UFC 3-220-01N
15 AUGUST 2005
effective procedure. Experience indicates that pervious materials can be
compacted to an average relative density of 85 5 percent with no
practical difficulty. For cohesionless materials, stipulate that the fill be
compacted to either a minimum density of 85 percent relative density or
95 percent of compaction effort, whichever giver, the greater density.
If it is necessary to use fill material having a tendency to swell, the
material should be compacted at water contents somewhat higher than
optimum and to no greater density than required for stability under
proposed loadings (Table 8-2.1). The bearing capacity and settlement
characteristics of the fill under these conditions should be checked by
laboratory tests and analysis. Swelling clays can, in some instances, be
permanently transformed into soils of lower plasticity and swelling
potential by adding a small percentage of hydrated lime (Chapter 16).
8-2.3.3
Compacted Rock. Compacted crushed rock provides an excellent
foundation fill. Vibratory rollers are preferable for compacting rock. Settlement of fill
under the action of the roller provides the most useful information for determining the
proper loose lift thickness, number of passes, roller type, and material gradation.
Compaction with an 89 kN (10-ton) vibratory roller is generally preferable. The rock
should be kept watered at all times during compaction to obviate collapse settlement on
loading and first wetting. As general criteria for construction and control testing of rock
fill are not available, test fills should be employed where previous experience is
inadequate and for large important rock fills.
8-2.4
Placing And Control Of Backfill. Backfill should be placed in lifts no
greater than shown in Table 8-2.1, preferably 200 mm (8 inches) or less and depending
on the soil and type of equipment available. No backfill should be placed that contains
frozen lumps of soil, as later thawing will produce local soft spots. Do not place backfill
on muddy, frozen, or frost-covered ground. Methods of compaction control during
construction are described in paragraph 8-3.
8-2.5
Fill Settlements. A fill thickness of even 1 m (3 ft) is a considerable soil
load, which will increase stresses to a substantial depth (approximately 2B, where B =
smallest lateral dimension of the fill). Stress increases from the fill may be larger than
those from structure footings placed on the fill. Use procedures outlined in chapter 10
to obtain expected settlements caused by fill loading. Many fills are of variable
thickness, especially where an area is landscaped via both cutting and filling to obtain a
construction site. In similar cases, attention should be given to building locations with
respect to crossing cut and fill lines so that the proper type of building settlement can be
designed (building may act as a cantilever, or one end tends to break off, or as a beam
where the interior sags). Proper placing of reinforcing steel in the wall footings (top for
cantilever action or bottom for simple beam action) may help control building cracks
where settlement is inevitable; building joints can be provided at critical locations if
necessary. The combined effect of structure (one- and two-story residences) and fill
loading for fills up to 3 m (10 ft) in thickness on sound soil and using compaction control
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