UFC 3-260-02
30 June 2001
(2) Exploration. In all instances, field and laboratory tests will be conducted to determine the
classification, moisture-density relations, expansion characteristics, and strength of the subgrade. If
stabilization of the subgrade is to be considered, other tests as required by TM 5-822-14/AFM 32-1019,
will be made, as well as chemical analysis and clay mineralogy determination. When a subgrade soil
that has a chemical stabilizing agent added but does not meet the 1.72-MPa (250-psi) compressive
strength requirement, the soil should be characterized with procedures for subgrades and be considered
simply as part of the subgrade. The engineer is cautioned that although the elastic layered method
requires only the modulus of elasticity and Poisson's ratio of the subgrade, such factors as groundwater,
surface water infiltration, soil capillarity, topography, drainage, rainfall, and frost conditions may affect
the future support rendered by the prepared subgrade or base course. Experience has shown that the
subgrade will reach near saturation, even in semiarid and arid regions, after a pavement has been
constructed. If conditions exist that will cause the subgrade soil to be affected adversely by frost action,
the subgrade will be treated in accordance with the requirements in Chapter 20. Subgrades and base
courses are grouped into three types with respect to behavior during saturation: low plastic soils
exhibiting little or no swell, swelling soils, and cohesionless sands and gravels. Special cases of
subgrade soil are discussed in Chapter 6.
(3) Modulus and Poisson's ratio. The modulus of elasticity and Poisson's ratio of subgrade
soils will be determined from repetitive triaxial tests on undisturbed samples when possible or on
samples prepared as close as possible to field conditions when fill is involved. The samples considered
should represent the worst anticipated condition in the field. The recommended test procedures are
outlined in Appendix K. The procedures are similar to those used for granular base (subbase) materials.
There are differences in details of the test procedures and presentation of results for cohesive and
cohesionless materials. These differences are necessary because of the sensitivity of cohesive soils to
moisture and the differences in the behavior as a function of the state of stress.
(a) For characterizing cohesive materials, the triaxial tests should be conducted at a
range of stress conditions. Tests should be conducted at confining stresses of 13.8, 27.6, and 41.4 KPa
(2, 4, and 6 psi), and at axial stresses applied that will result in a range of deviator stress from about 13.8
to 110 KPa (2 to 16 psi). From the composite curve, the resilient modulus used to represent the material
should be selected at a deviator stress of 34.5 KPa (5 psi). No well-defined relationships exist for
Poisson's ratio, but similar plots may be made and a representative value selected.
(b) For cohesionless soils, the confining stress in the triaxial tests should approximate
conditions in the subgrade. The minor principal stress in the subgrade is a measure of the confinement.
For cohesionless subgrade soils, it is considered appropriate to select properties at minimum values of
the first stress invariant and confining stress, since the general trends are applicable for cohesionless
subgrade soils, i.e., as the confining stress and the first stress invariant decreases, the resilient modulus
decreases.
(c) Basically, the same stresses should be used in the triaxial tests for characterizing
cohesionless material as are used for granular bases. Confining pressures of 13.8, 27.6, 41.4, and
68.9 KPa (2, 4, 6, and 10 psi) and axial stresses that result in principal stress ratios (F1/F3) of 2, 3, 4, and
5 should be applied. From the average relationship of resilient modulus versus first stress invariant, a
representative modulus value should be selected at a first stress invariant of 68.9 KPa (10 psi). A
representative value of Poisson's ratio should be selected from a composite plot of Poisson's ratio
versus principal stress ratio. If test results prove unreliable or are not available, the values of 0.4 for
cohesive and 0.3 for cohesionless materials may be used.
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