A Rig (or Two) and a Prayer

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It’s not often we have two of our rigs working on the same site together, but this week we needed to do just that.
Our client had originally requested a plate bearing test to determine the bearing capacity of the underlying geology for a new extension to their mosque.
We advised that a plate bearing tests was not the right test. Instead , using our Geoprobe DirectPush and Terrier percussive drill rigs, we were able to complete soil sampling, dynamic probing and SPTs in the chalk underlying the site. These data then allowed us calculate the ultimate bearing capacity for future foundation design.

When is an SPT not an SPT?

SPT Calibration - 2
SPT Hammer Calibration

The answer is when it’s not calibrated.

The Standard Penetration Test, or SPT, has been the mainstay of geotechnical site investigations in the UK for decades. The test is simple, quick and cheap and it’s particularly suited to the light cable percussion drill rigs that are so widely used in the UK.

What isn’t generally appreciated is that even a low tech test such as this requires proper calibration in order to yield consistent, reliable results. In fact Eurocode 7 requires the SPT hammer to be calibrated annually. Unfortunately very few drilling contractors bother to do this and, in our experience, few consultants know that they should be asking for it. Given that results from out of spec hammers can vary by as much as 50%, we think this is something that should be given more attention.

We’ve put our money where our mouth is and now all our SPT hammers are correctly calibrated and all our drill rigs come with up to date LEAA inspection certificates. If you’d like to know more about our drilling capabilities please contact Angus Gale – 01296 739433.