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The role of hydrothermal alteration in producing clay-rich rocks is discussed. Hydrothermal fluids derived from magmatic sources change rock lithologies to argillic rocks as a result of temperature, pressure and chemical effects. The grade of the argillization can vary from one in which only trace amounts of clay minerals are present to one in which there has been complete alteration into clay. Detailed geologic surface mapping and subsurface drilling are required to accurately delineate the extent and grade of alteration. Suggestions are presented for assessing the grade and amount of alteration. Two examples illustrate the effects of argillic alteration on the engineering design of excavated slopes. One example demonstrates a successful design where alteration effects were incorporated into the initial design stage. The second example illustrates where an inadequate geologic model underestimated the grade and extent of alteration, and a landslide of over four million tons of material resulted. Reappraisal of the geologic model enabled a successful mitigation and incorporation of the clay-rich lithologies to be designed, and permitted continued safe excavation at the site.

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