Stability of DPT hammer efficiency and relationships of blow-counts obtained by different DPT apparatuses

The blow counts of different DPT apparatuses and their relationships are analyzed by means of the energy-calibration method which uses a pile dynamic analyzer (PDA) energy measurement and the in-situ DPT tests. The stability of energy transmitted into penetration tips is discussed. The analytical results demonstrate that the DPT is a reliable site testing technique with good hammering efficiency. The energy transfer ratios (ETR) of different types of DPT are high with the average values more than 80%. The relationships of blow counts calibrated by ETR values are consistent well with those obtained by the in-situ tests. The blow counts of heavy DPT tests with a hammer of 63.5 kg is in average 2.5 times those obtained by super-heavy DPT with a hammer of 120 kg, and the influence of rod diameters can be neglected. The ETR values measure the energy transmitted into the rod through hammer drops, i.e., hammering efficiency. However, the actual energy transmitted into soil usually reduces due to the energy consuming of rods and should be measured at the tip. The technique is expected to be solved in the future.