Engineering and Construction

The analysis for the external stability is similar to a gravity retaining wall. For example, Figs. 11.12 and 11.13 (adapted from AASHTO, 1996) present the design analysis for external stability for a level backfill condition and a sloping backfill condition. In both Figs. 11.12 and 11.13, the zone of mechanically stabilized earth mass is treated in a similar fashion as a massive gravity retaining wall.

The following analyses must be performed:

1. Allowable bearing pressure. The bearing pressure due to the reinforced soil mass must not exceed the allowable bearing pressure.

2. Factor of safety of sliding. The reinforced soil mass must have an adequate factor of safety for sliding (F = 1.5 to 2).

3. Factor of safety of overturning. The reinforced soil mass must have an adequate factor of safety (F = 2) for overturning about Point O.

4. Resultant of vertical forces N. The resultant of the vertical forces N should be within the middle 1/3 of the base of the reinforced soil mass.

5. Stability of reinforced soil mass. The stability of the entire reinforced soil mass (i.e., shear failure below the bottom of the wall) would have to be checked.

Note in Fig. 11.12 that two forces (P1 and P2) are shown acting on the reinforced soil mass. The
first force P1 is determined from the standard active earth pressure resultant equation (i.e., Eq. 11.1).

The second force P2 is due to a uniform surcharge Q applied to the entire ground surface behind the mechanically stabilized earth retaining wall (i.e., Eq. 11.3). If the wall does not have a surcharge, then P2 is equal to zero.

Figure 11.13 presents the active earth pressure force for an inclined slope behind the retaining wall. Note in Fig. 11.13 that the friction d of the soil along the back side of the reinforced soil mass has been included in the analysis. The value of kA would be obtained from Coulomb’s earth pressure equation (Fig. 11.4). As a conservative approach, the friction angle d can be assumed to be equal to zero and then PH = PA. Note in both Figs. 11.12 and 11.13 that the minimum width of the reinforced soil mass must be at least 7/10 the height of the reinforced soil mass.