Schmertmann (1978) proposed the soil profiling chart shown in Fig. 2.4A. The chart is based on results from mechanical cone data in “North Central Florida” and also incorporates Begemann’s CPT data. The chart indicates envelopes of zones of common soil type. It also presents boundaries for density of sands and consistency (undrained shear strength) of clays and silts, which are imposed by definition and not related to the soil profile interpreted from the CPT results.

Also the Schmertmann chart plots the cone stress against the friction ratio, that is, the data are plotted against their inverse self. Fig. 2.4B shows the Schmertmann chart converted to a Begemann type graph. When the plotting of the data against own inverse values is removed, a qualitative, visual effect comes forth that is quite different from that of Fig. 2.4A. Note also that the consistency boundaries do not any longer appear to be very logical.

Schmertmann (1978) stated that the correlations shown in Fig. 2.4A may be significantly different in areas of dissimilar geology. The chart is intended for typical reference and includes two warnings: “Local correlations are preferred” and “Friction ratio values decrease in accuracy with low values of qc”. Schmertmann also mentions that soil sensitivity, friction sleeve surface roughness, soil ductility, and pore pressure effects can influence the chart correlation. Notwithstanding the caveat, the Schmertmann chart is very commonly applied “as is” in North American practice.

Douglas and Olsen (1981)

Douglas and Olsen (1981) proposed a soil profiling chart based on tests with the electrical cone penetrometer. The chart, which is shown in Fig. 2.5A, appends classification per the unified soil classification system to the soil type zones. The chart also indicates trends for liquidity index and earth stress coefficient, as well as sensitive soils and “metastable sands”. The Douglas and Olsen chart envelopes several zones using three upward curving lines representing increasing content of coarsegrained soil. The chart distinguishes where soils are sensitive or “metastable”.

Comparing the Douglas and Olsen chart (Fig. 2.5A) with the Schmertmann chart (Fig. 2.4A), a difference emerges in implied soil type response: while in the Schmertmann chart the soil type envelopes curve downward, in the Douglas and Olsen chart they curve upward. Zones for sand and for clay are approximately the same in the two charts, however.

A comparison between the Douglas and Olsen and Schmertmann charts is more relevant if the charts are prepared per the Begemann type of presentation. Thus, Fig. 2.5B shows the Douglas and Olsen chart converted to a Begemann type graph. The figure includes the three curved envelopes. The sleeve friction is limited to 160 kPa, which is a practical limit for actual conditions. Three of the labels in the original chart fall outside this limit (two were combined here to fit). Moreover, it is hard to accept that the areas indicated as metastable or sensitive are correctly identified in the original chart.

Obviously, plotting the cone stress versus the friction ratio, i.e., against its inverse self will easily lead to distorted conclusions from the graph.