Drilling and sampling of boreholes represents important components of virtually all hydrogeologic investigations and ground water monitoring programs. Drilling should cause as little disturbance of the subsurface as possible. Methods should be incorporated for identification of saturated zones and sampling of formation materials to characterize the subsurface and, subsequently, allow for proper monitoring well installation. Here we'll look at hollow-stem auger drilling.<
Hollow-stem augers are recommended for penetrating unconsolidated materials. Auger rigs generally are the smallest, lightest and most maneuverable. Each section or flight typically is 5 feet in length. A head is attached to the first flight and cuttings are rotated to the surface as the borehole is advanced. A pilot bit (or center bit) can be held at the base of the first flight with drill rods to prevent cuttings from entering. When the bit is removed, formation samples can be obtained through the auger using split-spoon or thin-wall samplers. Generally, the introduction of fluids is not needed; therefore, ground water quality alteration usually is avoided.
One of the major advantages of hollow-stem augers is that they allow for well installation directly through the auger into non-cohesive material. The inside diameter of the hollow-stem generally is used to specify size, not the diameter of the hole drilled. Appropriate clearance should be available to provide effective space for materials placement. If space is insufficient, bridging of the materials may bind the casing and auger together, resulting in the extraction of the well as the auger is removed. Additionally, insertion of a tremie pipe may be difficult.
The most widely available size is 3.25-inch (6.25-inch outside diameter, including the flights), which has been used to install 2-inch (2.378 outside diameter) monitoring wells; however, this allows limited access. It is doubtful that materials can be placed adequately at depths below 15 feet considering the relatively small amount of clearance offered. The minimum size that should be used for installation of 2-inch diameter casing is 4.25 inches; however, larger augers may be necessary. The Environmental Protection Agency recommends that the inner diameter of the auger be 3 inches to 5 inches greater than the outer diameter of the casing.
The depth capability of hollow-stem augering is dependent on site geology and the size of the rig and stem. In general, greater depths can be reached when penetrating clays than when penetrating sands; however, clays may cause the auger to bind, which limits depths. The size of the rig and stem affects the downward pressure and torque on the stem. Greater depths may be reached by smaller augers. Depths of 200-plus feet can be reached utilizing a 4.25-inch hollow-stem auger, whereas 10.25-inch augers can reach a maximum depth of approximately 75 feet.
Hollow-stem augering can present some disadvantages. It cannot penetrate cobbles, boulders and most rock formations. In some cases, obstructions can be pushed aside by spinning the augers in-place. When this is not successful, replacing the pilot assembly with a small tri-cone bit may allow penetration. Additionally, carbide-tipped cutting teeth have been developed for the upper portions of weathered bedrock, which may be useful when the unconsolidated/bedrock interface is the zone of interest.
Although augering generally allows for adequate identification of water-producing zones, the technique may cause clay and silt to smear on the borehole wall, preventing the identification of low-yield zones and hindering well development. This smearing may be beneficial if it serves to impede vertical ground water movement, which reduces the potential for cross-contamination between subsurface zones. However, the possibility of this circumstance occurring is unpredictable.
The use of hollow-stem augers may be hindered by “heaving sands,” which occur when a confined, saturated sand unit is encountered. Infiltration of the sand and water into the augers causes them to bind. Common strategies to alleviate this:
- Water may be added to maintain a positive downward pressure to offset the pressure of the formation.
- Drilling muds can be added to further offset the pressure.
- The lower portion of the auger may be perforated to allow formation water to enter. This will equalize the hydraulic pressure and prevent entrance of sediments. Screened augers have been developed for this purpose, although strength and structural integrity is lost.
- The pilot bit can be kept in place or a knock-out plug or winged clam can be added to the base of the hollow-stem to prevent infiltration.