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Energy Infrastructure and Climate Change: Protecting Erodible Slopes in Fire-Prone Areas

Posted by Katie Bocskor

Energy infrastructure is critical to the functioning of modern societies, and its protection against natural disasters and environmental threats is a top priority. Climate change exacerbates these disaster risks, with extreme weather conditions and wildfires being of particular concern, considering potential damage to the energy infrastructure and disruption of energy supply. Wildfires cause rapid, severe destruction, and, aside from damage to infrastructure, can impact our climate, vegetation, and atmosphere.

To measure the size and impact wildfires have, scientists use observations from several low Earth-orbit satellites, including the Copernicus Sentinel-3. These tracking satellites gather shortwave-infrared data combined with other techniques to differentiate between burned areas and other low reflectance covers such as clouds. The European Space Agency (ESA) compiles that long-term dataset to analyze global fire trends. According to the ESA, fire affects an estimated four million square kilometers (1.5 million square miles) of Earth´s land each year [1]. That is 400,000,000 hectares (990,000,000 acres) yearly—about half the size of the United States of America, an area larger than the country of India. The United Nations Environment Programme (UNEP) Rapid Response Assessment on Wildfires compiles findings from over 50 experts from research institutions, government agencies, and international organizations around the globe, and confirms that “wildfires are growing in intensity and spreading in range” with an “annualized economic burden for the United States to be between $71.1 billion and $347.8 billion USD” [2].

As wildfire seasons become longer and more extreme, the level of response must be increased. The United States and Canada have renewed an arrangement of cooperation to provide mutual aid during wildfire emergencies [3]. This, as Canada expects the 2023 wildfires, their worst on record, to continue into the winter season, with nearly 18 million hectares (44.5 million acres) burned to date through September 2023, as registered by the Canadian Wildland Fire Information System (CWFIS) [4].

In California, about 4.5 million hectares (11 million acres) have burned in wildfires over the past seven years, with an average of 485,623 hectares (1.2 million acres) per year [5]. The University of California´s Agriculture and Natural Resources (UCANR) Fire Network has studied post-incendiary erosion trends and offers a series of videos and other resources for wildfire and post-wildfire management. UCANR confirms the “chance for erosion is significantly greater and can result in mass movements of soil and water if vegetation has been burned off…with steep, hilly areas especially vulnerable” [6].

Project Case Study: Protecting Energy Infrastructure

The project owner had recently completed facility renovations and needed to repair and protect the slopes and hillsides surrounding the facility. However, the project owner had attempted other vegetated and non-vegetated erosion control methods in the past, and due to recent wildfires, drought cycles, and heavy erosion, those previous efforts did not achieve the desired level of long-term protection. The project owner and their contractor sought to meet initiatives from the U.S. Department of Agriculture (USDA) Forest Service for land management practices designed to mitigate erosion before and after severe fire events, and in evaluating possible solutions, the GEOWEB® Geocell Slope Protection System was identified for further evaluation and analysis. GEOWEB Geocells are three-dimensional cellular confinement products made from strips of high-density polyethylene (HDPE) welded together to create an expandable honeycomb-shaped structure. The geocells confine the soil or aggregate fill, minimizing movement and migration of the embankment materials by functioning as anchored containers in the upper soil layer. The geocell system resists sheet flow, preventing severe erosion and controlling rill and gully formation, especially in erosive post-incendiary soils.

Accordingly, the GEOWEB slope protection system was determined to have a high probability of achieving project objectives, and the facility owner ultimately selected the GEOWEB System utilizing tendon-based anchorage and ATRA® Tendon Clips (load transfer devices) in lieu of conventional staking techniques. The repair areas comprised two hillside areas of approximately 6,500 m2 (70,000 sf), with varying slope angles from 40-100°, and varying vertical heights from 6 m to 34 m (20 ft to 110 ft). After analysis of the site parameters, calculations for appropriate anchorage, planning for proximity to and around energy structures, it was determined that the 15 cm (6 in) GEOWEB Geocell with load transfer devices, integral connectors, high strength tendons, and earth anchors provided proper anchorage of the system without interference with existing infrastructure, as shown in Figure 1 below.

Fig. 1. Installation of tendoned GEOWEB Geocell System at energy facility.

The actual rock size chosen for the infill was based on slope angle and site hydraulic conditions, with earth anchor pullout strength determined by the Engineer of Record based on the manufacturer´s recommended factor of safety and site soil conditions. Aggregate infill was placed from crest to toe, using a rock slinger to assist workers and limiting the drop of the infill to prevent geocell wall distortion, as seen in Figure 2 below.

Fig. 2. A rock slinger helps workers place infill on steep slopes.

The tendon-anchored system with aggregate infill satisfied the needs of the project owner´s installation for long-term performance, burn protection, erosion control, and low maintenance while offering flexibility of fit without interference with the newly installed energy infrastructure.

GEOWEB Cellular Confinement System Options and Benefits

The GEOWEB Cellular Confinement Systems (CCS) offers a broad range of surface protection treatments for slopes that are subject to erosive forces. The inherent flexibility of the system, combined with a variety of adaptable anchoring techniques, permits the application of soft or hard armoring techniques to steep slopes. By ensuring the long-term stability and effectiveness of slope cover materials, underlying soils are protected, and customizable aesthetic objectives can be achieved. When slope reclamation and revegetation is desired, the geocell system provides the ability to fully vegetate slope surfaces that could not otherwise support plant life, with appropriate anchorage (based on specific site conditions) to hold the system to slope [7].

The GEOWEB walls, which contain the topsoil infill in a vegetated system, form a series of check-dams extending throughout the protected slope. Normal rill development, produced when concentrated flow cuts into the soil, is prevented since flow is continuously redirected to the surface. This mechanism also disrupts flow velocity and hence the erosive force of runoff. A predetermined depth of topsoil and the developing vegetative root mass is contained and protected within the individual cells. Roots become intertwined with the perforated cell walls, thereby creating an integrated, blanket reinforcement throughout the slope surface. In arid regions, it has been observed that the GEOWEB Geocells can enhance the development of indigenous vegetation by retaining a higher proportion of available moisture in the near-surface soil zone.

When vegetation is not appropriate or desired, aggregates or concrete infill may also be used for GEOWEB slope protection, stabilizing and protecting the surface. Aggregate infill reduces environmental impacts by allowing water infiltration on the slope face, reducing sheet flow runoff, and by precluding the need for irrigation systems, particularly in drought-prone areas, as might otherwise be required to maintain a vegetated slope cover. In this particular energy project installation, it was also chosen as a burn-protection zone around the newly installed energy infrastructure.

Protection of Energy Infrastructure Against Extreme Weather Events & Wildfires

The protection of energy infrastructure against extreme weather events and wildfires becomes increasingly more challenging as climate change exacerbates those threats. The National Park Service and the California Department of Forestry and Fire Protection (CAL FIRE) have started to collaborate with Indigenous communities to return traditional burning to the land as a wildfire prevention method [8]. Local tribes have helped to set prescribed burns in Yosemite National Park, among other wooded areas, as preventative protection against damaging wildfires like the Oak fire that burned over 8,000 hectares (20,000 acres) west of Yosemite National Park as shown in Figure 3 below.

Fig. 3. Oak Fire near Mariposa, California, photo courtesy NPR and David McNew/AFP via Getty Images.

In addition to prescribed burns, erosion control solution sets must perform uniformly with a balance of properties for robust and resilient performance, as highlighted in the Overview of Resilience Concept by Bruneau et. al [9]. Resilience in infrastructure includes qualities that help reduce vulnerability, minimize the consequences of threats, accelerate response and recovery, and facilitate adaptation to disruptive events. All of these are likely to be expressed as essential for the solution sought in pre- and post-incendiary erosion control, especially for energy sector infrastructure.

Within the resilience framework, the concept of robustness presents unique opportunities for innovative solutions such as the high-quality GEOWEB Geocells to be integrated into infrastructure designs and contribute toward achieving infrastructure resilience goals. Redundancy to maintain functional requirements in disruption occurs with the use of aggregate infill, serving as both fire protection and a permeable hard-armoring stabilization of the slope. The resourcefulness of the GEOWEB Geocells with tendons and ATRA Tendon Clips with specific engineering values allows resources to be mobilized in an effective manner, without interference to existing energy structure. The rapidity in response of The GEOWEB Geocell System allows infill to be placed and the slope to be stabilized quickly, achieving project goals in a timely manner with reasonable labor and equipment inputs from contractors.

GEOWEB Geocells should be considered an industry best practice option for slope stabilization and a solution for pre- and post-incendiary erosion control on hilly, dry terrain prone to wildfire, drought, and erosion. The type of system as shown below in Figure 4 is an appropriate option for permanent and resilient erosion control at similar energy infrastructure sites around the world.

Fig. 4. Aggregate infill of geocell system at energy facility.

References

Transforming Transportation Infrastructure: Protecting Road and Bridge Embankments with Geocells

Posted by Ashley Beyer

workers installing geocells on highway slope

In a rapidly changing world, maintaining and improving our transportation infrastructure’s resilience and sustainability has become a critical concern for civil engineers. Climate change and increasing frequency of natural disasters present an ongoing challenge to the durability of our infrastructure. In the context of road and bridge embankments, protecting these structures can be of paramount significance to the safety and welfare of the public. These structures are often subjected to fluctuating environmental conditions, heavy traffic loads, and must be able to withstand major storm events to protect embankment materials from soil washouts and the long term damaging effects of erosion. So how can civil engineers meet these growing demands without compromising sustainability or longevity? Increasingly, engineers are turning to geosynthetic solutions, such as the GEOWEB® Soil Stabilization System—a low-maintenance and eco-friendly solution for long-term protection of road and bridge embankments.

In many cases, the GEOWEB Geocells offer a flexible, durable, and environmentally responsible alternative to traditional construction materials that can accommodate a wide range of infill materials, including soil, aggregate, or concrete, to establish hard or soft armor, as necessary, for protection as well as aesthetics. As we explore the capabilities of the GEOWEB Geocells, we will find that this solution not only addresses some of today’s most pressing infrastructure challenges but also paves the way toward a more resilient and sustainable future.

Improving Bridge Resilience with the GEOWEB Geocells at the I-90 Mississippi River Bridge

Bridge stability relies heavily on the long-term integrity of its abutments. Any vulnerability or weakness in these components can lead to structural failure with a potentially disastrous outcome. Therefore, prioritizing the design and construction of resilient abutments is crucial to ensure the stability and longevity of bridges.

In 2013, the I-90 Mississippi River Bridge, or the Dresbach Bridge, underwent a significant reconstruction project led by SRF Consultants. The aim was to replace the old bridge on the Wisconsin/Minnesota border and improve the interchange between Highway 61/14 and I-90 to enhance traffic safety and provide better access for motorists.

The GEOWEB System played a crucial role in two different applications. The GEOWEB sections (4-inch-depth, mid-sized cell) were utilized on slopes directly beneath the bridges and around structural supports, ranging from 2H:1V to 1.5H:1V, with heights up to 45 feet. These sections, filled with aggregate, were custom-produced in a tan color selected to blend in with the local aggregate color, improving visual aesthetics. Standard black GEOWEB sections  (6-inch-depth, mid-sized cell) were used around the bridge abutments on slopes varying from 2.5H:1V to 3.5H:1V, reaching heights of up to 48 feet. These sections were filled with topsoil and covered with an erosion control blanket to support vegetation.

dresbach bridge geocell installation

To secure the GEOWEB sections to the slopes, 18″ ATRA® Anchors were employed, with the anchor pattern adjusted to suit different slope characteristics, section depth, and infill material. The Presto Geosystems engineering team provided calculations and recommendations for anchor spacing in each of the 11 application areas.

The implementation of the GEOWEB Soil Stabilization System in the Dresbach Bridge project was completed in the fall of 2016. This versatile system continues to provide robust slope protection, offering benefits for both aesthetics and long term project resilience. The Dresbach Bridge project serves as a testament to the versatility and effectiveness of the GEOWEB System in achieving slope stabilization and erosion control objectives around a vital piece of U.S. transportation infrastructure.

Fortifying the Roadway Embankment along River Road in Lewiston, Maine

Roadway embankments are another critical component of transportation infrastructure that face similar challenges. Erosion, washouts, and landslides can all lead to road failure, posing significant safety risks and disruptive repair costs. To protect against these risks, the GEOWEB Soil Stabilization System adds a valuable layer of protection. The three-dimensional honeycomb-like structure confines infill materials and protects slopes from sheet flow runoff, washouts, and shallow translational failures that can occur during and following major storm events.

River Road in Lewiston, Maine, faced a similar challenge due to its steep 1:1 slope and heavy traffic, including constant truck flow. To ensure the long-term safety and integrity of the road, the River Road Reconstruction project was initiated, which included pavement reconstruction, shoulder widening, and construction of a stabilized vegetated slope, among other upgrades.

before and after installing geoweb geocells vegetation

One significant hurdle was constructing a 45-degree vegetated slope, with the risk of erosion heightened during spring seasons due to rainfall and snowmelt. To maintain the slope’s stability and the road’s integrity, the GEOWEB Vegetated Slope Protection System was chosen for soil stabilization and erosion control.

The system’s installation process utilized geogrid lifts for enhanced slope stabilization. The 3D cellular GEOWEB system, combined with a tendon system, held the topsoil in place on the steep slope, enabling sustainable vegetation growth and mitigating severe erosion risk.

The GEOWEB system was filled with rocks at the slope’s toe for a strong foundation, with larger rocks on top for added support. Moving upwards, cells were filled with topsoil and covered with a turf reinforcement mat to promote grass growth, demonstrating the pivotal role of the GEOWEB geocells in providing stability, soil confinement, and vegetation support.

A Step Forward in Sustainable Infrastructure

In the face of modern infrastructure challenges, civil engineers need solutions that are not only resilient but also sustainable. GEOWEB Geocells provide a dynamic response to these demands. They offer significant benefits in terms of durability, adaptability, and environmental consciousness, making them an optimal choice for modern bridge and roadway embankment projects.

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Dam Structure Safety Installation and Repair Using Advanced Geosynthetic Technology

Posted by Ashley Beyer

Written By: Samantha Justice, P.E.

scenic view of damDams and Spillways Are a Critical Part of U.S. Infrastructure

With estimates of 91,804 structures nationwide, dams and spillways are essential for controlling flooding, water distribution, and providing hydroelectric power. However, these structures cannot last forever. The average age of dams and spillways in the U.S. is now 61 years​ (USAFacts)​​, significantly over the typical 50-year lifespan of these structures. Aging infrastructure can lead to serious consequences if safety precautions are not taken or measures are not implemented to address identified problems promptly. Continual inspection and upkeep are crucial for any dam manager.

The 2021 Infrastructure Report Card by the American Society of Civil Engineers rated the condition of U.S. dams with a “D” grade, highlighting the pressing need for repairs and maintenance​ (Home)​. State and federal regulations provide a framework for assessing and maintaining dam and spillway structures, requiring at least yearly audit inspections to identify areas needing repair or replacement. Performing these repairs can help extend the lifetime of dams, maintaining essential services without excessive costs or increased failure potential.

Understanding Areas of Concern for Existing Structures

The vast majority of America’s rivers and lakes have existing dams and spillways, and as such, very few new structures are being built. With new construction, safety measures can be incorporated during the design phase to extend the lifetime of the project and help prevent failures. The true threat is with existing structures that have gone past their intended lifetimes or have seen areas of potential failure.

A recent example of the potential for catastrophic damage due to a dam failure is the 2017 Oroville Dam crisis in Northern California. Extremely heavy rainfall over a number of days raised the level of Lake Oroville, increasing the flow over the main spillway to above-average levels.  Almost immediately, damage was observed in the lower half of the spillway, with a large section of the concrete path collapsing. The emergency spillway was utilized to help prevent further damage to the main spillway, however, excessive erosion occurred to the emergency spillway path, and emergency repairs were subsequently required to address damage in both spillway areas. Further damage occurred when more rainfall increased the lake level yet again, including blocking the downstream river and requiring the immediate shutdown of the Oroville hydroelectric power plant. Luckily, total collapse of the dam did not occur, but more than 180,000 residents of the Feather River Basin were required to evacuate for multiple days, and over the next year, more than 1,000 people worked more than 2 million hours to rebuild the spillways to ensure the safety of downstream communities.

With the passage of the 2021 Infrastructure Investment and Jobs Act, states will have access to funds to complete repairs and upgrades of aging dams and spillways before failure can occur. The failure at the Oroville Dam was preceded by rejection of a 2005 upgrade proposal to build a concrete emergency spillway that could have handled the high water flows seen in the 2017 event. Re-evaluating existing structures to ensure that they are still able to withstand 100-year and 500-year flood events is crucial to the longevity of the dam network within the US. Maintaining both upstream and downstream dam faces and spillways is an ongoing process, fighting against wave action and erosion, as well as any potential impact damage caused during storm events. Even simple maintenance of roads and work pads over dams can have a lasting effect on the health of these structures by allowing workers access to inspect and repair the structures quickly and easily.

GEOWEB® Geocells Are a Repair Solution for Dams and Spillway Sites

GEOWEB geocell technology is a versatile geosynthetic system that can be used to create long-term solutions for many of the common dam and spillway problem areas. Geocells function as the support structure for unpaved roadways, capable of supporting maintenance and repair vehicles. They also function as surface erosion control solutions, preventing the formation of rills or the collapse of unstable soils due to water flow, wave action, and storm events.

charleroi dam geocells

GEOWEB geocells can be placed on the upstream face of a dam structure to mitigate the effects of wave action on the dam, supporting existing riprap areas, or replacing them entirely with vegetation, gravel or concrete. The flexibility of the GEOWEB system allows for the use of mixed infill materials, such as topsoil above normal water levels for grass growth and small aggregate below the water level for erosion prevention. Comprised of high-density polyethylene (HDPE), GEOWEB geocells are formulated for long term durability to resist weathering, chemical attack, and ultraviolet radiation, and are therefore suitable for use in applications where the material will be subjected to cyclic wetting and drying, permanently submerged, or full sun exposure. The material is not prone to degradation or corrosion due to environmental factors, and can be placed on the downstream face of, or within, a spillway structure. The system is also compatible with concrete infill to accommodate extremely high flow velocities. For comparison, Table 1 summarizes allowable velocities and shear stresses for various channel lining alternatives.

Comparison of allowable velocity and shear stress for channel lining alternatives

In emergency spillway areas, topsoil infill with vegetation can be used to allow for a natural camouflaged look, while still preventing erosion and uncontrolled water flow, and outperforming traditional unreinforced channel lining alternatives.

Staging areas and maintenance roads are also integral parts of a dam site, and when necessary, these features provide vital access and adequate ground support for vehicles and heavy equipment to perform inspections, routine maintenance, and repairs. The GEOWEB system can be used in a variety of load support applications, including unpaved access roads, laydown areas, and parking lots. Reinforcing these roads means significantly reduced maintenance requirements, reduced rutting, and access to areas that might otherwise be unable to support heavy loads due to soft soil conditions. Minimizing stresses on top of dam structures is critically important to preventing the formation of cracks or slumps within the structure that could lead to failure. The GEOWEB road system can be integrated with the slope protection system on the upstream and downstream faces of the dam for a continuously protected berm from water, vehicle, and impact loads.

mud lake dam geocells

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.

Related Articles and Case Studies

Mud Lake Dam Rehabilitation
Olivenhain Dam Power Line Access

References

United States Department of Agriculture, Natural Resources Conservation Service, (2007) Part 654 Stream Restoration Design, National Engineering Handbook, Chapter 8, Threshold Channel Design, (viewed 23 March 2022 and available https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17784.wba as a link directly to Chapter 8). Table 8-11 “Allowable velocity and shear stress for selected lining materials” referenced from 8-37.

Colorado State University, Engineering Research Center (2009) Hydraulic Testing and Data Report for GEOWEB 30v with Concrete, research summary courtesy Presto Geosystems, (viewed 23 March 2022 and available Colorado State University Testing PDF)

Green Retaining Walls Protect an Advanced Wastewater Treatment Plant from a 500-Year Flood Event

Posted by Ashley Beyer

GEOWEB Green Retaining Wall

Flood Protection Plan

To meet federal requirements for flood mapping of levee-protected areas, a levee reconstruction project for the Indianapolis Southport Advanced Wastewater Treatment (AWT) plant along Little Buck Creek was part of a more extensive Deep Rock Tunnel Connector project—one of the largest combined sewer overflow projects for the City of Indianapolis.

The project included plans to protect the Southport ATW plant and wastewater-processing pond from a 500-year flood event from an adjacent creek and river. To accomplish this, a wall system designed on the creek side of the levee would have to maintain a narrow profile to increase the water capacity of the creek.

A Natural Erosion Protection Solution

Flood events and high water flow from the adjacent creek caused significant toe erosion of the levee embankment along the north side of the wastewater treatment plant. The AWT required a long-term soil stabilization solution to combat erosive forces from Little Buck Creek’s varying depths and flows. The creek flows as low as a 1-foot depth with velocities of 3 feet per second (fps) to as high as 8 fps with a depth of 12 to 15 feet during a flooding event.

The project engineer preferred a wall system with native vegetation along the levee that would be robust enough to withstand erosive forces from the creek. They chose the GEOWEB® Vegetated Retaining Wall System to reduce the environmental impact, protect the levee from scour and erosion, and satisfy regulatory requirements.

Construction of the Levee Wall

levee wall being constructed with geocells

Working within a limited footprint to maintain a narrow profile, the engineer designed the GEOWEB Retaining Wall as a gravity structure. Installers filled the back cells of the GEOWEB System with aggregate to promote drainage and placed a mixture of topsoil and #2 stone in the front outer cells to support vegetation and provide stability and resistance to soil loss during larger storm events.

Wall Dimensions & GEOWEB Green Wall Attributes

  • Wall length: 1,500 feet; Wall height: 5 to 12 feet
  • Open fascia cells allow infiltration of stormwater.
  • Green fascia panels blend with natural environment.
  • Flexible wall performs well in soft soil environments; conforms well to a waterway’s geometry.
  • The GEOWEB HDPE material is unaffected by water contact.

Performance Update

Since its installation in 2012, the GEOWEB green wall continues to provide vital protection to the Southport ATW plant and wastewater-processing pond from major storm events. Significantly more economical than the U.S. Army Corps of Engineers’ (USACE) conventional riprap solution, the GEOWEB walls are a practical alternative for levee applications.

With native vegetation, the GEOWEB levee wall proved to be an attractive solution that effectively minimized environmental and permitting impacts.

Living Green Walls + Low Impact Development

An attractive alternative to MSE block walls or riprap, walls built with the GEOWEB geocells offer a green aesthetic and low-environmental impact approach to designing retaining walls. The GEOWEB Retaining Walls conform well to landscape contours, are resistant to environmental degradation, and install 25-30% faster than MSE block walls. The GEOWEB System also offers design flexibility for a variety of wall configurations, including gravity, reinforced, and steeped slopes.

Create Strong, Long-Lasting Mechanical Connections Using the New ATRA® Wall Key The new ATRA Wall Key is the most effective device for connecting the GEOWEB Retaining Wall sections, ensuring the long-term success of your project. Made of non-reactive, chemically inert high-density polyethylene, the ATRA Wall Key provides a more secure and permanent mechanical connection over staples or zip ties, and they are the only geocell connector specifically designed for use in exposed wall face applications.

The innovative ATRA Wall Key includes an integrated washer at the base of the handle to provide coverage of the I-slots, frictional barbs for an improved interlock with the GEOWEB sections, and an ergonomic handle with S-shaped contouring for ease of installation.

Formulated to withstand weathering and ultraviolet radiation, the ATRA Wall Keys will not corrode or photodegrade, even when exposed to harsh environments. Securing sections with the ATRA Wall Keys is faster than using staples or zip ties, requires no tools, and can be completed by one installer with one easy turn.

Design Support for Retaining Walls

Presto Geosystems offers fast and easy-to-use resources and tools for designing GEOWEB Retaining Walls. Let our Design Engineering Team prepare a complimentary project evaluation for your next project. We also offer free licenses for our GEOWEB® MSE Design software for retaining wall applications.

 

Creep is not a factor for geocell load support

Posted by Katie Bocskor

Written by: Bryan Wedin, Chief Engineer

An accurate understanding of creep resistance is essential to proper material selection when using polymers, and in the case of geocells, this science is being misapplied.

The definition of creep deformation is “the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stress.”

This potential failure mode creates fear and uncertainty among designers wherever the possibility of creep factors exists. Yes, creep can occur in almost all materials including plastics, metals, and concrete. In cases such as bridge and building design, it is important to properly understand creep factors and account for creep in engineering calculations. However, in the case of designing with geocells for load support, creep factors have no relevance.

What causes creep?

In order for creep to occur, two factors must be present:

1) A constant load applied to the area and

2) A sustained deformation of the geocells.

Creep only applies when there is a sustained load on a material for an extended period. In a case of repeated on- and off-loading, this type of deformation would be governed by fatigue, not by creep, because it is not a constant applied load.

The second required factor for creep to occur is an ability to undergo sustained deformation of the material. When a polymer has a load applied, the molecules of the material start to pull apart and stretch. This leads to elongation of the material in one direction and typically results in a thinning of the material’s thickness.

Creep not a factor in load support

Now, consider a geocell load support application. The geocell material is expanded to cover the project area on-site and then an infill material is placed into the cells. At this point, there is not an applied load or deformation occurring in the material.

Next, the infill material is compacted. This compaction applies a load to the cells, but this load is removed as soon as the compaction equipment is no longer positioned over the cells.

As an individual cell is loaded, it exerts an outward force while each of the adjacent cells pushes back on it (passive resistance) and prevents any sustained deformation. Therefore, at the time of compaction, there is neither a constant load nor is there a sustained deformation. Thus far, the material is successfully installed without any creep effects.

 

After the geocell load support system has been installed, the two types of loads that will affect the system are driving (live) and stationary (parked) loads. When a vehicle drives over a geocell system, the load is applied vertically. As the geocell distributes the load laterally, there is a temporary load applied to the geocell material. The load is not a sustained load, and therefore, would not have a creep effect.

In the case of stationary loads, the load is continually applied to the geocell, so it meets the first criteria for creep. Due to the pressure from all adjacent cells surrounding the loaded cell(s), there is no ability for the cells to move enough to have any appreciable sustained deformation. Therefore, creep cannot affect this scenario.

ASTM D6992 creep test is not applicable

Those who make claims about creep potential in geocell load support applications have cited ASTM standard methodology as the reasoning for concern.

ASTM standards provide an accepted means for standardizing testing to be able to directly compare products. It is important to review the intention and scope of a test to ensure that it is appropriate and will give relevant results. The Stepped Isothermal Method (SIM) is used to accelerate creep testing. ASTM D6992 uses SIM to predict the expected deformation of geosynthetics over time when used for reinforcement applications. This method can be effective; however, it is not suitable for a polyethylene geocell evaluation.

ASTM D6992 5.3 Note 1 states, “Currently, SIM testing has focused mainly on woven and knitted geogrids and woven geotextiles made from polyester, aramid, polyaramid, poly-vinyl alcohol (PVA), and polypropylene yarn and narrow strips.”

Additionally, the note continues with a warning against expanded scope of the test saying, “Additional correlation studies on other materials are needed.” While this test has applicability for geogrids and geotextiles, the test is not intended for evaluating geocells and correlations for polyethylene have not yet been established.

Further, D6992 cannot be considered in isolation.

D6992 states, “Results of this method are to be used to augment results of Test Method D5262 and may not be used as the sole basis for determination of long-term creep and creep-rupture behavior of geosynthetic material.”

This reinforces the importance of reviewing each test standard to ensure that the product is within the scope of the test and that the results are relevant and complete. In the case of geocell evaluation, using ASTM D6992 is inappropriate as it has not been properly correlated to provide accurate evaluation of polyethylene and without ASTM D5262, it provides an incomplete overall evaluation of the product.

HDPE’s long history of success and repeatability

High Density Polyethylene (HDPE) has been used as the industry standard material for geocells since it was invented over 40 years ago. HDPE has been extensively researched by independent scientists across multiple industries, allowing for a comprehensive understanding of its performance capabilities. Using a virgin HDPE material allows for direct verification of resin consistency through laboratory testing to ensure that each manufacturing location and production lot have consistent material performance. This laboratory verification also allows for the comparison of the material to independent scientific results and does not rely solely on manufacturer’s claims.

Challenges with Fabricated Inelastic Blend (FIB)

A few geocell manufacturers are promoting a Fabricated Inelastic Blend (FIB) to cut manufacturing costs and increase material stiffness utilizing recycled and other unpublished polymer materials. These FIB-based materials can vary widely, even for the same product. Due to the vast number of combinations possible with FIB materials, they pose two key problems when included as a material choice: validation and consistency.

Because of the unpublished nature of the blending mixture there is no way to validate this material in comparison with published testing. Any testing of FIB materials must start from the beginning without any experience to rely on for long-term performance.

The second concern with FIB materials is controlling consistency of the blend. Because each FIB blend is so variable, there is no way for a third-party tester to fully determine consistency of the blend between different manufacturing plants or even between different production lots. This inability to determine consistency creates uncertainty because there is no way to determine if there has been improper blending or changes to material blend.

Manufacturers using FIB materials promote the advantages of increased material stiffness. This stiffness is often a function of multiple generations of recycling. It is important to review the differences between elastic and inelastic materials and how they affect geocell performance. An elastic material is able to undergo a deformation (strain) and then spring back to its original state without permanent (plastic) deformation.

Conversely, an inelastic material does not undergo immediate deformation, but rather ends in catastrophic (complete) failure. Many of engineering’s worst failures have resulted from catastrophic failures of inelastic materials that were subjected to unexpected loads. This absolute nature of inelastic failure puts projects at great risk because it does not give indication prior to collapse. With elastic materials, as material limits are reached, the material will stretch and yield prior to complete material failure. This yield zone allows for changes to loadings prior to catastrophic failure.

The mobilization of soil strain and geocell strain occurs from not only deformation of infill, but deformation of subgrade materials. For reasonable ranges of geocell stiffness, subgrade deformations will cause the strain to occur in the geocell system. For a given strain stemming from subgrade deformation, a stiffer geocell will realize larger tensile stresses, both in its wall and especially at the seam, which will result in seam rupture and system failure.

The material stiffness is not the most critical point in geocell performance. It is a combination of stiffness, tensile strength, seam strength, and passive resistance of adjacent cells. Published data shows that strain in geocell cell walls is on the order of less than 0.5 to 1.0%. At such low strains, the effect of creep should be ignored for all practical purposes. Properties such as seam strength, strip flexibility, environmental stress cracking resistance, and passive resistance from adjacent cells play a much larger role than stiffness of the material. Also, at these strain rates, HDPE (including virgin mixes, most recycled and other polymer alloy geocell) stress-strain behaviors are similar.

In load support applications, loadings are transient and quite small due to the stress-distributing behavior of the pavement material and geocell mattress effect, which further compounds the irrelevance of creep in reinforcement applications. Thus, overall system strength is not related to any performance factors that are tied to creep. Therefore, the focus should be on how much strain is mobilized in geocells.

Some manufacturers have examined theoretical 2% to 5% strain rates, both of which are far above actual field conditions, and therefore, not applicable. Five percent strains in load support applications are not applicable since compacted granular materials fail at much lower strain rates. At 2% strain, granular materials would fail within a geocell, as well as outside of the confined system, due to significant rutting and deformation. The geocell material would not be the failure point, and a stiffer geocell would not affect the failure of granular materials at 2% strain.

Geocells used in load support applications prevent high strains from occurring due to the very nature of the geometry, its confining behavior, and passive resistance of adjacent cells. Evaluating geocell material strength beyond reasonable strain values is not relevant for subgrade reinforcement as it contradicts actual measured data and represents conditions outside of practical design and application. The basis of comparison should focus on stress-strain behavior at very low strains – less than 1.0%. This information is readily available from large-scale laboratory tests, field tests, and numerical modeling.

True HDPE Performance vs FIB Results

FIB materials bring a new uncertainty to the geocell market. These materials are of unverifiable composition so connecting material to performance is nearly impossible. Ultimately, these FIB materials beg your trust in their performance touting their unnecessary creep resistance. They hide the truth that creep resistance comes at a cost – inelastic material that can fail catastrophically at the seam. FIB material prioritizes a single material property of the geocell at the expense of a uniformly designed system with measurable material consistency and applied field testing.

After 40 years, HDPE continues to be the industry standard material for geocells. Presto Geosystems proudly pioneered the use of HDPE material in its GEOWEB® Geocell products due to proven performance and reliability of that material.

Over these four decades, GEOWEB Geocells have been used in load support projects worldwide without a single failure due to creep effects. Although this consistent performance is impressive, it is not surprising given that creep forces are irrelevant in these applications.

Advancing Rail Resilience: How Geosynthetics Help Achieve CRISI Objectives for Robust and Stable Infrastructure

Posted by Ashley Beyer

train along track stabilized with geoweb geocells

Discover the Latest CRISI Rail Infrastructure Funding Opportunities: Apply Before the May 2024 Deadline

 

The U.S. Department of Transportation is bolstering rail infrastructure advancements through the Consolidated Rail Infrastructure and Safety Improvements (CRISI) program. With a recent allocation of $2.47 billion, the CRISI program is set to significantly impact rail safety, efficiency, sustainability, and reliability across the United States.

This funding initiative is designed to support a variety of projects that are pivotal to enhancing the nation’s passenger and freight rail systems. It represents a call to action for rail industry professionals, including engineers, planners, and project managers, to leverage this opportunity to advance their rail infrastructure projects.

The deadline for application submissions is 11:59 p.m. ET, May 28, 2024. Professionals in the rail sector are urged to prepare their proposals that align with CRISI’s mission to improve the rail infrastructure’s overall landscape.

For a comprehensive overview of the application process and to assess project eligibility, stakeholders are encouraged to review the Fiscal Years 2023-2024 Notice of Funding Opportunity (NOFO) available through the CRISI program. This funding presents a pivotal chance for those involved in rail infrastructure to gain the support and resources needed to propel their projects forward.

The GEOWEB® Soil Stabilization System (Geocells): A Proven Solution for Rail Infrastructure

Mainline Ballast Reinforcement

geoweb geocells installed for mainline ballast reinforcement

The GEOWEB Rail Ballast Stabilization System stands out as an innovative solution for addressing ballast stabilization challenges, creating a more resilient and stable layer underneath the track. The 3D geocellular system yields unparalleled performance and construction benefits, surpassing the capabilities of 2D methods like planar geogrids or Hot Mix Asphalt (HMA), especially in areas with soft subgrades.

The performance of the GEOWEB system is backed by extensive research and rigorous field testing at renowned institutions such as TTCI and Oregon State University. It has demonstrated its ability to reduce settlement and track displacement under the strain of heavy freight loads on soft subgrades, and has already been adopted for use in railway track beds by international authorities in other advanced nations, such as Network Rail in the United Kingdom, with their recent published guidance on “The Use of Geocells in the UK Railway Track Bed”. Additionally, SmartRock testing by the University of Kansas revealed significant reductions in ballast abrasion, movement, and rotation, as further evidence the life of the ballast can be extended when the right geosynthetic product is incorporated into the project design.

Bridge Approaches, Crossings, Diamonds: Ballast Reinforcement in High-Stress Areas

Areas like bridge approaches, diamonds, turn-outs, and crossings face immense stress and usually require a lot of upkeep. The GEOWEB Soil Confinement System helps lower the need for maintenance in these challenging spots. It strengthens the ballast layer, reduces movement and deflection, and cuts down on maintenance in these crucial transition zones.

GEOWEB Geocells: BABA-Approved

Last year, the White House provided guidance on the Build America, Buy America (BABA) initiative. BABA specifies certain products must be manufactured in the United States to qualify for federal funding under the IIJA.

Selecting the GEOWEB System for enhanced track stabilization allows projects to achieve improved resilience and longevity, ensuring compliance with the standards set by the CRISI program, the Infrastructure Investment and Jobs Act, and Build America, Buy America. Presto Geosystems is ISO 9001 certified, and the GEOWEB Soil Stabilization System is 100% U.S. made. (A copy of our Certificate of Registration can be provided upon request.)

Need Assistance with Your Rail Projects?

Presto Geosystems offers free project planning support for all GEOWEB Geocells applications in rail projects. Our experienced engineers are ready to assist with project evaluations to ensure your project’s success from start to finish. If you’re dealing with challenges related to soil stabilization or looking for innovative track stabilization solutions, please reach out to us.

Request Free Project Evaluation

White House Provides Clarification on Build America, Buy America (BABA)

Posted by Ashley Beyer

truck on partially infilled geoweb geocellsThe White House released guidance on the Build America, Buy America (BABA) initiative, an important component within the $1.2 trillion Infrastructure Investment and Jobs Act (IIJA) from 2021. BABA stipulates that certain products must be manufactured in the U.S. to qualify for federal funding in infrastructure projects and emphasizes the use of domestically produced construction materials.

As the faucet opens for IIJA projects, make sure your project has certainty and you are building with quality materials you can trust, 100% made in the USA.

BABA Highlights:

  • Scope: The BABA guidelines apply to federally funded infrastructure projects, including those under the IIJA.
  • Material Categories: BABA focuses on three primary categories: iron and steel products, manufactured products, and construction materials. Notably, the list has been expanded to include engineered wood but excludes coatings, paint, and bricks based on feedback.
  • Made in America Criteria: To wear the “Made in America” badge, a product must be produced in the U.S., with at least 65% of the cost of its components sourced domestically. This will further increase to 75% in the calendar year 2029.
  • Included Materials: The guidance specifically lists plastic and polymer-based products, non-ferrous materials, glass, fiber-optic cable, engineered wood, drywall and lumber.

Implications for Infrastructure Development

For manufacturers involved in infrastructure projects, these guidelines carry weight. The inclusion of polymer-based products, in particular, sheds light on the growing importance of innovative geosynthetic solutions in federal projects.

With BABA’s focus on polymer-based products, the GEOWEB® Soil Stabilization System offers a reliable solution for project stakeholders looking to utilize proven, U.S.-made geosynthetic products that align with federal directives.

Ascertaining Whether a Manufacturer Meets BABA Requirements

As the industry begins navigating this new terrain, project stakeholders can conduct their own screening-level due diligence to confirm if a specific product is manufactured in the U.S.

For example, one approach would be to determine if the manufacturer holds an ISO 9001 Certification, and if so, request a copy of their Certificate of Registration. The Certificate of Registration will list the address of the manufacturer’s production facility, and it will also identify which specific products are manufactured at that location.

We are pleased to share that Presto Geosystems is ISO 9001 certified, and that the GEOWEB® Soil Stabilization System is 100% U.S. made! (A copy of our Certificate of Registration can be provided upon request.)

 

sign up for presto geo p3 portal Request Free Project Evaluation

 

 

Geocell Technology Proves Effective in Solving Soil Stabilization Challenges for Solar Farms on Underutilized Lands

Posted by Ashley Beyer

photo of solar panels on solar farm with blue sky and clouds background

With the increasing demand for clean energy, there is a growing interest in repurposing underutilized lands for solar farm developments, particularly abandoned mines, capped landfills, brownfields, and other unused areas. These locations offer a unique opportunity to transform unused spaces into sources of renewable energy, and can be particularly enticing because they are often situated near established transmission infrastructure. This makes the interconnection process simpler and more cost-effective than connecting to remote greenfield sites. In addition to contributing to the shift toward sustainable energy sources, the development of solar farms on underutilized lands can create jobs, generate revenue, and bring new life to areas that have been neglected or forgotten.

However, poor soil conditions can pose significant challenges for solar farm developers. To ensure the long-term success of solar projects, factors such as erosion control, stormwater management, and site access must be carefully considered during the design and construction phases, especially when repurposing underutilized lands for solar farm developments where the site conditions may be less than ideal.

GEOWEB® Geocells: A Versatile Site Development Solution for Solar Projects

Geosynthetics, specifically geocells, can be highly effective in mitigating the challenges posed by poor soil conditions during the development of solar farms. By reinforcing the soil and providing a stable base for access roads and balance of system (BOS) components, geocells can help distribute loads evenly and prevent soil erosion. Geocells can also be used to improve stormwater management, drainage, and filtration, ensuring that the solar farm site remains stable and functional in wet conditions.

Proper planning and execution, including the use of geosynthetics, can contribute to the long-term success of solar projects, reducing maintenance costs over time and minimizing environmental impact. In this article, we will discuss two projects that utilized the GEOWEB geocells in the development of solar farms.

Building a Solar Farm Site Access Road Using GEOWEB Geocells

installing geotextile and geocells for solar site access road

Residents of Brandywine, Maryland, recognized the benefits of redeveloping a closed quarry site into a community solar farm. However, poor soil conditions made it extremely challenging for crews and machinery to access the site for construction and future maintenance.

The EPC contractor for the project contacted Presto Geosystems and local material supplier Colonial Construction Materials to devise a solution that would meet their needs. To support heavy equipment during the construction phase and to ensure the required bearing capacity for emergency vehicles in accordance with local and state regulations in the long term, they opted for the GEOWEB® Load Support System with a vegetated infill to construct a permeable access road leading to the solar farm.

With the on-site support of Colonial Construction Materials, crews deployed the GEOWEB geocells over a non-woven geotextile to construct a geosynthetic-reinforced foundation layer for the unpaved road. The geocells were then infilled with a mixture of on-site material, imported stone, and topsoil to build a vegetated roadway capable of supporting heavy vehicle loads.

The GEOWEB geocells afforded the EPC contractor and project owners the ability to beneficially reuse on-site material to reduce imported material volumes, thereby offering a significant savings to the overall project construction costs. Moreover, using a permeable access road instead of a paved road provided the added advantage of decreasing the overall impermeable surface area at the site, in turn reducing runoff and associated stormwater management requirements, and bringing even more savings in terms of both up-front and long-term operations and maintenenance costs.

GEOWEB Slope Protection System: Protecting Solar Developments Against Major Storm Events

The Spotsylvania Solar Farm, a massive 617 megawatt utility scale solar farm covering 6,350 acres, posed unique erosion protection challenges that required a permanent stabilization solution. A sloped area leading into one of the larger detention ponds on the site was experiencing severe erosion due to concentrated stormwater flows.

Following multiple unsuccessful attempts to stabilize the surface using conventional erosion and sediment control practices (including hydroseeding, sod/staples, turf reinforcement mats (TRMs), etc.), the contractor opted for the GEOWEB Slope Protection System, citing cost and performance as the major determining factors. The GEOWEB system cell walls allow water to flow throughout the system while holding the soil in place, preventing soil loss and gullies.

The GEOWEB system (mid-size cells, 6-inch depth) was successfully secured over the 2:1 slope utilizing TP-225 tendons (woven polyester, 5100 lb. break strength) anchored to a buried deadman pipe and fastened to the cell walls using the patented ATRA® Tendon Clips – which provide twice the pull-through strength of any other tendon-based load transfer device. The ATRA Tendon Clips lock into the GEOWEB cell wall for the most secure connection on the market, and together with the tendons, can be preassembled at the top of slope prior to expanding for fast and easy installation.

After installation, the slope was hydroseeded and covered with a straw-coconut erosion control blanket. The GEOWEB® 3D Slope Protection System provides a structurally stable environment for topsoil and sustainable vegetation through a structured network of interconnected cells. The 3D GEOWEB system confines and reinforces the vegetated upper soil layer, and over time, will facilitate root mat entanglement with cell wall perforations, even further increasing system resistance to erosive and sliding forces.

first photo shows geocells installed on slope leading down to pond. second photo shows the same area vegetated with grass

The 3D GEOWEB system at the Spotsylvania Solar farm has held up to multiple high-intensity rain events, including the remnants of Hurricane Ian, which impacted the region with heavy rain and storms in September of 2022. The system will continue to provide robust erosion protection against similar major storm events in the future, allowing the Spotsylvania Solar Farm to generate reliable power for the local community.

Design Support & Resources for the GEOWEB System Applications

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

 

 

A Week of Celebration and Inspiration: Engineers Week 2024

Posted by Ashley Beyer

engineers week image

“Welcome to the Future!”: Engineers Week 2024

From February 18 to 24, 2024, the engineering community will come together to celebrate Engineers Week. This year’s theme, “Welcome to the Future!”, is a nod to the incredible advancements that have been made and a look forward to the innovations yet to come. It’s a week to celebrate, reflect, and inspire the next generation of engineers.

The Roots and Relevance of Engineers Week

Initiated in 1951 by the National Society of Professional Engineers (NSPE), Engineers Week has grown into a global celebration. It acknowledges the vital role engineers play in progressing our society. The week aligns with the birthday of one of history’s great engineers, George Washington, who was also a surveyor. This connection underscores the deep roots and enduring impact of engineering in our world.

Why “Welcome to the Future!” Matters

This year’s theme emphasizes the forward-looking essence of engineering. It’s not just about honoring past achievements; it’s about shaping the future. Engineers are instrumental in developing innovative solutions to some of the world’s most complex challenges, from climate change to advancing technology in renewable energy and communications. This week is an opportunity to showcase how engineering keeps us moving forward, turning today’s dreams into tomorrow’s reality.

Inspiring the Next Generation

A core aspect of Engineers Week is inspiring young people to explore engineering. With activities like Introduce a Girl to Engineering Day and various educational outreach programs, Engineers Week aims to spark curiosity and passion in the minds of potential future engineers. By showcasing the diverse and impactful careers in engineering, the week helps to cultivate a more inclusive and innovative future for the profession.

Innovating for the Future: The Role of Tools in Engineering Progress

During Engineers Week 2024, with its forward-looking theme “Welcome to the Future!”, we’re reminded of the importance of innovative tools in shaping the engineering landscape. The Presto Geo P3 Project Planning Portal is one such tool, designed to support engineers in navigating the complexities of modern project planning. It reflects our commitment to facilitating collaboration and enhancing efficiency in geosynthetic engineering projects. As we celebrate this week, our gratitude goes out to the engineering community worldwide, whose dedication inspires us to develop resources like the Presto Geo P3 Portal. It’s through collective efforts and shared tools that we can look forward to a future where engineering continues to achieve new heights.

Using Geosynthetics to Stabilize Soils in a Harsh Environment

Posted by Katie Bocskor

By Dhani Narejo, PE, Bruno Hay, and Bryan Wedin, PE

Mine Site Erosion Problems

One of the largest nickel mining sites in the world is located on the South Pacific island of New Caledonia. Due to the size of the mining project and the terrain of the site, significant cut-and-fill work for civil engineering structures was unavoidable.

Mine Site Erosion

FIGURE 1: A typical progression of erosion at one of the slopes.

Given the magnitude of the site, the challenge of safeguarding the structures against erosion is formidable. Inaction is not an option due to the sensitive nature of the structures, environmental concerns, and a keen desire by the owners to protect the environment. A typical example of the erosion at the site is the slope in Figure 1. Such slopes require continuous maintenance if the erosion problem is not addressed. In some cases, erosion can cause interruption in the mobility of materials and personnel at the site.

Several erosion-control measures had been successfully used at the site, including riprap and concrete. An alternate erosion control system was desired by the owner that would meet the following objectives:

  • Be cost-effective,
  • Require little or no maintenance,
  • Utilize local labor and materials,
  • Have a design life exceeding 50 years.

Soil, topography, weather

FIGURE 2: A simple representation of ultrabasic soil profile in the island.

Ultrabasic soils cover about one-third of New Caledonia, where large deposits of nickel are found. Peridotites and serpentines–the parent rocks of these soils–formed 1.5-65 million years ago during the Tertiary period.

The chemical weathering of these rocks over thousands of years and subsequent erosion have resulted in a soil formation of the general nature shown in Figure 2. Ultrabasic soils are rich in iron and magnesium, yet are deficient in nutrients to support vegetation. These soils are fragile in structure and easily erodible, especially when the dense vegetation at the surface is disturbed by fires, mining, or construction activities.

The topography of the site is generally hilly and mountainous. Slopes vary continuously from steep to gentle and from fully vegetated to barren. There are numerous water runoff features on the island. There are large areas of unstable soils and mass movement as shown in Figure 2. As a result, soil erosion is a challenging engineering problem in this region.

The weather pattern is cyclonic, with a single cyclone dumping up to 800mm (31 in.) of rain within 24 hours. Significant rainfall from at least three major events has affected the island during the past 50 years.

Tropical Cyclone Anne dropped 714mm (28 in.) of rain within 24 hours in 1988. In 1969, Tropical Cyclone Colleen recorded 214mm (8 in.) of rain in 4 hours. In January 2011, Tropical Cyclone Vania brought a rainfall of 50mm (2 in.) per hour for several hours. The rainfall intensity for a 6-hour, 100-year storm is on the order of 400mm (16 in.) in this region. The annual number of cyclones can range from 2-10.

Table 1 presents the 10 wettest storms recorded on the island (through 2010).

The unstable nature of the soils, together with the hilly terrain and cyclonic weather, presented unique engineering challenges for the soil erosion problems.

Sustainable Solutions

FIGURE 3: Gravel used as the infill in the geocell

The contractor, having installed liner systems at the site, maintained a long and successful relationship with the mining company and was well aware of the challenges associated with protecting the slopes from erosion in this environment.

The owner suggested the potential of geocell applications to develop a conceptual solution to the erosion problems. The solution involved covering the slopes with geocells, three-dimensional structures made of high-density polyethylene (HDPE), designed to contain and stabilize infill material.

The recommended infill material consisted of a byproduct waste aggregate from the mining operation. A nonwoven, needle-punched (NW-NP) geotextile separation layer was also recommended. Figures 3 and 4 present the proposed gravel infill and the geocell, respectively.

FIGURE 4: Expanded and connected geocell sections partially infilled

The owner accepted the contractor’s proposed solution as a more cost-effective answer than previous methods. The geosynthetic solution would require little to no maintenance during the effective design life and was visually appealing.

The proposed gravel infill was available as a waste material at no cost. The installation could be performed by local labor with little technical support and training by the manufacturer. However, the owner required that an independent design engineer prepare a design for the proposed solution.

The primary design considerations included:

  • Minimum thickness of the geocell,
  • Veneer stability,
  • Type of the separation geotextile,
  • Hydraulic response during a storm, and
  • Infill procedures.

Due to length constraints for this article, only the thickness and veneer stability are discussed here. Important design conditions for the site related to thickness and veneer stability included:

  • A maximum slope angle of 45 degrees,
  • A 6-hour probable maximum precipitation of 39mm (1.5 in.),
  • A Maximum slope length of 20m (65.5 ft), and
  • Presence of clay soils.

The geocell thickness was the most challenging factor during the design phase because of the long slope lengths and steep angles. As the thickness of the geocell increased, the driving force due to the infill weight increased, which led to higher anchorage requirements.

Alternatively, as the geocell thickness was decreased, more water could penetrate the clay soil, which could potentially jeopardize the effectiveness of the geocell system. After a detailed analysis, a geocell thickness of 100mm (4 in.) was selected to provide effective coverage and minimize anchorage requirements.

The anchorage requirements are explained with this veneer stability equation:

Where FS = factor of safety against veneer instability, Cr = required anchorage (kPa), h = thickness of the geocell (m), β = slope angle (degrees), δ = geotextile-subgrade friction angle (degrees).

A factor of safety of 1.4 was used, which is typical for slope stability analysis. The friction angle between the geotextile and underlaying site clay was base on GRI Report #30 (Koerner and Narejo, 2005). Figure 5 provides the relevant figure from this report.

A friction angle of 28 degrees was used in the calculations. Density of gravel, γ, was 20 kN/m3. Slope angle, β, varied from 26-45 degrees. The required anchorage, Cr, depends on the slope angle β for the known or assumed values of FS, h, δ, and γ. For the β value of 45 degrees, the required anchorage is 1.2 kN/m2.

FIGURE 5: Historical data for geotextile-clay shear strength (Koerner & Narejo, 2005)

The concept is simple and is based on the soil containment function of the geocell and the separation function of the geotextile.

For geocell installations, two anchorage methods that include stakes and tendons are typically evaluated. In the design phase, galvanized No. 4 rebar provided the most cost-effective solution. The rebar spacing was determined based on actual site load tests. Fifteen locations were identified for the field load tests. The rebar intended for use was hammered into the slope and a downward pull load was applied parallel to the slope. The load was increased until either maximum load capacity was reached or the rebar broke or pulled out of the ground. Testing determined that a maximum anchorage of 100kg or 0.98kN could be used for a single rebar anchor. From this value, the spacing of the stakes was determined.

Installation

FIGURE 6: Installation of the geocell in progress

The contractor recontoured the slopes where there was significant damage caused by erosion. A 6oz. NW-NP geotextile was installed on the slope as a separation layer between the existing subgrade layer and the gravel infill material. Cellular confinement sections were installed over the geotextile.

Starting from the top of the slope, the sections were expanded down the slope and filled with waste aggregate (Figure 6). The installation was completed within the target time.

Performance

In 2011, just weeks after the completion of the first phase of the project, Tropical Cyclone Vania dropped a total of more than 600mm (24in.) of rain within a 24-hour period. The site was further affected when, within 24 hours of Vania’s impact, a magnitude-7 earthquake hit a nearby island. This was a real-life test for a geocell installation on steep slopes, some up to 45 degrees.

The slope coverage performed as designed, with little or no erosion even on the steepest of the slopes. These successes were in keeping with previous results experienced by the manufacturer’s customers around the Pacific Rim—that the cellular confinement performs consistently under wet and seismic conditions.

Project Summary

For difficult and complex site conditions, cellular confinement applications can provide powerful protection against soil erosion.

The concept is simple and is based on the soil-containment function of the geocell and the separation function of the geotextile. A thin layer of overburden soil contained within the cell is enough to protect unstable slopes.

This protection is possible even on steep slopes if proper engineering procedures are followed and, most critically, provided that engineering design solutions are used only for the specific material and manufacturing characteristics of a cellular confinement material.

The engineer’s experience with the proposed design solution, that of the contractor with the site, and that of the manufacturer with previous projects in the region all contributed to the project’s success. The decision to use waste material as the infill during the design phase was crucial and limited project costs.

The materials installed on the initial phases of the slopes have already experienced dozens of heavy rainfalls and at lease one earthquake. This case history shows how geosynthetics can be engineered to solve complex problems at a significantly lower cost when compared to traditional solutions.

References: George Koerner and Dhani Narejo, “Direct Shear Database of Geosynthetic-to-Geosynthetic and Geosynthetic-to-Soil Interfaces,” Geosynthetics Research Institute, GRI Report #30, June 14, 2005.

Dhani Narejo, P.E., Principal at Care Engineering LLC in Conroe, Texas is a member of Geosynthetics Magazine’s Editorial Advisory Committee.

Bruno Hay, is Business Manager at FLI Pacifique SNC in New Caledonia.

Bryan Wedin, P.E., is Chief Civil Engineer with Presto Geosystems in Appleton, Wisconsin.