Pokeno (SH1) Slip Repair – DuraGreen

When oversaturation caused an 83m section of unstable land to slip right beside State Highway 1 just south of Pokeno, access became restricted to the northbound intersection with Pioneer Road. To blend in with the local surroundings, ASM looked to reinstate the slope using a naturally vegetated solution. To blend in with the original slope angle, most of the area would be reinstated using site material with a hydroseeded layer of topsoil, but in the centre of the slip, a 40m section required a more permanent retaining wall solution. Added drainage was also required at the cut face to further prevent oversaturation, with a protection matting for the vegetated slopes to be placed to protect and enhance grass growth. WSP approached Cirtex looking at DuraGreen as a likely option for the main retaining structure.

DESIGN

DuraGreen Soil Bags are supplied prefilled and made to order with a specified mix of soil, fertiliser and seed, the DuraGreen Wall System is also ideal for constructing tightly curved or variable gradient vegetated walls. To prevent further oversaturation, DuraFlow Q drainage composite would be placed on the cut slope and tied into the drainage aggregate. The upper and side slopes would be reinstated with hydroseeded topsoil beneath a layer of BioCoir. This acts as a protective barrier, preventing rain from washing seedlings away as well preventing soil erosion until the roots could establish. A fall protection fence would also be placed along the top of the wall just behind the face using the Sleeve-It pre-engineered post anchoring system.

Approx. 40m long and 4m high with a 2.4m return, this 70o slope would use 5.5m StrataGrid tieback lengths placed in 400mm lifts.

INSTALLATION

With an access track created through the neighbouring farm, the slip material was removed just o site, and the cut face was lined with DuraFlow Q. The site was lined with geotextile before the first 400mm layer of compacted GAP65 and StrataGrid was placed. A layer of larger drainage aggregate including a perforated, corrugated subsoil drain was placed just above the proposed toe of the slope, with the first lift of DuraGreen beginning from the layer above.

As the surface can be used once the aggregate is compacted to the specified design, the retained structure could be used as a work platform as each layer rose. This allowed for the use of smaller machinery without seriously impeding traffic along State Highway 1. Sleeve-It units were placed within the fill on the top layer, allowing the handrail to secured close to the edge of the wall. To help the MSE wall blend naturally into the local surround, site won fill was used to reshape the slopes above. Site won fill was also used to reshape the slopes above and to the side of the wall, with each slope hydroseeded and covered in BioCoir to allow grass to grow through.

By using a combination of DuraGreen, StrataGrid, DuraFlow, Sleeve-It and BioCoir, Cirtex were able to offer ASM a complete, robust retaining wall package that will soon blend in with the local farmland.

The DuraGreen system provided the following benefits to the project:

• Naturally vegetated to blend in with the surrounding environment
• Flexible construction materials allowing multiple slope angles
• Proven and reliable technology