15 AUGUST 2005
draining materials are compacted by saturating the material and operating crawler-type
tractor, surface or internal vibrators, vibratory compactors, or other similar suitable
equipment. The specifications generally will prohibit the use of rock or rock-soil
mixtures as backfill in this type of construction. However, when the use of backfill
containing rock is permitted, the maximum size of the rock is given in the specifications
along with maximum lift thickness, loading, hauling, dumping, and spreading
procedures, type of compaction equipment, and method of equipment operation. The
specifications should prohibit the use of rock or rock-soil mixtures as backfill in areas
where heavy equipment cannot operate. Rock / soil mixtures having greater than 8 to
10 percent binder should be prohibited in all areas. In the case of backfill containing
rock, the density is not generally specified. Obtaining adequate density is usually
achieved by specifying the compaction procedures. The specifications may require that
these procedures be developed in field test sections.
Specifications may also require specific equipment and procedures to ensure adequate
bedding for round-bottom structures such as tunnels, culverts, conduits, and tanks.
Procedures normally specified for placement of bedding for these types of structure are
discussed in paragraph1.3.
Backfill Against Structures. The specifications will state when backfill
may be placed against permanent concrete construction with respect to the time after
completion; this time period is usually from 7 to 14 days. To provide adequate
protection of the structures during backfill construction, the specifications require that
the backfill be built up symmetrically on all sides and that the area of operation of heavy
equipment adjacent to a structure be limited. Also, the minimum thickness of
compacted materials to be placed over the structures by small compaction equipment,
such as vibratory plate or rammer type, will be specified before heavy equipment is
allowed to operate over the structure. The specifications require that the surface of the
backfill be sloped to drain at all times when necessary to prevent ponding of water on
the fill. The specifications also provide for groundwater control, so that all compacted
backfill will be constructed in the dry. Where select, freedraining, cohesionless soils of
high permeability are required in areas where compaction is critical, the specifications
list gradation requirements. Gradation requirements are also specified for materials
used for drains and filters.
Unusually severe specification requirements may be necessary for backfill
operations in confined areas. The requirements may include strict backfill material type
limitation, placement procedures, and compaction equipment.
Protection from Freezing. It is not the policy of the Government to
inform the contractor of ways to accomplish the necessary protection from freezing
temperatures. However, to ensure that adequate protection is provided, it may be
necessary to specify that the contractor submit detailed plans for approval for such
STABILIZATION OF SUBGRADE SOILS