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Pervasively-Altered Rocks in the Glenbrook Creek Watershed: Effects of Slope Stability, Erosion, and Sediment Composition on Lake Tahoe's Water Quality
AdvisorWatters, Robert J.
Geological Sciences and Engineering
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Investigations of the Glenbrook-Spooner alteration zone, a small Tertiary hydrothermal system east of Lake Tahoe, Nevada, hosted in trachyte, granite, and diorite, show that hydrothermal alteration intensity and mechanical properties of rock are quite variable, but a strong correlation between the two can be discerned for trachyte. Because of a progressive drop in strength properties with an increase in alteration intensity, a large exposure of altered trachyte in highway and road slopes, and the overall proximity to Glenbrook Creek which drains into Lake Tahoe, these slopes may be a substantial source of sediment and solutes in the Lake.Both field and laboratory methods were employed. Eleven windows were chosen that represented the range of lithology and alteration. Field work included geologic and window mapping, rock quality designation measurements, and sample collection. Laboratory analyses included point load strength index testing, uniaxial compression strength testing, slake durability, density determination, calculation of Hoek-Brown rock mass strength, particle size distribution analyses to determine coefficients of uniformity and curvature, XRD analyses, infrared spectroscopy analyses, and thin section analysis to quantify alteration intensity. Rocks were classified using Rock Mass Rating (RMR) and Geological Strength Index (GSI).Alteration intensity and mineralogy were determined by using thin section analysis, supplemented by XRD and infrared spectroscopy to identify very fine-grained clay minerals and micas. Thin sections were described and mineralogy was characterized to quantify alteration intensity, which was represented in terms of "percent of alteration minerals."Interpretations were made by graphing percent of alteration minerals against each of the measured and calculated mechanical rock properties in turn. By fitting a best fit line or curve to the data, general relationships between alteration intensity and rock strength were determined.Trachyte displayed a strong negative correlation between alteration intensity and mechanical properties including point load index strength, slake durability index, density, rock mass rating, and calculated Hoek-Brown rock mass strength. No correlation between alteration intensity and mechanical properties was observed for coefficients of uniformity and curvature. This was likely due to sampling error as fines likely washed out of detritus piles.Granite displayed indications of a negative correlation between alteration intensity and mechanical properties, although alteration intensity varied within 2% for all three samples, well within the margin of error for thin section analysis. Without more varied data, little can be said about a correlation.Diorite displayed a strong negative correlation between alteration intensity and mechanical properties including point load index strength and density. No correlation between alteration intensity and mechanical properties was observed for slake durability index, coefficients of curvature and uniformity, rock mass rating, and calculated Hoek-Brown rock mass strength. The lack of correlation for these properties results from only two data points, which is statistically-meaningless.