DWR_2801677 - Laserfiche WebLink

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