Geoscience Reference
In-Depth Information
Table 11.2. Computation algorithm for the degraded bearing capacity and the seismic settlements
of strip foundations restingon the surface ofa clay crustover liquefiable subsoil
Design Requirements
FS
deg
≥
FS
o
(
=
1
.
0
−
1
.
5
)
ρ
=
ρ
st
+
ρ
dyn
≤
ρ
all
Degraded (end of shaking) Static Factor of Safety
FS
deg
=
min
5
.
14
C
u
/
q
FS
c
−
s
deg
1
2
γ
BN
γ
+
γ
HN
q
B
2
C
u
H
−
γ
HB
+
FS
c
−
s
deg
=
qB
φ
=
tan
−
1
[
(
1
−
U
)
tan
φ
o
]
U
=
0
.
50
1
+
a
σ
v
o
σ
vo
+ ∆
σ
v
a
=
1
−
250
ρ
dyn
B
2
.
0
Seismic Settlements
1
FS
c
−
s
deg
2
.
50
ρ
dyn
=
ρ
o
·
a
max
T
2
Z
liq
·
N
ρ
o
=
0
.
016
·
Symbols
q
=
average contact pressure offooting
T
=
predominant excitation period
ρ
st
=
static settlement corresponding to q
N
=
number of cycles corresponding to
a
max
B
=
width of footing
Z
liq
=
thickness of liquefiedsubsoil
σ
vo
=
verticaleffectivegeostaticstressatdepth
equal to
B
below the soil crust
H
=
thickness of soil crust
γ
=
buoyant unit weight of subsoil (soil crust
and liquefiable soil)
∆
σ
v
=
additional vertical stress caused by sta-
tic loading (e.g.
q
) at the same depth, on the
footing axis
ϕ
o
=
friction angle of liquefiable soil (before
shaking)
N
γ
,
N
q
=
staticbearingcapacityfactorsforthe
liquefiedsoil(forthedegradedfrictionangle
)
a
max
=
(effective)seismicgroundacceleration,
at the base of the liquefiable soil layer
ϕ
To provide insight to the beneficial effect of the clay crust, Figure 11.24 shows the
variation of seismic settlements and degraded factors of safety against the non-
dimensional thickness of the clay crust
H
/
B
, for a typical strip foundation with
=
.
=
B
3
0m, undrained shear strength of the clay
C
u
40kPa and the two different
