Study On The Machanical Properties Of Loess Considering Stress History And Depositional Environment

Taking Luochuan loess standard profile as the object of study, differences of physical mechanics properties of loess which vary with depth are analyzed systematically on the basis of the remarkable achievements on loess and environment study by former researchers. Study on Loess and environment is organically combined with research on Loess mechanics properties based on taking structural behaviors of Loess and its strength properties. The compressibility and shear strength of the loess with considered the structural property and collapsibility of Loess with considered structural property are taken on study, and stress-strain relation of loess with structural properties and stress history were considered is established. These belong to basis study, but it is a certain kind of supplement and expandedness to the field of loess study and also a driving force to the development of study on environmental soil mechanics. Its academic meaning in the field of loess mechanics study is obvious, and it will provide a certain kind of academic support to the choice of parameter in the design and construction of projects such as high slope engineering, highway tunnels, railway tunnels and underground constructions in loess area. It is also an important guide and practical significance to the study on loess engineering mechanics. The main achievements obtained in the thesis are as follows:(1)The major physical, mechanical properties and collapsibility of loess-fossil soil of Luochuan standard profile all appear different wavy changes with depth. The wave crest and trough appear in the fossil soil, overlying or underlying loess layers. The curve of Cone penetration test on-site confirmed this phenomena clearly, and it also showed that there exist obvious structural differences between loess and fossil soil, among the fossil soil layers formed in different times and the loess layers formed in the same times.(2)Combined with the achievements obtained by former researchers on loess and environment, and through the analysis tests of particles which are treated by the way of disperse and non-disperse, the new understandings of the content of clastic particles in loess is obviously more than in fossil soil while the content of clay particles is on the contrary, and the content of grain in fossil soil is far more than that in loess are obtained. Combined with the analysis of electric mirror scanning pictures, the formative process and reason of the sequence structure of loess-fossil soil and structural differences of Luochuan standard profile are discussed, and paleoclimate gyration with character of dry-cold and warm-wet is pointed out to be the most crucial actor for the formation of the structural differences between loess and fossil soil.(3)By the analysis of high pressure consolidation test, it is discovered that loess-fossil soil of the standard profile are all over-consolidated. This phenomena is caused by the environmental change and consolidation history after the normal accumulation of loess, namely. After the formation of initial structure of loess, structures of loess and fossil soil were strengthened in different degrees, where exists"structural strength of later reinforcement", under the conditions of different stress affecting time and climate gyration with character of warm-wet and dry-cold.(4)Compared with the tests of High Pressure Consolidation, Direct Shearing and Triaxial shear, the feasibility and applicability of the method for obtaining structural strength of later reinforcement are analyzed. It is found that the direct shearing test is affected by the test pressure, and there exists limitation in obtaining the structural strength of later reinforcement and the index of shear strength. But the tests of High Pressure Consolidation and Triaxial shear are feasible methods because the structural strength of later reinforcement obtained from them agrees with each other.(5)Quantitative relation between environmental factor, stress history and structural strength of later reinforcement is established. Then the relation between structural strength of later reinforcement, physical and mechanical property of sequence of loess-fossil soil is quantitatively studied. Environmental factor and stress history are introduced to the research field of loess mechanics.(6)The entire saturation essence of undisturbed soil sample, are the totally loss of structural strength of later reinforcement and partly damage of initial structural strength. Therefore, the sensitive degree towards water of the structure of loess and fossil soil can be concluded by the high pressure consolidation test with undisturbed soil sample, remolded soil sample and saturated soil sample.(7)The causes of loess collapsibility are analyzed from two aspects: environment and stress history, which could be explored from internal cause and external cause. The internal causes are the structural strength of loess will be reduced when meets water, and the differential initial water content and structural water sensitivity formed by the changes of paleoclimate and environment and the time of stress-effects. The fossil soil formed in the warm and humid environment, had a relatively long effect by the water, a large initial water content; the structural water sensitivity and collapsibility are relatively small. Loess formed in the dry and cold environment, had a relatively short effect by the water, a small initial water content; the structural water sensitivity and collapsibility are relatively large. The external causes include the changes of water and loading. Under the condition of saturation, loess and fossil soil will begin to appear collapsible deformation when the applied load is large enough to destroy the structural strength of later reinforcement.(8)Through the analysis of Mohr strength envelop and stress-strain property, the character of exertion of the structural strength of later reinforcement is discussed. It is believed that the initial section of Mohr strength envelop is the elastic deformation stage which is correspond to the exertion of initial structural strength; the middle section of Mohr strength envelop is the elastic-plastic deformation stage which is correspond to the variation from exertion to completely lost of structural strength of later reinforcement;the last horizontal section of Mohr strength envelop is the plastic deformation stage which is correspond to the completely lost of structural strength of later reinforcement, and the soil comes into the plastic-flow state .(9)Considering the exertion character of the structural strength of later reinforcement in the process of bearing loading and introducing the parameters which can show the exertion-degree of the structural strength of later reinforcement, the Duncan–Chang model is modified, has a more extensive applicability. But because the Duncan- Chang model and all of its modified models are all not suitable for the most harden type stress-strain relation, the logarithmic model considering the loess structural damage is brought forth, which, describes this stress-strain relation very well.