Environmental Engineering Reference
In-Depth Information
Figure 12.24.
Thin-layer correction factor K
H
for determination of equivalent thick-layer CPT resist-
ance (Youd et al., 2001; modified from Vreugdenhill et al., 1994; Robertson and Fear,
1995, reproduced with permission of ASCE).
Theoretical as well as laboratory studies indicate that CPT tip resistance is influenced
by softer soil layers above or below the cone tip. As a result, measured CPT tip resistance
is smaller in thin layers of granular soils sandwiched between softer layers than in thicker
layers of the same granular soil. The amount of the reduction of penetration resistance in
soft layers is a function of the thickness of the softer layer and the stiffness of the stiffer
layers.
Vreugdenhil et al. (1994) first studied this, and Robertson and Fear (1995) and Youd
et al. (2001) have modified that analysis to recommend the correction factor shown in
Figure 12.24.
In this figure, H
thickness of the interbedded layer in mm; q
cA
and q
cB
cone resist-
ances of the stiff and soft layers, respectively; and d
c
diameter of the cone in mm.
As for the standard penetration test, the CPT clean sand base curve in
Figure 12.22
is
for 3% cyclic strain.
Figure 12.25
shows the curves for 3%, 10% and 20% cyclic strain.
For sites just to the left of the 3% curve, cyclic liquefaction or cyclic mobility are more
likely than flow liquefaction. Flow liquefaction is likely left of the 20% curve.
