Because of the complexity of unsaturated soil

traditional three-phase model for unsaturated soil is oversimplified

which is unfit for interphase analysis and determination of stresses in soil. Considering a special structure in unsaturated soil

in this work

soil water system is divided into six phases and its bearing structure

named generalized soil structure

is defined. Then the differences between saturated and unsaturated soils are concluded: the bearing structures and pore liquids are different. Based on the similarity of bearing structures

a set of variables to depict the structural properties of soils is defined as generalized structural properties to describe the properties of the bearing structures of different soils. The rationality of some variables is analyzed by explaining some practical phenomena

e.g. the essence of collapse phenomenon is the weakening of the bearing structure of soil. Two representative interphase interactions are discussed and the physical significance of neutral stress is revealed

and the suction stress is redefined. Finally

it proves that the total skeleton stress in the grain is the “effective stress” for unsaturated soil.