Geoscience Reference
In-Depth Information
For each test, the critical seismic coefficient (k h ) cr was defined at the state when the
calculated safety factor became equal to unity. Theoretical lateral earth pressures
acting on the backface of wall were calculated by the Mononobe-Okabe method
(Okabe, 1924; Mononobe and Matsuo, 1929) assuming a single soil wedge for the
conventional walls and by the two-wedge method for the reinforced soil-type
walls, as described by Horii et al. (1994). In both methods, earth pressures
due to the self-weight of the backfill were assumed to be hydrostatically
distributed along the wall height, and those due to the surcharge applied at
the top of the backfill were assumed to be uniformly distributed. This
assumption of hydrostatic distribution was employed because it was broadly
used in the current practice to design soil retaining walls in Japan.
The theoretical safety factors against overturning were obtained by
assuming that overturning occurred around the toe of the base part of the wall.
The bearing capacity for the conventional walls was evaluated by assuming the
subsoil thickness to be sufficient to cause boundary-free subsoil failure, despite
the fact that the actual thickness of subsoil layer was limited to 200mm. On the
other hand, the ultimate failure of the reinforced soil-type walls due to the bearing
capacity failure was not considered; in other words, the maximum allowable
vertical contact load at the bottom of the facing was set equal to the bearing
capacity of the subsoil layer (RTRI, 1997).
For the cantilever wall having a wall base overlain by the backfill, a virtual
vertical backface was assumed within the backfill, and the portion of the backfill
located above the wall base and between the back face of facing and the virtual
backface was regarded as a part of the wall.
As mentioned before, the shear resistance angle f of the backfill and
subsoil layers was set equal to f peak (
¼
51
8
) obtained from the PSC tests
mentioned above.
It is very likely that the friction angle along the bottom face of the rigid
base is equivalent to the simple shear angle of friction f ss ¼
arctan(t/s) max along
the horizontal failure plane. The ratio of the simple shear peak friction angle f ss
to the peak angle of f peak ¼
s 3 Þ max } from the PSC tests
having the vertical s 1 direction, both obtained for air-pluviated Toyoura sand, is
around 3/4 (Tatsuoka et al., 1991). Considering the effect of the sandpaper glued
on the surface of the wall base, therefore, the frictional angle d b at the interface
between the subsoil and the wall base was assumed equal to 3/4f peak (
arcsin{
ð
s 1 2
s 3 Þ=ð
s 1 þ
¼
38
8
)in
the calculation of safety factor against sliding.
Similarly, with ignoring the effects of strength anisotropy, the frictional
angle d w at the interface between the backfill and the wall facing with sandpaper
was set equal to 3/4f peak . For the cantilever-type wall, the d w -value along the
assumed virtual vertical backface was also set equal to 3/4f peak , because with d w
set equal to f peak , the safety factor equal to unity could not be obtained until the
seismic coefficient became unrealistically large.
Search WWH ::




Custom Search