Geoscience Reference
In-Depth Information
which is to a major part compensated by grout injection. All these aspects cause
deformations in the direct environment, and they have to be taken into account in a
project design. During construction, specific monitoring is required to take timely
measures to prevent serious damage.
?
?
?
0
0
0
0
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
Zone
I
- sand and soft hard clay
- average workmanship
Zone
I
- sand and soft hard clay
- average workmanship
I
I
I
II
II
II
Zone
II
- very soft to soft clay
- limited depth
- significant depth below
bottom excavation
- construction difficulties
Zone
II
- very soft to soft clay
- limited depth
- significant depth below
bottom excavation
- construction difficulties
1
1
1
1
III
III
III
distance from excavation
maximum depth of excavation
distance from excavation
maximum depth of excavation
distance from excavation
maximum depth of excavation
distance from excavation
maximum depth of excavation
? =
? =
? =
? =
2
2
2
2
s
s
s
Zone
III
- very soft to soft clay to
significant depth
Zone
III
- very soft to soft clay to
significant depth
settlement
maximum depth of excavation
settlement
maximum depth of excavation
settlement
maximum depth of excavation
settlement
maximum depth of excavation
s =
s =
s =
s =
3
3
3
3
soft to medium clay stiff clay and cohesive sand cohesionless sand
soft to medium clay stiff clay and cohesive sand cohesionless sand
Figure 15.3 Environmental effect: settlements near excavations (Peck)
Since this environmental effect is rather case specific, only some general views
are available from experience. Peck (1969) has collected information of many
projects in the past and for the settlement adjacent to open cuts as a function of the
distance from the excavation, he suggested distinguishing three cases related to the
type of soil, shown in Fig 15.3. The data refer to excavations with a depth varying
between 6 and 23 metre where standard piles or sheet piles with cross bracing and
tiebacks have been used. Rankin (1988) published a comprehensive overview of
various empirical approaches for the assessment of surface movements due to soft-
ground tunnelling. In general, he adopts a Gaussian-curved bowl shape
characterised by a maximum depth, a length and a width. The position of the centre
of the bowl is 2
z
0
behind the tunnel face, and the border of the bowl is 2
z
0
ahead of
the tunnel face, where
z
0
is the depth of the tunnel. For cohesive soils, the
maximum settlement
w
max
is assumed to be related to the volume loss into the
tunnel. It does not hold for granular soils or stiff clays (see Fig 15.4). An
approximate trend can be obtained from case history data:
w
max
= (0.1
R
2
/
z
0
)
1.18
,
where
R
is the tunnel radius and
z
0
the depth of the tunnel axis. However the
experience so far with tunnelling induced surface settlements reveals a large
scatter. A recent state of the art review is presented by Mair (2011)
B
SOFT
-
GROUND TUNNELLING TECHNIQUES
There are various tunnelling techniques. In rigid soils, one may apply an open
boring front and even use explosives, but in soft permeable soils a shield is
required to keep soil and water under control and retain the front by a (pressure)
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