Construction on soft and expansive subgrades: South Station Road Pavement Rehabilitation

The Project

In 2021, Ipswich City Council (ICC) undertook pavement rehabilitation on a section of road along South Station Road, Silkstone. The original pavement geotechnical investigation and testing were undertaken in the middle of the 2019 calendar year. A subsequent deep lift asphalt pavement design (with no geosynthetics) was produced based on current Austroads and Council pavement design guidelines. The design considered the subgrade CBR results and projected future traffic design loading, taking into account the highly expansive subgrade environment identified at the site, which required adequate pavement cover to manage the undesirable impacts associated with this type of behaviour.

The Challenges

When construction began on site, significant problems arose with the soft subgrade. Further in-situ subgrade CBR testing, using the Dynamic Cone Penetrometer (DCP), revealed that the existing subgrade CBR was worse than anticipated. This necessitated additional subgrade treatment, deeper excavation, and a thicker pavement to meet the proposed design.

Project construction personnel from the Council expressed concerns regarding the potential impact on project timing, increased disruption to the community, and the need for possible relocation of underlying road utilities. These factors would substantially inflate project costs, exceeding the allocated budget. As a result, the Council's pavement design engineer was enlisted to reassess the proposed design and provide an appropriate and practical solution to mitigate the risks associated with soft and expansive subgrades.

Expansive subgrades are comprised of highly plastic soil or clay that undergoes volume changes in response to fluctuations in water content. These soils expand when exposed to excessive moisture and contract during water evaporation. This continuous cycle of wetting and drying induces soil movement, posing significant geotechnical and structural challenges to civil infrastructure. Even without traffic loading, expansive subgrades beneath sealed road pavements exhibit sinking and uneven rising behaviour, leading to pavement cracks and failures.

In recent decades, geosynthetic reinforcement solutions have been widely employed to enhance the roads' resistance against environmental loads. These solutions offer cost-effectiveness compared to conventional methods such as soil replacement or lime stabilisation; and by other means to control the subgrade moisture conditions. Moreover, geosynthetics are environmentally sustainable and have demonstrated resilience in full-scale field trials conducted globally, including local research in Australia.

Our Solution

The Council's pavement design engineer contacted Global Synthetics to seek technical assistance. They requested a geosynthetic reinforced pavement design solution that would address the identified construction risks, ensure adherence to industry standards for the desired pavement performance, and help manage construction costs within the allocated project budget.

After carefully reviewing the project-specific information and collaborating closely with the Council's pavement design engineer, Global Synthetics' specialist pavement engineer proposed a geosynthetic solution using Combigrid® and Secugrid®. The solution involved placing a layer of Combigrid® laid and welded biaxial geocomposite, with the geotextile component integrated between geogrid bars, directly on the poor subgrade. This layer was combined with a calculated depth of gravel material for subgrade stabilisation using SecuCal® CBR software, enhancing the subgrade's bearing capacity and reducing the overall pavement depth. Additionally, an extra layer of Secugrid® laid and welded biaxial geogrid was incorporated within the pavement for stiffening the unbound pavement layers, effectively managing the expansive subgrade behaviour by delaying and minimising longitudinal cracks.

The recommended reinforced design using Combigrid®/Secugrid® allowed the Council to construct the road with its original thickness, eliminating the need for deeper excavation or additional treatment. It also addressed concerns regarding proof rolling and constructability, as well as fulfilled the requirements for expansive soil management.

The Secugrid® Geogrid (shown on the right photo) provided reinforcement, stabilisation, and stiffening by confining lateral movements of the granular layer. On the other hand, the Combigrid® Geocomposite (shown on the left photo) integrated a geotextile between the Secugrid® geogrid bars, reducing the risk of geotextile delamination. It offered reinforcement, stabilisation, filtration, and separation, all in a single product.

The Results

By utilising the high stiffness reinforcement and separation benefits of Combigrid® Geocomposites and the reinforcement and stiffening properties of Secugrid® Geogrids, the Council successfully completed the construction works on time and achieved significant cost savings, keeping the project well within the allocated budget.

The Benefits and Features of Combigrid® and Secugrid® Geosynthetics:

• Combigrid® Geocomposites offer cost and time savings by combining two functions in one product, making installation easier and faster in a single operation, without the risk of recoil due to its flat structure.

• Immediate interlock with fill material (no construction-related strain).

• Excellent stress/strain behaviour (high tensile stiffness).

• High radial stiffness and high cyclic tensile stiffness at low strains.

• Resistant to installation damage, chemical and biological attack.

• Quick and easy to install.

• Wide width rolls to reduce waste and increase speed of construction.

In summary, Combigrid®/Secugrid® geosynthetic reinforced structures have proven to be an effective total system, providing stability against soft and expansive subgrades, and withstanding environmental and dynamic traffic loads.

We would like to thank Ipswich City Council (ICC) for their support in selecting the right products for this project.

Choosing the right geogrid for your application depends on various factors. For assistance in selecting the appropriate product for your individual project, please contact our specialist engineering team at Global Synthetics.