Use of CPTu to Estimate Equivalent SPT N60

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Courtesy of ASTM International

Although the CPTu offers many advantages over the SPT, it may be desirable in some instances to use SPT-based experience of soil behavior. An algorithm is presented here for estimation of equivalent N60 values directly from the CPTu without resort to soil sampling. The proposed algorithm is based on data and trends reported in the literature; the algorithm is tested against new data obtained for cross-calibration of the CPTu/SPT in a wide variety of soil types, penetration resistances, and depths. As part of the evaluation of the algorithm, replicate trials of both SPT and CPTu were carried out. The data show the CPTu is five times more precise than the SPT. Further, the equivalent N60 derived from the CPTu using the proposed algorithm is shown to be at least as reliable as values directly determined by the SPT; the much improved precision of the CPTu outweighs the uncertainty in the CPTu/SPT calibration, and the calibration in itself averages the testing error of the SPT. The proposed algorithm is tested for bias against depth, soil type, penetration resistance, and friction ratio; the algorithm is unbiased. Cumulative probability density functions are given for repeatability of the SPT, the CPTu, and an estimation of equivalent SPT values from the CPTu.

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