Civil Engineering Reference
In-Depth Information
P
=
P
×
γ
p d
;
p k
;
G fav
;
or
P
p k
;
P
=
γ
p d
;
Re
The values of both
γ
G; fav
and
γ
Re
for each design approach, taken from Table
5.1,
are presented in Table
8.2.
From the table it is clear that the question is only of concern when using Design Approach 2. For the
other design approaches both
γ
G; fav
and
γ
Re
are 1.0 and thus P
p;d
will be equal to P
p;k
.
The prevailing view (Schuppener,
2008
) is that the passive thrust for gravity walls should be treated as
a resistance, thus even the concern when using Design Approach 2 is addressed. Further, it could be
argued that once consideration has been given for an unplanned excavation, the contribution to stability
from any passive resistance will be small and could, conservatively, be ignored.
However, the question gains a bit of additional confusion when considering embedded walls. Recalling
that the purpose of a partial factor of safety is to allow for the uncertainty in the derived characteristic
value, then any design action (e.g. active thrust) determined from a soil property (e.g. angle of shearing
resistance,
φ
′
) will contain the same degree of uncertainty in its value as any other action (e.g. passive
thrust) which is determined from the same soil property. In the case of an embedded wall where the same
soil exists on both sides of the wall, this means that the uncertainty in the active thrust (taken up by the
application of the permanent, unfavourable partial factor,
γ
G,unfav
) must be the same as that in the passive
thrust. We must therefore consider the passive thrust also to be a permanent, unfavourable action. This
follows from the
Single Source Principle
in which actions coming from the same source must be combined
with a single partial factor of safety. Indeed EN 1997-1:2004 states the following:
NOTE: Unfavourable (or destabilising) and favourable (or stabilising) permanent actions may in some
situations be considered as coming from a single source. If they are considered so, a single partial
factor may be applied to the sum of these actions or to the sum of their effects.
Table 8.2
Values of
γ
G; fav
and
γ
Re
for each design approach.
Design Approach
1
2
3
Combination 1
Combination 2
γ
G; fav
1.0
1.0
1.0
1.0
γ
Re
1.0
1.0
1.4
1.0
Table 8.3
Values of
γ
G; fav
,
γ
G; unfav
and
γ
Re
for each design approach.
Design Approach
1
2
3
Combination 1
Combination 2
γ
G; fav
1.0
1.0
1.0
1.0
γ
G; unfav
1.35
1.0
1.35
1.0
γ
Re
1.0
1.0
1.4
1.0
