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Table 3.8: Influence of OCR on the Cyclic Stress Ratio Required to Cause Liquefaction and Large Strains in
Silt Specimens.
Sample
Undisturbed
Tacoma silt
Undisturbed
Hayden Island
silt
Undisturbed
CRL silt
Undisturbed
Fern Hill silt
Reconstituted
Tacoma Silt
Reconstituted
Corvallis Silt
Reconstituted
Adair Silt
Reconstituted
Bonnie silt
Undisturbed
French Silt
Site 1
Undisturbed
French Silt
Site 2
Atterberg Limits
2
Water
Percent Fines CRR
Results
N = 20
Content
Void Ratio OCR
(%)
#200
r
LL PI
2 µm
u = ε D.A. = ε S.A. =
Sieve
100% 5% 3 5% 3
0.65 – 1.06 1 0.19 0.17 0.21
27 - 48 - NP 27 – 100 4 – 22 0.89 – 1.15 1.5 – 2.0 0.24 0.28 0.39
0.88 2.0 – 2.5 ~0.34 - ~0.39
38 - 62 - 1.03 – 1.12 1.6 – 1.9 0.28 0.31 0.33
38 - 62 31 – 54 1 – 14 2 – 71
1.73 1.0 0.23 0.25 -
1.03 – 1.12 2.0 0.36 0.37 -
- - ≤ 2 80 5 - 1 0.24 0.24 0.24
0.78 - 0.82 1 0.17 0.17 0.17
30 – 31 - - - -
0.8 1.5 – 2.0 0.24 0.24 0.25
0.60 – 0.82 1.0 0.22 0.20 0.23
22 – 30 33 – 38 7 – 11 98 – 100 24 – 29 0.61 – 0.79 1.5 0.27 0.24 0.28
0.83 – 0.95 1.0 ~0.26 1 ~0.24 1 ~0.25 1
33 - 35 44 – 45 8 – 9 99 – 100 53 – 57 0.93 – 0.96 1.5 - 2.0 ~0.42
1
~0.41 1 ~0.41 1
29 15 90 – 93 6 - 7 0.79 – 0.80 1 0.28 1 0.27 0.27
19 - 26 22 – 32 5 – 8 - 19 - 21 0.47 – 0.65 -
0.26 –
0.32
0.23 –
0.33
28 - 42 30 - 50 7 – 15 - 17 - 28 - ~0.27 ~0.29 -
NOTES
1 Extrapolated value.
2 CRR N=20 is the cyclic stress ratio required to reach the specified pore pressure ratio (r u ) or strain ( S.A. or D.A. ) in 20
load cycles.
3 ε D.A. is the double amplitude axial strain and ε S.A. is the single amplitude axial strain.
-
The laboratory results presented in Figure 3.11 demonstrate that although the silts
are capable of liquefying during cyclic triaxial testing, the CSR required to induce
full liquefaction in the number of loading cycles of interest (roughly 10 to 50
cycles) are somewhat higher than those reported for loose clean sands under
equivalent loading conditions. It is interesting to note the influence of stress
history on the cyclic resistance of the silt. The effect of the overconsolidation ratio
(OCR) on the cyclic liquefaction resistance has been noted in several studies of
sandy soils (Yamazaki et al. and Ishihara, in PHRI 1997). Data from laboratory
testing of a silty sand demonstrates that the increase in cyclic resistance increases
with the square root of the overconsolidation ratio (Figure 3.12). The data is
plotted as (τ l /σ c ′) N= 20 cycles, where τ l is the uniform cyclic shear stress required to
case liquefaction in 20 cycles of loading (i.e., N = 20), and σ c ′ is the consolidation
pressure prior to cyclic loading.
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