INFLUENCE OF HAMMER TYPE ON SPT RESULTS. TECHNICAL NOTE

The authors of this technical note examine a relatively uniform sand site in Chicago. Standard penetration test (SPT) values from samples driven using a drilling rig instrumented with an automatic hammer were consistently lower than those from samples driven using a safety hammer. The automatic hammer is much more efficient, delivering greater energy per blow. The results, which were averaged at each depth, revealed that SPT values using an automatic sampling hammer were 75 percent of those recorded using the older-style safety hammer. The trend was consistent throughout the sand profile. This case agrees with the literature that indicates that SPT results using automatic hammers can be expected to be approximately two-thirds of those recorded using safety hammers.