Civil Engineering Reference
In-Depth Information
j
8
14
h
× 82.4
=
47.1 kPa
P
w
1
= 188.4 kN
i
P
w
2
= 141.3 kN
d
u
u
82.4 kPa
82.4 kPa
(a)
(b)
(c)
Fig. 8.19
Example
8.5.
Solution:
With the assumption that the excess head is linearly distributed around the length of
the pile within the water zone, the formula for u, the water pressure on both sides at
the pile toe, is
2
(
h d j d i
d h i
)(
)
+ − −
+ − −
γ
w
u
=
=
82 4
.
kPa
(
2
j
)
The assumed diagrams for water pressure on each side of the wall are shown in Fig.
Eurocode 7 Part 1 states that in silts and clays, water pressures must be considered to act and, unless
a reliable drainage system is installed, the groundwater table should be taken as a coincident with the
ground surface of the retained soil. The resulting water pressures are considered as geotechnical actions
(permanent, unfavourable), and the appropriate partial factors of safety are selected and applied to the
net water pressure acting to yield the design water pressure.
Thenault (
2012)
looked into the effect that passive pressure has on the design depth of embedment
when considered as an unfavourable action, as a favourable action or as a resistance. Her results quanti-
fied the significance that the depth of the GWT has on the design depths achieved between the three
approaches and showed that careful consideration of how to deal with passive pressure is required when
the GWT is taken, as Eurocode 7 states, at the ground surface.
8.7 Braced excavations
When excavating a deep trench, the insertion of shuttering to hold up the sides becomes necessary. The
excavation is carried down first to some point X, and rigidly strutted timbering is inserted between the
levels D to X (Fig.
8.20a
).
As further excavation is carried out, timbering and strutting are inserted in stages, but before the tim-
bering is inserted, the soil yields by an amount that tends to increase with depth (it is relatively small at
the top of the trench).
