Assessment And Improvement Of Thermal Conductivity Of Unsaturated Soils Under Coupled Thermo-hydro-mechanical Conditions

Most of the natural soils are under unsaturated conditions. Unsaturated soils are commonly characterized as a three-phase mixture constituted by solid particles and pores filled with liquid and gas. Researches on the structure and physical or mechanical properties of unsaturated soils are of great importance to many engineering problems, such as the unsaturated seepage problem of earth dam, rainfall-induced landslide problem, debris flow disaster, deep geological disposal of high level radioactive waste, consolidation of soft soil, seepage and deformation problem of deep sediments, assessment of water-sealed oil storage underground, and so forth. Especially, with a continual demand growth for safely exploiting of the nuclear energy nowadays, the deep geological disposal problem of high level radioactive waste is getting more concerns from geotechnical field. An engineering barrier system constituted by buffer/backfill materials, needs to be constructed between the waste container and the surrounding rock for the sake of safe and stable operation of high level radioactive waste repository. The unsaturated buffer/backfill material is involved in a multi-field coupling process of a complex multi-phase system under the engineering environment of repository. The multi-field coupling process includes heat release from decay process of high level radioactive waste, seepage of groundwater and hygroscopic expansion of buffer/backfill materials. Therefore, the physical and mechanical properties of buffer/backfill materials under coupled thermal-hydro-mechanical(THM) conditions, are the important foundation to the performance assessment and design optimization of high level radioactive waste repository. This wok focuses on the thermal conductivity of unsaturated soils on the engineering background of deep geological disposal of high-level radioactive waste. The evolution mechanism and constitutive model of thermal conductivity under coupled THM conditions was studied with great effort by the macroscopic equivalent method and micromechanics approach. Main factors affect the thermal conductivity of unsaturated soils were analyzed under coupled THM conditions, meanwhile, the evolution of thermal conductivity during the coupled THM process was observed with the proposed models. Finally, assessment and control measurement of thermal conductivity for unsaturated soils under coupling THM conditions were analyzed. Main research work and achievements are as follows:(1) Depending on analysis and summarization of research results on the microstructure features of unsaturated soils, the pore size distribution were chosen to describe pore structure of unsaturated soil. Differences of pore size distributions for various soils were analyzed with a comparison. Results show that clays like bentonite have a significant multiple pore micro-structure, however the pore size distribution curve of them generally exhibits a bimodal pattern. It is also illustrated that different types of pores have distinct performances on physical or mechanical properties and engineering responses of unsaturated soils. Mathematical model on the thermal conduction problem under coupled THM conditions was derived based on energy conservation equation with a discussion on the effect of THM coupling process on the thermal conductivity.(2) Characterized unsaturated soil as a three-phase mixture, an effective thermal conductivity model was developed based on the structural connections of pores and the solid phase and the series-parallel arrangements of multiphase fluids in the pore system. This model can take into account the effects of mineral compositions of solid, porosity, the degree of saturation, temperature, and pressures of the fluid phases on the thermal conductivity of the mixture, and is available for revealing the evolution mechanism of thermal conductivity of unsaturated soils under coupled THM conditions. Parameterization of the model was discussed in detail. The proposed model was comprehensively verified by five sets of laboratory data on the MX-80, FEBEX, Kunigel-Vl and GMZ01compacted bentonite materials with different dry densities, water contents and mineralogical compositions, and good agreements were obtained between the model predictions and the laboratory measurements. The evolutions of model predictions against porosity and saturation were discussed with a comparison with Wiener bounds and Hashin-Shtrikman bounds. The results show that the model predictions strictly fall within Wiener bounds, and obey the Hashin-Shtrikman bounds in wide ranges of porosity and saturation.(3) Based on the microstructure and the pore size distribution characteristic, a micro-mechanical thermal conductivity model depending on various homogenization schemes was proposed here for unsaturated soils by virtue of Young-Laplace equation(wetting phase preferentially occupy small pores) and the basic solution for the problem of ellipsoidal inclusions in an infinite homogeneous matrix. The proposed model can properly take account of the combined effects of porosity, saturation, the particle and pore shapes, distribution of pores and interaction performances between the pore spaces on the thermal conductivity. The models developed for different pore size distributions(the unimodal pore size distribution and the typically bimodal distribution) with various homogenization techniques were studied with a comparison. The shape and distribution parameters of the model were discussed in detail. The micro-mechanical model was comprehensively verified by five sets of laboratory data on the MX-80, FEBEX, Kunigel-Vl and GMZ01compacted bentonites, and great agreements were obtained between the model predictions and the laboratory measurements. Comparison between the performances of the series-parallel model and micro-mechanical model on the laboratory data shows that micro-mechanical model obtained better predictions with less parameters and had clearer physical mechanism. Comparison between the performances of the models for unimodal and bimodal distribution illustrated that bimodal model gives better predictions than unimodal model, but the improvement is limited for bentonites. The evolutions of predicted thermal conductivity against porosity, saturation, and shape variables were observed with proposed models, and the result showed that predictions with MT method, IDD method and self-consistent approach, except the dilute method, are strictly fall within Wiener bounds and Hashin-Shtrikman bounds.(4) With a summarization on modification measurments and their mechanism for engineering properties of unsaturated soils, this work focuses on the improvement measurments of thermal conductivity and assessment of the effects of various measurments on thermal conductivity. Impact of artificial compaction and moisture content control were considered in both the series-parallel and micro-mechanical models with state variables. Based on the solution of inclusion problem, a two-step homogenization method was proposed to evaluate the thermal conductivity of bentonite-sand mixture with an application to predict thermal conductivity of mixture of Kunigel V1bentonite and silica sand. Combined with the experimental study of THM Mock-up testing of bentonite carried out by French Atomic Energy Commission(CEA) for nuclear waste disposal, the proposed models were applied to observe the evolution of thermal conductivity under THM coupling conditions and the coupling effects of thermal conductivity on the temperature distribution.