Design Guideline: EnviroGrid® Cellular Confinement Retaining Walls

Two basic types of retaining walls are cantilever and gravity. Most cantilever retaining walls are made of cast-in-place, steel reinforced concrete. This type of structure is able to retain the earth behind it by virtue of its internal strength and rigidity. Reinforced concrete retaining walls are expensive to build, cannot be built in very cold weather and are not especially attractive. These walls are brittle and if stresses resulting from differential settlement exceed the strength of the concrete, cracks develop affecting the structure's stability and appearance.

 

Gravity retaining walls are constructed principally of soil that is stabilized with man-made materials such as Envirogrid® cellular confinement. This type of structure is able to retain the earth behind it by virtue of its weight. Gravity retaining walls are typically inexpensive to build and can be built in nearly all weather conditions. In addition, they have a degree of flexibility that allows them to adjust to small amounts of differential settlement without suffering structural damage.

 

Envirogrid® cellular confinement can also be used to create a steepened slope. Steepened slopes are slopes that are constructed much steeper than the soil alone would allow. A steepened slope made with EnviroGrid® can be considered to be a retaining wall if it has a face inclination greater than 25º.

 

 

Steepened Slope

 

An EnviroGrid® cellular confinement retaining wall or steepened slope can be constructed in almost any situation where rapid change of grade is desired. EnviroGrid® confines the soil or other fill material allowing the material to behave as a reinforced mass.

 

EnviroGrid® cellular confinement can be used in both fill and cut applications. In a project where the structure is built with fill material, it is usually more cost-effective to use EnviroGrid® in conjunction with a geogrid. The geogrid acts as a tieback in the reinforced zone. EnviroGrid® assumes the role of the facing element, such as concrete blocks in a modular retaining wall. Unlike modular block, EnviroGrid® cellular confinement is easy to handle, flexible and can be planted with grass or shrubbery to give the face a natural look.

 

 

In cut situations, the exposed face is unstable because the soil holding it in place has been removed. Some of this weight needs to be replaced, but in a much smaller area. In this circumstance it is often more economical to use EnviroGrid® cellular confinement in lieu of other methods such as gabion baskets. When used in this manner, the stack of filled EnviroGrid® panels acts as a near vertical, heavy, reinforced mass.

 

 

A gravity retaining wall must have sufficient weight and width or be otherwise supported so that it does not overturn or slide forward due to external forces being exerted upon it. The wall must also be able to hold together as a unit in order to function. That is, the wall must be stable with respect to both the external forces that might cause it to fall and the internal forces that might cause it to lose its shape and/or disintegrate.

 

 

EnviroGrid® cellular confinement retaining walls must be designed to be stable with respect to four potential external failure modes: global stability, base sliding, overturning and bearing capacity.

 

 

Global stability refers to the stability of the wall, the soil behind it and the soil below it. The design engineer must be certain that the entire area including the wall does not collapse. A thorough soil analysis must be performed to eliminate the possibility of global failure.

 

 

Global Stability Failure

 

Base sliding refers to the outward movement of the bottom of the retaining wall as a result of the lateral forces generated by earth pressure and, if present, water pressure. The force resisting base sliding is the friction between the fill in the bottom layer of the EnviroGrid® cellular confinement and the foundation soil beneath the bottom layer. The designer may increase the front-to-back dimension of the wall if calculations show that the resisting force is less than required. This will increase the area available to develop the resisting force. A second option would be to use a fill with greater frictional characteristics.

 

 

 

Overturning refers to the tipping over of the retaining wall as it rotates about the toe of the structure. The overturning force is the sum of each destabilizing force times its moment arm. The stabilizing force, or righting moment, is the product of the weight of the retaining wall and its moment arm, which is the horizontal distance from the toe to the center of the gravity wall. If calculations show that the righting moment is less than required, one option is to increase the front-to-back dimension of the wall, thereby increasing its overall weight and the magnitude of its moment arm.

 

 

Overturning Failure

 

 

Bearing capacity refers to the ability of the foundation soil to support the weight of the retaining wall placed upon it. The analysis is the same as for shallow foundations. It is necessary to increase the area of the base if calculations show that the soil beneath the wall is too weak. This will decrease the pressure (force per unit of area) on the foundation. Another option is to increase the depth into the ground of the retaining wall, thus increasing the ability of the foundation soil to resist the imposed weight.

 

 

Bearing Capacity Failure

 

For each of these considerations, the resisting or stabilizing forces must exceed the forces that would cause failure by a predetermined factor of safety for each of these considerations. The selected factors of safety should reflect the consequences of failure and the designer's confidence in the accuracy of the input parameters. The following factors of safety are normally used in design of gravity retaining walls:

 

 

 

 

 

If the minimum front-to-back dimension of a wall that uses EnviroGrid® cellular confinement is at least 0.6 times the wall height, the above factors of safety will be achieved in almost any design.

 

 

Internal stability refers to the ability of the individual parts of the wall to act as a single unit. The wall must be designed so that the individual pieces of the wall do not pullout, separate or slide apart. In a modular block wall, then designer must be concerned with the potential of the tieback failing under tension or pulling out from the soil or fascia. Also, a failure can occur if the fascia bulges out. The only internal stability consideration for walls consisting of EnviroGrid® cellular confinement and soil is the potential for sliding between panels. If a factor of safety of 1.5 or greater is not achieved with the initial design, the sections need to be made longer to increase the surface area or the fill material needs to be changed to one with greater frictional angles.