SPT Hammers calibrated to satisfy BS EN ISO 22476-3

Geotechnical Engineering have recently completed their program of calibrating all our SPT hammers, to bring us into accord with the requirements of BS EN ISO 22476-3, which lay down the conditions (applicable from July 2007) that the hammer 'energy ratio' has to be known and a certificate of calibration be available "if N values are to be used for the quantitative evaluation of foundations or for the comparison of results".

All our SPT hammers (including those used by our Pioneers, P60, P45 and Terrier rigs) will be re-calibrated on a yearly basis, or when they have been damaged or repaired, and a new calibration certificate placed in our files.

We are also insisting that any Subcontract Drillers brought onto site by ourselves have their equipment calibrated on the same basis as part of their subcontract terms and conditions.

Clients, to whom we have supplied SPT results and the calibrations for the hammers that undertook those tests, have been surprised by the change in values of the corrected N(60) SPT values, especially where we have had more than one drilling rig on site.  In this case it is critical to correct all the 'raw' SPT N values to N(60) , as this eases the interpretation of soil strength parameters across the site.

As a standard measure, we will be reporting the SPT N values in our Factual Reports and on our Borehole Logs as the 'raw' data, but also identify which hammer (they are all individually identified) took which specific SPT test, and supply a copy of the Calibration Certificate for each hammer used on that site so that our Clients can apply the corrections themselves.

If a Client so instructs, we can then amend our Reports and Logs to reflect the corrected SPT N(60) values, and put a note in the report or on the log to the effect that this is 'interpreted' data supplied by the Client and included at their request.

The Standard also makes the recommendation that Clients can ask for a 'new' calibration to be undertaken at the beginning of any site works.  We are happy to supply that service, but would make a charge for re-calibrations outside of our normal yearly schedule.

if you would like to discuss the implications of compliance with the new Standard, please contact Stuart Tod or Liz Withington who will be pleased to go through the process with you.