Geosynthetics October/November 2020 - 12

Reinforced embankments for the causeway for a North Wales bridge project

As an alternative to sheet-pile walls,
we proposed to construct reinforced
earthwork embankments using uniaxial
geogrids and a specific granular material
for the embankments' starter layers
to allow the installation to take place
directly in open water. The temporary
causeway was required to extend 263 feet
(80 m) into the tidal estuary from the
southern shore and 66 feet (20 m) from
the north (Figure 3).

Design considerations
Design parameters
The reinforced soil slopes with uniaxial
geogrids were designed to increase the
stability of the slopes so that heavy plant

FIGURE 2 Pont Briwet historic timber trestle bridge

FIGURE 3 Plan view of bridge and approaches
12

machinery could travel on the access
roads, 3.3 feet (1 m) away from the edge
of the embankment.
The height of the embankments
ranged from 6.6 to 13.5 feet (2 to 4.1 m)
with slope angles of 1V:1.5H and 1V:2H.
The causeway working platforms were
required to support large-tracked piling
rigs and cranes, in addition to an 882-ton
(800-tonne) mobile crane required to lift
the bridge beams into place.
The top level of the temporar y
embankments was at 17.1 feet (5.2 m)
AOD (above ordinance datum) and
maximum water level was assumed at
13.9 feet (4.3 m) AOD. For the causeway,
geogrids were used for the stability of the
slopes and for the basal reinforcement of
the working platform to support the
pressures applied by the cranes and piling rigs.
The top level of the causeway was at
10.5 feet (3.2 m) AOD with seabed varying from 3.3 feet (1 m) AOD to -8.2 feet
(-2.5 m) AOD and maximum water level
assumed at 8.5 feet (2.6 m) AOD. During
the temporary works, a woven geotextile
(wraparound) was used to protect the
slopes from erosion and to prevent the
fines from being washed out with the
constant changes of water level.
The main challenge for the design of
the reinforced embankments was to keep
the distance from slope edge for piling
rigs and cranes to 3.3 feet (1 m), with a
maximum slope angle of 1V:1.5H. The
variation of the water levels due to the
tidal activity added a further complication. For the design, it was considered
that during the construction the causeway would be completely submerged
on various occasions yet needed to be
back in use immediately after the tidal
events. In the design, we included some
reinforced areas on the ramp approaches,
above the predicted flood levels, to allow
the tracked plant to remain safe during
these events.

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Geosynthetics October/November 2020

Table of Contents for the Digital Edition of Geosynthetics October/November 2020

Geosynthetics October/November 2020 - Cover1
Geosynthetics October/November 2020 - Cover2
Geosynthetics October/November 2020 - 1
Geosynthetics October/November 2020 - 2
Geosynthetics October/November 2020 - 3
Geosynthetics October/November 2020 - 4
Geosynthetics October/November 2020 - 5
Geosynthetics October/November 2020 - 6
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Geosynthetics October/November 2020 - Cover3
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Geosynthetics October/November 2020 - Geosynthetics Conf. Brochure 1
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