As siting or expanding landfills becomes increasingly difficult, you might consider the use of mechanically stabilized earth walls for adding airspace.

By Thomas M. Yanoschak

Mechanically stabilized earth (MSE) walls offer engineers the ability to construct near-vertical earthen walls, reducing the amount of land required for embankment construction. Whereas a typical unreinforced soil might be able to achieve maximum stable slopes of between 3:1 (H:V) and 2:1, or 18.4° through 26.6°, the use of soil reinforcement allows construction of slopes of 1:3 (71.6°) and even 1:6 (80.5°). The allowable slope of a reinforced embankment is determined by the quantity and placement of reinforcing elements and also the type of facing selected for the wall.

Example: Solid Waste Application

Figure 1. Plan View and Cross-Section of a Hypothetical Landfill at Capacity

Figure 2. Possible MSE Wall Configuration

Figure 3. Alternate MSE Wall Design

Figures 1 through 3 illustrate how incorporation of an MSE wall can increase the waste-disposal capacity of an existing landfill without significantly increasing its footprint.

Figure 1 shows a plan view and a cross-section of a hypothetical landfill that has reached capacity. The sideslopes of the 1,000- x 1,000-ft. landfill are 3:1 (18.4°) with a height of 143 ft. The fill is bordered by a 55-ft.-wide perimeter berm (crest width of 25 ft., sideslope width of 30 ft.). The gross volume of the landfill is approximately 2.4 million yd.³

Figure 2 shows a possible MSE wall configuration in which the outer toe of the perimeter berm is kept constant but soil reinforcement is used to increase the outer slope of the perimeter berm from 3:1 (18.4°) to 1:3 (71.6°). An MSE wall with a height of approximately 18 ft. will maintain the crest width of the perimeter berm at 25 ft. but allow a continuation of the interior sideslope of the berm that would be lined to provide an additional 17 ft. of disposal depth throughout the landfill. In this example, approximately 700,000 yd.³ (29%) of additional volume is gained.

Figure 3 is an alternate MSE wall design in which the edge of the waste is kept constant in order to meet minimum buffer requirements for the landfill or to avoid regulatory requirements for a lateral expansion. The MSE wall with a 1:3 (71.6°) outer slope and extending 30 ft. above the existing perimeter berm maintains the 25-ft. crest width and the current limit of waste. Although this design requires a wedge of structural fill within the limits of waste, it allows an additional 30 ft. of disposal depth, resulting in approximately 1,070,000 yd.³ (45%) of added volume after subtraction of the structural wedge volume.

MSE Wall Design

While metal strips and high-strength polyester geotextiles have been used in MSE soil reinforcement, by far geogrids–a geosynthetic material consisting of connected parallel sets of tensile ribs with apertures of sufficient size to allow strike-through of surrounding soil, stone, or other geotechnical material–are in common use today. Geogrids can be uniaxial or biaxial, depending on the orientation of the load-carrying ribs, and either flexible or rigid, depending on the method of manufacture. Uniaxial geogrids are designed to carry tensile loads along one principal direction, while biaxial geogrids are designed to carry tensile loads in two principal directions located 90° apart.

Biaxial geogrids are used for wrapping the face of the MSE wall and as secondary reinforcement near the face since they are more capable of resisting shallow, multidirectional-type failures that might occur near the face. Biaxial geogrids are also used in soft-soil conditions where additional bearing capacity is desired, such as beneath structures or roadways located over soft soils.

Rigid geogrids are manufactured from sheet plastic, usually high-density polyethylene for uniaxial geogrids and polypropylene for biaxial geogrids. The geogrid is formed by punching holes in the sheet and cold-stretching (post-tensioning) the sheet in the direction of strength. Uniaxial geogrids are therefore stretched in one direction, while biaxial geogrids are stretched in two.

Flexible geogrids are made of high-tenacity polyester yarns sprayed with a protective coating, such as latex or polyethylene, to facilitate handling and to avoid ultraviolet and chemical degradation of the polyester yarns.

Probably the most important design property of a geogrid, and possibly the most confusing, is its strength. Strength values are often given in the machine direction and cross—machine direction and usually represent minimum average roll values. The machine direction of the geogrid typically exhibits the greater strength.

Geogrid product literature often lists the wide-width strip tensile strength of the product as determined by ASTM D4595 (ASTM, 1994), also known as the ultimate tensile strength (T_ult). This value, however, should never be used directly in the design of long-lasting structures, such as MSE walls forming a perimeter berm around a landfill, that will have to remain in place for many years. The wide-width value should be reduced by factors that account for creep deformation, installation damage, and chemical/biological degradation. This reduced-strength value is termed the long-term design strength (LTDS) and can typically equal half of the wide-width strength.

The Geosynthetic Research Institute (GRI) has developed a methodology for determining LTDS for geogrids. This methodology, known as GRI-GG4, has been adapted for rigid geogrids (GRI-GG4a) (GRI, 1993a) and flexible geogrids (GRI-GG4b) (GRI, 1993b). The LTDS for flexible geogrids is calculated as follows:

LTDS = T_ult [1/FS_ID x FS_CR x FS_CD x FS_BD x FS_JNT]
where
FS_ID = partial factor of safety for installation damage,
FS_CR = partial factor of safety for creep deformation,
FS_CD = partial factor of safety for chemical degradation,
FS_BD = partial factor of safety for biological degradation, and
FS_JNT = partial factor of safety for joints (seams and connections).

FS_ID accounts for damage that can occur to the geogrid during transportation, installation, equipment traffic, fill placement, and fill compaction. Values can be obtained by comparing the before- and after-installation tensile strengths of exhumed samples. Values, however, are dependent on backfill type and installation conditions.

FS_CR is determined from a standardized tension creep test and is a measure of the highest load the geogrid can sustain without exceeding 10% strain over 100 years.

FS_CD accounts for degradation of the geogrid due to chemical exposure, ultraviolet exposure, and hydrolysis effects on polyester.

FS_BD is a means by which the effects of bacterial and fungal resistance can be taken into account. This value is usually 1.0 for most geogrids, indicating no reduction in strength, due to the inherent resistance of the geogrid materials or coatings to biological activity.

FS_JNT allows reductions in strength due to the seaming or joining of geogrid layers to be accounted for in the LTDS calculation. Most MSE wall designs require that a primary reinforcement layer of geogrid be installed perpendicular to the wall from a single roll without joints or seams. In this case, FS_JNT would be 1.0.

The methodology for calculating the LTDS for rigid geogrids is similar to that used for flexible geogrids, except for the addition of a partial factor of safety for junction strength of the geogrid, FS_JNT, in the denominator of the equation.

Most geogrid manufacturers have performed testing on their products to determine suitable values for the above reduction factors. Manufacturers’ recommendations for these values should be used in the design, provided that the MSE wall will not experience any unusually harsh conditions during installation or the life of the wall. Harsh conditions might warrant project-specific testing.

The tensile modulus of a geogrid is also an important strength property in design of MSE walls. High-tensile moduli are desirable since they allow development of high-tensile strengths at low deformations. The design strength of geogrids should be adjusted to ensure that deformations produced at the anticipated stresses do not lead to excessive deformation that could jeopardize the stability of the wall or affect aesthetics. Design based on the LTDS typically yields acceptable deformations, although this should be checked on a case-by-case basis.

The tensile forces developed within an MSE wall are transferred from the soil to the geogrid by the frictional resistance and bearing stresses developed between the geogrid and the soil. The amount of stress transferred to the geogrid is dependent on the geogrid’s geometric structure and the surrounding soil properties. The amount of embedment length of the geogrid beyond the potential failure surface is affected by the ability of the geogrid to interlock with the surrounding soil.

Stress transfer between geogrids and backfill soils can be evaluated in the laboratory using pullout tests. Generally, granular backfill soils exhibit greater stress transfer and therefore are preferred for MSE wall construction. The cohesive component of shear stress, such as in clayey soils, is often ignored in MSE wall design in order to include an additional factor of safety in the calculations.

Many other properties are reported by geogrid manufacturers in their product specifications. Although these properties are important to ensure product conformance with specified values, they are generally not used directly in wall design, unlike the strength and geogrid-soil interaction properties previously described. These properties may include rib/strand count, aperture size, open area, thickness, weight, carbon black content, flexural rigidity, junction strength, and junction efficiency.

MSE Wall Stability

Whether a geogrid is uniaxial, biaxial, rigid, or flexible, it develops tensile stresses within the MSE wall in essentially the same manner. A pullout force is exerted on the geogrid as a result of the steepened face of the MSE wall that would otherwise fail if the geogrid were not present. This pullout force is transferred to the geogrid by means of the frictional resistance between the geogrid and the soil on both the upper and lower surfaces of the longitudinal and transverse ribs and by means of bearing resistance against the front surface of the transverse ribs. To counteract the pullout force, resisting forces are developed within the geogrid in the same manner as the pullout forces, except that they act in the opposite direction. For a stable condition to exist, resisting forces must exceed pullout forces.

The wedge of soil that would otherwise move outward is held in place by the geogrid. The length of geogrid located between the face of the wall and the failure plane, designated as pullout length, is where the pullout tensile stress (TP) is developed within the geogrid. To keep the unstable wedge of soil from moving outward, an equal or greater resisting tensile stress (TR) must be developed within the stable soil mass within the resisting length of the geogrid. If TR<TP, then the geogrid embedded in the stable soil will pull out and the wall will fail.

Another important concept is that the pullout and resisting stresses developed within the geogrid must not exceed the tensile strength of the geogrid. If this were to occur, the geogrid would either excessively deform or break and the wall would fail.

Actual MSE wall designs will often have several geogrid layers of varying length, tensile strength, and vertical spacing. This complexity in design is necessary to account for many different types of failure planes that can develop behind the wall. A geogrid designed to resist failure along a particular failure plane might not be adequate to resist failure along a different, but equally probable, failure plane. To ensure that all potential failure planes are addressed, designers use iterative slope-stability programs, such as STABGR (Duncan and Wong, 1984), that can evaluate numerous failure planes quickly. Some geogrid manufacturers also produce proprietary software for MSE wall analysis, such as StrataWall by Strata Systems Inc. The designer adjusts the geogrid strength, spacing, and length until an acceptable minimum factor of safety against failure is achieved, typically 1.5 for static conditions and 1.0 for dynamic and seismic conditions.

Failures can be classified as either internal or external, depending on whether or not the failure extends through the reinforced soil mass. The wall designer, often the geogrid manufacturer, is responsible for ensuring internal stability, while the landfill design engineer is responsible for ensuring external stability. Table 1 lists several factors that affect the internal and external stability of MSE walls.

Facing Options

By far, the MSE wall-facing option used most often in solid waste applications involves extending the biaxial geogrid layers beyond the edge of the wall and wrapping them over the layer of soil that is placed over the geogrid. Welded wire forms are used to ensure that the layer is constructed at the proper setback from the previous layer and to aid in the placement of geosynthetics. Secondary biaxial geogrid is then rolled out parallel to the wall face, such that a portion of the geogrid overhangs the welded wire forms. Erosion control matting is placed over the biaxial geogrid for soil retention. The next lift of soil is placed and compacted over the biaxial geogrid and erosion control matting to the top of the welded wire forms, and the free end of the geogrid is wrapped over the top of the soil lift. The facing materials are structurally tied into the wall by the primary uniaxial geogrid layers that are extended to the wall face. The process is repeated for each subsequent lift of soil.

The advantages of the facing option include its low cost when compared with the precast concrete panel and decorative block options and its ability to be hydroseeded to support vegetative growth. Another advantage of this design is its ability to absorb the wall movements previously described without noticeable cracking. The primary disadvantage is the need for periodic maintenance to remove large, deep-rooted vegetation that could distort or damage the facing components and primary geogrids.

Economic Analysis

Table 2 lists some of the factors that must be evaluated when determining the economic feasibility of an MSE wall.

The cost of MSE walls increases exponentially with height because of increased geogrid lengths, decreasing spacings at the base of the wall, and increased fill volumes. Accurate cost estimating requires input from consultants, geogrid manufacturers, and contractors who are experienced in MSE wall applications. Construction costs should include land, material, installation, contractor markup, and bonding costs, as well as a contingency. The cost of making modifications to the existing landfill components should also be evaluated. In situations where additional waste will be placed over a previously placed final cover, the cost of removing the final cover should be accounted for.

Construction quality-assurance requirements for MSE wall construction typically include observations and testing for proper excavation to suitable soils, proper placement of backfill to prevent damage to geogrids, adequate compaction of backfill, soil properties, strength of geogrids, placement of geogrids, and facing installation.

Operational factors that should be included in an MSE cost analysis include additional maintenance requirements, such as the periodic removal of unsuitable vegetation from the face, periodic inspections for wall integrity, and additional leachate pumping costs. The MSE wall will also limit access to the landfill. Traffic will most likely be required to enter and exit the landfill from one or more permanent ramps built into the design or from locations where the MSE wall does not exist. Operational difficulties resulting from narrow working faces and the necessity for long and frequently changing haul roads should be evaluated.

A potential disadvantage is that once an MSE wall is in place, future lateral expansions become impractical. Retrofitting a wall to provide gravity drainage and lining the steep outer slopes of the wall present serious obstacles. Closure and postclosure costs that should be evaluated include final-cover construction costs and maintenance costs. Placing only 20 or 30 ft. of waste over a slope that has already received final cover might not be cost-effective.

Evaluation of Alternatives

Cost comparisons should be made once several landfill expansion options have been identified and thorough cost estimates have been prepared. At this stage it is helpful to present the cost information in the following format:

UnitCost = TC / NetCY

where

UnitCost = unit cost per cubic yard of developed airspace ($/yd.³),

TC = total cost of developing landfill expansion ($), and

NetCY = net solid waste disposal capacity of landfill expansion (yd.³).

Once this calculation is made, the various expansion options can be ranked according to UnitCost to determine the most cost-efficient alternatives. At this point, the landfill owner or operator should be able to determine whether the cost of developing the additional airspace can be absorbed by the current market conditions. Future market conditions should also be estimated for the period of time when the expansion will come on-line. Factors such as waste-volume increases/decreases and the opening or closure of competing landfills or transfer stations should be evaluated.

The lowest UnitCost, however, should not necessarily determine which expansion option to choose. The net airspace gained by an option should be considered. The owner or operator might be able to accept a lower profit margin if it will result in a substantial increase in airspace, therefore delaying the need for future expansions or development of a new landfill.

MSE walls provide the landfill owner or operator with an option for potentially expanding a landfill when the traditional lateral or vertical expansion is either not feasible or not cost-effective. Successful incorporation of an MSE wall in a landfill design, however, requires careful evaluation of the many design variables. A thorough and accurate cost analysis should be performed during the conceptual design phase to ensure that an MSE wall will be cost-effective.

References

ASTM. "Test Method D4595-86 (1994) Standard Test Method for Tensile Properties of Geotextiles by the Wide Width Strip Method." American Society for Testing and Materials, West Conshohocken, PA. 1994.

Berg, R.R., V.E. Chouery-Curtis., and C.H. Watson. "Critical Failure Planes in Analysis of Reinforced Slopes." Proceedings of Geosynthetics ’89 Conference. San Diego, CA. 1989.

Christopher, B.R. "FHWA Guideline for Reinforced Soil Design, Theory and Practice." Federal Highway Administration, Washington, DC.

Duncan, J.M. and K.S. Wong. "STABGM — A Computer Program for Slope Stability Analysis with Circular Slip Surfaces and Geogrid Reinforcement: Users Manual." Virginia Polytechnic Institute and State University, Blacksburg, VA. 1984.

GRI. "GG4a: Determination of Long-Term Design Strength of Stiff Geogrids." Geosynthetic Research Institute, Drexel University, Philadelphia, PA. 1993a.

GRI. "GG4b: Determination of Long-Term Design Strength of Flexible Geogrids." Geosynthetic Research Institute, Drexel University, Philadelphia, PA. 1993b.

Koerner, R.M. Designing With Geosynthetics, Fourth Edition. Prentice Hall, Upper Saddle River, NJ. 1997a.

Tensar Corporation. "Guidelines for the Design of Tensar Geogrid Reinforced Soil Retaining Walls." Tensar Technical Note TTN:RW 1. 1988.

Guest author Thomas M. Yanoschak, P.E., is project manager with Camp Dresser & McKee in Raleigh, NC.