Geoscience Reference
In-Depth Information
2.1.1 Design Parameters
The fill, which was to be sourced from an adjacent quarry, was a silty fine sand
(Thanet Sand). Its design parameters were
;
and g
sat
¼
20
:
2kN
=
m
3
A series of 30-cm
£
30-cm laboratory shear box tests was commissioned to
measure the frictional shearing resistance between the various specified
geosynthetic materials and this fill.
The selected lining system was 1-mm-thick modified low-density
polyethylene (LDPE) geomembrane protected by a 700-g/m
2
polypropylene
needle-punched geotextile. The critical interface shearing angle (f
ls
) for this
combination was found to be 20
f
0
cv
¼
31
8;
c
0
¼
0
;
g
opt
¼
19
:
3kN
=
m
3
, for the range of design normal pressures tested.
Two strengths of high-density polyethylene (HDPE) geogrid reinforcement
were selected: Tensar 40RE (type 1) and Tensar 80RE (type 2). These are
manufactured from extruded sheets and orientated (stretched) in the machine
direction. Their Index QC strength measured in accordance with International
Organisation for Standardisation, 1993 in the longitudinal direction is 40 kN/m
and 80 kN/m, respectively, and rib thicknesses (t
r
) 0.7mm and 1.3mm and (t
b
)
1.9mm and 3.6 mm, respectively (Fig. 2). Shear tests on these two grid types with
the chosen fill material indicated friction angles in excess of 26
8
8
. This gives a
coefficient of soil interaction:
tan 26
8
m
¼
8
.
0
:
8
tan 31
This is typical for this combination of geogrid and fill type.
2.2 Global Stability
The shear box testing confirmed that the critical potential external failure
mechanism for Lake 1 was sliding over the geomembrane lining system. It was
Figure 2
Geometry of the grid reinforcement.
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