Shear strength, compressibility and permeability of embankment materials and soil foundations - Geotechnical Engineering of Dams - page 254

Figure 6.24. Vertical components of pressures acting on a cone penetrometer - Campanella and

Robertson (1988).

It can be seen that N KT varies from about 12 to 20 when correlated with the laboratory
undrained strength (the average of TC, TE and DSS tests), and 12-22 when correlated to
corrected field vane strength; the latter larger variation due probably in part to scatter in
the vane shear strengths.
N K and N KT values show a much larger range for overconsolidated clays (e.g. Figure
6.25 ). N K values of 15-20 are commonly adopted for overconsolidated clays.
It can be seen from the above discussion that the static cone penetrometer or piezocone
can be used to estimate undrained strength, but the value of the correlation factor N K or
N KT is uncertain, and strengths cannot be estimated with a high degree of accuracy. The
potential error is higher in overconsolidated clays.
In practice the static cone should only be used for assessment of undrained strength in
slope stability problems in clay when it has been calibrated on the site by comparison with
good quality laboratory undrained strengths.
The published correlations are too variable for the cone to be used alone for undrained
strength assessment in slope stability other than for preliminary design with an appropri-
ate factor of safety. In foundation design, where factors of safety adopted are usually
greater than in slope stability analysis, cones may be used alone, but conservative N K fac-
tors should be adopted.