Environmental Engineering Reference
In-Depth Information
Figure 6.25.
Cone factor N KT for normally consolidated and overconsolidated clays (a) Cone factor vs
laboratory undrained strength (Piezocone), (b) Cone factor vs corrected vane shear
strength (Piezocone). - Aas et al. (1986), reproduced with permission of ASCE.
Compacted earthfill in dams will be over-consolidated, so care should be exercised in
selecting the N K or N KT values. It should also be noted that the correlations between
undrained strength and core resistance are developed for saturated soil conditions. When
the CPT or CPTU is used in dams, the soils may be partially saturated above the phreatic
surface. In view of these problems, where the undrained strength is needed with some
accuracy, the CPT or CPTU should be calibrated against laboratory tests on samples from
the dam.
The CPT and CPTU have the advantage of providing a continuous profile of q c and
hence a continuous profile of undrained strength. The cone resistance is however affected
by the soil up to 10 to 20 diameters ahead of the cone, and may therefore under or over-
estimate the q c (or q T ) depending on the situation.
Based on a simplified elastic solution, Vreugdenhil et al. (1994) provided a method
to correct cone data for thin layers. They showed that the error in the measured cone
 
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