Geoscience Reference
In-Depth Information
a
0.2
Clay
0.1
Sand
0
0
0.2
0.4
0.6
0.8
1
V/V
uo
b
0.2
Clay
0.1
Sand
0
0
0.2
0.4
0.6
0.8
1
V/V
uo
Fig. 10.3. Comparison of the undrained (a) and drained (b)
H
-
V
and
M
-
V
sections
of the EC8 bearing strengthsurfaces for stripfoundations at the ground surface
Figure
1
0
.3
comparessectionso
fth
eEC8surfaces
.T
he
se
ctionsarealongit
ud
inalsection
in the
V
0 plane.
It isnotable that, intermsofnormalisedparameters, the foundation onclay has agreater
volume within thebearing strength surface.
,
M
boundary with the
H
=
0 plane, the
V
−
H
boundary in the
M
=
2.1. ACCELERATION INDUCED REDUCTION INVERTICAL
FOUNDATION STRENGTH
The terms in the brackets on the bottom line of Equation (10.3) show how the vertical
bearing strength, in the absence of shear and moment loading, is reduce
d
by horizontal
acc
ele
ration inthesoilbeneaththeshallowfoundation. Thereduction of
V
withincreas-
ing
F
isplottedinFigure10.4,thelefthandsidefortheundraine
d
caseandtherighthand
for the drained case. In the diagram
V
end
is the value to which
V
, under zero shear and
moment, is reduced by the horizontal acceleration
in
the ground beneath the foundation.
Thi
s fi
gure shows first that the range of values for
F
undrained
is quite different from those
for
F
drain
ed
.
Thelimitingconditionwhen
V
end
becomeszerogivesthemaximumpossible
value for
F
.
Themaximumvaluefor
F
drained
is1.04andfor
F
und
rai
ned
is3
.6
0.Figure10.4alsoshows
that for the drained case there is a rapid decline in
V
once
F
is below about 0.9. This
might be a manifestation of the “fluidisation” effect noted by Richards et al. (1993)
when discussing the effect of horizontal acceleration on the bearing strength of shallow
foundations in sand.
