Solid Stem Augers

Samples obtained from solid-stem auger-drilled holes usually are a mixture of the penetrated materials. Material samples that are deposited around the surface perimeter of the hole are representative of the penetrated materials; howevei; they are a mixture of several feet of the lithology encountered by the drill head. With experience, the driller can feel some textural changes while drilling at the approximate depth at which these changes are encountered in the hole. These notations, combined with collected samples, make a fairly reliable lithoto-gic log. Periodic reaming of the hole is necessary to ensure that auger returns are representative of the penetrated zone. Disturbed auger samples may be used for determinations of particle-size distribution, specific gravity, and lithologic or minerabgic analysis, as an inexpensive analytic procedure. Disturbed materials may be repacked for hydraulic-conductivity tests if no undisturbed samples arc available, but the validity of such tests is questionable. If disturbed materials are used, the following need to be considered when applying the hydraulic-conductivity test results to the field situation: (1) material returned to the surface by auger drilling rarely represents actual particle-size distribution in penetrated sediments, especially when auger drilling in saturated materials; (2) loose, granular materials encountered in the hole probably will be more prevalent than fine-grained materials in the sample appearing at the surface, because fine-grained materials may be left behind; and (3) further segregation of particle sizes occurs rapidly as granular materials are vibrated; in most cases, coarser materials are continuously returned ahead of finer materials. Thus, generally, a sample returned to the surface will not represent true lithologic conditions through the entire depth of the hole, nor can it be considered entirely representative of conditions at any given depth.

In addition to collecting disturbed-cuttings samples delivered at the surface when drilling with solid-stem augers, drive-core and even undisturbed Shelby-tube samples (see p. 72) can be collected at the bottom of the hole. However, sloughed or caved materials first need to be removed from the bottom of the hole.

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