Hydraulic And Mechanical Behaviour Of Unsaturated Expansive Soils And Its Elastoplastic Modelling

Many countries are planning to bury high-level radioactive nuclear wasters in sealed repositories located deeply in geology formations. Engineered barriers made up of highly compacted expansive clays are considered for isolating the nuclear wastes at great depth. Bentonite is used as buffer materials or backfill materials because of its low hydraulic conductivity, high swelling property and good self-sealing capacities, etc. Predictions of the compression behaviour and the swelling behaviour of bentonite and sand-bentonite mixtures due to water uptake are the important technical issues to be resolved both in theory and in engineering application for the deep nuclear wastes disposal system. It requires a constitutive model for predicting the hydraulic and mechanical behaviour of unsaturated expansive soil based on the further study to the coupled effect of deformation and water retention behaviour of bentonite and sand-bentonite mixtures.In the thesis, on the basis of reading the related literatures, the experiemental study on the hydro-mechanical behaviour of unsaturated bentonite and sand-bentonite mixtures and its constitutive modelling are conducted, and the main conclusions are shown as follows:1. The compression tests are completed on saturated bentonite and sand-bentonite mixtures with different preparations. The test results show that the compression curves show a bilinear shape, which is different from that of ordinary normally consolidated clays. The coefficient of lateral earth pressure of saturated bentonite at-rest is obtained by K0-oedometer; and the K0 value is greater than that of ordinary clays. The change in the lateral stress is obtained during the water uptake tests on compacted sand-bentonite mixtures which are transited from unsaturated to saturated states. The compression curves of compacted specimens saturated by the water uptake test and the vacuumized methods tend to be consistent in the relatively high stress range. Thus the vacuumized method for saturating the specimens can be adopted to save time for performing the water uptake test. The concept of skeleton void ratio is used to judge whether the sand skeleton is formed in sand-bentonite mixtures, and to obtain main factors affecting the compression behaviour of the mixtures.2. The undrained compression tests are producted on unsaturated bentonite with lateral stress being measured under vertical loading. The deformation characteristics of unsaturated bentonite and the change in lateral stress during tests are obtained under K0-compression stress state. According to the test results, the swelling index trends to be constant independent on the initial state; and for specimens with equivalent initial water content the yield stress increases with increasing suction and the compression curves tend to be consistent in the relatively high stress range; and to specimens with equivalent initial dry density the compression indexes increase with increasing water content; moreover, the relationships between the ratio of total lateral stress to total vertical stress with the degree of saturation and total vertical stress are obtained.3. The water retention behaviour of unsaturated bentonite and sand-bentonite mixutures are investigated. The filter paper method is adopted to measure the suction of Kunigel V1 Na-bentonite. According to the test results, the soil-water characteristic curves (SWCC) between degree of saturation and suction shift to the right with decreasing void ratio, and the drgree of saturation and void ratio present the good linear relation with almost the same gradient under different constant suctions. To measure the SWCC of the mixture in the wide range of suction, a pressure plate apparatus and the filter papers are used at lower and higher suctions respectively. The obtained SWCC shows that the results obtained from the two methods can be considered as consistent.4. By using a suction-controllable oedometer for unsaturated soils, a series of wetting tests at constant net stresses and compression tests at different constant suctions are performed on compacted sand-bentonite mixtures to study the coupled hydraulic and mechanical behaviour of unsaturated expansive soil. The wetting test results indicate that the swelling deformations have the relation with suction variation and suction paths to specimens at the initial state. Moreover the swelling deformation increases with decreasing the void ratio during wetting test for specimens with equivalent initial water content and the same suction path. The compression test results indicate that the swelling indexes trend to be constant; and the yield stress increases and compression index decreases with increasing the suction. The results also demonstrate that the mixture wetted to saturation and subsequently dried to the target suction has the lower yield stress than that wetted directly to the same suction.5. The Barcelona Basic Model (BBM) by Alonso cannot predict the mechanic behaviour of unsaturated expansive soil, and the Barcelona Expansive Model (BExM) for unsaturated expansive soils is complicated and the micro parameters and the coupling function from micro-structural strain to macro-structural strain are difficult to be determined. In this thesis, an elastoplastic constitutive model is developed from the macro-view for predicting hydraulic and mechanical behaviour of unsaturated expansive soils based on the existing hydro-mechanical elastoplastic model for unsaturated non-expansive soil. The model takes into consideration the coupled effect of the degree of saturation and the void ratio. The concept of Equivalent Void Ratio Curve (EVRC) is introduced in the model. The coupled model totally requires 13 parameters which can all be determined from element tests. The predictions are performed on the test results obtained by the previous compression tests and published in the literature by Zhan and Ng (2006), which include the swelling tests under constant net stresses, isotropic compression tests and triaxial shear tests under constant suctions. The comparisons between measured and predicted results indicate that the model can quantitatively predict the hydraulic and mechanical behaviour of unsaturated expansive soils.