30 June 2001
c. Placement of LVDT Measurement Clamps.
(1) Measure the diameter as accurately as possible at the location of the LVDT clamps for
calculation of radial strains. Place the lower LVDT clamp in the specimen at approximately the
lower third point of the specimen. A "jig" or gauge rods have been used successfully to assist in
placing the clamps. The lower LVDT clamp generally holds the LVDT body. Repeat the procedure
for the upper clamp being careful to align the clamps so that the LVDT core matches the LVDT
body. It is essential that the clamps lie in a horizontal plane and their spacing be precisely known
for calculating the axial strain. Again, gauge rods or a "jig" in conjunction with a small level have
been used successfully for this operation. With the clamps in a position and secured by the
springs, a small amount of epoxy (a "5-minute" epoxy has been used; rubber cement was found
unacceptable) is placed on top of the four contact points and allowed to dry.
(2) Install the LVDTs and connect the recording unit. Generally, 0.10-millimeter
(0.040-inch) LVDTs are used for radial deformation, and 0.25-millimeter (0.100-inch) LVDTs are
used for axial deformations. Balance the vertical spacing between LVDT clamps or check gauge
rods for secure contact, and record LVDT readings and spacing. Remove gauge rods and assemble
triaxial chamber. Any shifting of LVDT clamps during chamber assembly will be noted by LVDT
reading changes and can be accounted for.
d. Resilient Testing. The resilient properties of granular materials are dependent primarily
upon confining pressure and to a lesser extent upon cyclic deviator stress. Therefore, it is
necessary to conduct the tests for a range of confining pressures and deviator stress values.
Generally, chamber pressure values of 0.014, 0.027, 0.041, and 0.069 MPa (2, 4, 6, and 10 psi)
are suitable. Ratios of F1/F3 of 2, 3, 4, and 5 are typically used for the cyclic deviator stress.
Tests should be conducted in an undrained condition with excess pressures relieved after
application of each stress state. The testing procedure is as follows:
(1) Balance the recorders and recording bridges and record calibration steps.
(2) Apply about 0.014 MPa (2 psi) axial load Fd as a seating load simulating the weight of
the pavement and ensuring contact is maintained between the loading piston and top cap during
(3) Condition the specimen by applying 500 to 1,000 load repetitions with drainage lines
open. This conditioning stress should be the maximum stress expected to be applied to the
specimen in the field by traffic. If this is unknown, a chamber pressure of 0.034 to 0.069 MPa
(5 to 10 psi) and a deviator stress (F1 - F3) twice the chamber pressure can be used.
(4) Decrease the chamber pressure to the lowest value to be used. Apply 200 load
repetitions of the smallest deviator stress under undrained conditions, recording the resilient
deformations and load at or near the 200th repetition. After 200 load repetitions, relieve any pore
pressures, increase the deviator stress to the next highest value, and repeat procedure over the
range of deviator stresses to be used.
(5) After completing the stress states for the initial confining pressure, repeat for each
succeedingly higher chamber pressure.
(6) After completion of the loading, remove the axial load, apply a vacuum to the
specimen, release the confining pressure, and disassemble the triaxial chamber.