30 June 2001
PROCEDURE FOR DETERMINING RESILIENT
MODULUS OF SUBGRADE MATERIAL
L-1. GENERAL. The objective of this test procedure is to determine a modulus value for subgrade
soils by means of resilient triaxial techniques. The test is similar to a standard triaxial compression
test, the primary exception being that the deviator stress is applied repetitively and at several stress
levels. This procedure allows testing of soil specimens in a repetitive stress state similar to that
encountered by a soil in a pavement under a moving wheel load.
L-2. DEFINITIONS. The following symbols and terms are used in the description of this procedure:
F1 = total axial stress
F3 = total radial stress; i.e., confining pressure in the triaxial test chamber
Fd = F1-F3 = deviator strain; i.e, the repeated axial stress in this procedure
,1 = total axial strain due to Fd
,R = resilient or recoverable axial strain due to Fd
,R1 = resilient or recoverable axial strain due to Fd in the direction perpendicular to ,R
MR = Fd/,RI = resilient modulus
2 = F1 + 2F3 = Fd + 3F3 = sum of the principal stresses in the triaxial state of stress
F1/F3 = principal stress ratio
Load duration = time interval over which the specimen is subject to a deviator stress
Cycle duration = time interval between successive applications of a deviator stress
L-3. SPECIMENS. Various diameter soil specimens may be used in this test with the specimen
height at least twice the diameter. Undisturbed or laboratory molded specimens can be used.
Methods for laboratory preparation of molded specimens and for backpressure saturation of
specimens are given in the following paragraphs.
L-4. PREPARATION OF SPECIMENS. Specimens shall have an initial height of not less than 2.1
times the initial diameter, though the minimum initial height of a specimen must be 2.25 times the
diameter if the soil contains particles retained on the No. 4 sieve. The maximum particle size
permitted in any specimen shall be no greater than one-sixth of the specimen diameter. Triaxial
specimens 35.5, 71, 102, 152, 305, and 381 millimeters (1.4, 2.8, 4, 6, 12, and 15 inches) in
diameter are most commonly used.