Geoscience Reference
In-Depth Information
The grids were cut to design length on site. Bodkin joints were used to
avoid wastage from end of roll off-cuts. These lengths were abutted against the
inside face of the shutter and nominally joined by cable ties to avoid any gaps
opening during installation. The internal former was then placed against the grid
face and the geotextile was wrapped around the internal face of the former. A
geotextile overlap of 50 cm was specified. The silty sand fill was placed and
compacted in lifts to a depth of 60 cm. This fill was found to have sufficient short-
term cohesion that the former could be carefully raised and the resulting void
filled, by hand, with coarse material without any slumping of the geotextile face.
The grid wraparound face was next returned over the coarse fill and
connected to the next grid layer using a bodkin. The free end of the upper grid
was then hand-tensioned using a steel beam.
The face of Lake 1 was slightly reprofiled by local steepening to
accommodate 20-cm horizontal ledges at the top of each 2.4-m lift on which the
shutter could be seated. The overall slope remained at 70
. (The geomembrane
that extended up the rear of the reinforced soil block also required its own
temporary shuttering.)
These details resulted in relatively slow outputs of around 40-50m
2
of
completed face area per day using two gangs. In order to improve this, the
contractor developed a face detail that replaced the shutter and plywood former
with an internal steel mesh former. This was produced by site cutting 5-mm-
diameter steel mesh sheets and bending them into “U”-shaped units 60 cm high
by 28 cm wide. These units were positioned to act as a permanent face former
(Fig. 10)
and then filled with the coarse fill. Because the steel mesh aperture was
20 cm by 20 cm, the vertical face of the unit was lined with geogrid Type 1 before
filling. The top of the unit was cross braced using steel tiewire.
The geotextile was then wrapped up the rear face of the filled unit and the
bulk fill placed behind. The main grid length was then wrapped up the front face
of the unit and bodkined and tensioned as normal.
The alternative method proved a little quicker, particularly for the higher
levels, although the alignment, while acceptable, was less consistent.
The reinforced soil slopes of the lake are constructed with approximately
60,000m
2
and 80,000 m
2
of geogrids Types 1 and 2, respectively, supplied in 50-
m by 1.3-m rolls. They were constructed in approximately 3 months
(Fig. 11).
8
3.3 Lake 2: Precast Concrete Block Face
The water level of Lake 2 was designed to fluctuate with that of the surrounding
chalk aquifer, and so no geomembrane lining was necessary. About half of the
slope length of Lake 2 was formed from the existing chalk quarry face.
Reinforced soil construction took place in the winter and spring of 1997. In
order to simplify and accelerate installation, the contractor, with assistance from
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