State surfaces may be given by interpolation of experimental data or may represent the integration of linear or nonlinear elastic constitutive equations. Anelastic behavior may also be considered if only loading is taken into account to ensure the uniqueness of the surface. State surfaces are usually defined in the (p, q, s) space, where p, q, and s represent, respectively, the excess of mean stress over air pressure, the deviatoric stresses, and suction. An alternative approach assumes similar constitutive equations but defined in the space (p′, q), where p′ represents the effective pressure. This latter approach is extended to the space (p′, q, s) and improved to carefully deal with the role of suction, in conjunction with the confining pressure, in controling soil expansion (swelling).

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