Dual Effective Stresses Of Unsaturated Soils

Principle of effective stress is the most basic and most important principle in soil mechanics. Terzaghi is known as the father of the modern soil mechanics, one of his important contributions is the “principle of effective stress”. Because of the effective stress principle, soil mechanics made a fundamental change and separated from the general mechanics, and became an independent discipline. Although the Terzaghi’s effective stress equation has the advantages of simple form and convenience, yet, it will cause certain deviation in some cases. Many scholars are committed to improving the calculation formula of effective stress so as to make it play a better role in engineering practices.Unsaturated soils are widespread in the earth’s surface. They are closely related to the civil and architecture engineering, and have drawn great attention of engineers, and a lot of theoretical and experimental researches were carried out. The successful application of effective stress principle in saturated soils urges the study of effective stress principle of unsaturated soils inevitable. By now, however, the knowledge and theory concerning effective stress principle of unsaturated soils are very limited due to the complexity of the three-phase compositions of unsaturated soils.The calculation of effective stress of unsaturated soils is the key point of unsaturated soil mechanics. It is the foundation and precondition for studying the other properties of unsaturated soils. At home and abroad, some achievements have been made in the principle of effective stress in unsaturated soils, and some formulae of effective stress calculation were presented.However, the vast majority of them are based on a single effective stress(soil skeleton stress) to control the deformation and strength of the soil where the deformation of the soil particles is ignored. They considered that the deformation of the soil under the action of stress is only caused by the failure of the soil structure. This is obviously inconsistent with the actual deformation characteristics of the soil. Meanwhile, due to the existence of suction in unsaturated soils, the calculation of effective stress becomes more complicated. The calculation of grain suction is the key point in the study of effective stress of unsaturated soils. The existing calculation models about the intergranular suction in unsaturated soils assume that that the soil particles are in point contact, and the influence of the cementing material on the suction is neglected. This induces the non-ignorable error in the calculation of intergranular suction.In view of the above problems, we do some theoretical research. First, taking into account soil particle cementing area, we calculate the size of the intergranular suction in unsaturated soils. Second, considering the deformation of the soil body under the action of stress, and the hypothesis of the principle of effective stress in porous media is introduced. The total strain of the soil is divided into two parts, the main body strain and the structural strain, the deformation of the soil body is controlled by the effective stress of the body, and the deformation caused by the structural failure is controlled by the effective stress of the structure. Combined with the existing effective stress principle of unsaturated soils, we obtain the dual effective stress formulation of unsaturated soils. Finally, the presented effective stress formula is verified by the case study and applied to several conventional problems of soil mechanics.