Energy Measurement in Standard Penetration Tests

The standard penetration test (SPT) is a widely used in situ test method worldwide that can evaluate soil properties based on the blow counts (N-value). The N-value depends on soil properties, and the energy transferred to the drill pipe during hammering. Currently, European and American scholars generally believe that variation in the amount of hammer energy transmitted to the drill pipe due to different types of drop hammer systems is the primary factor that leads to variations in N-value. In China, there is a lack of research on the quantitative energy transfer efficiency of the drop hammer system based on test data from a penetration test instrument. In this study, an in-situ test in Jiangsu Province was performed at a test site using standard penetration test instruments that are commonly used in China. Corresponding time history curves and strain, acceleration, force, velocity, energy and penetration degree data were obtained through the stress wave test. The propagation law of the stress wave and energy in a drill pipe was analyzed, and the energy transfer efficiency of the domestic SPT system was measured. In the stress wave test, most of the measured hammer energy efficiency was between 74.5 and 84.5%, and the measured average energy was 0.3723 kJ; the average energy efficiency was 78.7%; the standard deviation (SD) of the energy efficiency was 3.82, and the coefficient of variation (CV) of energy transfer efficiency(E_R) was 4.9%. The average energy efficiency of 78.7% can be considered to be the energy efficiency of the domestic SPT system. The calculated results reported in this article can be used to improve the quantitative level of domestic investigation. Based on the calculated E_r, the results obtained from different SPT systems at home and abroad can be corrected.