(c) Reveal any significant variation in soil and rock conditions that would have a bearing on seepage flow, location and depth ofwells, or depth ofcutoff. Continuous wash or auger boring samples are not con­sidered satisfactory for dewatering exploration as the fines tend to be washed out, thereby changing the character of the soil.

b. Rock coring. Rock samples, to be meaningful for groundwater studies, should be intact samples ob tained by core’ drilling. Although identification of rocks can be made from drill cuttings, the determina­tion of characteristics of rock formations, such as fre­quency, orientation, and width ofjoints or fractures, that affect groundwater flow requires core samples. The percent of core recovery and any voids or loss of drill water encountered while core drilling should be recorded. The approximate permeability of rock strata can be measured by making pressure or pumping tests of the various strata encountered. Without pressure or pumping tests, important details of a rock formation can remain undetected, even with extensive boring and sampling. For instance, open channels orjoints in a rock formation can have a significant influence on the permeability of the formation, yet core samples may not clearly indicate these features where the core recovery is less than 100 percent,


U. S. Army Corps of Engineers

Figure 3-1. Geologic profile developed fromgeophysical explorations,


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