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(6) A vacuum source with a bubble chamber (optional) and regulator.
(7) Specimen forming jacket.
f. Recommendations. It is also necessary to have a fast recording system for accurate
testing. It is recommended, for analog recording equipment, that the resolution of the parameter
being controlled be better than 1.5 percent of the maximum value of the parameter being measured
and that any variable amplitude signals be changed from high to low resolution as required during
the test. If multichannel recorders are not available, by introducing switching and balancing units,
a single-channel recorder can be used.
N-5. PREPARATION OF SPECIMENS AND PLACEMENT IN TRIAXIAL CELL. The following
procedures describe a step-by-step account for preparing remolded specimens. Generally, for base-
course materials, 152-millimeter- (6-inch-) diameter specimens are required with the maximum
particle size being limited to 25 millimeters (1 inch) in diameter.
a. Material Preparation. The material should be air-dried and subsequently sufficient water
added to bring the material to the desired compaction water content (usually field condition).
Sealing the material in a container for 24 hours prior to compaction will allow the moisture to
equilibrate. For well-graded materials, it may be necessary to break the material down into several
sieve sizes and recombine for each layer to prevent serious segregation of material in the specimen.
If the compaction effort required to duplicate the desired testing water content and density is not
known, sufficient material for several specimens may have to be prepared. The compaction effort
required will then be established on a trial-and-error basis.
b. Specimen Compaction. Generally, base-course materials are compacted on the triaxial cell
baseplate using a split mold. If the particles are angular, two membranes may be required: one
used during compaction and the second placed after compaction to seal any holes punctured in the
membrane. A successful procedure has been to use a Teflon-lined mold and a thin sheet of
wrapping paper instead of a membrane. Often the density is sufficiently high and the water
content such that effective cohesion will permit a free-standing specimen to be prepared. In this
case, the wrapping paper is carefully removed and a membrane substituted. In most cases, impact
or kneading compaction is used. Vibratory compaction is only permitted on uniform materials
where segregation is not a problem. The specimens should be compacted in layers, the height of
which exceeds the maximum particle size.
(1) It may be necessary to place a thin layer of fine sand in the bottom layer to provide a
smooth bearing surface. Likewise, after compacting and trimming the topmost layer (it may be
necessary to remove large particles from this layer), fine sand can be sieved on the surface to fill in
the voids and provide a smooth bearing surface for the top cap.
(2) The top cap should be centered and lightly tapped to level and ensure a good smooth
contact of the cap on the specimen. A level placed on top of the cap is used to check leveling.
The forming mold is then removed, the membrane placed using a membrane stretcher and sealed
with O-rings or a hose clamp, and a vacuum applied. Leakage should be checked by using a bubble
chamber or closing the vacuum line and observing if a vacuum is maintained in the specimen.
Specimen dimensions should be measured to determine density conditions. A B-tape has been
found most useful for diametrical measurements.