Design Guideline: EnviroGrid® Cellular Confinement Soil Stabilization

Introduction

When traffic loads are applied to a soil subgrade, the soil will not deform or rut if the shear strength of the soil exceeds the applied loads. The strength of the soil is a function of such characteristics as its angle of internal friction, its cohesion and its degree of compaction.

Most road and parking systems consist of one or more layers of good quality fill materials placed and compacted on soil subgrades. The fill materials allow the system to support traffic loads that the soil, by itself, would not be able to withstand. The function of the layer(s) of base material is to distribute the imposed loads over a large area, thereby reducing the pressure (load divided by area) which is transferred to the subgrade. The base material is able to distribute the loads because the individual aggregate particles lock together. Applied loads are transmitted through the base material both as vertical and horizontal forces.

If the horizontal (lateral) forces push the base material sideways, rutting develops resulting in a thinner layer less able to resist additional load applications which then leads to failure. Even a good quality base material with the proper internal strength and interlocking of individual particles can be forced to move laterally. The poor quality subgrade in contact with the base material does not provide the required friction at the interface to restrain the movement.

Designing with Envirogrid® Cellular Confinement

EnviroGrid® cellular confinement filled with a base material acts as one layer in a multi-layer road system. A broadly accepted method used to analyze and design multi-layered road systems is a two-step procedure developed by AASHTO (American Association of Highway and Transportation Officials).

STEP ONE

The engineer determines the necessary overall strength of the road system, which is called the required Structural Number (SN). The SN is a function of three factors:

1. Soil Support Value (SSV). The strength of the subgrade soil is determined by one of a variety of standard methods. Through the use of equivalence tables, the subgrade is used to select the appropriate Soil Support Value.
2. Equivalent Axle Load (EAL). The expected traffic loads over the life of the system are tabulated. These include H²O loading (20-ton trucks with a given wheel configuration), lighter trucks, autos, etc. Using a table developed by AASHTO, each type of loading is converted to a common, single measure based on the impact that loading is expected to impose upon the road system. The common measure is a single 18,000 lb axle load and is called the Equivalent Axle Load.
3. Regional Factor (RF). This factor accounts for the susceptibility of the subgrade soils at the construction site to conditions of moisture and temperature. The Regional Factor, which typically ranges from 0.5 to 3.0 in the forty-eight contiguous states, can be selected from a map developed for this purpose.
The engineer enters these three factors into a monograph developed by AASHTO that determines the required SN.

STEP TWO

Select the base materials and the thickness of the layers of those materials which, when combined, will provide an SN equal to, or greater than, the required SN. Each base material is assigned a Structural Coefficient (SC), which is related to the ability of the material to spread applied loads. It has been conservatively determined that the SC for EnviroGrid® cellular confinement filled with granular material such as sandy soil is 0.35. A better load-bearing fill material would increase the EnviroGrid® structural coefficient. In the following table are structural coefficients for various fill materials and EnviroGrid® cellular confinement filled with sandy soil and the resulting equivalent layer thickness:

*Filled with sandy soil

Multiplying the SC of a given material by the thickness of the layer of that material, in inches, determines the contribution of that layer toward the required SN. For example, if the required SN is 2.90 and the engineer wants the top layer of the road system to be 2 inches of asphalt or concrete, they could make either of the following selections for the remainder of the base:

1. 15" of crushed stone(15 x .14) + (2 x .41) = 2.92
2. 6" EnviroGrid® with sandy soil   (6 x .35) + (2 x .41) = 2.92

Alternatively, if the engineer knows how much of a base material is normally used in a given design, they can substitute EnviroGrid® cellular confinement for that material in relation to their structural coefficients. For example, EnviroGrid® filled with sandy soil has five times (.35 / .07 =5) the support value of sandy soil without EnviroGrid®. Professor Robert Koerner provides an example which shows that the use of 8" EnviroGrid® increases the bearing capacity of sandy soil by 13 times (Designing with Geosynthetics, Koerner, Fourth Edition). As such, 4" EnviroGrid® cellular confinement filled with sandy soil has the same load bearing strength as 20 inches of sandy soil without EnviroGrid®. Therefore, if a road design calls for 18 inches of a sandy soil fill, the engineer could reduce that amount to a 4" EnviroGrid® with the same type of fill and have a stronger base.

The designer can add local fill materials to the above table with the appropriate AASHTO structural coefficients to calculate the savings using EnviroGrid® cellular confinement. Examples of such locally available materials are crushed shell in coastal areas, river gravel in mountainous areas and high quality limestone in other areas.

A complete description of the AASHTO design procedure, as well as design software are available from AASHTO @ (202)624-5800 or at www.aashto.org.