Drilling Engineers’ fleet of geotechnical drilling rigs—which includes tree CME-55s, three CME-75s and a Geoprobe 7822DT—is well equipped for auger drilling. The company is able to outfit each rig with hollow-stem augers or continuous flight augers. Work crews can deploy a variety of sampling equipment in the hollow-stem augers: CME’s Continuous Core Sampling System, heavy-duty split spoon sampling, Shelby tubes and California barrels (standard and modified). The company’s fleet has the capacity to drill at angles.
In auger drilling, the drilling rig turns a screw to push into the earth. As a bit at the bottom of the drill string cuts into the subsurface, the turning of the helical blades pulls material from the borehole to the surface. Auger drills work well in semi-consolidated sediments, but not in consolidated sediments.
Because auger drills rely on the mechanical spinning of the screw to move material out of the borehole, they do not work below the water table where water and other non-solid materials slip off the flights.
There are two types of auger stems, each with a specific application. Solid-stem augers work well for advancing and cleaning a bore hole, but their turning motion mixes material from different strata. Because of this mixing, samples from solid-stem augers are of limited use for sampling.
Hollow-Stem Auger Drilling
Hollow-stem augers provide greater opportunities for sediment sampling and well construction. As the name suggests, the center of the stem is hollow. During drilling, a rod fills the hollow and provides structural integrity to the auger.
Once the drill reaches sampling depth, the drilling crew can remove the rods and insert one of several sampling devices—CME’s Continuous Core Sampling System, heavy-duty split spoon sampling, Shelby tubes and California barrels (standard and modified). The drilling rig drives the coring device into the sediment to obtain an undisturbed sample below the auger bit.
The work crew records each sample in the drilling log and takes them back to the laboratory for testing.
Hollow-Stem Auger Drilling and Well Installation
Hollow-stem augers also provide an excellent opportunity for building wells. While the auger is in place in the ground, the drilling crew can slide sealed sections of PVC casing—along with screens and a bottom cap—into the auger’s empty core. Once the well is positioned, the work crew begins lift to the auger out of the ground, flight by flight.
As each flight is removed from the ground, the crew fills in the empty space in the borehole with permeable materials like filter-packed sand around the screened sections and impermeable materials like bentonite clay cement grout around the solid casing.
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