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Western Dam Engineering <br />Technical Note <br />8 <br />constructed of coarse-grained earth fill or rock fill can be <br />drilled and sampled using any of the appropriate <br />methods described above. Risk of contaminating and <br />plugging filters and drains should be considered; <br />therefore, if possible use an auger drilling method <br />because drilling mud is not required. If auger drilling is <br />impossible due to flowing ground conditions, consider <br />using a casing advancer method and keep the casing full <br />of water or drilling mud at all times. If gravel, cobbles, <br />and boulders are encountered, consider using the sonic <br />drilling method or the core drilling method. Embankment <br />shells constructed with rock fill may contain large <br />boulders in which refusal may be encountered when <br />using auger or even sonic drilling methods in some cases. <br />A flexible investigation plan with contingencies for <br />potential problems should be considered. For example, in <br />the case of refusal, a pilot hole can be made with a core <br />drill string to investigate whether refusal is due to a <br />discrete boulder, in which case the auger string or sonic <br />string could then ream the pilot hole and advance the <br />test hole through the boulder. <br />Drilling and Sampling the Foundation <br />Subsurface investigations of the foundation material can <br />be performed either through the embankment or outside <br />the dam footprint. Either location has precautionary <br />considerations. We have discussed the care needed to <br />drill through the embankment and that is also reiterated <br />in the paragraph below. Investigations within the <br />foundation outside the dam footprint also need to <br />consider the risk of blowing out foundation material. <br />Drilling or excavating test pits near the toe, especially <br />when the reservoir is near full pool, could provide an easy <br />exit for foundation pore pressures, resulting in the <br />potential for heave or blowout of the foundation. Larger <br />excavations such as test pits, especially those below the <br />water table, could exacerbate this risk. Offsetting borings <br />a distance away from the toe, lowering the reservoir if <br />possible, and drilling borings using fluid or mud help <br />reduce the risk. This should be evaluated based on the <br />specific foundation conditions expected at the site. <br />In cases in which the test hole will extend through the <br />embankment (hopefully the shell) and into the underlying <br />foundation, the materials to be drilled and sampled will <br />likely vary and may require different drilling and sampling <br />methods with depth. The investigation plan will need to <br />address how to safely drill through the embankment and <br />continue into the foundation. An example might be to <br />use the hollow-stem auger drilling method to advance <br />the test hole to the top of a rock foundation, then use the <br />auger as casing and seat the lead auger into the <br />foundation, then advance the test hole into the <br />foundation using the core drilling method. The <br />investigation plan, especially the drilling and sampling <br />approach, should be discussed with the driller prior to <br />start of the work so that potential equipment problems <br />can be identified. In many cases, the driller will offer good <br />ideas for conducting the drilling and sampling if they are <br />well-informed of the objectives of the investigation. <br />SPT Sampling for Liquefaction <br />SPT sampling for liquefaction evaluation is very <br />specialized procedure. Special care is required when <br />sampling very loose sands. By standard procedure, water <br />or mud is required in the hole when performing SPT <br />sampling. This limits the potential of causing flowing or <br />heaving in loose sands, which would disturb the in situ <br />test. As mentioned earlier, fluid is not recommended in <br />borings within embankment dams; however, compacted <br />embankment materials are not common targets of <br />liquefaction evaluation. Certain drilling methods may also <br />disturb the area, especially in loose soils. Precautions <br />should be implemented to limit drilling disturbance in the <br />target zone for liquefaction evaluation. This may include <br />switching drilling methods to auger methods in the target <br />zone, which generally results in the least disturbance, and <br />extracting drill strings at a slow rate to avoid a vacuum <br />effect. The procedure may also require corrections for <br />gravel content, which dictates the method to be used for <br />blow count. The Reclamation guidance manual on SPT <br />sampling (Reclamation 1999) should be referenced for <br />more detail. <br />Test Hole Backfilling <br />If the test hole is not being completed as a piezometer, <br />then it should be backfilled in such a manner to limit <br />potential for a weak zone or seepage paths in or around <br />the hole. Backfilling using cement/bentonite grout placed <br />using tremie (bottom-up) methods should be required in <br />or near the dam. Bentonite chips (also known as “hole <br />plug") can also be used. Bentonite chips expand upon <br />saturation, so the hole should be wetted as chips are <br />placed. Backfilling test holes using cuttings should never <br />be performed in holes within the embankment, <br />foundation, abutment, or toe area of dam, as this could <br />cause the surrounding soil to creep, deform, or result in <br />increased seepage. <br />Conclusions <br />Drilling and sampling for embankment dams should be <br />initiated with the preparation of a geotechnical