The standard Penetration Test (SPT) is a widely used soil exploration tool that involves using a SPT hammer to drive a split barrel sampler at the bottom of a drill string to obtain soil samples. The number of blows required to penetrate the last 12" is the “N value”, which is related to soil strength.
Why Measure the Energy Transferred by the SPT Hammer?
Several different types of SPT hammers are used to conduct Standard Penetration Tests. Their varying efficiencies influence the “N value”. The measured “N value” is standardized by multiplying it by the ratio of the measured energy transferred to the rod to 60% of the theoretical potential energy. The standardization compensates for the variability of the efficiencies of different SPT hammer types, and improves the reliability of soil strength estimates used in geotechnical applications.
N60 is what a safety hammer (cathead and rope) N value is estimated to be uncorrected. An auto hammer is estimated to be about 80% efficient. 1.333 times more than a safety hammer. The N value needs to be corrected to a normalized N60. We find that all SPT hammers, including auto hammers, can vary rig to rig, operator to operator, job site to job site.
On large or critical projects, energy testing may verify SPT performance to allow for increased design confidence and economy.
N60 = (ETR/60) x N
N60 = SPT blow count corrected for hammer efficiency
ETR = Hammer Energy Transfer Ratio
N = uncorrected (raw) SPT blow count
AASHTO – For a test to be satisfactory, an N-value of at least 10 must be obtained at each depth and a total of 50 measurements must be recorded for a particular sequence of depths