SPT Hammer efficiency - ASTM D4633

[Geosubhtech]

Dear All,

I just want to understand why there is a restriction in ASTM D4633 Clause 4.2 Note - 1 regarding the limitation of the SPT hammer energy evaluation of the hammer in between 10 to 50. In definition of the N- value it is understood that the blows required for 300mm penetration by ignoring initial 150mm penetration. So it is restricted not to conduct the tests in soft clay or very stiff clay and relative degree of packing in cohesionless soils. As per ASTM D1586 the refusal criteria mentioned are very different than the limitations in ASTM D4633. As per ASTM D4633 i can conduct the hammer efficiency tests up to 10 < N-value > 50. But in ASTM D1586 i can do SPT up to 100 (with other 3 refusal conditions). As the SPT-N is also inversely proportionated with hammer energy so it is not clear for getting the test done in some type of soils. So, here i am having Automatic hammers with hammer efficiency up to 92% so there is a requirement of reducing the hammer efficiency as per employer requirements. So, the tests were not getting qualified just in the name of limitation in ASTM D4633.

So, it would be very much helpful if i can get the expert's advice on this.

 

[GeoEnvGuy]

The note is based on the referenced study which states the following.

It was also observed that the N-value of the test interval has an influence on the effect that short rods have on the energy ratio calculated by the force-velocity method.

You should be looking at determining the average hammer efficiency for the machine you are using. This requires quality checking every blow in your records. In my practice I disqualify records which have low N values as well as N values influenced by gravels and cobbles part way through penetration.

In my opinion I would expect to see results in the 70% to 85% range for automatic hammers. Attached image below is from a 2008 study for SPT auto hammer efficiency.