The effect of global stability on MSE wall design
Mechanically stabilized earth (MSE) walls—gravity retaining walls constructed by reinforcing a mass of soil—have become common during the last 30 years. Just as steel bars are used to reinforce concrete, a material weak in tension, soil also is a material with little or no tensile strength. MSE walls typically have a facing constructed from masonry blocks or precast concrete panels/blocks connected to the soil reinforcement, which typically consists of geosynthetic sheets or metallic strips.
There are many reasons for building MSE walls, such as aesthetics, expected performance, design flexibility, or ease of construction. However, the principle reason is likely cost effectiveness. A study published by the Federal Highway Administration (FHWA) in 1995 found that an MSE wall would be expected to cost about half that of an equivalent castin- place concrete retaining wall.
Procedures for the design of MSE walls are well established, and are documented by the FHWA and the National Concrete Masonry Association. These procedures for assessing stability use limit equilibrium methods for analysis and design. The use of limit equilibrium is attractive because practitioners are experienced with its application, data required for analysis is relatively simple, and the results can be checked easily using charts or hand calculations.
Conceptually, these design analyses are divided into analyses of methods of potential wall failure.
Internal, external, facing, and global stability all should be considered in MSE wall design. Internal stability analyses examine a wedge of soil in the area where reinforcements will be placed (the reinforced zone) and determine the number, spacing, length, and strength of the reinforcements required. Analyses of external stability evaluate the effect of forces from outside the reinforced zone. These can include sliding of the wall and the bearing capacity and settlement of the foundation soils below the wall.
External stability often affects the length of the reinforcements needed. Facing stability looks at the local stability of the wall facing and the connection of the reinforcements to the facing. The results of facing stability may modify the spacing and strength of the reinforcement.
Global stability analyses are an application of conventional methods of slope stability to the wall and its environment.
The result of a global stability analysis typically is expressed in terms of a factor-ofsafety against mass instability. A factor-of-safety of one indicates a condition at failure, with higher factors-of-safety indicating greater stability. Acceptable factors-of-safety vary with the type of analysis—long-term, end-of-construction, or pseudo-static analyses—the consequences of failure, and the level of uncertainty about subsurface conditions.
For a typical wall, an acceptable factor-of-safety might be 1.3 for long-term global static stability. Global stability is properly a type of external stability analysis.
Analyses of internal stability look only at failure surfaces within the reinforced zone, and external stability analyses look only at failures outside the reinforced zone. Failure surfaces in a global stability analysis can be compound; they can pass through both the internal reinforcement and an area external to the reinforced zone. Global stability can affect the length, strength, and spacing of the reinforcement needed, and thus should not be separated from the design process.
To illustrate the effect of global stability on MSE wall design, following are case studies of two MSE walls where analyses of mass/global stability significantly affected the wall designs. Both walls were built successfully and are in service today; however, consideration of global stability required increased reinforcement lengths.
The case studies show that global stability can be a critical issue and suggest that this type of stability analysis should be an integral part of the design process.
Sites for the two walls are located within the Valley and Ridge physiographic province of Eastern Tennessee. Both walls are more than 20 feet tall and use geosynthetic reinforcements. One wall (Wall A) was constructed near the crest of a fill slope inclined at two horizontal to one vertical (2:1) and adjacent to a highway right-of-way.
The other wall (Wall B) was constructed adjacent to a stormwater detention basin.
Wall A was constructed in 2004 in upper East Tennessee and is about 29 feet tall. The wall was built with an adjacent soil fill slope above the right-of-way for an urban thoroughfare. The wall was built using segmental masonry blocks for the wall facing, with 26-foot-long geogrid reinforcements.
A triple-zoned backfill was used within the reinforced zone. A narrow drainage zone of clean, hard, durable rockfill was placed directly behind the wall facing. The middle zone was crushed stone aggregate composed of hard, durable rockfill with fines. The rear zone used clayey soil fill behind the crushed stone.
Originally, the owner of the wall hired a geotechnical engineer to provide the design.
The initial design used crushed stone backfill in the reinforced zone and geogrid reinforcements with a length of 30 feet.
Subsequently, the excavating contractor sought to reduce the cost of the proposed wall, hired a subcontractor to build the wall, and hired another engineer to redesign the wall. The owner retained the original geotechnical engineer to evaluate redesign submittals and observe construction.
The initial redesign submittal also used a crushed stone backfill, but had geogrid lengths of only 13 feet. This redesign was rejected by the owner on the advice of the original geotechnical engineer. A second redesign submittal was prepared, this time with a geogrid length of 20 feet. Again, the redesign was rejected by the owner. In conversations with the redesigning engineer, it was found that global stability analyses were not being considered in the design process. A third redesign was prepared, one that considered global stability and resulted in the zoned fill around the reinforcement previously described. The zoned fill was used to reduce the cost of the backfill used for the wall. The owner accepted this redesign.
Table 1 shows the factors-of-safety against mass instability under long-term conditions for varying reinforcement lengths. A factor-of-safety of 1.3 was considered acceptable.
Wall B was constructed in East Tennessee in 2004 and is about 22.5 feet tall. The wall was built with an adjacent stormwater detention basin below the wall and a parking lot above the wall. The wall was built using precast concrete blocks for the wall facing with 21-footlong geogrid reinforcements. Crushed stone was used as backfill around the reinforcement.
Because of the adjacent detention pond, the design of Wall B needed to address water levels that might rise and fall.
Consideration of the possible water levels within the reinforced zone and consideration of global stability increased the geogrid lengths needed. Table 2 shows the factors-of-safety against mass instability with and without water, and with consideration of global stability. A factor-of-safety of 1.3 was considered acceptable.
The municipality where Wall B was built had a moratorium on the construction of MSE walls because of a perceived high number of MSE wall failures. Wall B was one of the first three walls built that ended this ban. Its design underwent an independent technical review and contributed to the development of technical guidelines for use by municipal engineers in reviewing new MSE walls.
Jessee A. Scarborough, P.E., is a geotechnical engineer with Bunnell-Lammons Engineering, Inc., in Asheville, N.C. He can be reached at email@example.com.