Slope Erosion Control

for

SLOPE EROSION CONTROL

The TerraCell^ŽCellular Confinement System is a three-dimensional,
“honeycomb” structure made of polyethylene.
It is designed to minimize and/or eliminate the effects
of the erosive forces of water and wind on exposed soils.

TerraCell^Ž is highly effective in protecting:

Highway and railroad embankments
Berms, dikes, and levees
Landfill slopes and caps
Natural, cut, or fill slopes

Mechanics Of Soil Erosion

Slopes are prone to soil erosion. It takes place when the forces of wind or water cause rills to form in the exposed soil. Over time these forces concentrate within the rills which accelerate the erosive process. The principal site parameters that determine the amount of erosion likely to take place are:

Angle of the slope
Height and length of the slope
Type of soil on the surface of the slope
Water flowing onto the slope from above

Historically, engineers have attempted to minimize the effect of these erosive forces by protecting slopes with vegetation, erosion control blankets, armor stone, etc. Unfortunately, vegetation or blankets are not effective except in mild conditions and armor stone is very costly, highly unattractive, and may create a potential hazard for children and animals.

The TerraCell Solution

For many erosion control situations, the TerraCell Cellular Confinement System can be substituted for more costly, conventional systems such as riprap, revetment mats, and gabions. The cells in TerraCell confine a fill material (soil, sand, aggregate) and protect it from being moved by wind or water.

Each cell acts as a small dam that allows water or wind to pass over the top while holding the fill in place, thereby dissipating erosive forces. The cell walls inhibit formation of rills, thus preventing the erosive process from developing. Also, a grass covered TerraCell slope can be mowed with standard mowing equipment.

In areas subjected to substantial erosive forces (very steep slopes with heavy flows), concrete filled TerraCell is the most effective solution. In this situation, TerraCell becomes an articulated concrete mat that conforms to possible differential settlement.

Also, in cases where external weight on the slope is needed to enhance the stability of the slope, TerraCell provides the additional benefit of keeping the fill material in place.

Economics

Slope protection using heavy armor stone tends to be very costly in terms of materials and the time consumed in installation, especially if the rock must be transported from off-site. Slope protection using TerraCell filled with locally available soils, aggregate, or concrete can be more effective than expensive alternatives and is usually less costly to maintain.

Designing with TerraCell

Once it has been determined that TerraCell is the most cost effective solution to a slope erosion control problem, it becomes necessary to select the proper size TerraCell and the most suitable fill material. This is accomplished by first determining the degree of the slope being protected and the conditions affecting the slope. Using this information, one can then select the most appropriate cell height, cell size, and fill material needed for the particular situation.

Anchoring the TerraCell

Proper anchoring of TerraCell to a slope is critical to how well the product performs. No matter which anchoring materials are selected, they must be left in place throughout the life of the project. The following factors must be considered when deciding what anchoring materials will perform best:

Degree of slope
Length of slope
External loads, such as snow
Angle of internal friction (ř) of the fill material and of the slope soil (only the smaller of the two will be used)

Unit weight of the material used as fill.
Height of TerraCell
Presence of a geomembrane on the slope

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 TerraCell from sliding down the slope. If the NSF is negative, then the friction force between TerraCell and the slope is sufficient to hold the system in place. Table 1 shows examples of calculating NSF.

TABLE 1

Net Sliding Force = [( H x L x g ) + ( L x SL)] x [sin w - (cos w tan Ř)]

NSF	H	L	g	SL	W	Ř
NET SLIDING	Height of	Length of	Unit Weight	Snow	Slope Inclination	Lowest Angle of
FORCE	Cell	Slope	of Fill	Load	(H to V)	Internal Friction of Soil

LBS/FT*

INCHES

FEET

LBS/FT³

LBS/FT²

SLOPE

DEGREES

56	4	20	125	40	1.75 to 1 (30°)	28° (silty sand)
353	6	100	125	40	1.75 to 1 (30°)	28° (silty sand)
-912**	4	100	125	40	2.00 to 1 (26.5°)	32°(crushed stone)

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

Anchoring Methods or Materials

Anchor Trench
The upper edge of the TerraCell should be buried in an anchor trench to prevent flow underneath. This also serves to anchor TerraCell 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 Ř}

Where Ř 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 also be toed in or attached to the upper panel, or anchored using another appropriate method.

Stakes

Staking TerraCell to a slope is the most common anchoring method used if there is no geomembrane present and the soil has adequate strength to retain the stakes. Steel reinforcing bars bent into a “candy cane” shape called J-hooks are the preferred type of stake. (CAUTION if the surface of the slope is covered with vegetation that will be mowed, then anchoring methods other than J-hooks, such as plastic clips, should be considered.) As a general rule: the length of the stake is three times the cell height. A typical staking pattern is shown in the drawing.

	Rings and Staples

	If conditions require that adjacent sections of TerraCell be joined together rather than butted up against each other, hog rings or staples can be used. Staples are normally attached using a pneumatic staple gun and industrial grade staples. Staples or rings are attached through each set of adjoining cells.

Tendons

Tendons are employed on steep slopes where additional support is needed, or where use of stakes is prohibited (rock base, geomembrane underliner). They are also commonly used when more than one section of TerraCell is needed to cover from top to bottom.


The three important characteristics of tendons are strength, resistance to elongation, and durability. Tendons usually consist of high strength polyester cord. The design load and spacing of the tendons is determined by the force to be supported. A large number of lighter tendons is preferable to a smaller number of heavier tendons. Batten strips or large washers at the bottom of the lowest section of TerraCell are essential to avoid stress concentrations.

Before opening the TerraCell sections, drill holes where the tendons are to be located. Then thread the tendons through the holes and through the washers or holes in batten strips at the bottom of the lowest panel. “Tie off” the lower end of the tendon. Stretch the TerraCell sections, taking care that the tendons do not come out of the holes.

Position the sections and then tie the tendons to a supporting structure above the crest of the slope. The structure could be a grade beam or other structure designed to support loads. Instead of a structure, the tendons could be tied to earth anchors installed beyond the crest of the slope.

The types of earth anchors most appropriate for use to secure TerraCell to the slope are:

Those which are pushed or driven into the ground and then pulled back a
short distance to cause the harpoon-like head to deploy, or

2. Dead Man anchors such as concrete block, PVC pipe, etc.

In order to avoid stress concentrations on the TerraCell, a large number of light anchors is preferable to a smaller number of heavy anchors. In addition, use of large washers or batten strips help relieve point stresses where the anchors are attached to the TerraCell.

Earth Anchor Using Dead Man System

Earth Anchor Using Harpoon-like System

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 TerraCell
             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.

Which Anchoring Method?

Given the resulting net sliding force (NSF) for two of the cases in Table I, the next step is to decide how to anchor the TerraCell. For the situation where NSF = 56 lb/ft, two common methods of anchoring the TerraCell are to toe it in or to stake it to the slope. For the situation where NSF = 353 lb/ft, the TerraCell could be supported by earth anchors with tendons.

Anchor
Trench:

For a Net Sliding Force of 56 lbs/ft, a factor of safety of 1.25, fill material in the trench with a soil unit weight of 125 lbs/cubic foot, and a f angle of 28 degrees; the required L x H is 1.1 square foot.

L X H =		56 X 1.25	=		1.1 sq. ft.
		125 X tan 28^o

A practical combination would be to bury the top edge of the TerraCell 1 foot deep and 1.1 foot back. Another practical combination would be to let L be 2.2 feet and let H be 0.5 feet.

Stakes

56 lb/ft is equivalent to 56 x 8 = 448 lb for the 8-foot wide panel. Using a factor of safety of 1.25 and a stake pull-out capacity of 60 lbs:

448 x 1.25	=	_{9.33 j-hooks, use 10 stakes per 8-foot panel width}
60

Tendons

353 lb/ft is equivalent to 353 x 8 = 2824 lb for the 8-foot wide panel. Using a factor of safety of 1.25 and a tendon tensile strength of 700 lbs:

2,824 x 1.25	₌	_{5.04 tendons, use 5 tendons per 8-foot panel width.}
700

If the tendons are tied to earth anchors, using the same number of anchors as tendons, and an additional factor of safety of 1.25 to account for uncertainties in the subgrade soil

2824 x 1.25x 1.25	₌	_{883, use anchors with a minimum pull-out capacity of 900 lbs/anchor}
5

Installation Frame

Sometimes it is necessary to pre-stretch and open the TerraCell prior to placing it on a slope. This is especially true if part or all of a section were going under water. In most cases, an installation frame can be built from common lumber (or PVC) and rebar.

Phone: +1.800.438.0027

Fax: +1.704.394.7946

6509-C Northpark Blvd.

Charlotte, NC 28216

e-mail: info@webtecgeos.com