Soil Characteristics

  1. Subsurface Exploration. (AREMA 8.5.2.2)
    a. Sufficient borings shall be made along the length of the structure to determine, with a reasonable degree of certainty, the subsurface conditions. Irregularities found during the initial soil boring program may dictate that additional borings be taken.
    b. The subsurface investigation shall be made in accordance with the provisions of AREMA Chapter 8 Part 22, Geotechnical Subsurface Investigation.
  2. Type of backfill.
    a. Backfill is defined as material behind the wall, whether undisturbed ground or fill, that contributes to the pressure against the wall.
    b. The backfill shall be investigated and classified with reference to the soil types described in AREMA Table 8-5-1.

    c. Types 4 and 5 backfill shall be used only with the permission of the Engineer. In all cases the wall design shall be based on the type of backfill used.
  3. Computation of backfill pressure. (AREMA 8.5.3.2a)
    a. Values of the unit weight, cohesion, and angle of internal friction of the backfill material shall be determined directly by means of soil tests or, if the expense of such tests is not justifiable, by means of AREMA Table 8-5-2 referring to the soil types defined in AREMA Table 8-5-1. Unless the minimum cohesive strength of the backfill material can be evaluated reliably, the cohesion shall be neglected and only the internal friction considered. See Appendix page A-6 for AREMA generic soil properties.
  4. Compaction.
    a. The backfill shall preferably be placed in loose layers not to exceed 8 inches in thickness. Each layer shall be compacted before placing the next, but over compaction shall be avoided.
    b. It is required that backfill be compacted to no less than 95% of maximum dry density at a moisture content within 2% of optimum and tested using Modified Proctor ASTM D1557.
    c. Fill within 100 feet of bridge ends or 20 feet outside culverts shall be placed and compacted to not less than 100% of maximum.
    d. No dumping of backfill material shall be permitted in such a way that the successive layers slope downward toward the wall. The layers shall be horizontal or shall slope downward away from the wall.