Vos (1982) suggested using the electrical cone penetrometer for Dutch soils to identify soil types from the friction ratio, as shown in Table 2.2 (Vos, 1982). The percentage values are similar but not identical to those recommended by Begemann (1965).
Robertson et al. (1986)
Robertson et al. (1986) and Campanella and Robertson (1988) presented a chart, which was the first chart to be based on the piezocone, i.e., the first to include the correction of cone stress for pore pressure at the shoulder according to Eq. 2.1.
The Robertson et al. (1986) profiling chart is presented in Fig. 2.6. The chart identifies numbered areas that separate the soil behaviour categories in twelve zones, as follows.
1. Sensitive fine-grained soil
2. Silty sand to sandy silt
3. Organic soil
4. Sand to silty sand
5. Clay
6. Sand
7. Silty clay to clay
8. Sand to gravelly sand
9. Clayey silt to silty clay
10. Very stiff fine-grained soil
11. Sandy silt to clayey silt
12. Overconsolidated or cemented sand to clayey sand
A novel information in the profiling chart is the delineation of Zones 1, 11, and 12, representing somewhat extreme soil responses, enabling the CPTU to uncover more than just soil grain size. The rather detailed separation of the in-between zones, Zones 3 through 10 indicate a gradual transition from fine-grained to coarse-grained soil.
As mentioned above, plotting of cone stress value against the friction ratio is plotting the cone stress against itself (its inverted self) modified by the sleeve friction value, distorting the results. Yet, as indicated in Fig. 2.7, the measured values of cone stress and sleeve friction can just as easily be plotted separately. The friction ratio is a valuable parameter and it is included as an array of lines ranging from a ratio of 0.1 % through 25 %.
Essentially, the Bq-value shows the change of pore pressure divided by the cone stress, qt (the cone stress is very much larger than the total stress). Directly, the Bq-chart (Fig. 2.8) shows zones where the U2 pore pressures become smaller than the neutral pore pressures (u0) in the soil during the advancement of the penetrometer, resulting in negative Bq-values. Otherwise, the Bq-chart appears to be an alternative rather than an auxiliary chart; one can use one or the other depending on preference. However, near the upper envelopes, a CPTU datum plotting in a particular soil-type zone in the friction ratio chart will not always appear in the same soil-type zone in the Bq-chart. Robertson et al. (1986) points out that “occasionally soils will fall within different zones on each chart” and recommends that the users study the pore pressure rate of dissipation (if measured) to decide which zone applies to questioned data.
The pore pressure ratio, Bq, is an inverse function of the cone stress, qt . Therefore, also the Bq-plot represents the data as a function of their own self values.
