Geoysynthetics August/September 2023 - 44

FINAL INSPECTION
Efficient and effective surface
stabilization with StrataWeb
and StrataGrid. Photo courtesy
of Cirtex.
Slope stability in Auckland, New Zealand
R
ed Hills is a 400-lot residential development situated in Westgate, Auckland, New Zealand.
This north-facing, elevated greenfield site sits on sloping land, with steep batters surrounding
each block of housing to create the maximum amount of usable land. With some slopes steeper than
1V:2H, there was a high risk of the topsoil and new planting eroding and sliding off the slope face.
A permanent stormwater pond was also installed, designed to naturally clean and discharge
rainwater into the surrounding farmland. However, during a large rain event within such steep
land, this pond is calculated to overflow. An overland flow path was designed, allowing excess water
to spill over a road, down a bank and into a natural stream. This required an even more robust
slope design than the steep batters. To help stabilize the soil and the plants across the site, Cirtex
Industries was asked to incorporate several geosynthetic solutions.
These steep, engineered batters support the houses above, so it's vital they are well vegetated
PROJECT HIGHLIGHTS
SLOPE STABILITY
IN AUCKLAND,
NEW ZEALAND
LOCATION
Westgate, Auckland,
New Zealand
GENERAL CONTRACTORS
Ross Reid Contractors Ltd.
Evergreen Landcare Ltd.
ENGINEERS
Candor3 and ENGEO Ltd.
GEOSYNTHETIC PRODUCT
StrataWeb
GEOSYNTHETICS
MANUFACTURER
Strata Geosystems
44
Geosynthetics | August September 2023
and protected from the elements, but once the subdivision is complete, there will be limited access
to these slopes. If the topsoil and vegetation were to slip and expose the batters, it would put the
properties above at risk, so the slope design proposal had to be a long-term, permanent solution.
To stabilize and retain soil on the steep batters, StrataWeb Geocell with a 11.8-inch (300-mm)
cell depth was specified over the embankments. An underlying, reinforcement layer of StrataGrid
was also used to help stabilize the geocell and prevent it from slipping.
To further stabilize the topsoil and develop native planting in the path of the stormwater pond's
overland flow, a layer of T-RECS Turf Matting was placed over the top of the StrataWeb and topsoil,
secured using Platipus ARGS Anchors. T-RECS helps confine and retain the topsoil and root
systems while a large volume of water is flowing over it.
Installed by contractors Ross Reid Contractors Ltd. and Evergreen Landcare Ltd,, StrataWeb was
secured in an anchor trench at the top of the slope. Platipus Anchors were used within the anchor
trench, to which the StrataWeb support tendons were attached. The geocell was then stretched out,
over the crest of the trench and down the steep bank. Along with the underlying layer of StrataGrid,
these components work together to prevent topsoil erosion and to resist sliding. The StrataWeb was
locked in place, then filled with topsoil, with the large, honeycomb cell structure giving enough
room for plant and tree roots to take hold.
On the banks within the overland flow path, an engineered, winding water course slows the
water velocity coming from the overland flow. This, in turn, helps protect the soil and vegetation,
further protecting the trees and native plants as they develop. G

Geoysynthetics August/September 2023

Table of Contents for the Digital Edition of Geoysynthetics August/September 2023