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