UFC 3-260-02
30 June 2001
where
Do = average diameter
Dt = diameter at top
DC = diameter at center
Db = diameter at bottom
(2) Compacting Specimens of Cohesive Soil. Specimens of compacted soil may be
trimmed, as described above, from samples formed in a compaction mold (a 102-millimeter-
(4-inch-) diameter sample is satisfactory for 35.5-millimeter- (1.4-inch-) diameter specimens),
though it is preferable to compact individual specimens in a split mold having inside dimensions
equal to the dimensions of the desired specimen. The method of compacting the soil into the mold
should duplicate as closely as possible the method that will be used in the field. In general, the
standard impact type of compaction will not produce the same soil structure and stress-
deformation characteristics as the kneading action of the field compaction equipment. Therefore,
the soil should preferably be compacted into the mold (whether a specimen-size or a standard
compaction mold) in at least six layers, using a pressing or kneading action of a tamper having an
area in contact with the soil of less than one-sixth the area of the mold, and thoroughly scarifying
the surface of each layer before placing the next. The sample shall be prepared, thoroughly mixed
with sufficient water to produce the desired water content, and then stored in an airtight container
for at least 16 hours. The desired density may be produced by either kneading or tamping each
layer until accumulative weight of soil placed in the mold is compacted to a known volume, or
adjusting the number of layers, the number of tamps per layer, and the force per tamp. For the
latter method of control, special constant-force tampers (such as the Harvard miniature compactor
for 35.5-millimeter- (1.4-inch-) diameter specimens or similar compactors for 71-millimeter-
(2.8-inch-) diameter and larger specimens) are necessary. After each specimen compacted to
finished dimensions has been removed from the mold, proceed in accordance with steps c through
e of (1) above.
b. Cohesionless Soils Containing Negligible Amounts of Gravel. Soils which possess little or
no cohesion are difficult if not impossible to trim into a specimen. If undisturbed samples of such
materials are available in sampling tubes, satisfactory specimens can usually be obtained by
freezing the sample to permit cutting out suitable specimens. Samples should be drained before
freezing. The frozen specimens are placed in the triaxial chamber, allowed to thaw after
application of the chamber pressure, and then tested as desired. Some slight disturbance probably
occurs as a result of the freezing, but the natural stratification and structure of the material are
retained. In most cases, however, it is permissible to test cohesionless soils in the remolded state
by forming the specimen at the desired density or at a series of densities which will permit
interpolation to the desired density. Specimens prepared in this manner should generally be
71 millimeters (8 inches) in diameter or larger, depending on the maximum particle size. The
procedure for forming the test specimen shall consist of the following steps:
(1) Oven-dry and weigh an amount of material sufficient to provide somewhat more than
the desired volume of specimen.
(2) Place the forming jacket, with the membrane inside, over the specimen base of the
triaxial compression device.
L-3
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