The hollow-stem auger is a form of continuous flight auger usually used for drilling monitoring wells in unconsoli-dated materials. The auger consists of a tubular steel center shaft or axle around which is welded a continuous steel strip in the form of a helix, also known as flight, as shown in Figure 9.15.1. As the auger column rotates and axially advances in the ground, the dug material is simultaneously conveyed to the surface by the helix.
The main advantage of hollow-stem auger drilling is that no drilling fluids or lubricants are used; therefore, no contaminants are introduced into the aquifer. In addition, the hollow stem of the auger allows sampling of soil material as the borehole is advanced and installation of casings and screens for monitoring wells when the required depth has been reached. The drill head, or cutting bit, lo cated at the bottom of the auger can be removed (tripped) through the center of the auger to the surface. This feature allows the auger to stay in place providing an open, cased hole into which samplers, downhole drive hammers, casings, screens, and other instruments can be inserted.
The hollow-stem auger cannot be used, however, in consolidated, rock, or well-cemented formations. In addition, depths are usually limited to no more than 150 feet, and vertical leakage of water through the borehole during drilling is likely to occur.
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