Influence of Hammer Type on SPT Results

The case of a relatively uniform sand site in Chicago is presented. Standard penetration test (SPT) values from samples driven using a drilling rig equipped with an automatic hammer were consistently lower than those from samples driven using a safety hammer. This is due to the higher efficiency of the automatic hammer, which results in greater energy delivered per blow. When averaged at each depth, the results indicated that SPT values using an automatic sampling hammer were approximately 75% of those recorded using the older-style safety hammer. The trend was consistent throughout the sand profile. This agrees with previously published relationships that indicate SPT results using automatic hammers can be expected to be approximately two-thirds of those recorded using safety hammers.