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Western Dam Engineering <br />Technical Note <br />5 <br />Photo 3.Core Drill Rig <br />Core Drilling Considerations <br />Pros Cons <br />Can drill through almost all soils <br />and rock; however, typically used in <br />rock <br />Drilling fluid such as air, water, or <br />mud is likely required <br />Casing can be used to provide <br />embankment protection <br />Generally more expensive; but <br />widely available in the US <br />Yields a continuous core Core sample may be disturbed and <br />recovery may be low in some <br />materials <br />Performing SPT tests are time <br />consuming <br />Mud Rotary <br />Mud rotary (aka rotary wash) drilling (ASTM D5783) is <br />probably the most common soil and rock drilling method <br />used in the United States for a wide range of purposes; <br />however, it is not recommended for drilling in and <br />around dams. Air can be used as a drilling fluid (ASTM D <br />5782); however, some type of drilling mud is typically <br />used. Mud rotary is appropriate for drilling in almost all <br />soil and rock materials. It is different from diamond core <br />drilling in that it uses a rock or tricone-type bit at the tip <br />of the drill string and therefore only produces cuttings <br />during drilling rather than a continuous core. <br />Mud is generally used to stabilize the hole rather than <br />casing. As the test hole is drilled, mud inside the hole is <br />maintained at a level near the collar of the hole to help <br />keep the hole open and form a mud cake on the walls of <br />the test hole. Drilling with a casing advancer (ASTM D <br />5872) is a method related to mud rotary drilling and uses <br />casing as the drill string, with a wireline tri-cone bit in the <br />lead casing. A casing advancer drill string is often used for <br />materials such as flowing sands or loose gravels that may <br />not allow the test hole to remain open with only the drill <br />fluid. In some parts of the country (the northeast for <br />example), this method is sometimes used with water <br />instead of mud as the fluid – commonly called cased <br />wash borings. Similar to that described for core drilling, <br />mud rotary should not be performed through <br />embankment dams due to the use of drilling fluid and the <br />potential to hydraulically fracture any encountered <br />weaknesses, even if a casing advancer is used. Similar to <br />auger drilling, mud rotary drilling itself does not produce <br />a sample, only cuttings. However, most sampling <br />methods can be used in mud rotary-drilled holes by <br />removing the drill string from the hole and lowering the <br />sampler. This includes the potential use of diamond core <br />strings to obtain continuous core samples. <br />In Photo 4, a truck mounted CME-75 drill rig is equipped <br />with a string of “N” rod and a tri-cone bit to drill into <br />alluvium. A string of “A” rod was used to conduct in situ <br />testing and obtain SPT samples. <br />Mud Rotary Drilling Considerations <br />Pros Cons <br />Can drill through almost all soil and <br />rock and a wide range of drilling <br />conditions <br />Drilling fluid such as air, water, or <br />mud is required. <br />Generally least expensive and most <br />widely available method in all parts <br />of US <br />Should not be performed through <br />embankments due to fluid pressure, <br />even if cased <br />Easy to switch out drill string to <br />obtain samples, although the drill <br />method itself only produces cuttings <br />Photo 4.Mud Rotary Drill Rig