The energy threshold in dynamic probing

Abstract

In dynamic probing tests, penetration is closely related to the potential energy of the hammer (nominal energy). This energy stems from the mass and free fall of the hammer after being released from a certain height. Penetration depends on energy, although only on a portion of that nominal energy that is effectively transferred to the rods (ENTHRU) and, more precisely, the energy that reaches the cone (ENTHRUcone). ENTHRU can be measured by monitoring the upper part of the drive rods. To calculate ENTHRUcone, ENTHRU needs to be corrected in three ways. Firstly, the energy loss in the energy transmission through the rods has to be subtracted, as well as the energy loss due to the skin friction of the rods along the soil around them. It is also necessary to add the energy due to the rod weight penetrating the soil. The main hypothesis assumed and later experimentally proved in this paper is based on the fact that ENTHRUcone has to be greater than a certain value or minimum energy (energy threshold: Th) in order to be able to cause penetration. After analyzing more than one hundred blows with different hammer mass and drop height, a small but consistent Th and a linear relationship between energy and penetration beyond it were found. The energy that really produces penetration (ENPEN) will be ENTHRUcone, minus Th. This allows for improved energy corrections and correlations between results from various kinds of penetration tests.