are no localized deep erosion slots <br />filled with pervious materials in the vicinity of the dam. Pos- <br />sible seismic refraction lines supplemented by borings would pro- <br />vide this information. <br /> <br />5.2 The permeabilities of the strata beneath the embankment <br />extending toward Bijou Creek appear to be very high. The test <br />data available are very limited. Additional borings and pump- <br />in tests should be made to determine the permeability of these <br />formations. Permeability of the uppermost or clay strata have <br />been determined by pump-in tests near the end of the slurry wall; <br />however, there is a zone of alternating sand, silts, and some clays <br />for which no permeability tests have been made. Permeabilities <br />of these irregular strata have only been estimated on the basis <br />of grain size comparison. Grain size data are notoriously un- <br />reliable for predicting the permeability of stratified materials <br />and in particular clay materials. Additional field tests should <br />be made to establish the range of permeability in the immediate <br />zone on the southwest side of the dam or note whether the data <br />have been merely estimated. <br /> <br />5.3 The shrinkage characteristics of the proposed clayey <br />materials for the core of the darn should be investigated because <br />the core above the joint use pool may be subject to drying for <br />long periods of time. Consideration should be given to alter- <br />nate core materials within the range subject to water level <br />fluctuations and long dry periods. <br /> <br />5.4 The expansion of the proposed plastic clay core material <br />should be studied in more detail particularly to determine pore <br />pressures or weakening upon eventual saturation. (The sample <br />tested is the least plastic). Less plastic clay materials should <br />be investigated as alternate core materials. particularly above <br />the joint use pool. <br /> <br />5.5 There appears to be no reason for the great width of <br />the clay core, particularly considering the limited amount of <br />. clay compared to the amount of more pervious borrow available. <br />consideration should be given to a much narrower core. <br /> <br />. 5.6 Consideration should be given to maintaining the alluvial <br />clays upstream of the darn intact for a distance of about 500 feet. <br />Consideration should also be given to limiting excavation of the <br />alluvial clay for core construction so that a minimum of two feet <br />of Clay remain to minimize pore pressures within the right <br />abutment. <br /> <br />7 <br />