15 AUGUST 2005
Placement of Backfill. Backfill construction is the refilling of previously
excavated space with properly compacted material. The areas may be quite large, in which
case the backfilling operation will be similar to embankment construction. On the other hand,
the areas may be quite limited, such as confined areas around or between and beneath
concrete or steel structures and areas in trenches excavated for utility lines. Prior to
construction of the backfill, the inspection personnel should become thoroughly familiar with
the various classes of backfill to be used. They should be able to readily identify the materials
on sight, know where the various types of material should be placed, and be familiar with the
compaction characteristics of the soil types.
Good Construction Practices, and Problems. Problems with placement of
backfill will vary from one construction project to another. The magnitude of the problems will
depend on the type of materials available such as backfill, density requirements, and the
configuration of the areas in which compaction is to be accomplished. Problems should be
expected during the initial stages of backfill compaction unless the contractor is familiar with
compaction characteristics of backfill materials. The inspector can be of great assistance to
the contractor during this period by performing frequent water content and density checks.
The information from these checks will show the contractor the effects of the compaction
procedures being used and point out any changes that should be made.
Backfilling Procedures. Problems associated with the compaction of backfill
can be minimized by following good backfilling procedures. Good backfilling procedures
include: processing the material (paragraph 8-3.4) before it is placed in the excavation; placing
the material in a uniformly spread loose lift of the proper thickness suited to the compaction
equipment and the type of material to be used; applying the necessary compaction effort to
obtain the required densities; and ensuring that these operations are not performed during
adverse weather. Proper bond should be provided between each lift and also between the
backfill and the sides of the excavation.
Compaction Equipment, Backfill Material, and Zones. The type of
compaction equipment used to achieve the required densities will usually depend upon the
type of backfill material being compacted and the type of zone in which the material is placed.
In open zones, coarse-grained soils that exhibit slight plasticity (clayey sands,
silty sands, clayey gravels, and silty gravels) should be compacted with either
sheepsfoot or rubber-tired rollers; close control of water content is required
where silt is present in substantial amounts. For sands and gravelly sands with
little or no fines, good compaction results are obtained with tractor compaction.
Good compaction can also be achieved in gravels and gravel-sand mixtures with
either a crawler tractor or rubber-tired and steel-wheeled rollers. The addition of
vibration to any of the means of compaction mentioned above will usually
improve the compaction of soils in this category. In confined zones, adequate
compaction of coliesionless soils in either the air-dried or saturated condition can
be achieved by vibratory-plate compactors with a static weight of at least 100
pounds. If the material is compacted in the saturated condition, good compaction