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Western Dam Engineering <br />Technical Note <br />2 <br />Poking the Bear: Drilling and <br />Sampling for Embankment Dams <br />Subsurface investigations that include drilling and <br />sampling are often used to obtain geotechnical <br />information about embankment dams and their <br />foundations. However, performing these intrusive <br />investigations does not come without risk and unique <br />considerations. Drilling could connect existing seepage <br />paths or weak zones within the embankment, or create <br />such by fracturing or disturbing the material. As such, <br />drilling with fluids within embankment dams is <br />particularly adverse. Drilling a hole in or near a dam to <br />collect in situ information and samples should therefore <br />only be performed if warranted, under supervision of a <br />driller and field engineer with experience drilling in <br />embankment dams, and executed based on a well <br />thought-out plan. The plan should include the purpose <br />and goals of the investigation and address concerns <br />associated with drilling through an embankment dam. <br />Intrusive drilling and sampling are usually conducted after <br />other non-intrusive investigations have been completed, <br />including review of existing information, mapping of rock <br />and soil exposures, and perhaps geophysical surveys. <br />Intrusive methods are prescribed after it has been <br />determined that analyses requiring site-specific <br />geotechnical information are warranted. <br />In the last issue of the Western Dam Engineering <br />Technical Note, we discussed ways to evaluate stability <br />and determine whether more in-depth analyses are <br />warranted. In this edition, we include some methods of <br />drilling and sampling embankment dams typically used to <br />support those studies. The purpose of this technical note <br />is to discuss general guidelines for evaluating and <br />selecting the drilling and sampling methods that are <br />currently available in most parts of the United States. <br />Additional, less common, methods exist that have not <br />been included. The discussion begins with general <br />sampling and drilling methods for embankment dams, <br />followed by guidelines for drilling and sampling in the <br />core of the embankment, the embankment shell, and the <br />foundation. <br />Importance of Investigation Plans <br />A geotechnical investigation for an embankment dam <br />that involves drilling and sampling in test holes should <br />begin with a carefully thought-out plan that addresses <br />the objectives and purpose of the work and lays out <br />detailed specifics of the drilling and sampling approach. <br />Most projects will have existing information available <br />about the dam and foundation that can be used to <br />provide an estimate of the type and location of materials <br />that might be encountered. This information will be <br />helpful to determine appropriate drilling methods, the <br />estimated depth, inclination, and diameter of test holes, <br />and the types of samples that should be obtained. The <br />contents of this article should be carefully considered <br />when selecting the hole location and drilling method. In <br />particular the reasons to not drill with fluid through the <br />embankment (and with it in the foundation) should be <br />heeded. <br />Drilling and sampling usually forms only a part of the <br />investigation plan. The investigation plan should also <br />consider site access conditions, utilities, health and safety <br />requirements, test hole completion and abandonment, <br />installation of instrumentation, in situ testing, sample <br />handling, storage of samples, transportation of samples, <br />and laboratory testing. The investigation plan should also <br />consider the target drilling and sampling depths and <br />expected phreatic surface. <br />Perhaps the most important part of the investigation plan <br />is to be flexible and have contingencies for unknown <br />conditions often encountered in the subsurface. <br />Flexibility can be achieved by having several different <br />drilling and sampling methods available with the drill rig <br />selected for the investigation. An example contingency <br />for drilling would be to have extra drill rods on the rig, <br />should the test hole(s) need to go deeper or have a string <br />of casing available for caving hole conditions. An example <br />contingency for sampling would be to have a variety of <br />samplers on the rig to select the one most appropriate <br />for the soils encountered. <br />Drilling and sampling in an embankment dam has <br />associated risk of damaging the embankment; therefore, <br />the investigation plan should include procedures to arrest <br />damage if it is observed or suspected. Drains, <br />embankment slopes, and piezometers can be monitored <br />to observe changes during the drilling and sampling. If <br />the embankment is considered particularly sensitive to <br />potential changes (i.e., the dam has shown signs of <br />instability, seepage, or piping) materials such as gravel, <br />sand, bentonite, etc. can be stockpiled near the test hole <br />with equipment for transport to the test hole, should <br />damage become apparent. Signs of embankment <br />disturbance may include increased seepage downstream <br />of the test hole, seepage turning turbid, or <br />sinkholes/depressions forming on the surface of the <br />embankment. At a minimum the accessibility of the <br />nearest source of emergency materials should be