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If I Had a Hammer... Automatic standard penetrometer hammers are becoming more common in the North Georgia area. These hammers undoubtedly provide more consistent test results than the older, cathead and rope manual hammers, and in time should become the norm for standard penetration testing. However, the manual hammers have proven to be reliable when used by properly trained and experienced drillers, and will continue to be used for the foreseeable future. Where the challenge for geotechnical engineers arises is in the fact that the automatic hammers are on average about 50 percent more efficient than the manual hammers. Because of this, the standard penetration resistance (N) values obtained using the automatic hammers are generally lower than those for manual hammers and must be adjusted accordingly, typically by a factor of 1.5. Most empirical design correlations in use today are based on N values obtained using manual hammers. Therefore, if the adjustment is not made then design based on N values obtained using an automatic hammer will be overly conservative and cost the client money. In summary, it is important for geotechnical engineers to be aware of the type of standard penetration hammer being used to obtain the SPT values for a given project. If it is an automatic hammer, then appropriate corrections must be made to the SPT values to avoid overdesign. June, 2000
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