Effects of SPT Equipment and Procedures on the Design of Shallow Foundations on Sand

by William D. Kovacs, (M.ASCE), Univ of Rhode Island, Kingston, United States,

Abstract: The effect on our computations of bearing capacity and settlement of shallow foundations suggests that as more efficient SPT hammers and procedures are used, our shallow foundation designs are becoming more and more conservative. Increased efficiency of running the SPT comes from evolving hammer types (from donut to safety hammer to the new automatic hammers) as well as the common non use of liners in present practice. Increased hammer efficiency and the non use of liners both tend to reduce the blow count from that which would be obtained if the original ASTM standard was used. However, with our present SPT standard (ASTM D-1586-92), a free-for-all with respect to equipment used to perform this in-situ test exists and as a result, a wide variation in resulting blow counts precludes the consistent correlation between bearing capacity and settlement with blow count. Factors that influence the delivered energy to the sampler, and hence the resulting blow count are described and show that the blow count can vary by a factor of three. One should recognize that the `SPT System' includes the Operator of the drill rig; the specific model of the Drill rig itself; the SPT equipment hardware used to perform the test; and the procedures used in performing the overall test (which includes drilling the hole and sampling). All of these four factors within the `System' contribute to differences in delivered energy. For a given `System,' the variables that most contribute to energy variation include the operator, the use or non use of liners, the hammer type, the anvil (sub)system, and the number of turns of rope if a cathead is used. In addition, the following non energy related variables greatly influence the blow count as well. These variables include: the type and especially the direction of flow in the fluid discharge bit used in drilling, and the care used to drill, clean, and maintain the water level in the drill hole (when sampling below the ground water table). Suggestions for decreasing the variability of delivered energy to the sampler are given along with equipment and procedures to use to further standardize the test. Finally, existing correlations between SPT blow count and bearing capacity and settlement of foundations on sand are examined noting how our design has changed over the past decades. If we are ever to revert back to our original correlations then we must return to the original energy that was used for the SPT in order to maintain accurate predictions of foundation behavior. On the other hand, if the profession chooses to accept the present (1994) higher energy delivered to the sampler, then correction factors and/or revised design charts need to be made available.

Subject Headings: Shallow foundations | Foundation settlement | Soil settlement | Penetration tests | Load bearing capacity | Offshore platforms | Standards and codes

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