Significance of Energy Transfer Ratio in Standard Penetration Test and Its Influence on Foundation Design

The majority of the cost and time overruns in infrastructural projects are due to concerns related to substructures. Every project needs a detailed geotechnical investigation for cost economics, safety and to arrive at a competent load-bearing stratum for construction of foundation. The standard penetration test is an in situ method which measures penetration resistance of a standard split-spoon sampler to dynamic impacts by a standard hammer and involves several corrections to arrive at the corrected N value. Energy correction factor is a field correction tool to arrive at a normalized N value for 60% energy transfer ratio (N60) from the SPT hammer to the drill rods. IS 2131 mentions hammer drop to be a free fall but does not emphasize the need for energy measurement. It is a standard practice and quality control procedure to calibrate every geotechnical SPT drilling rig to measure the energy transfer ratio (ETR). In the present study, ETR data of 40+ geotechnical drilling rigs with different driving systems across India are presented. Consequences of overprediction and underprediction of bearing capacity for different cases are briefly discussed. The importance of the measurement of ETR is appraised, and its influence on foundation design, cost economics and safety are explored along with case studies.