To provide a direct comparison between the Robertson (1990) profiling chart and the Eslami-Fellenius chart, three short series of CPTU data were compiled from sites with very different geologic origin where the soil profiles had been established independently of the CPTU. The borehole information provides soil description and water content of recovered samples. For one of the cases, the grain size distribution is also available. The CPTU-diagrams from the site are shown in Fig. 2.12. The soil and CPTU information for the specific points are compiled in Table 2.3. The three sites are:

1. North Western University, Evanston, Illinois (Finno 1989). CPTU data were obtained in a soil profile consisting of 7 m of sand deposited on normally consolidated silty clay. The piezometer was attached to the cone face (u1) and not behind the shoulder (u2). The method of converting the pore pressure measurement to the u2-value presented by Finno (1989) has been accepted here, although the conversion is disputed. For comments, see Mayne et al. (1990).

2. Along the shore of Fraser River, Vancouver, British Columbia (personal communication, V. Sowa, 1998). A 20 m thick mixed soil profile of deltaic deposits of clay, silt, and sand. The first four data points are essentially variations of silty clay or clayey silt. The fifth is a silty sand.

3. University of Massachusetts, Amherst, Massachusetts (personal communication, P. Mayne, 1998). A soil profile (Lutenegger and Miller 1995) consisting of 5 m of a thick homogeneous over consolidated clayey silt. This case includes also information on grain size distribution. The borehole records show the soil samples for data points Nos. 3 through 7 to be essentially identical. Notice that the u2-measurements indicate substantial negative values, that is, the over consolidated clay dilates as the cone is advanced.

For each case, the soil information in Table 1 is from depths where the CPTU data were consistent over a 0.5 m length. Then, the CPTU data from 150 mm above and below the middle of this depth range were averaged using geometric averaging, preferred over the arithmetic average as it is less subject to influence of unrepresentative spikes and troughs in the data (a redundant effort, however, as the records contain no such spikes and troughs). The CPTU data were analyzed by the Eslami-Fellenius (1996) and the Robertson (1990) profiling methods and the results are shown in Fig. 2.13.

Evanston data:

The first three samples are from a sand soil and both methods identify the CPTU data accordingly. The remaining data points (Nos. 4 through 7) given as Silty Clay in the borehole records are identified as Clay/Silt by the Eslami-Fellenius and as Clay to Silty Clay by the Robertson method, that is, both methods agree with the independent soil classification.

Vancouver data:

Both methods properly identify the first four data points to range from Clayey Silt to Silty Clay in agreement with the independent soil classification. The fifth sample (Silty Sand) is identified correctly by the Eslami-Fellenius method as a Sand close to the boundary to Silty Sand and Sandy Silt. The Robertson method identifies the soil as a Sandy Silt to Clayey Silt, which is essentially correct, also.

Amherst data:

Both methods identify the soils to be silt or clay or silt and clay mixtures. Moreover, both methods place Points 3 through 7 on the same soil type boundary line, that is, confirming the similarity between the soil samples. However, the spread of plotted points appear to be larger for the Robertson method; possibly due that its profiling does not consider the pore pressures developed by the advancing penetrometer (but for the pore pressure on the shoulder, of course), while the Eslami-Fellenius method does account for the substantial negative pore pressures that developed.