Velocity Measurement of Free-Fall SPT Hammer

Abstract

Field measurements of the velocity just before impact of a Borros AB Standard Penetration Test (SPT) automatic trip hammer indicate that the average delivered energy is 99% of the theoretical available, free-fall energy. It is expected that other trip hammers used in Europe, Scandinavia, and Japan also give close to 100% of the theoretical energy during the performance of the SPT. In contrast, the delivered energy varies from 25% to 80% in U.S. practice where the cathead and rope system is used to 100% of that delivered when trip hammers are used. Since it has been shown theoretically and experimentally that the SPT blow count is inversely proportional to the delivered energy, it follows that for the same soil conditions, blow counts will vary where there are differences in delivered energy. Thus, engineers are cautioned in the interpretation of SPT data from countries where trip hammers are in use. Blow counts may differ by 200% or 300% from U.S. practice.