Study On Engineering Behaviors Of Expansive Soils And Its Treatment Measures

The engineering problem of expansive soils is a technical bottleneck in highway construction in our country, therefore combining the real engineering, studying the characteristics and treatment technology of expansive soils has important academic meanings and engineering values.The engineering properties of expansive soils have been studied synthetically by combining laboratory testing, field testing, in situ monitoring, theory analysis and numerical modeling, and then the corresponding analysis methods and engineering countermeasures for expansive soils have been put forward. It will provide scientific basis for construction in expansive soil region, at the same time, it will push forward the development of unsaturated soil mechanics and expansive soil mechanics further.The main work in the thesis is as following:1. The main damages caused by expansive soils during the construction of highway have investigated, and the disease characters and reason of highway in expansive soil zone have been revealed. It will deepen understanding to harm of expansive soils, and it will provide direction for treatment of expansive soils.2. The indices that can reflect and characterize swell-shrink mechanism and property were analyzed. The analysis indicates that 5 indices such as liquid limit, plastic limit, clay particles, free swell ratio, swell-shrink total ratio can reflect and characterize swell-shrink mechanism and property, hereby these 5 indices have been suggested as identification and classification indices of expansive soils, and a new approach, i.e. fuzzy comprehensive evaluation model for identification and classification of expansive soils has been established. The new model has been validated by experimental findings and it has been shown that evaluation results are more objective and valid adopting the new approach to identify expansive soils3. Experiments on the physical and mechanical properties of expansive soil have been investigated systematically, and the variation laws and its influencing factors for compacting properties, swelling-shrinkage behavior, intensity and deformation behavior of compacted expansive soils have been summarized. It is helpful to understand the engineering properties of expansive soils.4. Considered the rainfall infiltration characteristic of expansive soil slopes, a simplified equivalent model considered fissures for calculating rainfall infiltration in expansive soil slopes has been developed, so an effective approach for analyzing infiltration in fissure clays has been obtained. The simplified equivalent model divides the soil slope into two layers according to the weathering extent of the soil mass with depths. Layerâ… is an intensive weathered one where moisture can be fully evaporated or rapidly absorbed and the moisture movement parameters take into account the strengthening in soils permeability caused by fissures. Layerâ…¡is an unweathered one where the soils are undisturbed basically and the moisture movement parameters of homogeneous soils are applicable. The moisture movements in unsaturated expansive soil slopes suffering precipitation were simulated numerically using simplified equivalent model, and the simulation results have shown that the moisture movement in the expansive soil slope under rainfall permeation mainly takes place in the extensively weathered Layerâ… .5. Considered the deformation and failure feature of expansive soil slopes, a simplified method for stability analysis of expansive soil slopes has been suggested. The simplified model divides the soil slope into two layers according to the weathering extent of the soil mass with depths. Layerâ… is an intensive weathered one where depth can be determined by the depth of soil layer influenced by climate and shear strength parameters should adopt the residual strength values. Layerâ…¡is an unweathered one where the soils are undisturbed basically. The stability of expansive soil slopes were simulated numerically, and the simulation results have shown that the expansive soil slope failure usually is characterized by shallow layer failure.6. Based up the limit equilibrium method,a set of methods that take the effect of rainfall permeation into account to analyze the stability of unsaturated soil slopes has been developed. The proposed method has a consideration that the moisture content in a soil slope after permeation exhibits a field distribution and the shear strength of the soil is the function of degree of saturation. A prediction model, based on mechanical analysis, of slope stability due to permeation of rainwater has been proposed. The prediction of slope stability according to weather forecasting can be realized as the following steps: first, the seepage field may be computed by numerical method, second, the strength distribution of slope can be derived from the seepage field by the strength theory of unsaturated soil, third the slope stability is analyzed by limit equilibrium method or finite element method, at last, the relationship between the safety factor of slope stability and rainfall parameters such as intensify and duration of rainfall can be obtained. It will provide a new thought for prediction of slope stability and scientific basis for treatment of expansive soil slopes.7. A large field test of expansive soil landslide induced by artificial rain has been performed. It successfully reappears the common shallow slide phenomena in expansive soil slopes instability, and the inherent law of expansive soil landslide induced by rainfall infiltration has been revealed.8. Based on above testing, theory analysis and numerical modeling, the engineering countermeasures for expansive soils have been investigated. The structural shape of expansive soil embankment and the standard of controlling compaction of expansive soil for constructing embankment are suggested.A combining measure with engineering revetment and vegetation revetment for expansive soil cutting slopes has been suggested. These research achievements of this thesis have been successfully applied in several constructions, such as Xiangjing Expressway, Heluye Expressway, Zhoulu Expressway, New Hankou Airdrome and 2319-â…¡engineering, and good technical and economic effect have been obtained, and it will enjoy broad applications.