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 Novel Polymeric Alloy strips that are expanded on-site to form a honeycomb-like structure which may be filled with sand, soil, rock or concrete.[1][2]
Contents |
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.[3] 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. [4][5] These early efforts led to the civilian commercialization of the product by the Presto Products Company.[6] to produce the first cellular confinement system from high density polyethylene (HDPE) that was light weight, strong and durable.[7] 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 in cooperation with Presto Products also started during the 1980s.[8][9][10][11]
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.[12] In terms of the effectiveness of confinement, geocells have more attractive features due to its 3D structure than any other planar geosynthetic reinforcement.[13] 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).[14] 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),[15] slope stabilization and erosion control, channel lining systems (Engel, P. & Flato, G. 1987) (Simons, Li & Associates, 1988) (Wu & Austin, 1992) and other innovative uses.
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).)[16]Recent research in the last few years on geocell reinforcement for roadway applications - reflected by some 40 published papers - has been conducted at the University of Kansas as well as at other leading research institutes around the world, 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).[17]
Research was conducted on HPDE geocells as well as geocells manufactured from a novel polymeric alloy (NPA), called Neoloy®, developed by PRS[18]. NPA 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).[19]While HDPE is the commonly used material for geocells, leading researchers have questioned its suitability for long term applications (Leshchinsky, et al, 2009).[20]. This concern is backed up by “facts on the ground” as HDPE geocells are rarely used in critical applications, such as in the base layer of major highways and railways, subject to long-term heavy static and dynamic loading.
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).[21] 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).[22]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)[23]; 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)[24].
Laboratory studies, full-scale moving wheel tests, and field demonstrations (cosponsored by US DOT Department_of_Transportation as well as state DOTs) 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, while the design methods developed and calibrated in this research can help engineers design future roadway applications using geocells (Han, et al. 2011).[25] 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.”[26]
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 all 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 or when will it fail?
The lifespan of geocells in slope protection applications, for example, is less critical as vegetative growth and root interlock stabilize the soil. 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 usually have a short 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, requiring verifiable long-term durability.
Standards for geocells have not kept pace with the developments in the field of testing for material sciences, ignoring ASTM and ISO methods for testing, verification and quality assurance of polymer plastics, such as TMA - Thermomechanical analysis, DMA - Dynamic Mechanical Analysis, Stepped Isothermal Method (SIM) and CTE - Coeffecient of Thermal expansion. These methods are particularly suited for predicting long-term behavior and accumulated plastic strain in a geosynthetic under loading under different mechanical stresses, frequencies and temperatures. These widely accepted testing methods are used by the pipe, automobile, electronic, military, security and construction industries. Geomembrane testing, for example, utilizes accelerated test methods, which use temperature to stimulate aging over time to evaluate their durability.
Unfortunately, these ASTM/ISO procedures commonly utilized by many other industries to evaluate performance have not been adopted by the most of geocell industry. Current standards evolved from the world of 2D planar geosynthetics. These do not fully reflect the composite behavior of 3D geometry in soil, nor do they 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.
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:
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.
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.
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.
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 when suitable and granular, while the outer faces enable a green or tan fascia of the horizontal terraces/rows utilizing topsoil. Walls also can be used for lining channels and in cases of high flow, it is required that the outer cells contain concrete or cementious slurry infill.
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.
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.
|
||||||||||||||||||||||||||||||||||||||