Cellular confinement

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A cellular confinement system being installed on an experimental trail in south-central Alaska.
Wood matrix after installation in Wrangell–St. Elias Park in Alaska.
Geocells

Cellular Confinement Systems (CCS)—also known as geocells, turf reinforcement mats (TRM), and erosion control blankets (ECB)—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.[1] Typical cellular confinement systems are made with ultrasonically welded high-density polyethylene (HDPE) strips or Novel Polymeric Alloy—and expanded on-site to form a honeycomb-like structure—which may be filled with sand, soil, rock, gravel or concrete.[2][3]

History of cellular confinement[edit]

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.[4] 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. [5][6] The US Army Corps of Engineers (1981) in Vicksburg, Mississippi, had experimented with a number of confining systems, from short pieces of sand-filled plastic pipes standing on end to cubic confinement cells made from slotted aluminum sheets to prefabricated polymeric systems called sand grids and then, cellular confinement systems. Today cellular confinement systems are typically made from HDPE strips 50.0 to 200 mm wide and approximately 1.2 mm thick. They are ultrasonically welded along their width at approximately 300 mm intervals and are shipped to the job site in a collapsed configuration (see picture above).

These early efforts led to the civilian commercialization of the first cellular confinement system known as Geoweb® by the Presto Products Company.[7] The cellular confinement system was made from high density polyethylene (HDPE) that was light weight, strong and durable.[8] This new Geoweb 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.[9][10][11][12]

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.[13] In terms of the effectiveness of confinement, cellular confinement systems have more attractive features due to its 3D structure than any other planar geosynthetic reinforcement.[14] Since this early work, the results of large-scale triaxial test on isolated cellular confinement systems 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).[15] 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),[16] slope stabilization and erosion control, channel lining systems (Engel, P. & Flato, G. 1987) (Simons, Li & Associates, 1988) (Wu & Austin, 1992) and other innovative uses.

Available theory[edit]

In terms of design, soil stabilization and roadway systems are quite complex to assess. If we adapt the conventional plastic limit equilibrium mechanism as used in statically loaded shallow foundation bearing capacity the failure mode is interrupted by the CCS. For such a failure to occur, the soil in a particular cell must overcome the side friction, punching out of it, thereby loading the sand beneath the level of the mattress. this in turn fails in bearing capacity, but now with the positive effects of the small surcharge loading and typically higher-density conditions. The relevant equations are given in Koerner.[17] A recent comparison of methods are given in Neto, et al.[18]

Cellular confinement systems are also used in constructing the facing of mechanically stabilized walls (MSE) along with geogrid, geotextile or geostrap reinforcement. In such cases the cellular confinement systems are built up in a pyramid fashion with the reinforcement embedded between layers at designed intervals. Design in this regard follows standard procedures as given in FHWA.[19]

Recent marketing plays in cellular confinement technology[edit]

Despite the effectiveness of the CCS technology, particularly in slope and channel applications, its use in base reinforcement of paved roads and railways was thought to be limited by some. (Yuu, et al. 2008).)[20] Recent research in the last few years on CCS reinforcement for roadway applications 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 CCS reinforcement, evaluate its effectiveness in improving roadway performance, and develop design methods for roadway applications (Han, et al. 2011).[21]

Sponsored research was conducted on HPDE cellular confinement systems as well as cellular confinement systems manufactured from a novel polymeric alloy (NPA), called Neoloy, developed by PRS.[22] NPA is claimed to be a composite polymeric alloy based on nano-fibers (polyester and nylon) in a polyolefin matrix. The NPA purports to combine the properties of polyethylene and polyester, thus enabling a more effective use of CCS in new critical applications, such as reinforcement for earth retention, load support in pavements and railroads and more. (Leshchinsky, et al., 2009).[23] HDPE remains the commonly used material for CCS, but sponsored researchers have questioned its suitability for long-term applications (Leshchinsky, et al., 2009).[23] This concern is not backed up by “facts on the ground” as HDPE CCS are often used in critical applications, such as in the base layer of major highways and railways, subject to long-term heavy static and dynamic loading.

CCS made from NPA are marketed as significantly better in ultimate bearing capacity, stiffness, and reinforcement relative to CCS made from HDPE. NPA CCS are sold as having showed better creep resistance and better retention of stiffness and creep resistance particularly at elevated temperatures, although....even if true...this would be unimportant as creep is not a factor in CCS paving design due to passive adjacent cell pressures and the elevated temperatures are not a factor in buried base support design.

Application vs. long-term performance[edit]

HDPE-based CCS 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 CCS creep over time and under loading, the question is; what is the rate of degradation, under which conditions, how will this impact performance, and when will it fail? Thousands of load support applications show that HDPE remains the safe and predominant choice.

The lifespan of CCS 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 CCS. 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. HDPE has a life of over 50 years and unlike other materials, is inert and stable.

Development of standards for testing CCS[edit]

Standards for CCS have been in place for more than 30 years.

ISO procedures commonly utilized by many other geosynthetics industries to evaluate performance have not been adopted by some of the CCS industry. Current standards evolved from the 2D planar geosynthetics. These do not fully reflect the composite behavior of 3D geometry of CCS, nor do they test long-term parameters such as: dynamic loading, permanent plastic deformation, effect of temperatures, environmental durability, etc. That said, new standards for CCS were proposed and under discussion by leading experts in geosynthetics in ASTM technical committee D-35. Under the leadership of industry experts such as Bryan Wedin, proposed changes are reviewed. The goal is to set new industry standards that more accurately reflect 3D cellular confinement system geometry and material performance in the field rather than lab tests of individual strips and virgin materials that are typically used for geomembrane products today.

How it works[edit]

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 CCS 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.

At the job site they are placed directly on the subsoil's surface or on a geotextile filter placed on the soil's surface and propped open in an accordianlike fashion with an external stretcher assembly. This section expands to a 5 m by 10 m area and consists of hundreds of individual cells, each approximately 250 mm in size. They are then filled with various soil types and compacted using a vibratory hand-operated plate compactor. Sometimes a final step for roadways involves spraying the surface with an emulsified asphalt (approximately 60% asphalt in a 40% water suspension) at the rate of approximately 5.01/m2. Of course, other types of infill materials are possible depending on site-specific conditions.

Applications[edit]

Roadway load support[edit]

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.

Steep soil slope and channel protection[edit]

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. They can be built as an underlying mass or as a facing.

Earth retention[edit]

CCS 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 have been used to reinforce soft or uneven soil foundations for large area footings, for retaining wall strip footings, for load sharing of covers over pipelines and other geotechnical applications.

Reservoirs and landfills[edit]

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[edit]

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 CCS 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.

Additional details[edit]

  • CCS strip widths, hence the on-site height, come in various sizes from 50 to 300 mm.
  • CCS walls are usually made from textured or structured HDPE sheet so as to increase frictional resistance against the infill soil from displacement.
  • CCS have also been made from hybrid HDPE materials, low density polyethylene and nonwoven heat-bonded geotextiles.
  • CCS walls are typically perforated so as to allow for drainage from one cell to another.
  • On steep slopes CCS can have a steel cable extending through the central region up the slope and anchored to, or within, a concrete plinth so as to resist downgradient sliding of the system.
  • The backfilling of CCS on long and wide slopes is quite labor intensive. Construction equipment called phneumatic sand-slingers or stone-slingers have been used advantageously.

See also[edit]

References[edit]

  1. ^ Geosynthetics in landscape architecture and design
  2. ^ State of California Department of Transportation, Division of Environmental Analysis, Stormwater Program. Sacramento, CA."Cellular Confinement System Research." 2006.
  3. ^ Managing Degraded Off-Highway Vehicle Trails in Wet, Unstable, and Sensitive Environments, US Department of Agriculture in conjunction with USDOT, Federal Highway Administration. Page 28. October 2002.
  4. ^ Webster, S.L. & Watkins J.E. 1977, Investigation of Construction Techniques for Tactical Bridge Approach Roads Across Soft Ground. Soils and Pavements Laboratory, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS, Technical Report S771, September 1977.
  5. ^ Webster, S.L. 1979, Investigation of Beach Sand Trafficability Enhancement Using Sand-Grid Confinement and Membrane Reinforcement Concepts – Report 1, Geotechnical Laboratory, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS, Technical Report GL7920, November 1979.
  6. ^ Webster, S.L. 1981, Investigation of Beach Sand Trafficability Enhancement Using Sand-Grid Confinement and Membrane Reinforcement Concepts – Report 2, Geotechnical Laboratory, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS, Technical Report GL7920(2), February 1981
  7. ^ Prestogeo.com
  8. ^ Webster, S.L. 1986, Sand-Grid Demonstration Roads Constructed for JLOTS II Tests at Fort Story, Virginia, Geotechnical Laboratory, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS, Technical Report GL8619, November 1986.
  9. ^ Engel, P. & Flato, G. 1987, Flow Resistance and Critical Flow Velocities for GEOWEB Erosion Control System, Research and Applications Branch – National Water Research Institute Canada Centre for Inland Waters, Burlington, Ontario, Canada, March 1987
  10. ^ Simons, Li & Associates, 1988, Full Scale Hydraulic Studies of GEOWEB Grid Confinement System for Minimizing Embankment Damage During Overtopping Flows, Report to Presto Products Co., February 1988
  11. ^ Bathurst, R.J, Crowe, R.E. & Zehaluk, A.C. 1993, Geosynthetic Cellular Confinement Cells for Gravity Retaining Wall – Richmond Hill, Ontario, Canada, Geosynthetic Case Histories, International Society for Soil Mechanics and Foundation Engineering, March 1993, pp. 266-267
  12. ^ Crowe, R.E., Bathurst, R.J. & Alston, C. 1989, Design and Construction of a Road Embankment Using Geosynthetics, Proceedings of the 42’nd Canadian Geotechnical Conference, Canadian Geotechnical Society, Winnipeg, Manitoba, October 1989, pp. 266–271
  13. ^ Bathurst, R.J. & Jarrett, P.M. 1988, Large-Scale Model Tests of Geocomposite Mattresses Over Peat Subgrades, Transportation Research Record 1188 – Effects of Geosynthetics on Soil Properties and of Environment on Pavement Systems, Transportation Research Board, 1988, pp. 2836
  14. ^ Yuu, J., Han, J., Rosen, A., Parsons, R. L., Leshchinsky, D. (2008), “Technical Review of Geocell-Reinforced Base Courses over Weak Subgrade,” The First Pan American Geosynthetics Conference & Exhibition proceedings (GeoAmericas), Appendix VII, Cancun, Mexico
  15. ^ Bathurst, R.J. & Karpurapu, R. 1993, Large-Scale Triaxial Compression Testing of Geocell-Reinforced Granular Soils, Geotechnical Testing Journal, GTJODJ, Vol. 16, No. 3, September 1993, pp. 296303
  16. ^ Bathurst, R.J. & Crowe, R.E. 1993, Recent Case Histories of Flexible Geocell Retaining Walls in North America, International Symposium on Recent Case Histories of Permanent Geosynthetic-Reinforces Soil Retaining Walls, Tokyo, Japan, November 199
  17. ^ Koerner, R. M. (2012). Designing With Geosynthetics (6th ed.). Xlibris Publ. Co. p. 914. 
  18. ^ Neto, J. O. A; Bueno, B. S; Futai, M. M (2013). "A Bearing Capacity Calculation Model for Soil Reinforced With a Geocell". Geosynthetics International. 20, No. 3: 129–142. 
  19. ^ "Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes". FHWA GEC011 (U.S. Dept. of Transportation). 1 and 2: 129–142. November 2009. 
  20. ^ Yuu, J., Han, J., Rosen, A., Parsons, R. L., Leshchinsky, D. (2008) “Technical Review of Geocell-Reinforced Base Courses over Weak Subgrade,” The First Pan American Geosynthetics Conference & Exhibition proceedings (GeoAmericas), Appendix VII, Cancun, Mexico
  21. ^ Han, J., Pokharel, S.K., Yang, X. and Thakur, J. (2011). Exploring Geocell Technology for Roadway Applications. Accepted for publication in Roads and Bridges
  22. ^ PRS.com
  23. ^ a b Leshchinsky, D. (2009) “Research and Innovation: Seismic Performance of Various Geocell Earth-retention Systems,” Geosysnthetics, No. 27, No. 4, 46-52
  • "WES Developing Sand-Grid Confinement System," (1981), Army Res. Ver. Acquisition Magazine, July–August, pp. 7–11.

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