PENETRATION TEST COMPARISONS: MODIFIED CALIFORNIA VERSUS STANDARD PENETRATION TEST SAMPLERS

Penetration sampling data from the California Geological Survey's (CGS) Seismic Hazard Mapping Program (SHMP) geotechnical database are used to compare blow count information collected using Standard Penetration Test (SPT) samplers with blow count information collected using Modified California samplers (MCS). The standard measure of penetration resistance used in evaluating liquefaction susceptibility is SPT blow counts. However, many consultants and other practitioners elect to use other samplers when collecting penetration resistance data. There exist relationships and conversion factors that can be used to convert MCS blow counts to SPT-equivalent blow counts. The relative abundance of penetration test data in the SHMP database allows for a relatively robust examination of the validity of these existing relationships.

We compare penetration test measurements that should be similar. This is accomplished by comparing penetration test measurements from the same borehole, which are within 5 vertical feet of each other, and in the same lithologic layer. A similar comparison is made between pairs of SPT penetration tests and MCS penetration test pairs under the same conditions. To remove the effect of overburden pressure due to the difference in depth between samples, calculated blow counts that are adjusted for energy delivery and overburden pressure [(N₁)₆₀] are compared. Data comes from either the San Francisco Bay area (SFBA) or the Los Angeles Basin (LAB) and is extracted from geotechnical reports on file with cities, counties and other state or local agencies.

We present regressions of MCS vs SPT, MCS vs MCS and SPT vs SPT for both SFBA and LAB. Scatter observed in all regressions is large and penetration test values have typical standard errors of approximately ± 11. The large scatter appears to mostly reflect both the natural variability of the geology and non-uniformity in sampling. The regression slopes (conversion factors) calculated for MCS to SPT for the SFBA and LAB differ from those routinely used by CGS, however, there appears to be differences between consultants as to the exact definition of the MCS sampler and actual sampler size is often not reported, which might affect these results.