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Western Dam Engineering Technical Note 4 density, consolidation (in the case of soil cores), rock quality (in the case of rock cores), and strength properties. Typical Drilling Methods Six general methods of drilling are presented herein that are typically considered in the investigation of embankment dams. The methods include auger drilling, core drilling, mud rotary drilling, Odex drilling, sonic drilling, and Becker drilling. A number of other methods exist; however, these six have been found to be most commonly used in the United States. Refer to the referenced ASTM standard for detailed information for each drilling method. Auger Drilling Auger drilling is one of the most commonly used methods of drilling, and for embankment dams should be the preferred method, if possible due to its low risk of embankment damage, cost, versatility and availability. Many drill rigs can be set up to use solid flight auger (ASTM D1452) and hollow stem auger (ASTM D6151) strings. Auger drilling requires a high torque and low revolution speed rig, and with care depths of 200 to 300 feet can be drilled. Auger drilling does not require the use of drilling fluid; however, in some materials water may be poured into the test hole to aid in removing cuttings. Auger drilling is appropriate for fine-grained soils, granular soils, and weak rock, but may have difficulty advancing in very dense and/or very coarse-grained granular material due to friction. It is not appropriate for strong rock. Auger drilling allows for split spoon (SPT), Shelby tube, and thick-walled tube sampling. Photo 2.Auger Drill Rig A track mounted drill rig using a 6-inch hollow stem auger (HSA) is shown in Photo 2. The string of 6-inch HSA has an inside diameter of 3 ¼ inches. This configuration allows use of the auger string as casing. Sampling and deepening the hole below the lead auger can be accomplished by switching to mud rotary or core drill strings. Note the added precautions necessary, particularly access and safety, when drilling on the steep incline of an embankment slope. Auger Drilling Considerations Pros Cons Can drill through loose/soft to dense/hard material and possibly weak rock Not suitable for strong rock or some larger diameter granular material (i.e., cobbles and boulders) HSA casing provides some embankment protection Auger string may deviate more easily than other drilling methods Drilling fluid not required Generally less expensive; available in most parts of the US HSA casing allows easy access for most sampling techniques including core barrel [although test hole not filled with mud per ASTM standard of SPTs (ASTM D1586)] Core Drilling Core drilling (ASTM D2113), also referred to as diamond core drilling, can be used to obtain continuous sample cores of soil and rock, although this method is most commonly associated with drilling and sampling in rock. Drilling fluid, if required, can be air, water, or mud. SPT samples can be obtained from the inside of the drill string when the core barrel has been removed; however, this presents a risk of damaging the diamond bit. Because drilling fluid (water, mud, or air) is used, this method should not be used within embankments. If used within a rock foundation below an embankment, the hole should be cased within the embankment zone with the casing seated in the underlying bedrock, and preferably drilled through an outer shell zone. In Photo 3 below, a track mounted core rig is shown advancing a test hole at an angle of 25° from vertical. The drill rod on the pipe rack is “HQ” size (4-inch diameter hole) from a wireline core string, which allows retrieval of the core samples as the test hole is deepened without having to trip out the drill string.