Well-graded, free-draining granular material is usually specified for permanent-placed soil reinforced walls. Lower-quality materials are sometimes used in reinforced embankment slopes. Experience with cohesive backfills is limited. However, low strength, creep properties, and poor drainage characteristics make their use undesirable. Some current research is focused on the use of cohesive soil backfills.
The following gradation and plasticity limits have been established by the AASHTO-AGC-ARTBA* Joint Committee Task Force 27 for mechanically stabilized embankments:
|
U. S. sieve size |
Percent passing |
|
4 in (100 mm) |
100 |
|
No. 40 |
0-60 |
|
No. 200 |
0-15 |
|
Plasticity index (PI) less than 6 percent |
It it recommended that the maximum particle size be limited to /4 in (19 mm) for geosynthetics and epoxy-coated reinforcements unless tests show that there is minimal construction damage if larger particle sizes are used.
Metallurgical slag or cinders should not be used except as specifically allowed by the designer. Material should be furnished that exhibits an angle of internal friction of 34° or more, as determined by AASHTO T-236, on the portion finer than the No. 10 sieve. The backfill material should be compacted to 95 percent of AASHTO T-99, method C or D, at optimum moisture content. See Art. 8.5.7 for backfill requirements that are important in relation to the durability of the steel reinforcement.
On-site or local material of marginal quality can be used only with the discretion and approval of the designer.
