AASHTO TP 46, where the LVDTs are placed on the outside of the chamber (Fig.
B4). Deformation of the specimen is inferred from the movement of the loading
piston rod. Our experience has shown that there can be significant differences be-
tween the measurements made from the piston rod and that made on the speci-
men. The difference has been attributed to friction as the rod slides through the
cover plate. The LVDT barrels are mounted on the top ring and the tips of the
spring loaded cores protrude to the bottom ring. An alignment jig was used during
setup to ensure a uniform gage length for each test. The LVDTs have a range equal
to or greater than 5% strain over the gauge length. The 2.8-in.- (71-mm-) diam.,
6-in.- (152-mm-) tall specimens have a gauge length of 3 in. (75 mm) and the 6-in.-
(152-mm-) diam. and 12-in.- (304-mm-) tall specimens have a gauge length of 6 in.
(152 mm).
Radial displacements were measured with three noncontacting displacement
transducers called multipurpose variable impedance transducers (multi-VITs) (Fig.
B3). Brass targets were glued on to the specimens around the middle of the speci-
men. Each multi-VIT was calibrated with the aluminum foil targets and calibration
curves of voltage vs. distance obtained. The multi-VITs are mounted on rods that
bolt to the base of the triaxial cell. The position of each transducer is adjusted by a
micrometer, which reacts against the spring loaded rod to which the transducer is
attached. Early in the research, the three measurements are recorded and averaged
from which the radial strain and Poisson's ratio are calculated. Later, we recorded
each signal separately.
Repeated Load Actuator
Load Cell
Ball Seat (Divot)
Chamber Piston Rod
Steel Ball
13 mm (0.5 in.) Minimum Diameter for Type 2 Soils
51 mm (2 in.) Maximum
38 mm (1.5 in.) Minimum Diameter for Type 1 Soils
LVDT Solid Bracket
LVDT
Thompson Ball Bushing
Cell Pressure Inlet
Cover Plate
O-Ring Seals
Sample Cap
Porous Bronze Disk
or Porous Stone
Chamber (Lexan or Acrylic)
Sample Membrane
Specimen
Tie Rods
Porous Bronze Disk
or Porous Stone
Sample Base
Base Plate
Vacuum Inlet
Vacuum Inlet
Solid Base
Figure B4. Typical AASHTO triaxial chamber with external LVDTs and load cell.
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