Processing of Borrow Area Soils / Backfill Materials
The construction of subsurface structures often requires the construction of elements of the structure within or upon large masses of backfill. The proper functioning of these elements are often critically affected by adverse behavioral characteristics of the backfill. Behavioral characteristics are related to material type, water content during compaction, gradation, and compaction effort.
While compaction effort may be easily controlled during compaction, it is difficult to control material type, water content, and gradation of the material as it is being placed in the backfill; control criteria must be established prior to placement.
1. Material type
Backfill material should consist of a homogeneous material of consistent and desirable characteristics. The field engineer must ensure that only the approved backfill material is used and that the material is uniform in nature and free of any anomalous material such as organic matter or clay pockets.
Stratified material should be mixed prior to placing to obtain a uniform blend. Excavated material to be used as backfill should be stockpiled according to class or type of material.
2. Water content
While water content can be adjusted to some extent after placing (but before compacting), it is generally more advantageous to adjust the water content to optimum compaction conditions before placing.
Adjustment of water content can be accomplished by aeration (disking or turning) or sprinkling the material in 12 to 18 inch layers prior to placing or stockpiling. If the material is stockpiled, provisions should be made to maintain constant moisture content during wet or dry seasons.
3. Ensuring gradation
Some backfill materials consisting of crushed rock, gravel, or sand require limitations on maximum and minimum particle-size or gradation distributions. Where materials cannot be located that meet gradation criteria, it may be advantageous to require processing of available material by sieving to obtain the desired gradation.