Every structure built relies on proper support and a strong foundation. A properly supported foundation is essential to longevity for any structure. In the geotechnical engineering field, accurate environmental data is king. Soil variances can impact the type of pile that is used, the method of implementation and the impact of nearby structures. Before starting construction there should be a thorough understanding of the soil conditions, strength and physical properties of the ground. Having this information will allow for an informed plan of action.
A common means of analysing soil strength and conditions is using standard penetration testing (SPT), a widely used geotechnical test used to estimate soil. This soil exploration tool uses an SPT hammer to drive a drill string with a split-barrel sampler attached at the bottom of the string. The split-barrel sampler recovers soil samples and the bottom of the borehole after it has been advanced to the required sampling depth. Using this test provides clarity to the engineers and is essential to understanding foundation conditions.
SPT hammer efficiencies vary, which influences the resulting N-values. For this reason, many authorities, including the Federal Highway Administration, in the US, require SPT hammer calibration. In addition, period calibration is required by many US Departments of Transportation.
ASTM D1586 recommends that a measured N-value be normalised to 60 per cent drill rod energy, N₆₀ by multiplying it by the ratio between the measured energy transferred to the rod and 60 per cent of the theoretical potential energy. This compensates for the variability in efficiency, and therefore, improves the reliability of soil strength estimates used in geotechnical designs.
To perform the SPT calibration, GRL Engineers attach an SPT rod, instrumented with strain gauges and accelerometers, to the SPT drill string rod. As the drill string is driven into the ground, the strain gauges and accelerometers obtain force and velocity signals with each hammer blow. The signals are transmitted to a pile-driving analyser that displays the force, velocity and energy transmitted to the drill string, calculates and displays the maximum transferred energy value, and stores the complete time record of force and velocity for all SPT hammer blows.
GRL Engineers typically acquire several SPT energy measurements per hammer at a given test location, in accordance with ASTM.
With this testing, GRL Engineers can provide a quantitative calibration report presenting transferred energies, energy transferred ratios and the SPT N₆₀ value for each sample interval tested.