2080
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Table 1. Geotehnical parameters
layer
thickness,
m
E
oed
,
MPa
E
d
,
G
d
MPa
k
o
`
º
c`,
kPa
N
SPT
1
man-
made
fill
0.6–2.8
-
-
-
-
-
-
2
silty
clay
11.4-16.2 7-16
56.5
19.5
0.6-
0.7
16-
23
51-
92
10-
34
3
sand,
gra-
vel
0.9-6.9
30*
-
0.4
35*
-
16-
33
4
clay
10.5-16
8.3-
11.7
54.5
18.9
0.7
9-
20
61-
113
11-
39
5
sand
6.7-7
40*
57.8
20.2
0.5
30*
-
23-
59
6
clay
21.7-27.8
-
71.2
-
-
14
141
-
* - values estimated from SPT tests.
where:
E
oed
– oedometric modulus, corresponding to 0.2-0.3 MPa
stress interval;
E
d
, G
d
– linear deformation modulus and shear modulus for
300 kPa stress;
k
o
– at rest earth coefficient;
`, c` - drained shear strength parameters;
N
SPT
– number of blows from SPT test.
Hydro-geological study emphasized two aquifers: a free
level aquifer (layer 3) and a second confined aquifer (layer 5).
The excavation will be 2 – 3 m below the groundwater level.
3 RETAINING STRUCTURE
3.1
Geometrical and technological characteristics
The deep excavation was retained using diaphragm walls 80 cm
thick, with variable length, from 20 m to 24 m. The lower level
of the wall remained constant (+60.50 m), while the upper level
varied according to the architectural details of the basement and
adjacent buildings (+84.50 m along the long sides and +80.50 m
along the short sides of the pit).
Figure 2 presents a layout of the diaphragm wall enclosure
and the final excavation levels.
Figure 2. General layout of the diaphragm wall enclosure
The natural ground level varies on the site around +87.0 m,
so for the working platforms required for the diaphragm walls
execution, a sloped excavation was realized. The final
excavation levels are +72.7 m and +70.7 m (for the 6 m thick
raft area), respectively, which led to a maximum excavation
level of approx. 16.3 m below the ground level.
It can be seen that the foundation level of the raft is in the
sand and gravel layer (layer 2).
The temporary support of the diaphragm wall was ensured
using 2 or 3 levels of anchors. The total length of the anchors
was comprised between 20 m and 25 m. In the corners were
used metallic struts and wale beams.
Figure 3 presents a cross section through the diaphragm wall
for the sides with 3 anchor levels.
Figure 3. Cross section through the diaphragm wall
For the anchors on 3 levels the characteristics are the
following:
Anchors level 1: +82.20 m
- inter-axis distance: ~ 1.75 m
- number of strands in each anchor: 4
- maximum pull-put force / anchor estimated by calculation
(ULS) = 200 kN
Anchors level 2: +78.90 m
- inter-axis distance: ~ 1.75 m
- number of strands in each anchor: 4
- maximum pull-put force / anchor estimated by calculation
(ULS) = 250 kN
Anchors level 3: +75.60 m
- inter-axis distance: ~ 1.20 m
- number of strands in each anchor: 6
- maximum pull-put force / anchor estimated by calculation
(ULS) = 320 kN.
As it can be seen on figure 3, the lowest level of the pit base
is 3.0 m below the groundwater level. The soil permeability and
the ground level differences led to a water flux in the enclosure
of about 90 l/s, unevenly distributed, being higher on the
Southern side. Considering these conditions, a dewatering
system was designed, comprising 12 wells disposed along the
enclosure sides.
3.2
Diaphragm wall calculation
Diaphragm wall calculation was done based on Eurocode 7 (SR
EN 1997-1:2004 and the Romanian National Annex SR EN
1997-1/NB). According to the National Annex in Romania, the
calculations were performed for design approaches 1 and 3,
approach 2 not being recommended by this document.
As well, according to the Romanian technical norm for
retaining structures (NP 124-2010), the seismic action was
considered on the wall. The seismic coefficient was decreased
considering the temporary character of the retaining structure,
according to the same technical norm.
Figure 4 shows some of the results obtained for the stresses
in the diaphragm wall, corresponding to the side with 3 levels of
anchors, for ULS calculation. Calculations were performed
