Geocells

Geocells (also called Cellular Confinement Systems, or CCS's) are used in civil engineering for roadway load support, walls and steep slopes, channel protection and erosion control. They are typically made from ultrasonically welded high density polyethylene strips and expanded on-site to form a honeycomb structure which is subsequently filled with sand, gravel, locally available soil, or concrete.

Background

The US Army Corps of Engineers (1981) in Vicksburg, Mississippi, has 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, geocells. Today geocells 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 the following figure).

At the job site they are placed directly on the subsoil's surface and propped open in an accordianlike fashion with an external stretcher assembly. This section expands to a 5 by 10 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.

Available Theory

In terms of design for the above referenced soil stabilization and roadway systems, they 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 geocell system. For such a failure to occur, the sand in a particular cell must overcome the side friction, punch 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-desity conditions. The relevant equations are given in Koerner (2012) and a recent comparison of methods are given in Neto, et al. (2013).

Geocells are also used in constructing the facing of mechanically stabilized walls (MSE) along with geogrid, geotextile or geostrop reinforcement. In such cases the geocells 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 (2009).

Testing

Unfortunately, the ASTM/ISO procedures commonly utilized by most other geosynthetics to evaluate performance have not been formulated or adopted by the geocell industry. Current standards evolved from the 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 are under discussion by ASTM technical committee D-35. The goal is to set new industry standards that more accurately reflect 3D geocell geometry and amterial performance in the field rather than lab tests of individual strips and virgin materials that are typically used by geomembrane manufacturers.

Major Applications

Roadway Load Support - Geocells are regularly used for unpaved road support as well as for stabilization of parking and staging areas.

Walls and Steep Soil Slopes - Geocells can form the facing of mechanically stabilized earth walls and steep soil slope. They can be built in such a fashion so as to be a gravity mass themselves or be used as facing for geosynthetic reinforcement.

Additional Details

• Geocell strips width, hence the in-situ height come in various sizes from 50 to 300 mm.
• Geocell walls are usually made from textured or structured HDPE sheet so as to increase frictional resistance against the infill soil from displacement.
• Geocells have also been made from hybrid HDPE materials, low density polyethylene and nonwoven heat-bonded geotextiles.
• Geocell walls are typically perforated so as to allow for drainage from one cell to another.
• On steep slopes geocells 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 geocells on long and wide slopes is quite labor intensive. Construction equipment called phneumatic sand-slingers or stone-slingers have been used advantageously.

References

• Koerner, R. m. (2012) Designing With Geosynthetics, 6th Edition, Xlibris Publ. Co., 914 pgs.
• Neto, J. O. A., Bueno, B. S. and Futai, M. M. (2013), "A Bearing Capacity Calculation Model for Soil Reinforced With a Geocell," Geosynthetics International, Vol. 20., No. 3, pp. 129-142.
• "WES Developing Sand-Grid Confinement System," (1981), Army Res. Ver. Acquisition Magazine, July-August, pp. 7-11.
• "Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes," U.S. Dept. of Trnasportation, FHWA GEC011 - Vol. 1 and Vol. 2, November 2009.