Study On Settlement And Stability For Roadbed Of Expansive Soils

Expansive soil is a special clay soil, which has wide distribution in our country. Along with the large-scale construction of highway and fast development of the western part of China, it is important to study on the roadbed and slope problems in the districts of the expansive soil.By using the theoretical, experimental and numerical methods, the classification and identification of the expansive soils are studied and the settlement of the roadbed and the stability of slopes are analyzed in this thesis. The main contents are listed briefly as following.Firstly, based on the experiments and inspections of the expansive soils in Yichang district, the ingredients, structures, physical characters, the swell-shrink characters and roadbed diseases of the expansive soils are analyzed. The reasons of the charged particles and the conditions of the cationic conversion of the expansive soils are studied. The mechanism of the swell-shrink is discussed. The strength-decaying character is demonstrated by the experimental approach. Secondly, based on the experiment results of the expansive soils in Yichang district, the plastic-diagram method, multi-index evaluation and multi-variable linear function judgment for the expansive soils are proposed. This may be used for the judgments of the regional expansive soils. Also, the fuzzy evaluation method is brought forward. The sensitivity of the results of the fuzzy evaluation for the expansive soils to the weight vector is discussed. It can be demonstrated that the fuzzy evaluation method is a safe and accurate method by comparing with the other judgment results of the many methods including the code one. Thirdly, the rule of the settlement and the swell-shrink characters of the expansive soil roadbed under the load action are analyzed. The fundamental mechanical conditions of the swell-shrink and settlement are studied. The limiting equilibrium method that determines the influence depth of the expansive soils by the air is presented. The "increment method" for calculating settlement is proposed and the relative analytical model is given. The calculated expressions for the instantaneous settlement of the roadbed are deduced. The calculated expressions are presented where the settlement direction and incomplete reversibility for the swell-shrink settlement are considered. The settlement of the expansive soil roadbed in Yi-Huang highway is analyzed and the relationship of the roadbed settlement with time, stress path and the moisture content has been expressed by the increment method. This may be used to establish further a basis for the simulating the process of the expansive soil roadbed settlement.Fourthly, the limiting state equation for the settlement of the expansive soil roadbed is established, which can be used to express the direction of swell-shrink settlement and the main consolidation. A series of the calculated expressions for the settlement reliability index are deduced by the "JC" method and the iterative programs for calculating the reliability index are given. The analysis of the reliability and sensibility for some roadbeds shows that the failure of the expansive soil roadbed usually takes place in the soil hydrophilic expansion. The reliability degree would reduce with the increment of the expansive level. The swell-shrink settlement has a large proportion in the total settlement. The reliability degree of the settlement is very sensitive to the sampling of the expansive settlement coefficient ψep and insensitive to the calculated depth of the mail consolidation settlement. The calculated accuracy is satisfactory by taking the air-influence depth as the calculated depth of the roadbed settlement.Fifthly, based on the traditional reliability concepts, the concepts of the tolerance and fuzzy critical interval are introduced. The fuzzy limiting state of the settlement of the expansive soil roadbed and its relation with the reliability are defined. The relation between the tolerance and fuzzy critical interval, the span value of the fuzzy critical i...