Geoscience Reference
In-Depth Information
300
Kobe Port
Island
Italy
Avasin
250
200
USA
Borah Peak
150
100
Hokkaido
Morimachi
50
0
0
5
10
15
20
25
30
SPT
N
-value
Fig. 8.1. Unadjusted SPT
N
-value versus S-wave velocity relationship for recently
liquefied gravelly soils(Kokusho et al., 1995)
triggering lateral spread in gently sloping ground. The
N
-value of the loosely deposited
gravellayerswas5-9andtheS-wavevelocity90-160m/s(Andrus,1994).Besidesthese
cases,liquefactionofgravellysoilswasalsoreportedduringseveralearthquakes,suchas
the 1948 Fukui earthquake in Japan, the 1964 Alaskan earthquake, Chinese earthquakes,
etc. Figure8.1 summarizes SPT
N
-values versus
Vs
relationships of liquefied gravelly
deposits inrecent years.
Figure 8.2 exemplifies typical grain size curves of the gravelly soils liquefied recently.
They are actually the mixture of gravels, sands and sometimes even finer soils.
In Figure 8.3, the mean grain size
(
D
50
)
is plotted versus the uniformity coefficient
(
Cu
)forthesamegravellysoils.Sofar,theupperlimitsfor
D
50
and
Cu
areabout20and
300mm, respectively, but no limit may reasonably be justified, indicating that gravelly
soils can liquefy if they are loose enough no matter how well graded and how coarse
they may be. Dry densities of these gravelly soils are relatively high (1
m
3
for
.
7-2
.
0t
/
m
3
for debris avalanche soil in Hokkaido) due to
large uniformity coefficients, actually much higher than typically liquefiable loose sands
(e.g. 1
reclaimed soil in Kobe and 2
.
0-2
.
1t
/
m
3
for alluvial Niigata sand). Gravelly soils can be densely packed and
are normally believed to be stiffer and seismically more stable than sands. However,
gravelly soils as previously mentioned can sometimes have unexpectedly low
N
-value
and S-wave velocity.
.
4-1
.
5t
/
Itshouldalsobenotedthatpermeabilityofwell-gradedgravellysoilscannotnotbehigh
enoughtoretardpore-pressurebuildup,becausesandparticlesfillingthevoidsofgravels
keep the permeability no higher than sandy soils. For example, permeability coefficients
of the order of 10
−
2
-10
−
4
cm
s are reported for gravelly layers in Japan; reclaimed DG
fill of Port Island, Pleistocene gravel layer in Osaka and Holocene fluvial gravel near
Tokyo (JGSCommittee, 2001).
/
