Hollow Stem Auger Sampling in Unsaturated Materials

Core sampling through the hollow-stem auger in unsaturated materials is a relatively simple process. Unlike the problems of loose material left in the hole when solid-stem augers are pulled, the hollow-stem auger remains in the ground to act as a casing and hold out any potential caving materials. The only loose material remaining in the hollow-stem auger-drilled hole is 2-3 in. of material loosened by the pilot bit, that has not been moved up the auger flight. The method for sampling through the hollow-stem auger (fig. 27) is: drill the augers to the prescribed sampling depth; stop rotation; remove the hollow-stem auger adaptor cap and center-rod bolt that holds the rod-to-cap adaptor to the hollow-stem auger-adaptor cap; remove the center rod-to-cap adaptor; fasten a swivel to the center rod; and remove the rod from the hole with the wire-line winch. After the center rod has been removed, unscrew the bottom 5-ft section of the center rod that has the pilot bit and center-assembly plug attached and lay it aside. Next, attach the sampler to be used to another 5-ft section of center rod;

(A) Advancing hole to sampling depth using center-rod plug assembly and pilot bit

(B) Sampling depth—center— [ rod plug assembly removed

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  • C) Split-tube sampler in sampling position
  • C) Split-tube sampler in sampling position

Figure 27.—Soil-sampling technique using hollow-stem augers and split-tube drive sampler reattach this section to the last section of center rod removed from the hollow-stem auger; and lower the entire string to the bottom of the hole in preparation for sampling.

The following procedures are guidelines to sampling. R>r example, when drilling in a formation containing sand, with possibly some fine gravel, and a sample is required, the 3-in. solid- or split-barrel sampler with three 6-in. aluminum liners may be used. Assuming that the sample barrel is set on the bottom of the hole, 20 in. is measured above the top of the hollow-stem auger and an easily distinguishable reference mark is made on the drive rods. The method for collecting the drive sample is the same as that described for solid-stem auger-

drilled holes (page 50); all sampling methods described for solid-stem auger drilling are applicable to hollow-stem auger sampling. Note: When sampling is done through the hollow-stem auger, that is, when the center rod and pilot bit are removed, usually 2-3 in. of disturbed material remains from the drilling action of the pilot bit that has not been moved up the auger flights. After the sample has been collected and returned to the surface, the sampler has been dismantled, and the inner liners have been removed, the loose, contaminated material can be poured out of the upper 6-in. liner. However, in practice, the upper 6 in. of sample rarefy is used except for general lithologic logging of the hole, because it is the part of the sample that will be the most disturbed and possibly contaminated. If removing these few inches of disturbed material prior to collecting the sample is necessary (for example, if collecting a full-length, undisturbed sample using a Shelby tube is necessary), a 3-in. drive-core barrel equipped with basket retainer could be run into the hole and driven a few inches into the undrilled formation. When the sampler is removed, the hole bottom will be clean for sampling.

Hollow-stem augers offer another method of coring not previously described: rotary coring through hollow-stem augers. For instance, if auger drilling or sampling is done in unconsolidated materials and a refusal point for auger drilling is reached and it is not known whether this may be just a boulder or bedrock, attach a standard NX size or similar rotary core barrel to the center rod, mix a drilling mud, and proceed with a standard hydraulic-rotary coring method, using the in-place hollow-stem auger as surface casing through the overburden. The augers will provide a return conduit for the drilling fluids and cuttings.

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