Research On The Universality Of Unsaturated Soil Strength And Deformation Characteristics

It is known that the variation of suction in unsaturated soil leads to many complexities in soil behavior, for instance, the failure of the effective stress principle. Doubt was therefore cast on the possibility that the suction described traditionally may not be proper both conceptually and mathematically. Attempts are thus made in the thesis to describe the suction in a new way that leads to the establishment of a mathematical formulation for soil-water characteristics, a mathematical formulation for shear strengths and a mathematical formulation for volumetric deformation.The mathematical formulation for soil-water characteristics was derived based on the fact that suction is linked to the pore radius which is linked to the size of soil particles. Therefore the soil-water characteristics curve is related to the particle size distribution curve. Bearing in the mind that the size of soil particles will not become infinite small, i.e., it will reach a certain limit. The particle size distribution curve traditionally cannot reflect this fact, a new particle size distribution curve is therefore proposed. The derived mathematical formulation for soil-water characteristics has two salient features. Firstly, the suction will reach a limit as the degree of saturation reaches zero which remove the unreasonable suction infinity given by many SWCCs in the past. Secondly, the soil-water characteristics curve can have multiple smooth steps if the soil particle size distribution curve has multiple peaks.As the degree of saturation decreases to a certain value, the some soil particles are not completely covered by the water film. The capillary potential force only acts on the water covered part of the soil particle body. In other words, the water-particle interaction area varies as the degree of saturation changes. The true suction needs to reflect the effect of water-particle interaction area reduction. A new theory is thus established for the shear strengths of unsaturated soil to take account of the effect. The derived theory indicates that the shear strength will reach a peak then decrease as the soil dries if the soil is sand. Since the derived theory incorporated the contribution of structural cohesion for clay, the shear strength of unsaturated clay may not have a peak, or it may reach the peak when the soil is extremely dry.The unsaturated soils upon wetting may swell or collapse depending the soil types or void ratio of the soil. The particle structure of the unsaturated soil was considered to be in sub-stable and super stable states depending on the values of the void ratio. Both the sub-stable and super stable states are maintained by the suction. The sub-stable and super stable structure becomes unstable upon wetting because of the suction reduction. There may exist a stable state that neither swelling nor collapsing will happen upon wetting. Employing the concept of stable state void ratio, a general formula was obtained which can describe both the swelling and collapsing behavior of the unsaturated soil.The theories derived were all confirmed to be sensible by the existing experimental results. Some further experimental validations remain to be carried out yet.