DESIGN CONSIDERATIONS FOR A MECHANICALLY STABILIZED EARTH WALL.

The design of a MSE wall involves the following steps:

1. Check for internal stability, addressing reinforcement spacing and length.
2. Check for external stability of the wall against overturning, sliding, and foundation failure.

The general considerations for the design are:

1. Selection of backfill material: granular, freely draining material is normally specified.
However, with the advent of geogrids, the use of cohesive soil is gaining ground.
 
2. Backfill should be compacted with care in order to avoid damage to the reinforcing material.
 
3. Rankine's theory for the active state is assumed to be valid.
 
4. The wall should be sufficiently flexible for the development of active conditions.
 
5. Tension stresses are considered for the reinforcement outside the assumed failure zone.
 
6. Wall failure will occur in one of three ways

a. tension in reinforcements
b. bearing capacity failure
c. sliding of the whole wall soil system.
 
7. Surcharges are allowed on the backfill. The surcharges may be permanent (such as a roadway) or temporary.
 
a. Temporary surcharges within the reinforcement zone will increase the lateral pressure on the facing unit which in turn increases the tension in the reinforcements, but does not contribute to reinforcement stability.
b. Permanent surcharges within the reinforcement zone will increase the lateral pressure
and tension in the reinforcement and will contribute additional vertical pressure for the
reinforcement friction.
c. Temporary or permanent surcharges outside the reinforcement zone contribute lateral
pressure which tends to overturn the wall.
 
8. The total length L of the reinforcement goes beyond the failure plane AC by a length Le.
Only length Le (effective length) is considered for computing frictional resistance. The length LR lying within the failure zone will not contribute for frictional resistance (Fig. 19.15a).

9. For the propose of design the total length L remains the same for the entire height of wall H.
Designers, however, may use their discretion to curtail the length at lower levels. Typical ranges in reinforcement spacing are given in Fig. 19.16.

Figure 19.15 Principles of MSE wall design

Figure 19.16 Typical range in strip reinforcement spacing for reinforced earth
walls (Bowles, 1996)