Building Pacific Highway with Combigrid® Geogrid

Australia’s Pacific Highway connects the major metropolitan areas of Sydney and Brisbane. A major construction phase along the route has involved the Stewarts River Bridge project, which is part of the Coopernook to Herons Creek upgrade. But like many projects along this vital highway, the subgrade conditions (soft clay) complicated the work.

In road building, geosynthetics are used frequently to provide subgrade stabilisation and improvement. The placement of geogrids, such as NAUE Combigrid®, locks the aggregate, distributes tensile loads, and prevents the movement of fines when wheel loads pass along the road surface. This stabilisation is essential to long-term road performance, including reduced maintenance needs and the prevention of rutting.

For the Stewarts River Bridge project, the subgrade problem was actually discovered by accident: a piling rig could not be situated safely on site – not without subgrade improvement.

Thiess Contractors needed to support a Bauer BG28 piling rig, which has a loading pressure of approximately 167kPa while travelling and 317kPa when extracting piles. To support this extremely heavy equipment, the site design adopted a conservative sub-base CBR of 1%. Accommodating such a conservative design was necessary, but it initially seemed like it would be almost prohibitively expensive. Then NAUE entered the picture. Working with its Australian distribution partner Global Synthetics, NAUE presented an economical solution for stabilising the piling rig platform on the site’s weak soils: Combigrid®.

Combigrid® geogrids combine the reinforcement strength of Secugrid® geogrids with the outstanding separation and filtration functions of Secutex® nonwoven geotextiles.

With one, convenient, roll-delivered product, a project team can quickly install this single layer, multifunctional composite reinforcement material.

The selection of Combigrid® proved to be the difference at Stewarts River, allowing not only the construction but pre-serving the project’s cost goals. Utilising a factor of safety of 2.5 for all loading conditions, a 500mm well-graded gravel base course was placed over Combigrid® 40/40 (40kN/m strength @ 8% peak strain, 16kN/m @ 2% working strain). This included a 300mm-thick initial lift layer of clean rockfill base course over the Combigrid® layer and a 200g/m² nonwoven geotextile layer with 200mm of road base gravel. A maximum 2% fall was adopted to allow water runoff and maintain stability.

Additional design elements included a recommended 200mm minimum overlap of Combigrid® on lateral and transverse joints.

Being a cohesive subgrade, the Combigrid® prevented the up-ward migration of fines into the working platform.

Combigrid® provided the necessary elastic modulus increase along this stretch of the Pacific Highway, delivering a high load capacity within the working strain of the pavement. In addition, it increased the shear strength of the base course through the Secugrid® geogrid’s outstanding resistance to tensile forces. The pavement’s ability to tolerate the sustained load of the piling rig, movement of rig, and the fluctuating loads during operation of pile activities in-creased significantly.

Transportation in Australia rolls on.