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CRR
where:
q
c
2
3
0.00128
0.025
(0.17R
) (0.028 ) (0.0016 )
0.7
f
Rf
R (3-15)
σ
v'
1 f
q
' 0. 7
v
c
CRR
R f
q c
v ′
= generalized normalized cone resistance
= normalized liquefaction cyclic resistance ratio
= calculated friction ratio (percentage)
= CPT measured cone resistance (in atm units)
= vertical effective stress (in atm units)
CRR
MSF
CRR 1
(3-16)
A direct comparison of the two CPT methods is made in the design application
contained in Chapter 8.
3.4.2.3 Cyclic Resistance Ratio from Laboratory Tests
Cyclic tests (triaxial, simple shear, torsional shear) can be performed to determine the
cyclic behavior of silty and fine sandy soils. Undrained, stress- or strain-controlled tests
consisting of uniform sinusoidal loading are commonly performed. With respect to
common stress-controlled cyclic triaxial tests, loads are applied to the specimens until a
specified axial strain or number of loading cycles is reached. In numerous laboratory
studies of sandy soils, axial strains of 5% are generally achieved when the specimen first
reaches full liquefaction (defined as zero effective stress). An equivalent way of defining
full liquefaction is based on the pore pressure ratio (r u = 100%), as defined in Equation
3-17. Note that 5% axial strain and r u criteria for defining the onset of liquefaction do not
always occur in the same number of load cycles. The differences observed in the number
of cycles are generally minor for sandy soils, but increase with fines content due to the
relative low permeability of fine grained soils and testing limitations in measuring excess
pore pressures generated with rapid loading.
∆u
r u
*100%
(3-17)
σ' 3
The CRR values obtained from laboratory tests must be corrected to field CRR values
through the use of two correction factors. The first factor, c r , accounts for the fact that
cyclic triaxial compression and cyclic simple shear tests impose different loadings. The
cyclic simple shear tests are considered to be more representative of the field conditions
with vertically propagating shear waves. In order to relate cyclic triaxial shear (CRR tx )
data to cyclic simple shear (CRR ss ) data, the following equation has been formulated:
CRR c
ss
r
* CRR
where recommended values of c r have been compiled in Table 3.7 as a function of the
static lateral earth pressure coefficient (K o ).
tx
(3-18)
65