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… Read more »
Posts By: Katie Bocskor
Using Geosynthetics to Stabilize Soils in a Harsh Environment
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. 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 Ultrabasic soils cover about one-third… Read more »
Why Geocells Outperform Geogrids for Road Construction
Geocells (cellular confinement system CCS) offer a more effective and practical 3D design solution to load support challenges than multilayered 2D geogrid efforts. Geocells transfer applied loads instantaneously, delivering practical soil stabilization in a product that is fast and easy to install. How do geogrids work? Geogrids rely on rutting, displacement and lateral movement of the road material to activate the load support reaction of the product. As shown below, failure of the driving surface must occur before the geogrid reacts. As a result, rutting and soil displacement is a prerequisite reality to the system. Since the geogrid is two-dimensional, material not located directly within the plane occupied by the geogrid is free to move, shift and displace. It is essential that geogrids are placed in a flat or a pre-tensioned manner—but that is not practical in a construction environment. It is common to see geogrids unrolled over a prepared grade with an undulating surface. As aggregate is placed over the top of the geogrid, the material kinks and waves, further warping the 2D plane. The geogrid is rarely pulled tight during installation which does not allow full tension under load. Geogrids are difficult to… Read more »
Geosynthetics and PFAS: Understanding the Role of Polymer Processing Aids in Geosynthetics
Written By: Michael Dickey, P.E., Director of Presto Geosystems Like many other industries, geosynthetics manufacturers are navigating the rapidly evolving landscape of new per- and polyfluoroalkyl substances (PFAS) regulations. However, in the case of geosynthetic products, an interesting and seemingly paradoxical question emerges: Is it possible that the same products that have been designed to solve complex environmental problems, and even contain pollutants, could also be a possible contributing source of PFAS? In this article, we explore this question and discuss the historic role of polymer processing aids (PPAs) in the production of geosynthetics. What Does Intentionally vs Unintentionally Added PFAS Mean? Since the discovery of PFAS in the 1930s, these compounds have been widely used in manufacturing operations worldwide—both intentionally and unintentionally. In a recent article published by the American Bar Association, the concept of intentional versus unintentional use of PFAS is discussed, and in the case of the latter, the use of fluorinated PPAS used in thermoplastics processing is highlighted as a well-known unintentional PFAS source. How this concept relates to traditional geosynthetics manufacturing is discussed further below. Eliminating Polymer Processing Aids (PPAs) from Geosynthetics Production of geosynthetic products such as geogrids, geomembranes, and geocells commonly involves sheet… Read more »
Ballast Stabilization Using Geocells
The Often Overlooked Importance of Junction Efficiency as a Key Design Consideration A significant number of research studies have been carried out to investigate the benefits of using geocells in railway track bed applications. Combined with an ever-expanding list of successful projects from around the world, the benefits of using geocells in rail ballast stabilization is well-documented. Rail operators understand that durable track geometry starts with a solid foundation, and geocells have emerged as a powerful value engineering tool for reinforcing ballast and sub-ballast layers while optimizing layer thicknesses. Many practitioners may not be aware of the critical role that geocell junctions (both mechanical and internal) play in ensuring that the installed system performs in a uniform and consistent manner. In track bed stabilization applications, non-uniform junction performance can lead to differential settlement and localized subsidence—which in turn can lead to serviceability issues, damage to the overlying structure/pavement, and a reduction in overall design life. In essence, poor junction performance can nullify all the intended benefits of a geocell system. This article will succinctly discuss the different types of junctions present in geocell systems, failure mechanisms and test methods, and the concept of junction efficiency as a performance parameter. Types… Read more »
Conserving Natural Resources Using Geosynthetics
Written By: Cory Schneider, Environmental Scientist, Presto Geosystems Natural resources are finite, or at a minimum, can easily be consumed faster than they can be replaced. As such, the conservation of natural resources is a pragmatic endeavor. Geosynthetics—widely available materials used in construction, civil engineering, and environmental protection—can be useful in promoting the conservation of these resources. When used as intended, geosynthetics can enhance soil properties and reduce the demands placed on natural resources. Types of Geosynthetics Geosynthetics are typically made from synthetic polymers, such as polyethylene, polypropylene, and/or polyester, and are designed to be durable and resistant to weathering and other environmental factors. General groupings of geosynthetics include: geotextiles, geogrids, geomembranes, geocells, erosion control blankets (ECBs), and turf reinforcement mats (TRMs). Geotextiles (permeable) and geomembranes (impermeable) provide separation, while geogrids and geocells provide varying degrees of stabilization and confinement. ECBs and TRMs, made with a combination of natural and synthetic fibers, resist surficial erosion by preventing seed washout prior to germination. Application areas where these geosynthetic materials are used typically include: load support, slope, shoreline, and channel protection, and earth retention. Using Geosynthetics in Load Support Applications to Conserve Natural Resources In load support applications, geogrids, geotextiles, and geocells… Read more »
Understanding Hoop Stress in Geocells
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… Read more »
Sustainable Vegetated Channels = The Death of Rip Rap
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… Read more »
GEOWEB® Geocell Reinforcement Improves Structural Performance of Railway Track Beds
Every year, railroads dedicate a great deal of capital and resources toward creating and maintaining high-quality track profiles. Providing a well-designed track profile is the foundation on which a successful rail line operates. With ballooning rail traffic carrying heavier loads than ever and increased occurrence of extreme weather events, a stable track profile is essential for successful operation. GEOWEB® geocells have been used in the track bed for rail applications worldwide for more than 40 years. Through an interconnected honeycomb-like network, the HDPE-based GEOWEB Soil Stabilization System provides apparent cohesion and strength to materials that would otherwise be unstable over soft subgrades. Geocells stabilize the ballast, reduce vertical and lateral stresses, and limit ballast movement. Stabilization within the geocell system provides a longer lasting track profile that extends rail service life, while also reducing maintenance cycles and recurring maintenance costs. Research has shown that geocells reduce settlement of the ballast foundation and can reduce required cross-section thicknesses by up to 50%. This is particularly advantageous where track beds must be constructed over soft soils. The reduction in thickness leads to cost savings, along with an accompanying reduction in carbon emissions due to decreases in aggregate processing, transportation, handling, and installation…. Read more »
Protecting Environmental Geomembrane Covers With Suspended GEOWEB Geocells
Economic pressure, the desire for green solutions, and the intensification of climate extremes have converged to create a need for better methods to effect soil stabilization. Fortunately, a proven technology exists that addresses issues associated with these conditions and provides a more stable cover solution for landfill covers, lagoons, stormwater containment basins, and other geomembrane-covered systems. Soil, aggregate, and concrete protective covers over geomembranes can be secured against known gravitational, hydrodynamic, and seismic forces using the GEOWEB® Soil Confinement System. Soil and aggregate are commonly used as a protective cover over liners on slopes of 3H:1V or less. However, when slope gradients are greater, unconfined soil and aggregate covers are typically unstable and not used. In arid areas, cover depth may range from 75 mm (3 in) to 150 mm (6 in). Where conditions support vegetation, cover depth may range from 100 (4) to 600 mm (24 in) or greater where the final depth is a function of the characteristics of the desired vegetation. Regardless of cover depth, if an extreme rainfall event occurs that is 10%, or greater than what would typically be expected, soil mass increases, assumed friction angles decrease, and factors of safety for soil stability drop to… Read more »
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