Geoscience Reference
In-Depth Information
Fig.11.4. Analyticalpredictionofliquefactionresponseofstripfoundationsrestingona
clay crust (after Cascone and Bouckovalas, 1998). (a) Criticalthickness of clay crust,
and (b) bearing capacity degradation factor
γ
is the buoyant unit weight of the subsoil and
N
γ
,
N
q
are the bearing capacity
factors corresponding to
where
ϕ
.
Based on the above approach, the authors end up with two basic design parameters. The
first is the critical thickness of the soil crust
H
cr
beyond which failure occurs totally
within the clay crust, and consequently any partial or complete liquefaction of the sand
does not have any significant effect on the bearing capacity. For strip foundations,
H
cr
is
computed fromthefollowing analytical expression (Figure 11.4a):
H
B
28
C
∗
−
10
.
N
γ
cr
=
/
N
q
−
2
C
∗
(11.6)
2
1
+
with
C
∗
=
C
u
/(γ
B
)
.
The second design parameter is the bearing capacity degradation factor
ζ
=
Q
deg
which is expressed in terms of the above non-dimensional parameters
and the normalized thickness of the crust
H
ult
/ (
5
.
14
C
u
B
)
/
B
,as
⎧
⎨
⎫
⎬
1
.
00
N
γ
+
2
H
B
N
q
−
2
C
∗
1
ζ
=
min
(11.7)
1
+
⎩
⎭
.
28
C
∗
10
The variation of
ζ
versus the excess pore pressure ratio
U
and the normalized thickness
of the clay crust
H
/
B
are shown in Figure 11.4b, for typical values of the soil parame-
ters
C
∗
and
ϕ
o
. Bouckovalas et al. (2005) tried to refine the above solution, considering
the possibility of inclined slip planes within the clay crust, as well as a non-zero resid-
ual shear strength for the liquefied sand. Furthermore, they recasted the solution assum-
ing a reduced buoyant unit weight of the soil
γ
∗
=
γ
(
1
−
U
)
, instead of a degraded
