15 AUGUST 2005
Rock. The suitability of rock as backfill material is highly dependent upon the
gradation and hardness of the rock particles. The quantity of hard rock excavated at most
subsurface structure sites is relatively small, but select cohesionless materials may be difficult
to find or may be expensive. Therefore, excavated hard rock may be specified for crusher
processing and used as select cohesionless material.
Shale. Although shale is commonly referred to as rock, the tendency of some
shales to breakdown under heavy compaction equipment and slake when exposed to air or
water after placement warrants special consideration.
Some soft shales break down under heavy compaction equipment causing the
material to have entirely different properties after compaction than it had before compaction.
This fact should be recognized before this type of material is used for backfill. Establishing the
proper compaction criteria may require that the contractor construct a test fill and vary the
water content, lift thickness, and number of coverages with the equipment proposed for use in
the backfill operation. This type of backfill can be used only in unrestricted open zones where
heavy towed or self-propelled equipment can operate.
Some shales have a tendency to break down or slake when exposed to air.
Other shales that appear rock-like when excavated will soften or slake and deteriorate upon
wetting after placement as rockfill. Alternate cycles of wetting and drying increases the slaking
process. The extent of material breakdown determines the manner in which it is treated as a
backfill material. If the material completely degrades into constituent particles or small chips
and flakes, it must be treated as a soil-like material with property characteristics similar to ML,
CL, or CH materials, depending upon the intact composition of the parent material. Complete
degradation can be facilitated by alternately wetting, drying, and disking the material before
compaction. A detailed discussion on the treatment of shales as a fill material is given in
Marginal Materials. Marginal materials are those materials that, because of
either poor compaction, consolidation, or swelling characteristics, would not normally be used
as backfill if sources of suitable material were available. Material considered to be marginal
include fine-grained soils of high plasticity and expansive clays. The decision to use marginal
materials should be based on economical and energy conservation considerations to include
the cost of obtaining suitable material whether from a distant borrow area or commercial
sources, possible distress repair costs caused by use of marginal material, and the extra costs
involved in processing, placing, and adequately compacting marginal material.
The fine-grained, highly plastic materials make poor backfill because of the
difficulty in handling, exercising water-content control, and compacting. The water content of
highly plastic finegrained soils is critical to proper compaction and is very difficult to control in
the field by aeration or wetting. Furthermore, such soils are much more compressible than
less-plastic and coarse-grained soils; shear strength and thus earth pressures may fluctuate
between wide limits with changes in water content; and in cold climates, frost action will occur
in fine-grained soils that are not properly drained. The only soil type in this category that might