ACEGrid® Two Tree Hill

Problem

During the intense wet period of 2011 and 2013, Southeast Qld received record rainfall. Due to saturated soils, there were landslips creating many road closures in the rain affected areas. This particular failure occurred beside the Warrego Highway at Minden in the Lockyer Valley. Being a major highway between Brisbane and Toowoomba the risk of closure would have significant cost implication to industry and the traveling public. Emergency works were conducted to cut the slope back to a more stable angle which in part worked. Further rains lead to some localised failures along the new section that required a more detailed engineered solution.

Solution

After considering different options, the final design was to construct a geogrid reinforced soil structure with a vegetated face. Global Synthetics ACEGrid® geogrid was specified by the designer to provide the necessary reinforcement. To add extra stability to the toe of structure and to lessen the slope angle, a Global Synthetics Link® gravity gabion structure was installed at the base of the slope. To prevent rain drop impact and runoff scouring the finished surface of the slope, Propex® Landlok 300 turf reinforcement mat was selected for use on the face. Additionally, Bentofix® Geosynthetic Clay Liner (GCL) was installed along the crest of the structure as a water proofing layer to prevent surface water penetrating into the soil. This layer of Bentofix ® GCL will force any surface water running from further up the slope into the existing concrete cut off drain and be carried safely down to the base of the slope.

ACEGrid® Geogrid is constructed from high tenacity polyester which is proven to withstand constant load and suffer very low levels of long term creep (stretching of the geogrid). This allows any designer to use the most cost effective strength of product to meet the design requirements. The manufacturing process of ACEGrid® Geogrids ensures maximum interaction with the engineered backfill material and its flexible nature makes it well suited to quick and easy installation.

Link® Gabions are manufactured using the very best coating systems available to provide a highly durable product. The gabions were coated with a licensed wire technology - Galfan® alloy mischmetal that is proven to provide 3-4 time greater protection in similar environments as traditional zinc metallic corrosion protection. Additionally there is the option of having a PVC coating added to the Galfan coated wire which will further extend the life of the structure. Link® Gabions come flat packed and have a range of sizes to suit any wall design.

Landlok 300® Turf Reinforcement Mat is a high performance erosion control matting that permanently reinforces the root zone of the vegetation. Landlok 300 is manufactured by Propex in the USA and is specifically design to entrap soil within its structure while allowing good light penetration and maximum protection of the surface until vegetation is established. There are various strengths of Landlok available which are matched to the requirement of each application.

All of grades of Landlok utilise the patented X3 fibre technology that increases the effective surface area of the fibre and matting and greatly assists the interlocking of the matting with the soil and roots of the vegetation once established.

Bentofix® GCL is a proven alternative to a compacted layer of impermeable clay. Bentofix® is a layer of bentonite power sandwiched between 2 layers of geotextile and bound together as one material. Bentofix® allows a much thinner layer and greater control over the manufacture and installation of the product thereby providing improved waterproofing performance.

Global Synthetics provided the assistance to both the designer and contractor throughout the project from early design concepts to onsite construction issues.

The project was constructed over a 5 month period by the contractor with fine weather persisting, no major delays occurred in the construction sequence.