UFC 3-260-02
30 June 2001
in accordance with the methods specified for stabilized materials. Chemically treated soils having
unconfined compressive strengths less than that specified for subbases are considered to be modified
subgrade soils and should be tested under the provisions for subgrade soils. Most likely this will result in
using the maximum allowable subgrade modulus. Bituminous-stabilized materials should be
characterized in the same manner as bituminous concrete. Stabilized materials other than bituminous-
stabilized should be characterized using flexural beam tests or cracked-section criteria. Flexural
modulus values determined directly from laboratory tests can be used when the effect of cracking is not
significant and the computed strain based on this modulus does not exceed the allowable strain for the
material being used.
(a) The general approach in the flexural beam test is to subject the specimen to repeated
loadings at third points, measure the maximum deflection at the center of the beam (i.e., at the midpoint
of the neutral axis), and calculate the values for the flexural modulus based on the theory of a simply
supported beam. A correlation factor for stress is applied.
(b) Procedures for preparing specimens of and conducting flexural beam tests on
chemically stabilized soils are presented in detail in Appendix J.
(c) The stabilized material for the base and subbase must meet the strength and durability
requirement of TM 5-822-14/AFJMAN 32-1019. The strength requirements are as summarized in
Chapter 9.
(4) Subgrade soils. The modulus of the subgrade is determined through the use of the
repetitive triaxial test. For most subgrade soils, the modulus is greatly affected by changes in moisture
content and state of stress. As a result of normal moisture migration, water table fluctuation, and other
factors, the moisture content of the subgrade soil can increase and approach saturation with only a slight
change in density. Since the strength and stiffness of fine-grained materials are particularly affected by
such an increase in moisture content, these soils should be tested in the near-saturation state. Two
methods are available to obtain a specimen with this moisture content: the soil can be molded at
optimum moisture content and subsequently saturated or molded at the higher moisture content using
static compaction methods. Evidence exists that the resilient properties of both specimen types are
similar. It is not apparent whether this concept is valid for materials compacted at the higher densities;
therefore, for the test procedures presented herein, back-pressure saturation of samples compacted at
optimum is recommended for developing high moisture contents in test specimens.
(a) For cohesive subgrades, the resilient modulus of the subgrade will normally decrease
with an increase in deviator stress, and therefore, the modulus is determined as a function of deviator
stress. The modulus of granular subgrades will be a function of the first invariant. Procedures for
specimen preparation, testing, and interpretation of test results for cohesive and granular subgrades are
presented in Appendix K. For the layered elastic theory design procedure, however, the maximum
allowable modulus for a subgrade soil should be restricted to 207 MPa (30,000 psi).
(b) In areas where the subgrade is to be subjected to freeze-thaw cycles, the subgrade
modulus must be determined during the thaw-weakened state. Testing soils subject to freeze-thaw
requires specialized test apparatus and procedures. Where commercial laboratories are not available,
the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL), 72 Lyme Road, Hanover,
NH 03755, can conduct tests to characterize subgrade soils subjected to freeze-thaw.
(c) For some design situations, estimating the resilient modulus of the subgrade (MR)
based on available information may be necessary when conducting the repetitive load triaxial tests. An
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