Geoscience Reference
In-Depth Information
2
Dr~50%
σ
c
'=0.098 MPa
1.5
1
Postliquefaction
strength at ε
1
=25%
Preliquefaction
strength at ε
1
=15%
0.5
0
1
10
100
Uniformity Coefficient Cu
Fig. 8.23. Post-liquefaction or preliquefaction deviatoric strengthby monotonic
undrained tests(Kokusho et al., 2004)
the same relative density. In Figure 8.23, deviatoric stresses
q
at axial strain of 25%
evaluated by all post-liquefaction tests are plotted versus
Cu
of tested soils with open
circles. Data points for tests without cyclic loading (preliquefaction condition) defined
at 15% strain are also plotted on the same chart with open triangles. Drastic increase in
shear resistance for large strain with increasing
Cu
is evidently seen from the curves
approximating the plots whether or not the soil is subjected to preceding undrained
cyclic loading. The increase seems to occur mostly in the interval of
Cu
between 1.4
and 4.
Basedonthefindingsmentionedbefore,itcanbesaidingeneralthatwell-gradedgravelly
soils are prone to liquefaction corresponding to almost full pore-pressure buildup and
5% DA axial strain as much as poorly-graded sands, so long as their relative density
is the same. However, if strength at larger strain is concerned, the relative density is no
moreapertinentparameter.Instead,particlegradationrepresentedherebytheuniformity
coefficient
Cu
makes a big difference even for soils of the same relative densities. This
implies that well-graded clean gravelly soils are less prone to post-liquefaction failure
accompanying large deformation. Considering that the uniformity coefficients of natural
gravelly soils are normally larger than several tens as indicated in Figure 8.2, their post-
liquefactionundrainedstrengthcorresponding to25%axialstrainmaybejudgedatleast
eight times larger than poorly-graded sands according to Figure 8.23. This may be able
to explain why liquefaction-induced damage in gravelly deposits was less witnessed in
past earthquakes. Post-liquefaction large ground deformation such as cracks, differential
settlements, etc. is harder to develop in well-graded soils than poorly-graded sands even
after initial liquefaction with almostfullpore-pressure buildup.
