Characteristics And Mechanism Of Dynamic Residual Deformation Of Loess

Seismic settlement is one of the most common earthquake disasters on loess ground. It occurred many times during historical earthquakes and caused ground settlement and triggered landslides which caused loss of both human lives and properties. Using laboratory test on dynamic residual strain of loess can provide information on mechanism of seismic settlement of loess, understand effect of different factors and thus leads to a better evaluation of seismic settlement coefficient of loess which is necessary of better prediction of seismic settlement of loess ground.In this thesis, based on laboratory tests and theoretical analysis, the effect of dynamic load, soil physical parameters, soil unsaturation, grading and microstructure on development of dynamic residual strain is studied, furthermore, the study provide further understanding of seismic settlement of loess which is also compared with that of collapsibility of loess. Through this study, the following findings are obtained:1. The effect of parameters of dynamic load on dynamic deformation of loess is established. Based on a large amount of test data, it is proved that there is a close relationship between dynamic deformation of loess and parameters of dynamic load, mainly magnitude and cycles. Other conditions kept the same, the amount of residual strain and magnitude of dynamic load obeys a quadratic function without constant term. There is a natural logarithm relationship between residual strain and cycles of loading.2. The effect of dry density, unsaturated characteristics (using volume water content as indicator) and grading on dynamic deformation of loess is established. It is found, the amount of residual strain of loess under certain dynamic load decreases linearly with increase of dry density. Based on study of soil-water characteristics of loess, the effect of unsaturation on dynamic residual strain is investigated. The results show that change of volumetric water content has significant effect on the residual strain of loess, and their relationship can be expressed with a logarithm function. The change of grading is achieved by adding additional sand or clay particles into remoulded loess samples. Within the range of grading change studied in this thesis, increase of either clay or sand particles can reduce residual strain of loess under the same dynamic load. Though they lead to similar result, the roles play by clay and sand particles in microstructure scale are different. Based on particle composition, the effect of grading on dynamic deformation of loess can simply be express as a function of silt content.3. Based the analysis of microstructure, the mechanism of dynamic deformation of loess is validated. The pore distribution of samples before and after dynamic deformation tests shows that significant amount of dynamic deformation is well reflected in change of pore distribution. Based on analysis, it is proved that under certain dynamic load, it is often that those larger and also weaker pores collapse first, after that pores with secondary size will be crushed. Also, there is a square root relationship between loss of area of larger pores and amount of residual strain.4. A comparison of seismic settlement and collapsibility of loess is carried out in this thesis. It is found that collapsibility and seismic settlement of loess are based on the same microstructure basis, which is the existence of metastable pores with large size. But their mechanisms are different. As a result, no direct relationship between the two can be established.The results in this thesis can provide valuable information for prediction and treatment of seismic settlement of loess and subgrade deformation under traffic vibration.