MSE walls should be dimensioned as required by AASHTO. The soil reinforcement length must be at least 70 percent of the wall height, as measured from the leveling pad, but not less than 8 ft (2.4 m) for both strip and grid type reinforcement. AASHTO requires the reinforcement length to be uniform throughout the entire height of the wall. The specification does allow deviation from this uniform length requirement, subject to the availability of substantiating evidence.
MSE walls must be designed for both external stability and internal stability. The recommended minimum factors of safety in various areas of external stability are noted in AASHTO as follows:
|
External stability |
Factor of safety |
|
Overturning |
2.0 for footings on soil, >1.5 for |
|
footings on rock |
|
|
Ultimate bearing capacity |
>2.5 for Group 1 loadings, >2.0 if justified by geotechnical analysis |
|
Sliding |
1.5 |
|
Overall stability (deep-seated failure) |
>1.5 for bridge abutment walls; >1.3 for walls with static loads; >1.1 for seismic loads |
|
Seismic (overturning and sliding) |
>75% static safety factor |
In lieu of the overturning check, the eccentricity e of the force resultant R must be located such that e < L/6 where L is the base length (see Fig. 8.33). In calculating bearing
In regard to internal stability, AASHTO notes the following:
Factor of safety
>1.5
Allowable tension [see Eqs. (8.14a) and (8.14b)]
According to design life [see Eqs. (8.14a) and (8.14b)
>75% static safety factor
Figures 8.29 and 8.30 indicate the two basic failure modes for internal stability analysis— specifically, rupture or creep failure of the reinforcement and a pullout failure mode. These failure modes suggest the use of the tied-back wedge analysis approach depicted in Fig. 8.31, which represents the basic method of analysis included in the AASHTO specifications.
|
FIGURE 8.31 Parameters for tied-back wedge analysis. |
