Technical Bulletins

Before the CPT was widely used in practice, geotechnical engineers relied on the Standard Penetration Test (SPT). The SPT is performed by hammering an open-shoe sampler 18 inches into the ground. The number of blows required to drive the last 12 inches is recorded as the SPT N value (N60 is the equivalent number of blows at 60% hammer energy). The SPT N value has a lot of uncertainty. The CPT began to grow in the 1980s following the first ASTM CPT standard, but there was resistance from established engineers that were accustomed to using N-values and obtaining a soil sample. The CPT needed to be able to estimate the soil type to accelerate its adoption.