Robertson (1990) proposed a development of the Robertson et al. (1986) profiling chart, shown in Fig. 2.8, plotting a “normalized cone stress”, qcnrm, against a “normalized friction ratio”, Rfnrm, in a cone stress chart. The accompanying pore pressure ratio chart plots the “normalized cone stress” against the pore pressure ratio, Bq, defined by Eq. 2.2 applying the same Bq-limits as the previous chart (Zone 2 is not included in Fig. 2.8).
The normalized cone stress is defined by Eq. 2.4.
The numbered areas in the profiling chart separate the soil behavior categories in nine zones, as follows.
1. Sensitive, fine-grained soils
2. Sand [silty sand to clean sand]
3. Organic soils and peat
4. Sand to gravelly sand
5. Clays [clay to silty clay]
6. Sand – clayey sand to “very stiff” sand
7. Silt mixtures [silty clay to clayey silt]
8. Very stiff, fine-grained, over consolidated
9. Sand mixtures [sandy silt to silty sand] or cemented soil
The two first and two last soil types are the same as those used by Robertson et al. (1986) and Types 3 through 7 correspond to former Types 3 through 10. The Robertson (1990) normalized profiling chart has seen extensive use in engineering practice (as has the Robertson et al. 1986 chart).
The normalization is professedly to compensate for that the cone stress is influenced by the overburden stress. Therefore, when analyzing deep CPTU soundings (i.e., deeper than about 30 m), a profiling chart developed for more shallow soundings does not apply well to the deeper sites. At very shallow depths, however, the proposed normalization will tend to lift the data in the chart and imply a coarser soil than necessarily the case. Moreover, where soil types alternate between light-weight and soils (which soil densities can range from 1,400 kg/m3 through 2,100 kg/m3 ) and/or where upward or downward pore pressure gradients exist, the normalization is unwieldy. For these reasons, it would appear that the normalization merely exchanges one difficulty for another.
More important, the chart still includes the plotting of data against the inverse of own self. This is not necessary. A chart with the same soil zones could just as well have been produced with normalized cone stress against a normalized sleeve friction.
Accepting the Robertson (1990) normalization, Figs. 2.9A and 2.9B show the envelopes of the Robertson (1990) chart (Fig. 2.8) converted to a Begemann type chart. The ordinate is the same and the abscissa is the multiplier of the normalized cone stress and the normalized friction factor of the original chart (the normalized sleeve friction is the sleeve friction divided by the effective overburden stress). Where needed, the envelopes have been extended with a thin line to the frame of the diagram.
As reference to Figs. 2.4B and 2.5B, Fig. 2.9b presents the usual Begemann type profiling chart converted from Fig. 2.8 under the assumption that the data apply to a depth of about 10 m at a site where the groundwater table lies about 2 m below the ground surface. This chart is approximately representative for a depth range of about 5 m to 30 m. Comparing the “normalized” chart with the “as measured” chart does not indicate that normalization would be advantageous.
Other early profiling charts were proposed by Searle (1979), Jones and Rust (1982), Olsen and Farr (1986), Olsen and Malone (1988), Erwig (1988). CPTU charts similar to that of Robertson (1990) were proposed by Larsson and Mulabdic (1991), Jefferies and Davies (1991, 1993), and Olsen and Mitchell (1995). Robertson (2016) has further developed his CPTU classification approach to separate classification per grain size from the per soil response.
