Written By: Michael J. Dickey, P.E., Samantha Justice, P.E., Bryan Wedin, P.E.

When constructing roadways over soft soils and weak subgrades, geocells are one of the most powerful value engineering tools available to the civil engineering and construction industries today. Understandably, some engineers may be apprehensive about using a geosynthetic product for which they have an incomplete technical understanding. So, if you’ve ever wondered how geocells work in load support applications – and the relationship between lateral confinement and hoop stress – you’ve come to the right place.

Generally speaking, geocells can be used to alter the geometry of a soil pressure bulb beneath an applied load through a phenomenon known as the mattress effect. Key to the mattress effect is a physical mechanism unique to geocells known as lateral confinement. When a load is applied to a geocell-reinforced layer, lateral earth pressures develop within the infill material, which is confined laterally by the cell walls against movement, in turn developing upward shear resistance along wall interfaces throughout the interconnected network of cells. In essence, lateral confinement converts horizontal earth pressures into upward resisting shear forces.

When combined with suitable base reinforcement (i.e., an enhanced woven geotextile), it becomes possible to construct over very weak subgrade materials, including those with standard penetration resistance (SPT-N) values less than 2 blows per foot (CBR < 0.5%), where most planar geosynthetic products, such as geogrids, would otherwise fail.

Now, where does hoop stress come into play, and how does it relate to lateral confinement?

Hoop stresses develop within the cell walls as earth pressures propagate radially in response to an applied load at ground surface. In other words, the same earth pressures responsible for developing interface friction between the geocell and the infill material also result in hoop stresses within the cell walls. Although not perfectly cylindrical, geocells can be envisioned to behave similarly to an interconnected network of pressurized cylinders, wherein hoop stresses are a function of the net pressure that develops due to the internal and external pressures acting within and around each cell.

In this manner, radial pressures that develop within each cell are resisted by those that develop in the adjacent cells, and hoop stresses may be estimated using the classic equation for hoop stress for a pressurized thin-wall cylindrical vessel:

σ_H = p_net*(D/2t)

Where,

σ_H = hoop stress

p_net = net pressure = p_i – p_e

p_i = internal pressure

p_e = external pressure

D = geocell diameter

t = wall thickness

The internal active earth pressure in a cell directly beneath a point load can be calculated using Boussinesq’s point load stress equation. Concerning external, or “passive”, earth pressures in adjacent cells, Emersleben (2009) investigated the interaction between hoop stresses and passive earth resistance in geocell systems and observed that lateral pressures in adjacent cells decrease exponentially with increasing distance from the actively loaded, or “source” cell(s) – in effect, defining a pressure gradient. Based on Emersleben’s findings, it is possible to evaluate the net earth pressure that develops between the interior and the exterior of a cell wall, using the thickness of the cell wall as the distance between two points along the defined pressure gradient line.

Not surprisingly, the largest net earth pressures, and largest hoop stresses, occur in cells directly beneath the perimeter of the load footprint, the wheel contact area in the case of vehicle loads. Based on this, it is possible to estimate the maximum hoop stresses that would be expected to develop in geocells in response to standard AASHTO load conditions.

Accordingly, the table below summarizes the estimated hoop stresses that would be expected to develop under standard AASHTO load conditions in a 6-inch geocell-reinforced layer overlain by 2 inches of aggregate wearing course. The calculations assume a 9.5-inch diameter geocell infilled with coarse sand having an internal friction angle of 32 degrees.

As shown, the corresponding tensile forces that develop under working load conditions are relatively modest due to the lateral confinement effect of the adjacent cells. When compared to the typical yield strength for most high-quality HDPE geocells, the above-referenced tensile forces are well within the elastic region for the material, and would not be expected to undergo any permanent deformation or “creep” over time, even when subject to repeat traffic loads over many years.

With regard to strain, the elastic response of the geocell-reinforced layer will ultimately be governed by the elastic properties of the infill material and provided that suitable granular infill is used, the development of any significant strain in the cell walls will be heavily constrained by the effects of lateral confinement. Because of this, the actual strain that develops in the cell wall will be far less than the amount of strain represented on a typical stress-strain curve generated from laboratory tests such as ISO 10319 or ASTM D4595 where samples are subjected to tensile forces in an unconfined state.

This is not to say that hoop stress is not important. Development of hoop stress is essential for the proper engagement of the lateral confinement mechanism. Moreover, the ability to estimate hoop stresses under specific project circumstances can be useful as it allows designers to develop a preliminary (and very conservative) understanding that tensile forces in the cell wall will remain within the elastic region for the material (with the caveat that many laboratory test methods such as ISO 10319 ignore the effects of confinement, and therefore tend to overestimate strain levels).

In terms of long-term hoop integrity as it pertains to the cell wall (junctions are another matter altogether), dynamic mechanical analysis using a method such as ISO 6721 allows for more accurate characterization of expected material behavior under repeat dynamic loads at reduced strain levels. In general, provided the product is a high-quality HDPE geocell with a flexural storage modulus of at least 116 ksi (800 Mpa) and a 100-year durability rating (ISO 13438), the product can be expected to perform as intended throughout the life of the project.

References:

Emersleben A. et al (2009). Interaction Between Hoop Stresses and Passive Earth Resistance in Single and Multiple Geocell Structures. GIGSA GeoAfrica 2009 Conference, Cape Town 2 – 5 September 2009.

When extreme weather events occur, communities are often left to grapple with the devastating effects. An increase in extreme weather patterns, coupled with aging or inadequate infrastructure, amplifies the often dangerous and costly damage that ensues—especially for vulnerable communities living in low-lying areas.

According to a recent study, the United States could see a 26.4% increase in flood risk by 2050, which could cause significant damage to existing infrastructure. For this reason, it is vital to build resilience into infrastructure projects to mitigate climate risk and ensure the long-term reliability of critical infrastructure.

Incorporating geosynthetics into infrastructure can improve the ability of communities to withstand and recover from extreme weather events. For example, in hurricane-prone parts of the country, designing robust access roads along power transmission lines allows repair crews to safely and quickly restore power to communities that might otherwise be without electricity for days or even weeks. Geosynthetic products, such as geocells, can be used to construct reliable access roads along transmission infrastructure, which often traverse very remote areas with difficult terrain and very soft ground conditions. Through an interconnected honeycomb-like network, geocells confine and stabilize soils that would otherwise be unstable under loading.

When used in load support, slope stabilization, channel protection, and retaining wall applications, geocells are a powerful weapon against the long-term effects of climate change. This article discusses several examples where GEOWEB® geocells were successfully used to help communities adapt to, and recover from, extreme weather events.

GEOWEB Porous Pavements Used for Rebuilding Roads, Replacing Transmission Lines Damaged During Hurricane Michael in Florida’s Panhandle

Hurricane Michael caused extensive damage to Florida’s power grid network, leveling more than 100 transmission towers in a 34-mile right-of-way from Port St. Joe to Callaway. This right-of-way crosses swampy, remote, and hard-to-reach areas, making rebuilding the grid even more challenging. This extremely wet, muddy ground prevented repair vehicles from accessing the area. Helicopters were employed to transport the new steel towers installed on-site.

Accessing the lines for maintenance would require a stronger roadway to support heavy vehicles in the wettest areas. The GEOWEB Load Support System was used to make the roads operational and improve performance in saturated conditions.

The GEOWEB system was placed over an enhanced geotextile and filled with crushed aggregate to create access roads across critical wetlands and stabilize pole pads. The access roads and pads are permanent.

GEOWEB Geocells Repair Storm-Ravaged Trail & Maintenance Road

In the spring of 2018, several storms violently swept through areas along southern Maine’s coastline, devastating the beaches and trails of Fort Foster—a town-owned park in Maine. Known as “nor’easters,” these destructive storms form along the east coast, bringing strong winds, rain, and flooding to the New England states. As the storms rolled past, the damage was visible to Fort Foster and Kittery Point’s 2.1-mile-long shoreline walking trail and maintenance road, as well as on the slopes leading down to the beach.

The park’s goal was to repair the damage and protect the slopes, maintenance road, and recreational trail from future storm damage. The GEOWEB® Soil Stabilization System was chosen to restore and protect two sections of the park’s shorelines and trails.

By using the GEOWEB System, the park was able to armor the maintenance road, recreational trail, and slopes from future storm events. Since being installed in 2018, the GEOWEB Load Support and Shoreline Protection Systems continue to perform as designed, allowing the community to once again enjoy the trail system and local beaches.

GEOWEB Hard-Armored Shoreline Protection System Protects Vulnerable Riverbank from Erosive Forces

The extreme El Niño event caused the western Pacific to warm, developing atmospheric convection and increased rainfall. The storm events caused catastrophic flooding and severe erosion in the eastern equatorial region of Ecuador and Northern Peru. A portion of the Zarumilla River, located in the remote Tumbes region in Northwestern Peru, was experiencing severe erosion and required a shoreline protection solution to prevent further deterioration of the riverbank.

The GEOWEB® Shoreline Protection System was selected to protect the Zarumilla riverbank against future storm events. The GEOWEB system with concrete infill provides economical, hard-armored protection of slopes and channels exposed to high flow velocities and high shear stresses. The system has been proven to withstand sustained flow velocities over 36 ft/s (11 m/s) and shear stresses of 20.9 psf (1.0 kPa), outperforming rip-rap, gabions, and other conventional hard armor strategies.

The product was shipped to the site in September 2020, and installation was complete by December 2020. The project was completed on schedule and within budget. The GEOWEB® Shoreline Protection System is performing to expectations and will provide much-needed protection to the Zarumilla River and the communities that depend on it when the next major storm hits.

GEOWEB Geocells Used for Erosion Protection of Canal Floodwall

Erosion of the 17th Street Canal’s flood protection system was a major concern for the Southwest Louisiana Flood Protection Authority-East. The Authority required a solution to protect the canal’s slope against erosive forces and prevent a floodwall breach in the event of a tropical storm event.

To mitigate channel slope erosion, engineers chose the GEOWEB® Confinement System to stabilize the slopes along the Metairie side of the drainage channel. By confining the infill material, the GEOWEB system prevents flow from causing scour and erosion on slope surfaces.

The construction company installed approximately 380,000 square feet of GW40 (mid-cell size, six-inch deep panels) over a woven geotextile along the canal’s slope and infilled the cells with crushed aggregate sized based on research and testing at Colorado State University hydraulics lab.

The GEOWEB System:

• Allows the use of smaller, less expensive rock—even waste rock which decreases installation and transportation costs.
• Creates a permeable, cover when drainage is desired but vegetation cannot be established.
• Resists high velocities and tractive forces.

Design Support & Resources for the GEOWEB System Applications

The engineering team at Presto Geosystems works closely with engineers and project planners, offering free project evaluation services and on-site support. Our recommendations will deliver a technically sound, cost-effective solution based on four decades of accredited research and testing data.

Please contact our knowledgeable staff and network of qualified distributors and representatives to discuss your project needs today.

How long have you been with Presto Geosystems?

Time flies when you are having fun! I have been with Presto Geosystems going on 14 years. In that time, I got to work with some really great people and enjoyed speaking with Gary Bach on how geocells were invented back in the late 70s and early 80s.

Can you tell us a bit about your background?

I grew up in the Upper Peninsula of Michigan and enjoyed doing all things outdoors. My father was an Olympic ski jumper, so I had no choice but to strap on a pair of skis and follow in his footsteps. I became pretty good and was in three junior Olympics before a bad fall ended that part of my life. I grew up before video games, so we were always outside playing football, baseball or just riding bikes.

After a year at Michigan State University, I attended college at Michigan Tech University in Houghton, MI and graduated with a degree in Environmental Engineering. After graduation, I moved to Green Bay, WI, and worked for Foth & Van Dyke and then Robert E. Lee engineering consulting firms before joining Presto Geosystems.

What attracted you to the world of engineering and geosynthetics?

I was lucky enough to land a wonderful job out of college at Foth & Van Dyke in Green Bay, WI. Foth was a growing consulting firm at the time and the staff was always at the leading edge of technologies. When I started in consulting, geosynthetics were just becoming an option for roadway and earth retention projects. I got to see early on how engineers included geosynthetics in their projects and how the approval agencies accepted them. I guess I’m really showing my age…

What does your job entail? Can you take us through a day in the life as the Chief?

The best part of my job is that every day is different and it seems there are no “typical” days. Our products can solve so many soil stabilization problems that I am blessed to work with engineers all over the world on their challenging projects. It is rewarding to walk away from my computer at the end of the day knowing that the Geosystems Team and our worldwide distribution network work together to make a difference.

What do you like most about your job and/or what do you like most about this industry?

The people and relationship building through the years has been enjoyable. Our industry really works closely to ensure successful geosynthetic projects. I enjoy working on the AREMA, ASTM, and ISO committees and the discussions with the industry experts.

What is the most challenging part of your job?

How do you turn it off? When I first started consulting 35 years ago, we didn’t have the technology we have today. We didn’t even have AutoCAD or personal computers at the time! Everything was done by hand, and I think we understood the calculations better. When you left work, it allowed you to unwind. Today, it seem it is 24/7, especially with the advancements in technology. It can be a challenge to turn it off.

What do you enjoy doing when you aren’t out helping solve the world’s soil stabilization and erosion challenges?

My wife and I stay pretty active, especially in the summers. We live on a golf course, and you will always find us out playing or enjoying a cocktail on the 19^th hole! Family is a big part of our life. We are lucky to have four wonderful kids who have moved out of the house and are employed! They have given us three beautiful grandchildren (one more on the way in December) that we love to spend time with and spoil which is what grandparents are supposed to do.

What is your favorite place in the world to visit?

I have been lucky to have traveled all over the world for leisure and work. A few of my favorites include Austria, Switzerland, Australia, and Banff in Canada. If had to pick one it would have to be Punta Cana since my wife and I have the best of times when we visit. We have so many great memories there. We both have stressful jobs, so it is nice to get away, relax on the beach, enjoy some cocktails and let someone else do the cooking.

If you could meet anyone, living or dead, who would you meet?

This is an easy one but hard to decide which person. I am a huge old western movie fan. It is to the point that if I am watching tv and scrolling through the channels and come across one, my wife just gets up and goes to another tv. I would have to pick my top 20, throw them in a hat, and just pick one and would be fine.

What would you name the autobiography of your life?

Never a Dull Moment.

Rip rap is a common channel protection method because of its resistance to most flows with appropriate rock size; however, it does have significant drawbacks. Material can be expensive, not locally available, and placement requires heavy equipment. Additionally, rip rap channels are prone to regular maintenance, collection of debris and garbage, erosion at boundaries, undermining, and movement.

Naturally vegetated channels are grassed greenways that offer substantially lower maintenance and cost but are limited in their ability to resist moderate-high flows and shear forces even for short durations—unless the soils and vegetation can be stabilized.

High-Performing Vegetated Solution

The GEOWEB® Soil Confinement System offers protection to channels with continuous low flows—as well as moderate-to-high flow intermittent channels. The system’s honeycomb-like network creates check-dams that protect the soil layer from hydrological erosive forces and resulting erosion that impacts unconfined soils. Cell wall perforations lock up with the vegetative root for further stabilization.

The GEOWEB single-layer vegetated channels can withstand ~9 ft/s (2.7 m/s), more than doubling the resistance of typical unsupported vegetated channels (4 ft/s (1.2 m/s). The GEOWEB channels can withstand even higher velocities—as high as 30 ft/s (9m/s)—with an overlying Turf Reinforcement Mat (TRM) when fully vegetated. This is a significant improvement, essentially doubling performance resistance to shear stress and velocities for TRMs and Erosion Control Blankets (ECBs). Rigorous flume testing at Colorado State University (CSU) with varying shear stresses and flow rates substantiated resistance of high shear stresses up to 15.9 lbf/ft2 (77.6 kgf/m2).

Vegetated GEOWEB TRM System Compared to Rip Rap

The GEOWEB TRM System offers advantages over rip rap for vegetated channels:

• Streamlined Transition with Boundaries: Smooth integration with boundaries reduces undermining and erosion at boundary points
• Higher Permeability: Ideal for environmentally low-impact design
• Easier Integration in Landscape Plan: Less obtrusive and can be incorporated as a “soft” solution (i.e., grassed conveyances) with capabilities of some hard-armored systems

Sustainable Solution for Vegetated Conveyances

Sustainable vegetation in channels can be achieved with the GEOWEB TRM system. Applications that benefit include roadside ditches, stormwater channels, shoreline embankments, dams, spillways, and pond overflow systems. The GEOWEB/TRM system is a more sustainable—and environmentally friendly solution than rip rap for these common applications.

Presto offers full design and specification tools and a free project evaluation service to determine applicability.

We have a great team here at Presto Geosystems, so we thought it would be fun to share a bit more about each member through a new blog series. For our inaugural Meet the Presto Geosystems Team blog post, we are thrilled to introduce José Pablo George—Presto Geosystem’s International Business Manager (BDM).

Let’s get to know JP…

How long have you been with Presto Geosystems?

Time slides and blurs in a funny way. It seems so natural for me to be doing what I do with GEO, as if I’ve always been here and can draw deeply on decades of experiences, yet at the same time I still have the positive outlook and optimism of a freshly minted graduate. The year-over-year calendar and the bits of chrome in my hair suggest a compromise: this year marks my eighth year with GEO; call me a well-seasoned new-ish guy.

Can you tell us a bit about your background?

My specialty is in establishing, culturing, and maintaining business relationships, especially with our international clients. My former life and experiences pre-Presto support this well.

I was a Spanish teacher (secondary schools and technical colleges) for 17 years, and have the linguistic skills and cross-cultural skills thanks to that, as well as the “educating adults” part needed for onboarding new network agents for us. Presto initially hired me as a LATAM BDM, and I gradually took over most of the international world from there!

I´ve developed my chops with quality assurance and network training through the State of WI DOT, technical colleges, and Harley-Davidson corporate in their Quality Assurance programs and motorcycle training programs. I´ve studied Verbal Judo, am a legit Black Belt in Tae Kwon Do, and have negotiated labor-management bargaining contracts from both sides of the table. I don´t shy away from tough conversations, and although I´m usually the “quiet one,” my words have strength and I usually deliver them in a way to make them count.

Additional life experiences of growing up with my artist grandfather, working the family dairy farm, building houses from blueprints, solo motorcycle trips across the country, living and working abroad, and working the world´s most famous pottery kilns have all expanded my perspective, and make for interesting stories to share (another time).

What attracted you to the world of geosynthetics?

Initially it was softball and Spanish that attracted me to the world of geosynthetics. The former Geosystems Director and I played on a rec league softball team, and he kept trying to hire me for my Spanish skills. When two other Fortune 500 companies offered me positions (also due to my Spanish skills), I figured a career change merited a closer look. I decided to leave teaching for the corporate world, and am glad to have made the leap. Since starting with Presto, I have obtained my CPESC certification and have learned so much more about geosynthetics and engineered soil stabilization solutions. I´m even published and on patents now!

What does your job entail? Can you take us through a day in the life as an International Man of Myst…umm, I mean International Business Manager?

The day of an International Manager is always a mystery… yet also contains some comfortable stability. Every single day I am talking with contacts from around the globe. My mobile number is my WhatsApp, Viber, Signal, Teams, text, and Messenger number… and I use each of them to reach our agents and clients in the way that is most natural to them. Every day I get to use my Spanish language skills, and I have also learned a bit of Dutch, Setswana, Japanese, Portuguese, French, and German. Most days I am offering design and sales support, sourcing appropriate supporting resources for projects, and doing my part to keep our solutions present in the minds of engineers and clients. Most weeks I am presenting the details of one of our solution sets for a particular application, such as Infrastructure Resilience for Roads and Bridges, Channel Armoring, or Mine Slope Reclamation… sometimes in English, sometimes in Spanish.  Sometimes it means presentations… of whitepapers… to government entities… to International Erosion Control or Geosynthetics groups, and the like. Once upon a time, my job meant lots of travel to meet with clients IRL, not just virtually. I look forward to the travel again someday. Really, my job entails building relationships with the right connections. Sometimes I have to seek them out, sometimes connections find us. Either way, I must develop them into something.

What do you like most about your job and/or what do you like most about this industry?

What I like most about my job is the connection to people and places around the globe. The industry allows me to offer lasting, valuable solutions to real problems. We solve the most challenging soil stabilization problems, and give the world a strong foundation to build on. There is a bit of superhero stuff in that! What´s not to like?

Can you tell us a little about what it’s like working with international business partners?

Ha! Never before have I understood time zone differences and ultra-long project gestation times so well.  I organize my day to speak with different parts of the world at different times… Europe and the UK at the start of my day, LATAM and SADC throughout the day, Asia Pacific and MENA towards the end of the day… it is a lovely variety! Not always so clean cut, but that generalizes it some. I must exercise great patience… as anything international involves extra touches and tending. With all of our international projects and partners, it comes back to building and keeping strong relationships, as well as ease of use, reliability, and high performance. Our partners know we will do what it takes, and with integrity, to help them find success.

What do you enjoy doing when you aren’t out helping solve the world’s soil-stabilization and erosion challenges?

I love spending time in the forest, and am fortunate to live in a region that allows me to experience the changing seasons with unhindered access to the woods. My family and I love to camp, hunt, fish, ski, bike, hike, and enjoy time together outdoors. I hate ticks and mosquitoes, and most other things that try to bite me without my permission… but generally speaking will spend as much time as possible in the woods.

What is your favorite place in the world to visit?

There are so very many places I have yet to see!!! How can I pick just one?!

Tops on my list of having been: Cartagena, Colombia; Granada, Nicaragua; La Rioja, Spain; the Frankfurt area of Germany; and the Parque Manuel Antonio in Costa Rica.

Soon-to-be favorites include Paris and the wine country of France, and the Amsterdam area of the Netherlands.

An eventual favorite trip will be a summer camping trip to the Denali area of Alaska… and to Banff, Alberta, Canada. Maybe back to the Patagonia area of Argentina, this time to fish and not just for work… and to other parts of Japan beyond the Tokyo area, including Sapporo, Osaka, and Fukuoka.  So many adventures yet to be had!

If you could meet anyone, living or dead, who would you meet?

This is a tough one. At this juncture in time, if I had to pick just one person I´d like to meet, I would ask Keanu Reeves to accompany me for a long walk and talk at a dog park with our pets. He is also an International Man of Mystery, a “cool breeze over the mountains” that is aware of his celebrity status yet is generous and careful with his presence. I admire the way he sticks to his principles and manages to stay cool and look at the bright side, with kindness, humility, and gratitude. He models being a good human in a way the world needs right now. If Keanu turned me down, I would ask Steve Martin to share his mad banjo skills and a few jokes with my son, or Shakira to sing with my daughter.

What would you name the autobiography of your life?

The Next Adventure Awaits…