Unsaturated Soil Mechanics Characteristics Of Compacted Loess Moistening Deformation

Many engineering problems are associated with unsaturated characters of soils. In arid or semi-arid areas, where the ground water level is normally relatively deep, the compacted loess is in their unsaturated states. Future climate changes, which could potentially cause significant changes in the soil moisture regime, induce to many engineering problems, such that the settlement of compacted loess embankment due to rainfall is a problem related to unsaturated mechanical behaviour of compacted loess. In order that safe and cost-effective solutions to these engineering problems, the study on unsaturated mechenics of compacted loess is necessary.Based on the lab tests and theory and method of unsaturated soil mechanics, the unsaturated characteristic of compacted loess is discussed, including soil-water characteristic of compacted loess and mechanism of compaction, unsaturated permeability coefficient of compacted loess, the features of compressive deformations on loading and wetting paths, and elastoplastic constitutive model.To summarise the main contents which the current research aimed at addressing are as follows:(1) A review is given of the characteristic and experimental metod of soil-water characteristic curve, of the theory of unsaturated seepage and unsaturated permeability coefficient, of the mechanism of collapsible deformation of loess and wetting deformation of compacted loess, of the elastoplastic critical state-based framework for describing the constitutive behavior of unsaturated soils. This review presents the nessary background for the paper. Gaps in knowledge are disscused tegother with the specific objectives of the current research.(2) Soil water characteristic curve for compacted loess is studied by lab test, with the dry density of 1.61,1.65,1.71,1.74 g/cm3. The soil water characteristic curve for compacted loess with virous dry density is same as the typical curve shape of soil water characteristic curve, with the obvious 3 stages. The curves fit VG model nicely. All the 4 soil water characteristic curve intersect at one point. The results show that the volumetric water content of the point is 27.9% approximately, where the water content is 15.3%, almost being equal to optimization water content 16%, and the suction is 55kPa. The critical water content or critical suction is defined, and the parameter means that there exists a critical value which is independent of dry density or microstructure. Based the soil-water characteristic curve and compaction curve, mechanism of compaction is explained.(3) Unsaturated permeability coefficient of compacted loess is studied by the test system of the hydraulic conductivity under different dry densities. The relationship between matric suction and permeability coefficient shows that the permeability coefficient decreases with suction non-linearly, and increases with dry density. It is found that dry density has more effect on the permeability coefficient in the low matrix suction than in the high matrix suction. It is indicated that from test results, the relation between permeability coefficient and matrix suction can be fitted with Wind(1955) model, Gardner(1958) model, and Brooks-Corey(1964)model under certain conditions.Based on the Green-Ampt model, the general mechanism of rain infiltration is presented. The seeper time, the depth of wetting front, the infiltration rate, and the total infiltration displacement is calculated for various dry density compacted loess. The results show that the infiltration rate and the total infiltration displacement decrease with the increasing of dry density, and the seeper time decreases nonlinearly with the increasing of dry density, and the depth of wetting front decreases linearly with the increasing of dry density.The FEM simulations show the transient moisture profile after raining is made up of 3 sections, saturated section, transmitting section and wetting section. The soil is saturated in the saturated section, and water content decrease linearly along the depth with the less grads in the transmitting section, and water content fall to the initial values in the wetting section. After raining the water continue to migrate in the loess. The saturated state of saturated section changes to unsaturated state, and the water content of transmitting section and wetting section increases, resulting in the wetting front moving deeper. (4) A series of experiments including constant-water-content loading tests and wetting tests has been conducted by using oedometer apparatus to investigate the features of compressive deformations on loading and wetting paths. In constant-water-content loading tests, the compressive deformation is of 2 stages:the first stage is compression and the second is stabilized compaction deformation. In the space of compressive deformation, pressure and water content, the state surface of loading-wetting (dying) is a inclined plane with the increasing of water congtent and pressure.The coefficient of compressive deformation increases with the increasing dry density. The dry density has less influence on the coefficient of compressive deformation at lower water content, nevertheless, the influence is obvious with the increasing of water content.The results of wetting tests show that the elementary characteristics of moistening deformation, as of the consolidating deformation, is that it will be stable after some time. The increment of wetting deformation decreases with the advancing water content. The increment of wetting deformation increases with the advancing dry density at each wetting stage.(5)With the same confining stress, the strength and cohesion of the compacted loess incease with the increased suction, and the internal friction angle is approximately a content, which is close to the effective angle of internal friction of the saturated samples.Within the range of less suction(less than 50kPa), the parameter for water content change and volume strain in the isotropic compression tests controlling suction change significantly, but the change under the suction beyond 50kPa is small. The parameters for water content change and volume strain in the triaxial shrinkage test with controlled net mean stress change slightly with the increased net mean stress. The two parameters of the triaxial shrinkage test with controlled net mean stress are approximately 22% and 27% higher than the isotropic compression tests controlling suction. It means that stress paths have significant effect on the deformation and moisture change of unsaturated soil. The yield stress increases with the increased suction, but the yield suction is not a constant, decreasing with the increased net mean stress. For contrasted with known data, the mean is considered as the the yield suction. The dry density has significant effect on the strain and yield in the suction paths, but small in the load paths.The simple mode of the wetting deformation shows that wetting strain is less than 2%, and inceases with increased suction and decrased confining stress.