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Western Dam Engineering Technical Note 7 Becker Drilling Becker drilling (ASTM D 5781) uses an AP-1000 diesel hammer to drive double walled casing. Becker casing can be driven through almost all materials within the embankment dam and weak rock in the foundation. SPT and core samples can be obtained through the inside of the casing and other methods of drilling can be used to deepen the test hole, if required. Drill fluid is not required for this drilling method; however, some water may be poured into the casing to reduce friction. During drilling cuttings from inside the casing are carried to the surface using reverse circulation air and separated using a cyclone. In Photo 7 below, a truck mounted AP-1000 Becker rig and required pipe truck is shown set up on a test hole. The Becker casing on the pipe truck is 9-inch diameter, with an inside diameter of about 4 inches (with a crowd in bit). Note the drill pad size required for the Becker rig and pipe truck, which is about 120 feet long and 20 feet wide. The pipe truck is typically set up behind the Becker rig to allow safe handling of the heavy casing. Becker Hammer Drilling Considerations Pros Cons Can drill through loose/soft to very dense/hard material, and rock fill May not suitable for strong rock Casing provides embankment protection Generally more expensive and less widely available Yields continuous cutting samples Performing SPT are more difficult/costly than with HSA Drilling fluid not required Becker blow count can be correlated to SPT blow count Photo 7.Becker Drill Rig The drilling and sampling descriptions above provide limited information about the equipment. Refer to the ASTM standards for detailed descriptions and considerations to use when selecting drilling and sampling methods. ASTM D 6286 (Standard Guide for Selection of Drilling Methods for Environmental Site Characterization) may be useful for comparing drilling methods and selecting a preferred method for the investigations. Often the best source of information when selecting drilling and sampling methods is drilling company employees familiar with the equipment they use. If possible discuss your proposed drilling and sampling plan with the driller you plan to work with before heading into the field. If multiple drilling and sampling methods are to be used, it will be important to check the feasibility of the investigation plan with the driller. Drilling and Sampling the Core Zone The core zone of the dam refers to the generally fine- grained, low permeable zone of the embankment that is the primary seepage barrier. Drilling and sampling in the core zone of an embankment dam risks damage to the core. If possible, drilling and sampling in the core should be avoided. Alternatives should be considered to obtain the same or similar information by doing investigations at locations outside the core. For example, drilling through the shell or toe area is preferred if the primary objective is obtaining information on the foundation. If no alternatives are available, the preferred method of drilling in the core zone should be auger drilling. Auger drilling can be done without the use of drilling fluid (air, water, or mud) and therefore limits the risk of hydraulic fracturing of materials comprising the embankment core that could lead to internal erosion and piping damage. If drilling with water or mud must be conducted, limit the effective head to 0.5 psi per foot of vertical depth to reduce risk of hydraulic fracturing. Never drill in the cut- off trench, adjacent to outlet works or conduits, in or near known areas of seepage, and at locations above abrupt changes in the shape of the foundation. These may be areas of low stress that are more susceptible to damage due to drilling-induced disturbance or may be areas in which a drill hole could connect concentrated seepage paths. Drilling and Sampling the Shell Zone(s) Drilling and sampling in the shell(s) of an embankment dam may require the same considerations as drilling and sampling in the core, especially if the shell was constructed from fine-grained material. Shells