| GEOCELLULAR CONTAINMENT
SYSTEMS |
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1. CATEGORY
2.0 Bank Armor and Protection
2. DESIGN STATUS
Level II
3. ALSO KNOWN AS
Cellular Confinement Systems, Geocells, Sand Grid, Soil Reinforcement Systems
4. DESCRIPTION
Geocellular Confinement Systems are flexible, three-dimensional, high density polyethylene (HDPE) honeycomb-shaped earth-retaining structures that can be expanded and backfilled with a variety of materials to mechanically stabilize surfaces. Various sizes, colors, and depths are available. They can be used flat, as channel or slope lining, or stacked to form a retaining wall.
5. PURPOSE
Geocellular Confinement Systems (GCS) are a permanent erosion control practice for stabilizing slopes as steep as 0.5V:1H. They can be filled with rock, gravel, topsoil or a combination of materials, to form walls or line channels.
6. PLANNING
Useful for Erosion Processes:
Toe erosion with upper bank failure Scour of middle and upper banks by currents Local scour Erosion of local lenses or layers of noncohesive sediment Erosion by overbank runoff General bed degradation Headcutting Piping Erosion by navigation waves Erosion by wind waves Erosion by ice and debris gouging General bank instability or susceptibility to mass slope failure
Spatial Application:
Instream Toe Midbank Top of Bank
Hydrologic / Geomorphic Setting
Resistive Redirective Continuous Discontinuous Outer Bend Inner Bend Incision Lateral Migration Aggradation Conditions Where Practice Applies:
GCS can be used as a channel lining or a retaining wall, and are suitable for areas where vegetation alone is not sufficient to resist scour. They can be used in place of riprap, and vegetated quite easily. They can also be used to create low-flow crossings.
Complexity:
Moderate.
Design Guidelines / Typical Drawings:
Geocellular Confinement Systems are available in a variety of shapes and sizes and can serve several different bank protection needs. Three of the most common uses of geocellular confinement systems are gravity walls, channel linings, and low-flow crossings.
A variety of backfill materials can be used, including gravel, topsoil, a combination of gravel and soil, or concrete. Topsoil infill is generally selected for slope protection and the outer cells of retaining walls.
Gravity Wall: Geocellular confinement systems can be used to create gravity walls, which are nearly vertical retaining walls that rely primarily on their own weight for structural stability. The structure of a gravity wall should be tailored to site conditions, and a consultation with the manufacturer is highly recommended. Walls must have sufficient weight and width so there is no movement due to external forces. Maximum slope for walls is generally 2V:1H, although they have been installed as steep as 0.5V:1H and even 1V:1H in some cases.
The construction area should be cleared of vegetation. Bedrock can be left in place and geocells can be cut to fit around bedrock outcroppings. The base should be compacted. Some manufacturers specify that walls should have a subdrain at a minimum gradient of 1% (Presto, 2003b).
Channel Lining: Geocellular Confinement Systems can also be used to line and armor channels and banks. Specific guidelines and drawings are available from the manufacturers.
Geocellular Containment
System Retaining Wall Designs |
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Geocellular Containment
System Channel Installation |
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7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS
Geocellular Confinement Systems can be filled with topsoil, rock, or a combination thereof, and vegetated with woody cuttings, native grasses, or other suitable vegetation. The expandable panels create a structure that confines topsoil infill, protects and reinforces the plant's root zone, and permits natural subsurface drainage.
8. HYDRAULIC LOADING
Hoitsma (1999) reported a GCS retaining wall withstanding shear forces of 115 Pa on the Little Miami River.
9. COMBINATION OPPORTUNITIES
This technique can be used in conjunction with Live Staking, Vegetation Alone, Vegetated Riprap, Live Brushlayering and Erosion Control Blankets.
10. ADVANTAGES
Relatively easy to install, although some safety equipment may be required for laborers working on steep slopes. Can be aesthetically appealing.
11. LIMITATIONS
Requires appropriate aggregate or soil fill material. If fill material needs to be imported, cost and level of effort can be high.
12. MATERIALS AND EQUIPMENT
A variety of materials and equipment are necessary for installation; needs will vary based on exact design specifications. Construction materials include geocells, anchors, tendons, lumber, geotextiles, granular material for infilling, topsoil for infilling, and vegetative materials. Standard equipment necessary for installation include rakes, shovels, hammers, sledgehammers, spikes, nails, crowbar, levels, and string lines. Conventional excavators, front-end loaders, mini-excavators, and skid-steer loaders equipped with smooth edge buckets are normally used in earth moving activities associated with GCS installation. Infilling of aggregate materials into geocells can be accomplished using equipment, shovels, conveyers, chutes, or skips. Compaction can be accomplished using walk behind plate tampers and vibratory drum rollers. Large segments of geocells can be compacted using smooth drum and sheep foot riding compactors (Presto, 2003b).
13. CONSTRUCTION / INSTALLATION
Gravity Retaining Wall
Site Preparation: Remove existing vegetation. Excavate, shape and de-water the proposed slope and base. Compact and shape the slope that will be retained. Any weak or unstable soil at the base of the structure should be removed and replaced with compacted soil. Install filter fabric along side slopes and base if necessary. Granular base material should be placed at the base and compacted to 95%. For some granular material, a lower percentage compaction is acceptable if compaction to 95% is not feasible (Presto, 2003b).
Installation: Expand the first layer of geocells and anchor them into position. Install the subdrain, if desired, ensuring that a minimum gradient of 1% is maintained. Overfill the geocells with material. Compact the filled material in the geocells. If high channel flows are anticipated or high scour potential at the base of the wall, concrete can be used to infill lower layers. Continue to fill geocells with granular material until they are completely filled and compacted. Remove any excess material from the top of the geocells. If the wall is to be vegetated, appropriate soils should be added and compacted to 80-85% (see Special Topic: Optimal Soil Compaction) in the outer geocells. In order to add planting soil to outer geocells, place a board over those cells during overfilling or remove granular material from appropriate geocells after overfilling and replace with desired soil (Presto, 2003b).
For each additional layer of geocells repeat the steps outlined previously. Anchoring is not necessary for additional levels. Geocells should be held open for infilling with granular material using a stretching frame. Add additional layers until desired height is reached. When positioning additional layers, ensure that each layer has the appropriate setback and that vertical alignment of geocell ends are maintained. Do not over compact the material in additional layers, as lateral displacement of geocells can occur as a result of overcompaction. Do not use heavy equipment within .92 m (3 ft) of the wall base. The slope behind the gravity wall should be filled and compacted in lifts as additional layers of geocells are added (Presto, 2003b). Each layer should overlap any joints in the previous layer by at least 2 ft.
Channel Lining
Site Preparation: Level the surface of the slope, removing all stones, vegetation and debris. Fill and compact any gullies. Major obstacles such as boulders or bedrock can be left in place. Simply cut out panel around them. Excavate, shape and de-water the proposed channel section. The area should be excavated such that when the geocellular confinement systems are installed, the top of the system is flush with or slightly lower than the adjacent terrain or final grade. Place, compact and shape any required earth fill to design elevations and grades. Dig key trenches at the crest and perimeter of the slope as required. (Presto, 2003b) Install filter fabric, if needed, along channel bed and side slopes, and into key trenches, and secure the fabric.
Installation: Install anchors at the highest edge of the channel being lined with GCS at the predetermined spacing provided by the manufacturer. Install the GCS over the anchors and expand down the channel side slopes to the specified length. Anchor with manufacturer recommended stakes, or infill peripheral cells. Align and interlock adjacent sections of GCS by pneumatic stapling. For steep slopes, tendons and anchors should be used, and installed according to manufacturer recommendations.
Backfill: Once all tendons and anchors are securely in place, fill the cells with the chosen backfill. Limit backfill drop height to 1 m (3 ft). Fill from the crest to the toe. Overfill to allow for settling and compaction. The fill must be flush with the edge of the cells when installation is finished.
Apply surface treatments following the placement of infill. Surface treatments such as permanent seeding or willow staking may be used.
14. COST
General costs for installed GCS structures are $11 to $54 per m2 for channel lining and $149 to $240 per m3 for gravity walls. GCS structures tend to be less expensive than other techniques like riprap or concrete retaining walls (Crowe, 1995). GCS costs vary based on local costs of infill material, labor, and equipment. Costs of geocells very based on their use. Geocells come in a variety of shapes and with several different anchoring and securing devices. The type of infill being used can substantially change the price of the Geocellular Confinement System. Infilling with material obtained on-site is usually the cheapest, while infilling with concrete is the most expensive. Infill material should be selected based on site-specific criteria.
At the Miami River site referenced in the Case Study below, the treatment was easily and quickly put in place; however, the use of synthetic materials made it fairly expensive, approximately $20 per linear ft per 20 cm tall lift (Hoitsma, 1999).
In Alberta, Canada, the cost of geocell installation is approximately CAN $25 per m2, while as a comparison, gabion installation is CAN $50 per m2 (Cheng, 2003).
The Philadelphia District of the Army Corps of Engineers found that design and construction of a geocell wall on Molly Ann's Brook in New Jersey cost less than half that paid for an equal length of reinforced concrete retaining wall. They also found that a section of channel lining was completed at about 25% of the cost of a 1.4 m (54 in) riprap section of equal length (DePasquale et al., 2001).
15. MAINTENANCE / MONITORING
Proper monitoring and maintenance of GCS structures is important for proper functioning and reliability. GCS structures should be inspected after installation and after the first few rain events. Structures installed for channel protection purposes should be inspected for empty cells, anchoring problems, and for areas that are showing gaps between segments or between the geocells and the substructure media. Any empty cells should be refilled and compacted. If empty cells reoccur, then a new infill media should be used for those cells. Any problems with anchoring or gaps should be immediately repaired. Gravity walls should be inspected for loss of infill material and for any serious changes in angle or bulging of layers. Infill loss should be replaced and changes of angle or bulging could indicate structural problems and the wall should be reevaluated for structural integrity. Vegetation should be monitored for growth and establishment. If vegetation doesn't establish, reseed or replant as necessary.
16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE
Common reasons for structure failure include:
17. CASE STUDIES AND EXAMPLES
West Bouldin Creek, Austin, TX
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West Bouldin Creek,
Austin, TX |
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The following case study is from Hoitsma (1999):
A 240 m (787 ft) long stretch of the Little Miami River was experiencing rapidly fluctuating river levels and scour that had eroded the 9.1 m (30 ft) high riverbank to nearly vertical slopes. At this site, shear forces exceeded the stability of traditional bioengineered bank designs. Thus, engineers decided to build terraced geocells (retaining wall) over the stone foundation for the lower portion of the bank.
The 3 m (10 ft) high, 240 m (787 ft) long portion of bank that was treated with Geocellular Confinement Systems received 14 terraced rows. The GCSs, which were 3 m (10 ft) long, 1.2 m (4 ft) deep and 20 cm (8 in) tall, were stapled together within each row, backfilled with a soil/cobble mix, and compacted. Each row was set back 0.3 to 0.6 m (1 to 2 ft) to create terraces at slopes ranging from 1V:1.5H to 1V:2H. Once the GCS was installed, the exposed surface of each layer was seeded with a native seed mix and covered with a biodegradable coir erosion control fabric. This GCS installation on the Little Miami River has withstood shear forces of 115 Pa.
18. RESEARCH OPPORTUNITIES
Use of geocells as hard lining or substrate reinforcement for reconstructed streambeds.
19. REFERENCES
Cheng, F. (2003) Geo-Cells, A First for Alberta Transportation. TSB Newsletter, Volume 2, Issue 2.
Crowe, R.E., Sent, D.F., & Martin, S. (1995) The Protection and Rehabilitation of Dams Using Cellular Confinement Systems. Presto Products Company. Found at www.prestogeo.com
DePasquale, A.J., & Leatherman, D. (2000) Geocell Wall and Channel Protection for U.S. Army Corps Flood Control Project. In Proceedings of Conference 31, International Erosion Control Association.
DePasquale, A.J., Leatherman, D., & Thomas, R. (2001) Molly Ann's Brook Channel Protection System. Geotechnical Fabrics Report, January/February 2001
Hoitsma, T. (1999) Banking on Bioengineering. Civil Engineering, Vol. 69, Issue 1
Presto Products Company (2003a). The Geoweb® Earth Retention System Technical Overview. GRWERTO-11-Aug-03. Found at www.prestogeo.com
Presto Products Company (2003b). Geoweb® System Earth Retention Construction Package. GW/RWOOO-18-Dec-03. Found at www.prestogeo.com
