Standard Penetration Test (SPT) is one of the most widely used test for estimation of soil properties and correlations. However it details with its own set of disadvantages on field making its acceptance questionable in a number of cases.
The SPT N value greatly depends on the energy transferred by a hammer to the split spoon sampler attached at the base of drive rod and which collects the soil sample. The SPT N value is calculated based on the number of blows required to drive the sample for a total of 450mm for which the N value is calculated based on the last 300mm penetration. Since the hammer or its assembly to apply blows as well as the procedures are not really standardized, it leads to a huge variation in the observed N values i.e. measurements done with different setups and systems.
Even after all necessary precautions SPT N values reported as N60 needs to be standardized by application of various corrections or else may lead to serious errors in understanding of the reported N60 values. Geo Dynamics is equipped to measure the actual energy transferred to the SPT rod to evaluate the actual efficiency of the driving system. This is basically achieved by attaching Instrumented rod (on which accelerometers and stain gauges are mounted) to the SPT rods during the performance and measuring the energy transfer values with the SPT Analyzer. Based on these measurements, corrections to the N-value are applied to normalize the field SPT values to standard 60% energy transferred in to the rods
We can also monitor hammer performance for diesel and hydraulic hammers to measure their actual energy transfer using Pile Driving Analyzer (PDA). This is greatly useful for contractors to make decisions before purchasing expensive systems and also helps designers refine their calculations based on actual hammer efficiency recorded on site.
