Engineering and Construction

The construction of a mechanically stabilized earth retaining wall is shown in Fig. 11.9 to 11.11, as follows:

1. Excavation of key and installation of drainage system (Fig. 11.9). This photograph shows the
first step in the construction of a mechanically stabilized earth retaining wall. A key (slot) has
been excavated into the natural ground and a drainage system is being constructed at the back of
the key.

2. Construction of mechanically stabilized earth retaining wall (Fig. 11.10). A mechanically stabilized earth retaining wall is composed of three elements: (1) wall facing material, (2) soil reinforcement, such as strip- or grid-type reinforcement, and (3) compacted fill between the soil reinforcement, as follows:

a. Wall facing element. The wall facing elements, which are precast concrete members, are being installed as shown in Fig. 11.10. Other types of wall facing elements are often utilized, such as wood planks or concrete interlocking panels. The wall facing members are first aligned and then connected together by using steel dowels.

b. Geogrid. Figure 11.10 shows the black geogrid, which is a type of geosynthetic. Geogrids are usually composed of a high strength polymer used to create an open grid pattern. The large arrows in Fig. 11.10 indicate the width of the geogrid, which is considered to be the zone of mechanically stabilized earth. Geogrid is transported to the site in rolls, which are then spread out and cut as needed. Geogrid has different tensile strengths along the roll versus perpendicular to the roll and it is important that the highest strength of the geogrid be placed in a direction that is perpendicular to the wall face. The small arrow in Fig. 11.10 points to a splice, where the geogrid is overlapped. A smaller width of geogrid is used at the end of the wall (location of small arrow) because the required resistance is less at the wall perimeter. The geogrid is often attached to the wall facing elements and the vertical spacing of the geogrid is equal to the thickness of the wall facing elements.

c. Compaction of fill. The final step is to place fill on top of the geogrid and the fill is then compacted. Since this mechanically stabilized earth retaining wall only has a drain in the key, granular (permeable) soil is being used to prevent the buildup of pore water pressure behind or in the mechanically stabilized earth zone. For geogrid installed in cohesive (plastic) soil, a backdrain system is often installed.



The geogrid and compacted fill derive frictional resistance and interlocking resistance between each other. When the mechanically stabilized soil mass is subjected to shear stress, the soil tends to transfer the shear stress to the stronger geogrid. Also, if high stress concentrations develop in the mechanically stabilized soil mass, such as along a potential slip surface, then the geogrid tends to redistribute stresses away from areas of high stress.

Lightweight equipment must be used to compact the fill that is placed on top of each layer of geogrid. Heavy compaction equipment could push the wall facing elements out of alignment.



3. Final constructed condition (Fig.11.11). This photograph shows the final constructed condition of the mechanically stabilized earth retaining wall. The arrow in Fig. 11.11 points to a gap in the wall facing element, which can be used to install plants which will grow over the wall facing elements and eventually blend the wall into the surrounding vegetation.

The design analysis for a mechanically stabilized earth retaining wall is more complex than for a cantilevered retaining wall. For a mechanically stabilized retaining wall, both the internal and external stability must be checked as discussed in the next two sections.