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Energy measurements and correlations of the standard penetration test (SPT) and the becker penetration test (BPT) Sy, Alexander


The Standard penetration test (SPT) and the Becker penetration test (BPT) are two of the most widely used in-situ tests in North America. The SPT is most commonly used in sands and silty sands, while the BPT, being a large-scale penetration test, is more useful in gravelly soils. Both tests involve hammer impact on penetration rods, and the resulting penetration resistance or blow count is strongly influenced by the amount of hammer energy actually transferred into the drill rods. To make use of the large world-wide foundation performance data base currently available for the SPT, the BPT blow counts are commonly correlated to the SPT blow counts. Most of the existing correlations, however, have limited applications since they do not take into account the inherently variable output of the diesel hammer used in the Becker system and they ignore the soil friction acting on the Becker casing during driving. This research shows that the existing methods of SPT and BPT energy calibrations have serious shortcomings, and that a more fundamental approach of determining the transferred energy, based on force and acceleration measurements, should be adopted for both tests. The proposed approach provides a unified method of measuring transferred energy in the SPT and BPT, similar in principle to that currently used in dynamic testing of piles. At four research sites in Greater Vancouver, SPTs, BPTs and electric cone penetration tests were conducted. Dynamic measurements were also carried out which included force and acceleration near the top of the drill rods or pipes in the SPT and BPT, as well as bounce-chamber and combustion-chamber pressures in the double-acting diesel hammer during the BPT. An energy approach for correcting the measured BPT blow count to a reference energy level, similar in concept to that used for the SPT, is proposed. Factors affecting the BPT blow counts are investigated including hammer combustion conditions, different drill rigs, and different pipe sizes. The test results confirm that the measured transferred energy is a fundamental and useful parameter for normalizing the BPT blow counts to account for the variable energy output of the diesel hammer. The effect of casing friction in the BPT is investigated by field measurements and numerical analyses. New BPT-SPT correlations are proposed which consider the energy transfer in both tests and which, for the first time, account for casing friction in the BPT. It is shown that the proposed BPT-SPT correlations provide a rational framework for determining equivalent SPTN60 values from measured BPT blow counts, and can be applied with some confidence to gravel sites for which the BPT has proven to be a most practical and economical testing technique.

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