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Table 3.6: Boundaries of Soil Behavior Type (Robertson and Wride 1997a, b)
Soil Behavior Type Index, I c Zone Soil Behavior Type
I c < 1.31 7 Gravelly sand to dense sand
1.31 < I c < 2.05 6 Sands: clean sand to silty sand
2.05 < I c < 2.60 5 Sand Mixtures: silty sand to sandy silt
2.60 < I c < 2.95 4 Silt Mixtures: clayey silt to silty clay
2.95 < I c < 3.60 3 Clays: silty clay to clay
I c > 3.6 2 Organic soils: peats
The simplified relationship that has been proposed for use in practice is provided
in Equation 3-11. It is recommended that this formula should be amended, where
needed, to fit the results of site- or region-specific geotechnical investigations.
Versions of this basic equation, modified with site-specific data have been proven
to be very reliable for use in liquefaction studies.
Fines
Content, FC
(%) 1.75(I
c
)
3.25
3.7
(3-11)
Figure 3.9 shows that for a given value of q c1N , the CRR increases with increasing
fines content; therefore, it is necessary to correct the in situ value of q c1N to an
equivalent clean sand value, (q c1N ) cs prior to the calculation of the cyclic
resistance ratio. This correction is achieved using the fines-dependent correction
factor, K CPT . The equivalent clean sand normalized CPT tip resistance is given as:
q
cIN
K
CPT
( q
cIN
)
cs
(3-12)
where:
∆q c1N = CPT tip correction for silty sands
(q c1N ) cs = equivalent clean sand normalized CPT tip resistance (this is
equal to q c1N + ∆q c1N )
q c1N
= measured tip resistance, corrected for overburden and
normalized
K CPT = 0 for FC 35%.
Thus the CPT correction can be expressed as a function of the measured data with
Equation 3-13.
K
CPT
q
cIN
( q
cIN
)
(1 - K
CPT
)
(3-13)
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