Section of TENAX TT geogrid reinforced soil structure
TENAX TT mono-oriented geogrids
TENAX MULTIMAT geomats
SH3, Region 3, RS 156, Mokau, Taranaki, New Zealand
Transit New Zealand
Beca Carter Hollings & Ferner Ltd, Auckland, New Zealand
McConnell Smith/Fulton Hogan/Burgess and Crowley
The project involves the construction of a new two way bridge over
the Mokau River to replace an existing old single lane steel bridge
and the realigning of the approaches to the bridge on State Highway
3 (SH 3). The southbound approach is located on the side of a slope
and required realigning to meet a specific design speed environment.
The road designation boundaries above and below the road had been
previously established. Therefore a retaining wall was required
to support a fill to enable the road to be realigned and widened
and remain within the road designation.
View from top showing placing and compaction of fill soil
Installation of TENAX TT geogrid ready for back wrapping
The realignment and widening of the road required filling over an
existing slope which exhibited signs of instability. Several options
were considered including a concrete retaining wall located at either
the top or the base of the slope.
The reinforced soil option was
selected as being the most cost effective to meet the boundary requirements,
and aesthetically combine with the rural and coastal environment.
BCHF carried out the design (refer Typical Cross Section) which
included a reinforced soil wall varying in face height between 1
and 8 m with a 2H:1V fill slope above the wall extending to the
finished road level.
The reinforced soil was built with 70°
face slope. The design included the use of ash and sand fill excavated
from site reinforced with Tenax TT 060 SAMP, TT 090 SAMP or TT 120
SAMP geogrids (depending on the retained height) at 400 mm spacing.
A drainage layer complying with TNZ F/2 specification was placed
at the rear of the wall and immediately above the foundation layer.
A 500 mm thick layer was placed as a foundation layer to support
the wall. The grids have a wrap around topsoil filled bag facing
which will be grassed and planted with small native flax to provide
an attractive but protective facing.
A small section of wall beneath
the bridge abutment will have a shotcrete facing. The walls are
designed for both static and seismic loadings as required by the
NZ Loadings Code (NZS 4203). It is noted that Mokau is located in
an area of New Zealand which is considered to have moderate seismicity.
The foundation soils comprised soft to firm silts and silty sands
over lying medium dense sand.
The foundation is located on the medium
The use of TENAX geogrids as part of the State Highway realignment
- Cost effective support to the base of a large fill within road
- Use of fill materials excavated from new cut slopes forming
part of the highway realignment works
- Fast construction, using standard earthmoving plant
- Creating aesthetic "environmentally friendly" vegetated
slopes in a rural and coastal location.