Design Guideline: EnviroGrid® Cellular Confinement Erosion Control

Erosion Control Application Overview

Many variables effect the installation and performance of EnviroGrid®, including slope grade, subsurface stability, infill material, rainfall and artificial watering conditions, hydraulic characteristics of ground water flow and sub-base anchoring quality. Due to the large number of factors, it is difficult to apply exact parameters to individual applications without depending on the engineering, design and environmental inputs of on-site professionals.

Basic Forms of Erosion Control Protection

1. Granular

• Reduces hydraulic energy by limiting forces within cells or under cells.
• Directs flow at the surface of the cell, eliminating flanking and undercutting.
• Controls individual particle movement caused by gravity and water flow.
• Results in a flexible and durable system.

2. Vegetation

• Reduces hydraulic energy by limiting forces within cells or under cells.
• Increases natural resistance and protects root system.
• Directs water flow over the top, rather than through the mat.
• Prevents gutting and rills.
• Helps reduce moisture loss.

3. Concrete

• Controls piping and undercutting by allowing cells to conform to sub-grade.
• Acts as a series of expansion joints, providing a flexible form.
• Develops vent structures where needed.
• Provides stability for steep slopes and for continuous flow channels.

Applications

SLOPES

Design of EnviroGrid® for slopes requires analysis of several site characteristics. The length, height and angle of the slope and the failure angle of the existing fill on the slope are important factors in determining the appropriate cell depth and anchoring design.

EnviroGrid® improves the performance of vegetated slopes by reinforcing root systems and directing hydraulic flows over the top of cells, with the cells acting as a series of check dams, thereby preventing formation of rills and gullies.

EnviroGrid® improves the performance of granular filled slopes by controlling the migration of fills that would otherwise be initiated by hydraulic and gravitational forces. This is accomplished by dissipating hydraulic energy in and underneath cells and by confinement of fill materials within cells.

CHANNELS

EnviroGrid® Cellular confinement systems offer a large array of methods for solving difficult situations with channel bottoms and slopes where minimal to severe erosive forces are at work with either intermittent or continuous flows.

Cellular confinement allows the use of various types of infill, including soil with vegetation, aggregate, concrete or a combination for unique and aesthetic applications.

1. Vegetative Soil
Confined vegetative soil performs exceptionally well in applications with low to moderate flows. EnviroGrid® enhances the performance of vegetation through reinforcing root zones and directing flows over the top of cells, thereby increasing the shear resistance of the fill and providing a finished site that is aesthetically superior when compared to conventional methods.

Soil infill with grass cover:

• Peak flow velocity less than 6 m/s (20 fps) and duration of peak flow less than 24 hours.

• Peak flow velocity less than 4.5 m/s (15 fps) and duration of peak flows less than 48 hours.

• Channel side slopes above high water level.

2. Aggregate
Aggregate performs well by allowing the use of different sizes for variances in flow velocities encountered from site to site. The EnviroGrid® provides an aesthetically pleasing and cost-effective alternative to large riprap or hard armoring by confining and improving the performance of smaller diameter, less costly aggregates.

Stone infill:

• Peak flow velocity less than 1 m/s (3.3 fps) - graded stone.

• Peak flow velocity less than 2 m/s (6.6 fps) - 38 mm (1 1/2") median stone size.

• Peak flow velocity less than 3 m/s (10 fps) - 125 mm (5") median stone size (EGA 40 EnviroGrid® sections).

• Peak flow velocity greater than 3.5 m/s (11.5 fps) - STONE INFILL NOT RECOMMENDED

3. Concrete
Concrete filled cellular confinement systems are a cost-effective alternative to traditional installed concrete lined channels. The flexible nature of the concrete filled cellular confinement system permits conformance with the sub-grade movement without the potential cracking and undermining associated with poured-in-place concrete slabs.

Installation costs are dramatically reduced through elimination of costly forms and other construction techniques typically related to concrete channel lining.

In areas with limited easements, stacked cellular confinement wall slopes along channels allow the use of vegetative, granular or concrete infill in the outer cells in order to create steeper slopes and to increase resistance to higher flow rates.

Concrete Infill:

• Peak flow velocity greater than 1.8 m/s (6 fps) and less than 6 m/s (20 fps) use 75 mm (3") EnviroGrid® sections.

• Peak flow velocity greater than 6 m/s (20 fps) and less than 7 m/s (23 fps) use 100 mm (4") EnviroGrid® sections.

• Peak flow velocity greater than 7 m/s (23 fps) use 150 mm (6") and 200 mm (8") EnviroGrid® sections.

NOTE: Anchor requirements are a function of depth of peak flow, bedslope gradient and self-weight of the lining system. The project engineer should base the design on the project specific information.

Suitable Cell Depths

In most erosion control applications, load bearing is not a major consideration. Therefore, the depth of the cell is generally determined by:

1. Size and weight of infill
2. Slope grade
3. Outside environmental factors
4. Economics

(INSERT ANGLE OF FRICTION DIAGRAM)

Anchoring

Proper anchoring of EnviroGrid® to a slope is critical to how well the product performs. Anchors should be left in place after installation. The number and type of anchors is determined by the following:

1. Subgrade density
2. Weight and type of infill
3. Length of slope
4. Slope grade
5. Environmental or external conditions, such as snow
6. Angle of internal friction of the fill material and of the slope soil (only the smaller of the two will be used)
7. Height of the EnviroGrid®
8. Presence of a geomembrane liner

Before selecting an anchoring method, it is first necessary to calculate the net sliding force (NSF) or the force which would have to be overcome to keep the EnviroGrid® from sliding down the slope. If the NSF is negative, then the friction force between the cellular confinement and the slope is sufficient to hold the system in place. The following table shows some examples of NSF calculation:

Net Sliding Force = [ (H x L x Y) + (L X SL) ] x [ sin w - (cos w tan Ø) ]

NSF = Net Sliding Force, H = Height of Cell, L = Length of slope, Y = Unit Weight of Fill, SL = Snow Load, W = Slope Inclination (H to V), Ø = Lowest Angle of Internal Friction of Soil

* Pounds per foot measured parallel to top of slope
** Indicates no special anchoring is required

Anchor Trench

The upper edge of the EnviroGrid® should be buried in an anchor trench to prevent flow underneath. This also serves to anchor the EnviroGrid® to the top of the slope. This method takes advantage of the weight of the soil on top of the buried cells. The following equation can be used to calculate the required length and height of the trench to resist the sliding force:

L X H = (Net sliding force x factor of safety) / (Unit weight of soil x tan Ø)

Where Ø is the angle of internal friction of the fill, or of the surface soil, whichever is lower.

If the slope is longer than the panel length, lower panels must be toed in or attached to the upper panel or anchored using another appropriate method.

Anchor Pins

Staking or pinning EnviroGrid® to a slope is the common anchoring method used if there is no geomembrane liner present and if the soil has adequate strength to retain the anchor pins. Steel reinforcing bars bent into "candy cane" shapes called J-Hooks are the preferred type of pin.

As a general rule the length of the pin should be three times the cell height. Typical detail drawings of pin locations are available.

Staples

If conditions require that adjacent sections of EnviroGrid® be joined together rather than butted against each other, staples can be used. Staples are normally attached using a pneumatic staple gun with industrial grade staples. The staples are attached through each set of adjoining cells. Adjacent panels may also be tied together with tendons.

Tendons & Restraint Pins

Tendons and restraint pins are employed on steep slopes where additional support is needed, or where use of pins is prohibited (rock base, geomembrane liner). They are also commonly used when more than one section of EnviroGrid® is needed to cover the slope from top to bottom.

The three important characteristics of tendons are strength, durability and resistance to creep. Tendons usually consist of high strength polyester webbing or cord. The design load and spacing of the tendons are preferable to a smaller number of heavier tendons. Batten strips or large washers at the bottom of the lowest section of EnviroGrid® are essential to avoid stress concentrations.

Installation Using Tendons

If the EnviroGrid® does not already have holes for tendons, drill the holes before expanding the EnviroGrid® sections. Measure and cut tendons to desired length (add approximately 10% for tying around restraint pins.) Tie the tendons to a supporting structure beyond the crest of the slope. This supporting structure may be a length of high-strength PVC pipe, a concrete beam or a set of concrete blocks placed in an anchor trench. An alternative system may consist of harpoon like earth anchors.

(INSERT DEAD MAN AND HARPOON SYSTEM ILLUSTRATIONS)

Whether or not tendons are utilized, EnviroGrid® should be placed beyond the crest of the slope to prevent surface water from undermining the EnviroGrid®.

At the top of the slope thread the tendons through the holes in the unexpanded EnviroGrid® sections. Measure and mark the perimeter of the area to be covered by the first section to be installed. If allowable, place anchor pins around the perimeter to hold the expanded sections into place. Expand and place the section, taking care that the tendons do not come out of the holes. Repeat this procedure for remaining sections.

The tendon must be tied, in tension, to a restraint pin or batten strip on the downhill side of the last cell wall. The use of washers or plates helps relieve point stresses. The use of restraint pins, batten strips, washers and plates helps to transfer the load from the EnviroGrid® to the tendons. Restraint pins, batten strips, washers and plates should be made from corrosion resistant materials such as galvanized steel, high strength plastic, etc.

(INSERT DRILLED HOLES ILLUSTRATION)

(INSERT ANCHOR PIN INSTALLATION WITH TENDONS)

STEP 1: Make 2 loops in the tendon.
STEP 2: Pull loop 1 partially through loop 2.
STEP 3: Insert the specified J-Hook anchor through loop 1 and drive J-Hook into the ground until the top of hook is level with the top of the cellular confinement section.
STEP 4: Pull both ends of tendon to close the loop and drive the J-Hook until the top of it is flush with ground surface.

Anchoring Method

Given the resulting net sliding force (NSF) for two of the cases in the previous table, the next step is to decide how to anchor the EnviroGrid®. For the situation where NSF = 0.8 kN/m, two common methods of anchoring of the EnviroGrid® are to toe it in or to stake it the slope. For the situation where NSF = 5.1 kNm, the cellular confinement could be supported by earth anchors with tendons.

Anchor Trench

Using the appropriate equation: (0.8 x 2) / (19.6 x tan 28º) L X H = 0.15 sq. m

A practical combination would be to bury the top edge of the EnviroGrid® 0.3 m deep and 0.5 m back. Another practical combination would be to let L be 0.75 m and H be 02 m.

Stakes

0.8 kN/m is equivalent to 0.8 x 2.56 = 2.0 kN for the 2.56 m wide panel. Using a factor of safety of 2.0 and stake pull-out capacity of 0.27 kN*.

(2.0 x 2.0) / 0.27 = 14.8 J-Hooks; use 15 stakes per 2.56 m wide width

Tendons

5.5 kN/m is equivalent to 5.5 x 2.56 = 14.1 kN for the 2.56 m wide panel. Using a factor of safety of 3.0 and a tendon design strength of 13.0 kN:

(14.1 x 3.0) / 13.0 = 3.25 tendons; use 4 tendons per 2.56 m panel width

If the tendons are tied to earth anchors, using the same number of anchors as tendons, an additional factor of safety of 1.25 is required to account for uncertainties in the subgrade soil. Stake pull-out capacity will depend upon several factors, including on-site soil conditions at their weakest and care with which the stakes are driven into the soil. Thus, the local engineer must evaluate and make a judgement as to what value should be used.

Infill Sections

A) Topsoil and vegetation: Steep slopes, berms, levees, chutes, aprons and spillways.
B) Sand and Granular: Suitable on gradual slopes.
C) Gravel (maximum diameter 760 mm): Channels, slopes, except for severe grades, moderate sheet flow.
D) Crushed Stone: Channels, slopes, except for severe grades, moderate sheet flow.
E) Concrete: Around bridges, severe slopes, high flow rate channels, spillways and chutes.

Geotextiles

Whether to use a geotextile under the EnviroGrid® is dependent on the subgrade and fill material. When the infill and subgrade are different, or if the subgrade is very soft or wet, a geotextile can provide a useful separation function by keeping the infill from migrating out from under the cellular confinement. However, using a geotextile can reduce significantly the friction along the plane at the bottom of the EnviroGrid® system, thus increasing the net sliding force. Thus, the decision whether or not to use a geotextile should be made carefully, after evaluating the benefits and costs.

Installation Procedures

Adjoining EnviroGrid® sections must be level and flush with each other. Overlap the sides of the EnviroGrid® sections and butt the ends together. Secure the adjoining sections to each other using a pneumatic stapler or other means as required by the job application.

1. Start site preparation for the cellular confinement earth retention system installation by removing debris, vegetative cover and any unacceptable soils from the installation area.
2. Replace any removed soils with acceptable materials and complete all earthwork in accordance with the job specifications, including toe in trenches when required for slopes or channel lining applications.
3. If a geotextile is required by the job specifications, installation should be accomplished in accordance with the manufacturer's recommendations.
4. Partially install stakes or J-Hooks, leaving a protruding length of the cell depth plus approximately 50 mm along the top edge of the area in which the cellular confinement is to be installed (or in anchor trench). A string or chalk line may be used to align staking locations and borders.
5. EnviroGrid® sections should be stretched past the designed length and then allowed to settle back to the desired length. Set the end of the EnviroGrid® sections over the previously installed stakes or J-Hooks and complete the installation by making the stakes or J-Hooks flush, or slightly below the cell walls.
6. Install the remaining stakes or J-Hooks as per the job specifications.
7. When the EnviroGrid® has been properly laid in place, the system should be infilled using the materials specified in the project specifications.
8. To prevent possible damage to the system, limit the drop height of the infill to no more than 1 m.
9. Infill should be placed in the EnviroGrid® from the top of the slope or channel to the base using a front-end loader, backhoe, bucket excavator or conveyor.
10. When using sand, granular or topsoil fills, overfill the EnviroGrid® sections by 25m to 50m to allow for settling and compaction.
11. Sand and granular fills should then be blade compacted to the top of the cells. Topsoil fill should be compacted with a loader or backhoe bucket with a tamper plate.
12. Concrete fills should be manually raked and machine finished.