The Bearing Capacities tabulated below are based on the Hiley Formula;
Bearing Capacity (tonnes) = Blow Efficiency x E/(s+2.54), where
E=Hammer Energy (kg.m), s=Final Set per Blow (mm/blow).
Blow efficency for a hydraulic hammer is typically around 80% and adding a safety factor of 2 permits the formula to be modified to - Bearing Capacity (tonnes) = 0.4 x E/(s+2.54).
Irrespective of hammer selection the Hiley formula is a simplified analysis and actual bearing capacity achieved will be significantly affected by pile type, length, mass and stiffness, in addition to prevailing
ground conditions.The Hiley formula was originally devised to estimate pile bearing capacity based on actual site measurements.
The Dawson Hydraulic Impact Hammer has a visible stroke marker on the side of the hammer. This marker can be calibrated to indicate maximum and minimum hammer energy outputs and any energy figure
between those two limits. In addition, it is possible to relate the hammer blow rate at final drive to hammer energy output as set out in the table and on the graph shown below providing hammer performance meets
with manufacturers specifications.
Please note that practical refusal is considered to be 10 blows/25mm. Driving in excess of 10 blows/25mm for more than 150mm of pile penetration or any driving in excess of 20 blows/25mm will invalidate
the hammer warranty.