Preliminary Study On The Validation Of Effective Stress Formula For Unsaturated Soils

Unsaturated soil is a multi-phase composed of solid,liquid and gas.And,unsaturated soils are widely distributed and display more complex behaviour compared with saturated soils in engineering practice.The theory of effective stress for unsaturated soil has been one of the most fundamental and important topics in the theoretical study of unsaturated soil mechanics.After nearly half a century of effort,a group of researchers derived many different formulas about effective stress of unsaturated soil.Unified effective stress theory raised by Shao L.T.is one of the different formulas which is based on a lot of experimental data and theoretical methods.Shao's equation is in view of the continuum mechanics theory,which not only has a certain theoretical basis but also clarifies the physical meaning of e ffective stress in the soil.It can combine the strength and deformation of saturated and unsaturated soils to establish a bridge between saturated and unsaturated soil mechanics.In order to verify the applicability of the formula,this paper validates it from the perspective of strength by a large number of test data.In this paper,the effective stress equation for triaxial and direct shear stress states is further deduced on the unified effective stress formula.The strength and soil-water characteristic of kaolin were investigated by triaxial instrument with digital image measurement system,pressure plate and filter paper method and a mass of domestic and foreign published test data were compiled to prove the theory in detail,and the following conclus ion are drawn:(1)The strength calculated by effective stress principle is in good agreement with the constant water content triaxial test data of unsaturated kaolin.The theory applies to predict the shear strength of silty sand,silty clay,weak expansive soil and many other typical soil samples and it also has a reasonable prediction for different sample preparation method(undisturbed or compaction),different strength test method(suction controlled or constant water),different test equipment(direct shear or triaxial),and different stress path(wetting and drying cycle or different stress state),all the difference of strength is due to the effective stress by these factors;(2)The filter paper method can be a very good engagement and complement on the data measured by pressure plate method and increases the suction range of kaolin.The four-parameter VG model is not only suitable for the kaolin's SWCC test data,but also suitable for the SWCC test data cited in the literature,and the parameters obtained from the model can well reflect the soil-water characteristic of soil;(3)The using of constant water triaxial tests of unsaturated soil is more time-saving than suction-controlled triaxial tests.The volumetric changes of soil during the shear damage can be more accurate calculation by the digital image measurement method and the changes of saturation also can be obtained.