Geogrid is a geosynthetic product used for stabilization and reinforcement in Civil Construction.
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Geogrids are classified as a type of geosynthetic because it is an engineered textile manufactured from polypropylene polymers.
However, geogrid differs from many of the other geosynthetics because it is less of a fabric and more of a plastic structured netting.
Geogrid is manufactured through extrusion. The extrusion process is very different than the way geotextile fabrics are weaved. The resulting product is optimized for vertical or lateral applications. This direction of strength determines the classification.
At Colonial, we supply both biaxial and uniaxial geogrids. Although there are a number of Civil Construction applications for uniaxial geogrid, we find that most of our customers need a geosynthetic solution for subgrade stabilization.
When it comes to poor subgrade, we will recommend biaxial geogrid nine times out of ten.
To understand why we recommend biaxial geogrids for so many construction projects, we&#;re going to explain some important facts including:
Geosynthetics have stabilized roads and soil for more than 4,500 years.
In fact, one of the first examples we have of using geosynthetics for subgrade support is the construction of the monumental Egyptian pyramids. Excavations of ancient Egyptian sites revealed the use of man-made mats woven from grass and linen.
In the days of the Pharaohs, these natural fiber mats stabilized haul roads during roadway construction.
The natural fibers rooted into the native soil and created a support system that improved road stability. Using geosynthetics allowed the Egyptians to haul heavy building materials across sandy soils and paved the way for the development of geogrid and geotextiles.
Now let&#;s fast forward several millenniums.
Geosynthetics manufacturers began to use advanced technological processes to develop geotextiles that solve the same subgrade support problems the Egyptians faced.
Synthetic fibers are weaved, spun bond and needle punched to produce woven and nonwoven geotextiles.
Then in the late s, Dr. Brian Mercer invented a process that revolutionized the synthetic fabric industry.
In , Dr. Mercer patented an extrusion process called the Netlon process. This new manufacturing method forces molten polypropylene polymers into a plastic net like structure.
This forever changes how nets, fences, packaging products and grids are produced.
Geogrid is born and the Civil Construction industry is forever changed.
Overtime, geosynthetic manufacturers tweak and perfect the process to adjust the strength of geogrid.
The direction of strength determines the classification of grid as either uniaxial or biaxial. Strength also determines the best application for each respective grid.
Even though we now use grid for subgrade stabilization, the first geogrids were used for tie back applications like retaining walls. Nowadays, you would select a uniaxial geogrid for vertical applications such as tie backs for MSE wall construction.
From here on we are going to concentrate only on biaxial geogrids for stabilization.
If you need further information about uniaxial geogrids our sales team will be happy to provide assistance.
In order to explain how biaxial geogrid improves the subgrade structure, we are going to define four important testing properties.
Those properties include:
MD and XMD are the two methods for testing most geogrid properties.
MD stands for the Machine Direction that the polymer moves through while in the extruding machine.
XMD stands for Cross Machine Direction, which is perpendicular to the machine direction.
Because biaxial geogrid must support weight across the entire plane, it is important for it perform well in both the in-plane of axis and cross plane of axis. In other words, in both latitudinal and longitudinal directions.
Aperture is one of geogrid&#;s defining properties. The geogrid&#;s aperture is the opening size of its net-like voids.
The size of the aperture determines how introduced aggregates (soil, stone or sand) will interlock, strike through or slide through the voids in the geogrid.
Junction Efficiency is another important property. This property demonstrates how strong the geogrid is at the cross section of its ribs. This cross section is also known as the node.
Junction strength is tested by the GRI GG2 Test Method, which is an industry standard testing method. During the test, geogrid is pulled at its nodes to determine its ultimate tensile or breaking strength.
Grid&#;s Junction Efficiency is expressed as a ratio of junction strength to strength of the rib. The resulting number is expressed as a percentage.
Minimum Rib Thickness is a distinguishing index property. This property tests for exactly what it sounds: the thickness of the geogrid&#;s ribs.
Let&#;s compare two biaxial Geogrids: TLG-11 and TLG-12
TLG-12 clearly has a thicker rib than the TLG-11. Logically, a thicker rib would be able to sustain a heavier load than a thinner rib. The results for Tensile Strength at 2% Strain, Tensile Strength at 5% Strain, and Ultimate Tensile Strength all test higher for TLG-12 than TLG-11.
Although you can draw this general conclusion, there are many other factors to consider when selecting the best geogrid for your site. Anticipated load, the quality of the subgrade and the type of aggregate are all important considerations that will affect the geogrid&#;s performance.
Lastly, let&#;s look at Ultimate Tensile Strength. This property is tested according to ASTM-D and determines the geogrids resistance to elongation when subjected to a load transfer.
Let&#;s compare TLG-11 to TLG-12 again.
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TLG-12 tests higher for Ultimate Tensile Strength for Machine Direction and Cross Machine Direction.
From these test results, you can generally conclude that TLG-12 has a higher load capacity threshold than TLG-11 and thus can handle greater strain before the subgrade is compromised.
Now let&#;s tie it all together.
If soil is too soft, it will bend and buckle when a heavy load is added. Geogrid stabilizes the soil by confining the compacted aggregate in its apertures. The apertures also allow aggregate to strike through the grid and interlock with the soils below. This increases the soil&#;s tension.
When a heavy load is introduced to the subgrade, the geogrid spreads that load of pressure over the whole surface area of the grid. Because biaxial geogrid has strength in both directions, the load is evenly supported across the surface. As the load is introduced, the layers of aggregate and grid are further compacted and the subgrade is reinforced.
Native soil conditions and the type of aggregate on site are two extremely important considerations that can greatly affect geogrid&#;s ability to stabilize a roadway.
If the native soil is very fine or has a high moisture content, the site may experience &#;pumping.&#;
This means multiple soil types are mixing and weakening the soil&#;s Core Bearing Ratio (CBR). When there is poor soil tension, a load transfer displaces the surrounding weak soils to nearby subgrade, resulting in ruts and low points.
If your site is experiencing pumping, you may need more than geogrid to reinforce the subgrade.
Geogrid provides subgrade support when compacted with the correct aggregate but when very fine and moist soils slide through the grid&#;s apertures, bearing capacity and compaction may be compromised.
To solve this issue, install a nonwoven geotextile fabric below the geogrid.
The nonwoven fabric filters the soil fines and separates them from the geogrid and aggregate base. As a result, the subgrade is further compacted and hydrostatic pressure is reduced.
Although you can install the geotextile fabric and grid separately, some manufacturers produce biaxial geocomposites. These composites have a biaxial geogrid thermally bonded to a nonwoven geotextile fabric. Geocomposites can be appealing because it makes installation of grid and geotextile fabric a one step process.
Geogrid is an integral product for Civil Construction and one that has evolved quite a bit over the years. As a result, there are a lot of options out there.
If you&#;re not sure what grid your site requires, let us help.
We&#;ll review the project plans or make a site visit to make sure you get the product you need.
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 . 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.
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