Cellular confinement
Encyclopedia
Cellular Confinement Systems (CCS, also known as geocells) are widely used in construction for erosion control
, soil
stabilization on flat ground and steep slopes, channel
protection, and structural reinforcement for load
support and earth retention. Typical cellular confinement systems are made with ultrasonically-welded
high-density polyethylene (HDPE) or Neoloy polymeric alloy strips that are expanded on-site to form a honeycomb-like structure which may be filled with sand
, soil, rock
or concrete
.
These early efforts led to the civilian commercialization of the product by the Presto Products Company. to produce the first cellular confinement system from high density polyethylene (HDPE) that was light weight, strong and durable.
This new cellular confinement system was used first for load support applications in the United States in the early 1980s; second for slope erosion control and channel lining in the United States in 1984 and; third for earth retention in Canada in 1986. Research on cellular confinement in these application areas also started during the 1980s.
Research by Drs. Bathurst and Jarrett discovered that cellular confinement reinforced gravel bases are “equivalent to about twice the thickness of unreinforced gravel bases” when placed over a saturated peat sub-base. Further, 1.25 mm (50 mil) HDPE performed better than single sheet reinforcement schemes (geotextiles and geogrids
) and was more effective in reducing lateral spreading of the infill material under loading than conventional reinforced bases. In terms of the effectiveness of confinement, geocells have more attractive features due to its 3D structure than any other planar geosynthetic reinforcement. Since this early work, the results of large-scale triaxial test on isolated geocells has demonstrated that cellular confinement imparts apparent cohesion to cohesionless compacted granular material on the order of 169 kPa - 190 kPa (3500 psf - 4000 psf).
Cellular confinement systems are now recognized as an important technology when applied to load support (Webster, 1986 and Bathurst & Jarrett, 1988) under roads and rail lines, gravity and reinforced earth retaining wall systems (Crowe, Bathurst & Alston, 1989), (Bathurst, Crowe & Zehaluk, 1993), slope stabilization and erosion control, channel lining systems (Engel, P. & Flato, G. 1987) (Simons, Li & Associates, 1988) (Wu & Austin, 1992) and other innovative uses.
Recent research in the last few years on geocell reinforcement for roadway applications has been conducted at the University of Kansas as well as at other leading research institutes, to understand the mechanisms and influencing factors of geocell reinforcement, evaluate its effectiveness in improving roadway performance, and develop design methods for roadway applications (Han, et al. 2011). Research was conducted on geocells manufactured from a novel polymeric alloy (NPA), called Neoloy®, developed by PRS. This novel polymeric alloy is a composite polymeric alloy based on nano-fibers (polyester and nylon) in a polyolefin matrix. The NPA combines the desired properties of polyethylene and polyester, thus enabling a more effective use of geocells in new critical applications, such as reinforcement for earth retention, load support in pavements and railroads and more (Leshchinsky, et al, 2009).
While HDPE is the commonly used material for geocells, leading researchers have questioned its suitability for long term applications. Large thermal contraction and expansion of cells due to daily seasonal temperature changes combined with high intrinsic thermal coefficient of the infill material could lead to progressive failure; Stress cracking of exposed facing could occur in low temperatures; low stiffness and strength may lead to significant creep having poor long-term stability (Leshchinsky, et al, 2009). ) Research demonstrated that NPA geocells have a lower thermal expansion coefficient and creep reduction factor, and higher tensile stiffness and strength than HDPE geocells.(Thakur, et al, 2010); and NPA increased the bearing capacity and reduced settlement of compacted sand base courses significantly more than geocells fabricated from HDPE (Pokharel, 2011, et al).
Laboratory plate loading tests on geocells showed that the performance of geocell-reinforced bases depends on the elastic modulus of the geocell. The geocell with a higher elastic modulus had a higher bearing capacity and stiffness of the reinforced base. Geocells made from NPA were found significantly better in ultimate bearing capacity, stiffness, and reinforcement relative to geocells made from HDPE (Pokharel, et al, 2009). NPA geocells showed better creep resistance and better retention of stiffness and creep resistance particularly at elevated temperatures, verified by plate load testing, numerical modeling and full scale trafficking tests (Pokharel, et al 2011).
Laboratory studies, full-scale moving wheel tests, and field demonstrations (cosponsored by US DOT Department of Transportation
as well as state DOTs) in this comprehensive research program have demonstrated clear benefits of NPA (novel polymeric alloy) geocell reinforcement in terms of increased stiffness and bearing capacity, wider stress distribution, reduced permanent deformation, and prolonged roadway life. Field demonstrations have shown that the NPA geocell is a viable option to reinforce silty sand in roadway construction. The design methods developed and calibrated in this research can help engineers design future roadway applications using geocells (Han, et al. 2011). This close cooperation and iterative research and development process between private industry and academia was cited by the editor of Geosynthetics magazine, as: “an example of how product development for the geosynthetics industry can be done effectively… and can further advance the geosynthetics industry into the 21st century with much success.”
, DMA - Dynamic Mechanical Analysis
, Stepped Isothermal Method (SIM) and CTE - Coeffecient of Thermal expansion
. These methods are particularly suited for predicting long-term behavior, accumulated plastic strain in a geosynthetic under loading under different mechanical stresses, frequencies and temperatures. These methods are supported by ASTM and ISO and widely accepted testing methods used by the pipe, automobile, electronic, military, security and construction industries. Current testing standards for geocells are based on tests developed for 2D planar geosynthetics. These do not fully reflect the composite behavior of 3D geometry in soil, or test long-term parameters such as: dynamic loading, permanent plastic deformation, effect of temperatures, environmental durability, etc. Therefore, new standards for geocells were proposed and under discussion by leading experts in geosynthetics in ASTM technical committee D-35. The goal is to set new industry standards that more accurately reflect 3D geocell geometry and material performance in the field rather than lab tests of individual strips and virgin materials that are used by most manufacturers today.
Confinement from adjacent cells provides additional resistance against the loaded cell through passive resistance, while lateral expansion of the infill is restricted by high hoop strength. Compaction is maintained by the confinement resulting in long term reinforcement.
Typical load support applications include reinforcement of base and subbase layers in flexible pavements, including: asphalt pavements; unpaved access, service and haul roads; railway substructure and ballast confinement; working platforms in intermodal ports; airport runways and aprons, permeable pavements; pipeline road support; green parking facilities and emergency access areas.
Erosion control
Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development and construction. Effective erosion controls are important techniques in preventing water pollution and soil loss.-Introduction:...
, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
stabilization on flat ground and steep slopes, channel
Channel (geography)
In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks.A channel is also the natural or human-made deeper course through a reef, sand bar, bay, or any shallow body of water...
protection, and structural reinforcement for load
Structural load
Structural loads or actions are forces, deformations or accelerations applied to a structure or its components.Loads cause stresses, deformations and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis...
support and earth retention. Typical cellular confinement systems are made with ultrasonically-welded
Ultrasonic welding
Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials...
high-density polyethylene (HDPE) or Neoloy polymeric alloy strips that are expanded on-site to form a honeycomb-like structure which may be filled with sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, soil, rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
or concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
.
History of Cellular Confinement
Research and development of cellular confinement systems (CCS) began with the U.S. Army Corps of Engineers in September 1975 to test the feasibility of constructing tactical bridge approach roads over soft ground. Engineers discovered that sand-confinement systems performed better than conventional crushed stone sections. They concluded that a sand-confinement system could be developed that would provide an expedient construction technique for building approach roads over soft ground and that the system would not be adversely affected by wet weather conditions.These early efforts led to the civilian commercialization of the product by the Presto Products Company. to produce the first cellular confinement system from high density polyethylene (HDPE) that was light weight, strong and durable.
This new cellular confinement system was used first for load support applications in the United States in the early 1980s; second for slope erosion control and channel lining in the United States in 1984 and; third for earth retention in Canada in 1986. Research on cellular confinement in these application areas also started during the 1980s.
Research by Drs. Bathurst and Jarrett discovered that cellular confinement reinforced gravel bases are “equivalent to about twice the thickness of unreinforced gravel bases” when placed over a saturated peat sub-base. Further, 1.25 mm (50 mil) HDPE performed better than single sheet reinforcement schemes (geotextiles and geogrids
Geogrids
A geogrid is geosynthetic material used to reinforce soils and similar materials. Geogrids are commonly used to reinforce retaining walls, as well as subbases or subsoils below roads or structures. Soils pull apart under tension. Compared to soil, geogrids are strong in tension...
) and was more effective in reducing lateral spreading of the infill material under loading than conventional reinforced bases. In terms of the effectiveness of confinement, geocells have more attractive features due to its 3D structure than any other planar geosynthetic reinforcement. Since this early work, the results of large-scale triaxial test on isolated geocells has demonstrated that cellular confinement imparts apparent cohesion to cohesionless compacted granular material on the order of 169 kPa - 190 kPa (3500 psf - 4000 psf).
Cellular confinement systems are now recognized as an important technology when applied to load support (Webster, 1986 and Bathurst & Jarrett, 1988) under roads and rail lines, gravity and reinforced earth retaining wall systems (Crowe, Bathurst & Alston, 1989), (Bathurst, Crowe & Zehaluk, 1993), slope stabilization and erosion control, channel lining systems (Engel, P. & Flato, G. 1987) (Simons, Li & Associates, 1988) (Wu & Austin, 1992) and other innovative uses.
Recent Developments in Cellular Confinement Technology
Despite the effectiveness of the geocell technology, particularly in slope and channel applications, its use in base reinforcement of paved roads and railways was limited due to the lack of design methods, lack of advanced research in the last two decades and limited understanding of the reinforcement mechanisms (Yuu, et al. 2008).)Recent research in the last few years on geocell reinforcement for roadway applications has been conducted at the University of Kansas as well as at other leading research institutes, to understand the mechanisms and influencing factors of geocell reinforcement, evaluate its effectiveness in improving roadway performance, and develop design methods for roadway applications (Han, et al. 2011). Research was conducted on geocells manufactured from a novel polymeric alloy (NPA), called Neoloy®, developed by PRS. This novel polymeric alloy is a composite polymeric alloy based on nano-fibers (polyester and nylon) in a polyolefin matrix. The NPA combines the desired properties of polyethylene and polyester, thus enabling a more effective use of geocells in new critical applications, such as reinforcement for earth retention, load support in pavements and railroads and more (Leshchinsky, et al, 2009).
While HDPE is the commonly used material for geocells, leading researchers have questioned its suitability for long term applications. Large thermal contraction and expansion of cells due to daily seasonal temperature changes combined with high intrinsic thermal coefficient of the infill material could lead to progressive failure; Stress cracking of exposed facing could occur in low temperatures; low stiffness and strength may lead to significant creep having poor long-term stability (Leshchinsky, et al, 2009). ) Research demonstrated that NPA geocells have a lower thermal expansion coefficient and creep reduction factor, and higher tensile stiffness and strength than HDPE geocells.(Thakur, et al, 2010); and NPA increased the bearing capacity and reduced settlement of compacted sand base courses significantly more than geocells fabricated from HDPE (Pokharel, 2011, et al).
Laboratory plate loading tests on geocells showed that the performance of geocell-reinforced bases depends on the elastic modulus of the geocell. The geocell with a higher elastic modulus had a higher bearing capacity and stiffness of the reinforced base. Geocells made from NPA were found significantly better in ultimate bearing capacity, stiffness, and reinforcement relative to geocells made from HDPE (Pokharel, et al, 2009). NPA geocells showed better creep resistance and better retention of stiffness and creep resistance particularly at elevated temperatures, verified by plate load testing, numerical modeling and full scale trafficking tests (Pokharel, et al 2011).
Laboratory studies, full-scale moving wheel tests, and field demonstrations (cosponsored by US DOT Department of Transportation
Department of Transportation
The Department of Transportation is the most common name for a government agency in North America devoted to transportation. The largest is the United States Department of Transportation, which oversees interstate travel. All U.S. states, Canadian provinces, and many local agencies also have...
as well as state DOTs) in this comprehensive research program have demonstrated clear benefits of NPA (novel polymeric alloy) geocell reinforcement in terms of increased stiffness and bearing capacity, wider stress distribution, reduced permanent deformation, and prolonged roadway life. Field demonstrations have shown that the NPA geocell is a viable option to reinforce silty sand in roadway construction. The design methods developed and calibrated in this research can help engineers design future roadway applications using geocells (Han, et al. 2011). This close cooperation and iterative research and development process between private industry and academia was cited by the editor of Geosynthetics magazine, as: “an example of how product development for the geosynthetics industry can be done effectively… and can further advance the geosynthetics industry into the 21st century with much success.”
Application vs. Long-term Performance
HDPE-based geocells have been successfully installed in thousands of projects worldwide. However, it is incumbent to differentiate between low load applications, such as slope and channel applications, and new heavy-duty applications, such as in the base layer of asphalt pavement structures of heavily trafficked motorways and highways. While, the vast majority of polymeric materials used in geocells creep over time and under loading, the question is; what is the rate of degradation, under what conditions, and how this will impact performance and/or fail? For example, the lifespan of geocells in slope protection projects with vegetative surface cover is less critical as established vegetative growth and root interlock stabilizes the slope soil mass. This in effect compensates for any long-term loss of confinement in the geocells. Similarly, load support applications for low volume roads that are not subject to heavy loading, are often of limited design life; therefore any minor loss of performance is tolerable. However, in critical applications such as reinforcement of the structural layer of asphalt highway pavements, long term dimensional stability is critical. The required design life for such roads under heavy traffic loads is typically 20-25 years, which require verifiable long-term durability.The Development of Standards for Testing Geocells
Until recently, standards and testing methods for geocells have not kept pace with the many developments in the fields of material sciences and polymer chemistry. Scientific techniques for testing, verification and quality assurance of plastics, such as TMA - Thermomechanical analysisThermomechanical analysis
Thermomechanical analysis is a technique used in thermal analysis, a branch of materials science which studies the properties of materials as they change with temperature....
, DMA - Dynamic Mechanical Analysis
Dynamic mechanical analysis
Dynamic mechanical analysis is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus...
, Stepped Isothermal Method (SIM) and CTE - Coeffecient of Thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
. These methods are particularly suited for predicting long-term behavior, accumulated plastic strain in a geosynthetic under loading under different mechanical stresses, frequencies and temperatures. These methods are supported by ASTM and ISO and widely accepted testing methods used by the pipe, automobile, electronic, military, security and construction industries. Current testing standards for geocells are based on tests developed for 2D planar geosynthetics. These do not fully reflect the composite behavior of 3D geometry in soil, or test long-term parameters such as: dynamic loading, permanent plastic deformation, effect of temperatures, environmental durability, etc. Therefore, new standards for geocells were proposed and under discussion by leading experts in geosynthetics in ASTM technical committee D-35. The goal is to set new industry standards that more accurately reflect 3D geocell geometry and material performance in the field rather than lab tests of individual strips and virgin materials that are used by most manufacturers today.
The Impact of Temperature on Performance
Even though geocells are installed underground it is a well known thermoplastic principle that temperature affects performance. Geosynthetics used in critical applications, such as geomembranes, routinely utilize accelerated test methods, which use temperature to stimulate aging over time to evaluate their mechanical durability. Unfortunately, ASTM/ISO procedures commonly utilized in other industries to evaluate performance at elevated temperatures, oxidation/UV resistance parameters (OIT/HPOIT), etc., have not been adopted by the geocell industry. This is due in part to the fact that in the past, HDPE geocells were used in the subbase/subgrade interface and the impact of thermal temperatures were of less concern. However, newly developed stronger and stiffer NPA geocells enable the use of geocells in the upper base layer of structural pavements underneath the asphalt layer. In this critical application, long-term performance factors, such as the impact of temperature on long-term performance are more critical.How it works
A Cellular Confinement System when infilled with compacted soil creates a new composite entity that possesses enhanced mechanical and geotechnical properties. When the soil contained within a geocell is subjected to pressure, it causes lateral stresses on perimeter cell walls. The 3D zone of confinement reduces the lateral movement of soil particles while vertical loading on the contained infill results in high lateral stress and resistance on the cell-soil interface. These increase the shear strength of the confined soil, which:- Creates a stiff mattress or slab to distribute the load over a wider area
- Reduces punching of soft soil
- Increases shear resistance and bearing capacity
- Decreases deformation
Confinement from adjacent cells provides additional resistance against the loaded cell through passive resistance, while lateral expansion of the infill is restricted by high hoop strength. Compaction is maintained by the confinement resulting in long term reinforcement.
Load Support
Cellular Confinement Systems (CCS) have been used to improve the performance of both paved and unpaved roads by reinforcing the soil in the subgrade-base interface or within the base course. The effective load distribution of CCS creates a strong, stiff cellular mattress. This 3D mattress reduces vertical differential settlement into soft subgrades, improves shear strength, and enhances load-bearing capacity, while reducing the amount of aggregate material required to extend the service life of roads. As a composite system, cellular confinement strengthens the aggregate infill, thereby simultaneously enabling the use of poorly graded inferior material (e.g. local native soils, quarry waste or recycled materials) for infill as well as reducing the structural support layer thickness.Typical load support applications include reinforcement of base and subbase layers in flexible pavements, including: asphalt pavements; unpaved access, service and haul roads; railway substructure and ballast confinement; working platforms in intermodal ports; airport runways and aprons, permeable pavements; pipeline road support; green parking facilities and emergency access areas.
Slope and Channel Protection
The three-dimensional lateral confinement of CCS along with anchoring techniques ensures the long-term stability of slopes using vegetated topsoil, aggregate or concrete surfacing (if exposed to severe mechanical and hydraulic pressures). The enhanced drainage, frictional forces and cell-soil-plant interaction of CCS prevents downslope movement and limits the impact of raindrops, channeling and hydraulic shear stresses. The perforations in the 3D cells allow the passage of water, nutrients and soil organisms. This encourages plant growth and root interlock, which further stabilizes the slope and soil mass, and facilitates landscape rehabilitation. Typical applications include: construction cut and fill slopes and stabilization; road and rail embankments; pipeline stabilization and storage facility berms; quarry and mine site restoration; channel and coastline structures.Earth Retention
CCS systems provide steep vertical mechanically stabilized earth structures (either gravity or reinforced walls) for steep faces, walls and irregular topography. Construction of CCS earth retention is simiplified as each layer is structurally sound thereby providing access for equipment and workers, while eliminating the need for concrete formwork and curing. Local soil can be used for infill, while the outer faces enable a green or tan fascia of the horizontal terraces/rows. In addition research by Leshchinsky, D. (2009) has demonstrated superior seismic stability of CCS systems in earthquake simulation tests.Reservoirs and Landfills
CCS provides membrane liner protection, while creating stable soil, berms and slopes, for non-slip protection and durable impoundment of liquids and waste. Infill treatment depends on the contained materials: concrete for ponds and reservoirs; gravel for landfill drainage and leachates, vegetated infill for landscape rehabilitation. Concrete work is efficient and controlled as CCS functions as ready-made forms; CCS with concrete forms a flexible slab that accommodates minor subgrade movement and prevents cracking. In medium and low flow-velocities, CCS with geomembranes and gravel cover can be used to create impermeable channels, thereby eliminating the need for concrete.Sustainable Construction
CCS is a green solution that makes civil infrastructure projects more sustainable. In load support applications, by reducing the amount and type of infill needed to reinforce soil, the usage of haul and earthmoving equipment is reduced. This in turn decreases fuel use, pollution and the carbon footprint, and at the same time minimizes on-site disruption from dust, erosion and runoff. When used for slope applications, perforated geocells provides excellent soil protection, water drainage and growth stratum for plants. The long-term design life of advanced CCS technology means that maintenance and the associated environmental costs are significantly reduced, as are long-term economic costs.Research
In the field and laboratory tests, CCS has proven to significantly increase the lifetime of the pavement and decrease road and railway substructure maintenance. Research on cellular confinement in these application areas focused on three ways: 1) use of triaxial or resilient modulus cells to investigate the confinement effect on apparent cohesion or reducing plastic deformation; 2) the use of laboratory model tests to investigate the reinforcement effect on bearing capacity and reducing settlement under static or dynamic loading; and 3) full-scale trafficking tests to investigate the overall effect, as reducing rut depth and prolonging road life. An extensive review of existing literature on 26 CCS research studies including triaxial compression tests, laboratory model tests, and field tests, researchers concluded that: 1) Geocell reinforced bases always performed better than unreinforced bases with the same thickness in terms of the bearing capacity under static and repeated loading; 2) Proper tensile and seam strengths are needed for geocells to provide effective reinforcement. Geocells made of high strength materials typically had higher bearing capacity; 3) There exist optimum values of the geocell height/width ratio and the loading area width/geocell width ratio; 4) Geocells showed excellent performance as compared with 2D planar reinforcement; 5) High-quality infill materials yield high bearing capacity of geocell-reinforced bases.See also
- GabionGabionGabions are cages, cylinders, or boxes filled with soil or sand that are used in civil engineering, road building, and military applications. For erosion control caged riprap is used. For dams or foundation construction, cylindrical metal structures are used...
, a historic precursor for both erosion control and defense - WashboardingWashboardingWashboarding is the name of the process which results in unmetalled roads developing a series of regular bumps with short spacing in the road surface...
. Cellular confinement acts as a solution to this common problem.