Comparison of energy measurement methods in the standard penetration test : final report, appendices I, II, III, and IV

Research sponsored by the US Bureau of Reclamation. The Standard Penetration Test (SPT) N-value is widely used in geotechnical engineering as a basis for foundation design and as the primary index of liquefaction resistance. The main factor affecting the N-value is the energy delivered to the rods by the hammer. As measured N-values are conventionally corrected to a standard energy level of 60% of the standard potential energy, it is necessary to measure the energy input. Research into the measurement of energy during SPT testing has been conducted since the 1970s. The early work was based on measurement of the force time history alone (F² method). In recent years, the Force-Velocity (FV) method) has been suggested to be superior to the F² method. It requires the measurement of acceleration in addition to force, with velocity determined by integration of the acceleration. As the values of energy obtained by the F² and FV approaches are often different, this research project was conducted to investigate the reasons for such differences and to develop recommendations for equipment, instrumentation and testing procedures, which would allow more accurate and consistent energy measurements in SPT. The work consisted of review of available energy measurement data, laboratory study of the energy transfer process and of the performance of the individual components of energy measurement systems and full-scale measurements under controlled conditions in the laboratory and in the field. The experimental work was carried out in conjunction with numerical modelling of the energy transfer process. The work confirmed that the FV method is the only consistent approach for measurement of stress wave energy transmitted from the hammer to the rod and sampler system in SPT testing. It is relatively straightforward to obtain reliable measurements of force time history but reliable determinations of the velocity time history are more challenging. A rigidly mounted 20,000g accelerometer in conjunction with a data acquisition system capable of sampling the acceleration signal at over 100 kHz was shown to result in high quality stress wave energy data. Both digital and analog integration of acceleration were found to give comparable velocity time histories provided attention was paid to compatibility of all mechanical and electronic components of the measurement system. Based on the experimental and analytical work, the UBC energy measurement research system was upgraded and limited field trials were carried out. The field trials also permitted some comparisons to results from a commercially available energy measurement system. With any system, reliable energy measurements require rigorous data quality assessment in the field by qualified personnel with an understanding of wave mechanics. Guidelines for such assessments are provided in the report. A simple field apparatus and test procedure capable of transmitting a reproducible impulse wave is proposed to allow the operator to check that the instrumentation is functioning properly. Finally, recommendations are provided for instrumentation and procedures that will allow reliable measurement of energy in the field.

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