In our on-demand webinar, Flexible Design Solutions for Resilient Dams and Spillways, Sam Justice, P.E., Civil Engineer for Presto Geosystems, explores how the GEOWEB® Soil Stabilization System helps address the unique challenges of dam and spillway applications through effective erosion control, slope protection, and load support solutions. The presentation highlights real-world design considerations, project applications, and strategies for improving infrastructure performance in demanding hydraulic environments.
To continue the conversation, our engineering team has compiled answers to some of the most frequently asked questions from the webinar. Read on for expert insights into how flexible design approaches can enhance the resilience, adaptability, and sustainability of dams and spillways for decades to come.
How do you know the aggregate roadway thickness is being reduced?
The GEOWEB System is used to reduce aggregate base thickness in road construction by improving load distribution and confinement. Depending on subgrade strength and loading conditions, reductions typically range from about 30% to 60%, with greater savings achieved on weaker soils (CBR < 3). This is possible because the geocell system increases the effective stiffness and shear resistance of the aggregate layer, allowing it to perform like a thicker unconfined section while using less material.
What thickness of GEOWEB Geocell material is commonly used for unpaved roads?
For unpaved roads, the cell depths are based on subgrade strength and loading conditions, with most applications using between 4-inch and 8-inch panels. We do not recommend standard tables or graphs since all projects have different site conditions. We provide free project evaluations no matter what the size of the project.
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Can on-site materials be used instead of engineered stone?
Yes. The GEOWEB System allows the use of on-site or low-quality materials as infill by providing confinement that improves their strength and stiffness. Instead of relying on high-quality crushed aggregate, soil such as sand, gravel, recycled materials, or even low-quality fills can be used effectively. The geocell structure limits lateral movement and distributes loads, enabling these materials to perform like higher-quality aggregates while reducing material costs and minimizing the need for import.
How does the GEOWEB System perform in cold weather?
The GEOWEB System performs well in cold weather environments due to the inherent properties of virgin HDPE, which remains flexible and durable at low temperatures without becoming brittle. We have been installing the GEOWEB System in extremely cold weather over the past 40 years with great success.
What happens if water freezes or shoves around the system?
Ice around the GEOWEB Geocell System, including ice shove, generally does not damage the material itself since HDPE remains ductile in cold temperatures. The main impact is from ice forces on the surface, which can displace weak infill, but confinement helps hold material in place. Aggregate or concrete infill performs best under these conditions, while freeze-thaw cycles may cause minor ground movement that the system helps distribute and control. As with any system, periodic inspection (e.g., in spring) is recommended.
Are the ATRA® Speed Stakes strong enough?
Yes. ATRA Speed Stakes are made from a special blend of HDPE. The speed stakes have undergone extensive laboratory testing to confirm strength and pull-out resistance. The speed stake is fast-to-install and has excellent pullout resistance due to the flanged arms and stake barbs. ATRA Speed Stakes are desired in wet or corrosive environments due to resistant properties. Long-term durability reduces lifetime maintenance costs and ensures the system will stay functional for the lifetime of the project.
What subgrade preparation is required?
Minimal preparation of the subgrade is needed and should consist of clearing vegetation and unsuitable materials and grading the surface to the required lines and drainage. A geotextile may be included if separation or reinforcement is required, depending on subgrade conditions.
What about very soft subgrade soils (CBR’s < 1%)?
For extremely soft or saturated soils, the project evaluation and preparation become more important. The surface should still be leveled as much as possible, but the GEOWEB System performs well in these conditions by creating a stiffened load transfer layer. Overall, the goal is to stabilize the subgrade enough to support installation while allowing the system to bridge soft areas and minimize rutting and excessive deformation.
Please remember that each project is site specific and the project evaluation will include the design subgrade of the system.
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What is the maximum slope angle the system can handle?
Once slope angles exceed roughly 70°, the system is no longer considered slope protection and is instead treated as a GEOWEB earth retention or retaining wall system, requiring full geotechnical design for internal and global stability. Ultimately, the allowable slope angle is governed by factors such as infill type, subgrade strength, hydraulic conditions, and anchorage capacity rather than the geocell alone.
Does infill material affect slope performance?
Yes. The GEOWEB system infill has an impact on slope performance because it directly controls shear resistance, erosion resistance, and overall stability. We have completed extensive hydraulic testing at Colorado State University hydraulics laboratory and have developed design software for different infill types (topsoil, aggregate, and concrete). We will work with you to determine the most appropriate infill type based on site specific conditions.
Is a geotextile required under the system?
No, a geotextile is not always required beneath the GEOWEB system, but it is commonly recommended depending on site conditions. When the subgrade soil is weak, fine-grained, or prone to pumping and contamination, an enhanced woven geotextile will provide separation, filtration, and reinforcement, improving overall system performance. However, if the subgrade and infill materials are similar, stable, and well-draining, the geotextile may be omitted to reduce cost without compromising performance. The decision should be based on subgrade strength, soil type, and long-term stability requirements.
When should geotextiles be avoided?
Each project is evaluated individually, and a geotextile is commonly recommended. It is very important to understand the subgrade including strength, composition, and hydraulic conditions. Geotextiles can be deleted when the subgrade and infill materials are similar in gradation and composition, meaning there is little risk of contamination or migration between layers.
We will work closely with the customer to determine whether a non-woven or enhanced woven geotextile is required. For projects with high fines and groundwater, we may recommend a heavy weight, non-woven geotextile and sand layer, which may provide a better solution. Each site is different and will be treated independently.
Can asphalt be installed over the GEOWEB system?
Yes. Asphalt can be installed over the GEOWEB Geocells, and we recommend a 2-inch wear surface to create a smooth, uniform base. This configuration provides improved load distribution and subgrade support while preventing surface irregularities or cracking in the asphalt, resulting in a stable and durable pavement systems.
Can concrete be placed over the system?
Yes, similar to asphalt, a wear surface should be applied. In some applications, such as load transfer slabs, parking areas, and slope/channel protection, the GEOWEB cells can be filled with concrete.
What is the role of the GEOWEB system under pavement?
The role of GEOWEB geocells under hard surface pavements (asphalt or concrete) is to provide load support, confinement, and stiffness improvement to the base layer, particularly over weak subgrades. By confining the infill aggregate, the GEOWEB system creates a semi-rigid mattress that distributes loads laterally, reduces vertical stresses on the subgrade, and minimizes rutting and differential settlement. This improves the effective modulus of the base course, allowing for reduced base thickness or extended pavement life.
Where is paving over the GEOWEB system commonly used?
Typical applications include access roads, dam and spillway roads, airports, temporary or permanent roadways.
Conclusion
We hope these answers have provided additional clarity on the topics discussed during the webinar. If you would like to learn more about GEOWEB Geocell solutions for dams, spillways, and other hydraulic structures, watch the full on-demand webinar.
If you have a project in mind, our engineering team can provide site specific evaluations and design support at no cost.
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Have more questions? Reach out, we’re always happy to help you find the right solution.
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