How do you know that you have quality data?

by Elizabeth Withington 06/01/2016 filed under General News

 

“When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do”. John Ruskin

 

To operate as a competent and trusted ground investigation contractor there are many “behind the scenes” procedures that need to happen to provide safe and reliable equipment to site to ensure that you, the Client, or you the Consultant, receives the quality data that reduces your risk.

 

Amongst these many procedures is the checking and calibration of the “SPT” hammer.  The Standard Penetration Test, the SPT, is carried out in primarily cohesionless soils to determine its strength and deformation properties.  The test is carried out to EN ISO 22476-3, and involves the dropping of a 63.5kg mass hammer onto a drive head from a height of 760mm.  The number of blows (N) necessary to achieve 300mm penetration of the sampler is the penetration resistance.

 

When the SPT N results are to be used for foundation design or comparison of results the actual energy delivered by the hammer drive weight onto the drive rods below is required.  The value is known as the Energy Ratio (Er). Energy loss occurs in the SPT hammer due to friction on the hammer shaft or on the impact with the hammer drive, and so the Er percentage of each individual hammer is required to be tested under the standard.

 

The measurement of the Er transmitted to the drive rods can be made using an instrumented section of rod positioned a specific distance below the point where the hammer impacts the drive head.  This instrumented section of rod is capable of measuring the vertical acceleration and the axial deformation induced in the rod, and requires a datalogger.  From these measurements a calculation is carried out to provide the Er as a percentage.

 

Over the Christmas shutdown Geotechnical Engineering Ltd annually takes the opportunity to ensure that all of our equipment is safe and calibrated.  Our SPT hammers have all been checked and calibrated and are ready to be used on our many geotechnical ground investigations throughout the UK.  Digital copies of the SPT hammer certificates are readily available for our Clients if required, please contact your appointed Contract Manager or email us on geotech@geoeng.co.uk .  




 
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