15 AUGUST 2005
having more than 10 to 20 percent finer than the No. 200 sieve, depending on
Site Problems. Small building areas or congested areas where many
small buildings or utility lines surround the site present difficulties in regard to
maneuvering large compaction equipment. Backfilling adjacent to structures also
presents difficulties, and power handtamping equipment must be employed, with
considerable care necessary to secure uniform compaction. Procedures for backfilling
around structures are discussed in this Chapter.
General. Guidelines for selecting compaction equipment and for
establishing compaction requirements for various soil types are given in table 8-2.1.
When fill materials have been thoroughly investigated and there is ample local
experience in compacting them, it is preferable to specify details of compaction
procedures, such as placement water content, lift thickness, type of equipment, and
number of passes. When the source of the fill or the type of compaction equipment is
not known beforehand, specifications should be based on the desired compaction
result, with a specified minimum number of coverage of suitable equipment to assure
uniformity of compacted densities.
Compaction Specifications. For most projects, the placement water
content of soils sensitive to compaction moisture should be within the range of - 1 to + 2
percent of optimum water content for the field compaction effort applied. Each layer is
compacted to not less than the percentage of maximum density specified in Table 8-2.2.
It is generally important to specify a high degree of compaction in fills under structures
to minimize settlement and to ensure stability of a structure. In addition to criteria set
forth in Chapter 8, consider the following factors in establishing specific requirements:
The sensitivity of the structure to total and differential settlement as related
to structural design is particularly characteristic of structures to be founded
partly on fill and partly on natural ground.
If the ability of normal compaction equipment to produce desired densities
in existing or locally available materials within a reasonable range of
placement water content is considered essential, special equipment
should be specified.
The compaction requirements for clean, cohesionless, granular materials
will be generally higher than those for cohesive materials, because
cohesionless materials readily consolidate, or liquefy, when subjected to
vibration. For structures with unusual stability requirements and
settlement limitations, the minimum density requirements indicated in table
15-2 should be increased. For coarse-grained, well-graded, cohesionless
soils with less than 4 percent passing the 0.075 Micron (No. 200) sieve, or
for poorly graded cohesionless soils with less than 10 percent, the
material should be compacted at the highest practical water content,
preferably saturated. Compaction by vibratory rollers generally is the most