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It has been found that G max applies to both the initial static monotonic loading and dynamic loading of
geomaterials (Jardine, et aL, 1991). Moreover, since very small strains have not yet generated any
excess porewater pressures, G max can be applied to both drained and undrained soil behavior. Once
G max is known, the shear modulus reduction curve can be derived from the plastic index (PI) of the
soils (Vucetic & Dobry, 1991). Given the shear modulus G, the corresponding Young's modulus E,
Bulk modulus B ! , and constrained modulus M' can be derived from the Poisson's ratio v. The range of
the value of Poisson's ratio v is from 0.1 to 0.2 at these strains.
118
0 0001 0 001
Shear Strain
Figure 3. Secant Shear Modulus Reduction
for Monotonic & Cyclic Loading
100 1000
Sicar Wsve Velocity, V s (ins)
Figure 4. Correlation for Unit Weight from
Depth and V s (Mayne, 2001)
The amplitude information of the collected shear waves can be used to analyze the material damping in
terms of damping ratio (D s ) at small-strains. Stewart & Campanella (1993) suggested that the spectral
ratio slope (SRS) method is the most reliable and consistent approach to obtain the damping ratio D s of
a soil layer by the SCPTu, but it still requires verification on different soil types to fully validate the
technique (Campanella, 1994). D s increases with the strain level, and the relationship between D s and
shear strain y c has also been correlated with the PI (Vucetic & Dobry, 1991).
LIQUEFACTION EVALUATION
Recent earthquakes in Gujarat, India (2001), Izmit, Turkey (1999), Chi-Chi, Taiwan (1999) and Kobe,
Japan (1995) have emphasized the importance of liquefaction hi geotechnical engineering because of
its significant destructive nature. Liquefaction predominantly occurs in sands to silty sands that are
difficult or impossible to sample. This warrants the use of field testing, especially the SCPTu, for
liquefaction analysis, since the SCPTu provides not only the tip resistance qr and shear wave velocity
V s , which can quantity the susceptibility to liquefaction (Robertson & Wride, 1998; Andrus & Stokoe,
2000), but also obtains the sleeve friction f s and porewater pressure u 2 that are used for characterizing
the soil types, layering, and geotechnical engineering parameters. Liquefaction notably occurs at depths
less than 20 meters, and research found that the presence and thickness of the overlying clay cap
significantly affects the liquefaction potential of underlying sand layers (Ishihara, 1985; Youd &
Gams, 1995). Evaluating the liquefaction susceptibility of a certain soil layer and its potential for
ground damage cannot therefore be confined to testing sandy soil samples only, but requires the
characterization of its overlying capping clay layer as well