The role of a biaxial geogrid in subgrade stabilization - Geosynthetics Magazine

Oct 15, 2024

Features | October 1, 2024 | By: Gretchen McInnes, P.E.

In the civil engineering discipline, the challenge of constructing durable and cost-effective roadways over problematic soils is a common yet complex issue. High-plasticity clay subgrades, known for their susceptibility to moisture and subsequent weakening, pose significant hurdles. Traditional methods of soil excavation or chemical treatment can lead to increased costs and project delays. However, with the increased utilization of geogrids, particularly the TerraGrid® TXG-7, a cost-effective and reliable solution, engineers have a trusted option to keep their projects on track.

The city of College Station, Texas, needed to widen Rock Prairie Road from an existing two-lane road to a four-lane road with bike lanes and sidewalks. Like many challenging roadway projects across Central Texas, the subgrade soil was saturated to such an extent that over-excavation did not reach stable ground. Efforts to remove and replace several feet of soil with better material were futile; the expensive, imported fill could not bridge over the underlying weak soil. The persistent saturation of the high PI (plasticity index) clay subgrade meant that traditional soil replacement methods not only were costly but also ineffective, risking project delays and budget overruns.

In search of a more viable and economical solution, the project team evaluated a geogrid solution. Among the various options available, the TerraGrid TXG-7 from Hanes Geo Components stood out as the most affordable and strongest geogrid to stabilize the weak areas. Biaxial geogrids (regardless of aperture shape) are polymer materials used to reinforce soil, providing structural support and improving load distribution. The decision to create a trial section of the project with TXG-7 was driven by cautious optimism, and the results exceeded expectations.

Brazos Paving, Inc., the contractor for the project, and Terracon, the geotechnical engineer, initially approached the trial with uncertainty. How would the geogrid section perform versus chemical stabilization on high PI clays? The trial section demonstrated remarkable performance. The geogrid provided the necessary reinforcement, distributing the loads more evenly across the weak subgrade and preventing further deformation. Encouraged by these results, the city decided to apply TXG-7 geogrid across the entire project, topped with a 6-inch base layer. This strategic move not only brought the project back on schedule but ensured that it remained within budget.

Understanding the mechanics of geogrids such as the TerraGrid TXG-7 is crucial to appreciating their effectiveness in subgrade improvement. The following features apply:

Tensile strength and load distribution: Geogrids are designed with a high tensile strength that allows them to distribute loads more effectively when placed over a weak subgrade. The geogrid creates a bridging effect, spreading the loads across a wider area and reducing stress on any single point. This distribution minimizes deformation and improves the overall stability of the roadway.

Interlocking and soil reinforcement: The structure of geogrids enables them to interlock with the aggregate base material. This interlocking enhances the shear strength of the graded stone, preventing lateral movement and providing additional stability. In high PI clay subgrades, this is particularly beneficial as it mitigates the effects of swelling clays on the pavement section.

Separation and filtration: Geogrids can also function as separators, preventing the mixing of subgrade soil with the base material. This separation maintains the integrity of the base layer, ensuring consistent performance. Additionally, geogrids can act as filters, allowing water to pass through while retaining soil particles, thereby reducing pore water pressure.

Reduction in required base material: By reinforcing the subgrade, geogrids reduce the amount of base material required to achieve the desired structural performance. This not only lowers material costs but speeds up construction, contributing to overall project efficiency.

“Geosynthetic-reinforced unbound base courses: Quantification of the reinforcement benefits,” a study by the Center for Transportation Research at the University of Texas at Austin, supports these findings. The research emphasized the effectiveness of geogrids in improving the performance of unbound base courses. It was noted that geogrids enhance the load-bearing capacity and reduce the deformation of the base course material, mitigating the effects of expansive soils. The study concluded that incorporating geogrids can lead to significant cost savings and longer-lasting roadways, validating the practical benefits observed in the field application where the TerraGrid TXG-7 was utilized.

The Federal Highway Administration (FHWA) has developed guidance for the inclusion of geosynthetics into mechanistic-empirical (M-E) pavement design. Testing of both triangular and rectangular geogrids was conducted at the Texas Transportation Institute (TTI) at Texas A&M University and was independently funded by the National Cooperative Highway Research Program (NCHRP). NCHRP Report 01-50 was published March 2017. As a result, researchers developed a subroutine for AASHTOWare® Pavement ME Design software, known as the Composite Geosynthetic-Base Course Model. This model standardizes the design benefits of different geogrid aperture shapes based on their strength rather than geometry, simplifying the design process for engineers.

The successful implementation of TXG-7 geogrid in this roadway project highlights several broader implications for civil engineering and infrastructure development, including:

Cost efficiency: The use of geogrids significantly reduces the need for extensive soil replacement, lowering material and labor costs. In this case, the switch to TXG-7 brought the project back within budget, demonstrating the economic advantages of TerraGrid over traditional methods.

Time savings: Projects involving high PI clay subgrades often face delays due to the challenges of soil stability. Geogrids expedite the construction process by providing immediate stabilization, helping to keep projects on schedule.

Environmental impact: Reducing the need for soil excavation and replacement has positive environmental implications. It minimizes disturbance to the natural soil structure and reduces the carbon footprint associated with transporting large volumes of material. Geogrids, being durable and long-lasting, also contribute to the sustainability of infrastructure projects.

The success of the TerraGrid TXG-7 by Hanes Geo Components in this project paves the way for broader adoption of geogrid technology in various construction scenarios. From roadways to railways and parking lots to airport runways, potential applications are vast. As engineers and project managers gain more confidence in the performance of geogrids, their use is likely to become a standard practice in addressing subgrade challenges.

The adoption of the TerraGrid TXG-7 geogrid for subgrade improvement in the described roadway project serves as a testament to the transformative potential of geogrid technology. By effectively addressing the challenges posed by saturated, high PI clay subgrades, TerraGrid TXG-7 not only ensured project success but also demonstrated the broader benefits of cost savings, time efficiency and environmental sustainability. As the construction industry continues to seek innovative solutions for infrastructure development, geogrids such as TXG-7 are poised to play a pivotal role in revolutionizing how we build and maintain roadways.

Gretchen McInnes, P.E., senior technical sales engineer at Hanes Geo Components, is based in Houston, Texas. She is a licensed professional engineer in the state of Maryland with over 20 years of experience in geosynthetic design of roadways, MSE berms, slopes and walls with a particular focus on design-build solutions. She earned her B.S. in civil engineering from The Georgia Institute of Technology.

All photos courtesy of Gretchen McInnes.

Rock Prairie Road Construction and Subgrade Stabilization

OWNER: City of College Station, Texas

ENGINEER: Terracon

CONTRACTOR: Brazos Paving, Inc.

GEO PRODUCT: TerraGrid® TXG-7

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