Work is a way of energy transfer between system and its surroundings. It is usually defined as the product of a force applied and the distance through which that force acts. In thermodynamics, work is done by all the macroscopic forces exerted on the system. These forces are associated with the macroscopic state variables of the system and always occur in conjugate pairs, for example pressure and volume, surface tension and surface area. Therefore, the work input equation derived from the energy balance equation provides a theoretical basis to deal with the relationship between a force and its conjugated deformation. It has greatly attracted researchers’ attention because it is beneficial to understand the mechanical properties of unsaturated soils. Houlsby1,2 derived the specific work input equation for both saturated and unsaturated granular material. He provided a valuable tool for the selection of the appropriate stress variables and showed that the selection was not unique. Extensive studies have then been conducted on the subject, and the effective stress of porous media for more complex problem has also been discussed (Borja,3 Coussy et al,4 Zhao et al,5,6 Broja and Koliji,7 Tamagnini,8 Li et al,9,10 Fuentes et al11). The work input into unsaturated soils is equal to the product of the generalized stresses and their conjugate strain increments, which includes the information about the mechanical and hydraulic behavior in unsaturated soils. It is therefore reasonable to use the work input equation to describe the interaction and mutual transformation caused by the general stresses and conjugated strain increments. The soil skeleton stress, which is work conjugated with the soil skeleton strain, can be defined as the effective stress. Thus, the work input equation is usually used to study the effective stress, which serves as the basis and starting point for constitutive modeling and strength analysis. Some of these studies have been proposed by Jommi,12 Vaunat et al,13 Wheeler et al,14 Sheng et al,15 Li,16,17 Nuth and Laloui,18 Buscarnera and Nova,19 Borja and White,20 Lu et al,21 Buscarnera and Einav,22 Lloret‐Cabot et al,23 Hu et al,24 Choo et al.