This article is the second in a three-part series about gravel grids:
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In this article, we go in depth about the pros and cons of using gravel grids.
In our research, many manufacturers made similar claims about gravel grids. We included some of these claims and a few observations of our own.
Gravel grids add structural support to gravel surfaces, preventing issues like gravel migration and rutting.
Typically gravel surfaces must be top-dressed with new gravel and/or regraded periodically. Gravel grids hold gravel in place, preventing the migration of the top layer of gravel laterally (sideways) as well as down into the gravel base or subgrade. Rutting is prevented as a result. These benefits lead to longer-lasting surfaces and lower maintenance costs.
Gravel grids can reduce the amount of gravel needed on a project and the depth of excavation required.
Gravel grids combined with gravel can have the same structural strength as a deep layer of gravel. For example, one manufacturer claimed that 1″ of gravel contained in their gravel grid system is equal in strength to a 3″-4″ of gravel layer. Other manufacturers made similar claims.
As a result, less gravel may be required to build the base and fill the grid when compared to a traditional gravel surface. Less gravel means less excavation, leading to saved time and money.
Gravel grids can create a permeable driving surface.
Permeable surfaces allow water to drain through and seep back into the water table rather than draining off the sides of the project area. Commercial gravel grids can even help control stormwater infiltration and create water storage.
Gravel grids can maximize the amount of driving surface on a property without running afoul of paving restrictions.
Some municipalities restrict how much impervious (non-draining) paving can be done on a property. Gravel grids can skirt this issue by providing permeable surfaces that are also drivable. This can eliminate the need for large, expensive drainage systems or retention ponds.
Some gravel grids meet accessibility standards.
The ADA defines an accessible surface as “stable, firm, and slip resistant.” Some brands have test data to demonstrate how their system meets this standard.
Gravel grids can give patios, walkways, and driveways a neat, uniform appearance.
Grids typically require clean crushed rock (gravel without fines). This results in a cleaner, more appealing look than a gravel surface built with minus crushed rock (gravel with fines).
Gravel grids can reduce driveway dust.
Because the cells are usually filled with a clean gravel, less fines may become airborne during dry weather.
Gravel grids may be more “green” than asphalt or concrete.
Some manufacturers claim that gravel grids are a greener alternative to asphalt and concrete, pointing to the use of recycled or recyclable plastics; the longevity of their materials; and the projected lifespan of the completed surfaces compared to typical gravel roads. The permeable nature of gravel grids also reduces the footprint of the paving surface. Additionally, some manufacturers claim that their systems naturally filter water before it passes back into the water table.
Gravel grids require precise installation.
While some manufacturers claim that installing gravel grids is quick and easy, the reality is that the installation process takes time and attention to detail. Excerpts from the GeoPave and Gravelpave² installation guides are shown below to give insight into the installation process.
One Seattle-based landscaper who works with a rolled mat product said, “A lot more care goes into installing it.”
The basics of installation for each grid system are the same: the project area must be excavated, leveled, and compacted. Then most systems require a gravel base to be installed, leveled, and compacted. The gravel grid system is laid out on top of the gravel base and filled with rock.
Some systems require additional steps, including the following:
With all of these steps in mind, you can see how installing a gravel grid can be time-consuming and labor-intensive.
Most gravel grids require permeable rock products, which are typically more expensive per ton than minus rock products.
A typical gravel driveway is built with one or more layers of minus rock products—for example, 1¼” minus for the base layer and 5/8” minus for the top layer.
On the other hand, most grid systems are built with a base layer of clean gravel, ranging from ¼” clean to 1½” clean. The top layer, aka the infill, is usually a clean gravel ranging from 1/8” to 1½” clean.
Clean rock products are generally more expensive due to the processing and resources required to remove the fines. Some customers may also choose to use a decorative gravel for the infill (e.g., salt and pepper granite, marble chips, rainbow rock), which will be even more expensive than a construction-grade gravel.
Gravel grids can be expensive, especially for large projects.
Of the brands we surveyed, prices ranged as followed:
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Keep in mind that these prices don’t include the cost of accessories like clips, stakes, edging, and geotextiles. These prices also don’t include shipping or tax, potential labor costs, or potential equipment or tool rentals.
To give a better of idea of the true cost of each system, we created a chart that compares each brand’s cost per square foot with the preparation and materials required.
To get a more accurate picture of the total cost of each gravel grid system, it’s important to examine the installation guide of your chosen product to determine all of the materials and tools you will need. You can use our guide to estimate the volume of gravel you’ll need.
Gravel grids are low-maintenance—but not no-maintenance.
In order to hide the cell structure completely, an extra layer of gravel must be spread on top of the grid, a process known as top-dressing. This means that there will be some degree of loose gravel migrating on the surface that needs to be swept back in place and/or topped off regularly.
Although gravel grids prevent rutting and potholes, these issues can still occur. Cells may need to be topped off with gravel occasionally. In the case of potholes, sections of grid must be emptied and removed to address problems with the gravel base or subbase.
You will also need to clear debris like leaves and trash to preserve the permeability of the surface.
Lifting and buckling can occur when a product is pushed past its limits.
Gravel grids may lift out of place, buckle, or warp when a product is pushed past its limits. For example, a local dealer told us that his clients used a light-duty product but drove heavy equipment on it, resulting in the product lifting and becoming wavy.
Improper installation can cause similar issues.
Not all products have the data to back up their claims.
One thing became clear when conducting research for this article: robust products cost more money. There are many knockoff products on the market that are much cheaper than the competition. But these brands may lack installation guides, test data, and customer support. They may also have issues with material composition and construction.
Here are examples of concerning issues we found:
The moral of the story: make sure the data backs up the claims.
This article does not represent an endorsement of any brand and is meant for educational purposes only. There is a possibility of errors in the data. If you notice any errors, please send us a note through our contact form.
Check out other helpful articles on our blog:
No. Geogrid strength does not correlate to in-ground performance, which has been demonstrated in multiple studies (Watts et al., & Giroud & Han, ). The ability of the geogrid to interlock and confine the aggregate subjected to traffic loading is fundamental to deliver enhanced performance of the mechanically stabilized layer (MSL). This enhanced performance can only be measured through full-scale in-ground trafficking testing. Geogrid characteristics such as rib shape, aperture shape, polymer type and material structure configuration, rib thickness, in-plane stiffness, junction shape, geogrid-aggregate compatibility and many other factors are contributors to the amount of interlock and confinement achieved, and therefore, enhanced performance of the MSL.
No, they don’t. Aperture shape and size are key parameters to allow interlock between the particles of the granular fill material and the geogrid. However, other geogrid properties such as the rib height, rib width, rib aspect ratio, rib material, geogrid in-plane stiffness, and relative location of the rib with respect to the center of mass of the granular fill particles are key to delivering improved performance of the mechanical stabilized layer (MSL). Having the right shape of rib is also fundamental to the performance of the system.
Geogrids must have a minimum tensile strength to make sure the product is not damaged during the installation and compaction of aggregate. However, no correlation has ever been shown to exist between tensile strength and the performance of geogrids used for roadbed applications. This has been demonstrated in multiple studies (Watts et al., & Giroud & Han, ).
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No, InterAx geogrids work differently than TriAx or BX geogrids because of InterAx’s unique structure, its wide range of aperture shapes and sizes, and the interactive outer layers. By having a geogrid structure with multiple aperture shapes and sizes, particles of different types of fill materials can interlock and interact more efficiently with InterAx. This geogrid/particle interaction is enhanced by InterAx’s interactive outer layers, which conform to the shape and angularity of the particles of the fill material. These characteristics enhance the load distribution onto the subgrade, mitigating subgrade deformation and early failure of the mechanically stabilized layer (MSL). Lastly, since different types of geogrids have different structures, material characteristics, and performance properties, geogrids are not interchangeable without revising the design.
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Biaxial geogrid was invented by Tensar in the late s. Biaxial geogrids have square or rectangular openings, called apertures. Different types of biaxial geogrids exist on the market, but for roadbed applications, testing has consistently shown that integral geogrids made by a process known as punched and drawn perform best. In , Tensar introduced multiaxial geogrids, known as TriAx. These geogrids have triangular apertures and ribs with a higher aspect ratio than biaxial geogrids. This geometry provides better interlock with the surrounding aggregate than biaxial geogrids, resulting in improved performance in stabilization applications. In , Tensar introduced InterAx geogrids. InterAx builds on the decades of knowledge and testing at Tensar to deliver a higher level of performance. Through thousands of hours and millions of dollars of research and development, we combined advanced materials science, a revolutionary new geometry, extensive performance validation testing, and an entirely new design approach to deliver better solutions that are more economical, longer lasting, more resilient, and more sustainable.
Geogrids are polymeric (plastic) construction materials used to confine and stabilize granular fill materials to increase their load distribution capabilities while reducing its potential to deform under load. Geogrids have openings called apertures, which allow for rock particles to strike through. This action is known as particle interlocking, and it allows for rock to become confined within the aperture.
Geotextiles are construction materials made from woven or non-woven polymeric (plastic) fibers. They can be used to provide filtration and separation, which restrains soil from mixing with adjacent materials due to dynamic forces. Geotextiles cannot confine granular fill materials, since there are no apertures to achieve particle interlocking.
Unless you are an expert in pavement design, it may be very difficult to evaluate all of the information that may be submitted to you – let alone find time to do so. Unscrupulous suppliers often try to take advantage of this fact by providing large quantities of information, even though it is not sufficient to show equivalency and may not even be relevant to the application. You can reduce the confusion by writing a project specific specification that includes the information outlined in the checklists we linked above – depending on the application (subgrade stabilization or designing a pavement).
Yes. There are different design methods and inputs for each design method. The allowable amount of permanent deformation also likely differs for both of these applications. They require different testing, and have significantly different failure criteria. Specifics on what to look for when comparing testing for subgrade stabilization and paved road design can be found in the FAQ below.
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Performance is not based on index properties of the geogrid, measured in a lab. It is determined empirically, using full-scale in-ground trafficking testing. Request a copy of the design inputs and the design method which were used as the basis of equivalency. Also ask for the full-scale testing, with the specific product being submitted (not another “similar product”), and documentation from an independent authority verifying that the testing and calibration were performed properly. Once you have this information, you will be able to compare performance related sections and evaluate equivalency.
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Performance is not based on index properties of the geogrid, measured in a lab. It is determined empirically, using full-scale in-ground trafficking testing. Require a copy of the third-party testing on which the supplier is basing its claim of equivalency. This testing should include Accelerated Pavement Testing (APT), performed on paved sections, following the procedures defined by NCHRP report 512. There should be multiple sections tested over different subgrade conditions and different pavement section thicknesses. Require a third-party review, by pavement design experts, verifying that the design values and methodology used are supported by testing for that product, and follow the guidance of AASHTO in R50-09 (“Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures”). Also require third-party field validation testing, following AASHTO T221-90 (), to demonstrate that the results obtained in the field support the assumptions used in the design methodology, and that deformations are in line with predictions.
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