| River embankments are the important parts of the flood control engineering system of China. Because embankments of China were built by filling along river generally, the choices of lines of embankments are restricted by condition of river. The foundations of embankmens are made of Quaternary Unconsolidated Sediments, and components and density of filled soils in embankment body are of heterogeneity, and so embankments scour is a very general and seriouse phenomenon in this process that embankments play a role seasonally or periodically. Therefore, research on seepage flow, deformation and stability of embankment under effects of river scouring has a significant meaning to the safe operation and management of embankments.Research on stability of embankment under effects of river scouring involves in two aspects: one aspect is relate to study on behavior of filled soils in embankment body and foundation, such as unsaturated behavior, the influence of repeated drying and wetting cycles on unsaturated soil, the compression and strength behavior of soils, anather is relate to study on scouring stability of the embankment and the riverbed on interaction among tractive force, seepage force and gravity stress. This thesis carries out a systematic study on the two aspects based on a typical embankment preject on Beijiang River in Guangdong. The main contents and results are listed as follows:1. A series of triaxial tests were carried out using double-cell triaxial system for unsaturated soils to investigate the behavior of volume change, yield and strength of unsaturated clays. The stress paths included drying with isotropic stress, isotropic compression and shearing with controlled suction. The test results show:(1) the clay exhibited an elasto-plastic shrinkage upon drying with low suction, indicating the existence of the suction-increase (SI) yield locus. (2) During isotropic compression, the yield stress increases with increase of suction and the loading-collapse (LC) yield curve changes parabolically. Increase of net normal stress has more significant effect on the behavior of volume change than that of matric suction. (3) The stress-strain relationship depends on the combinational style of net normal stress and matric suction, and suction enhances the stress-strain stiffness feature during triaxial shear test. (4) Under conditions of different values of constant suction, the effective friction angle is approximately a constant, and cohesion coefficient versus suction relationship is approximately linear.2. A series of tests are performed to measure the variation of deformation and shear strength of unsaturated clays subjected to repeated drying and wetting cycles so as to study the effect of repeated drying and wetting cycles on unsaturated soil by using unsaturated triaxial apparatus. The test results show:(1) Firstly, the matric suction of the same soil moisture in SWCC reduces, and shrinkage behaviors of soil enhance during the pre-yield stage of shrinkage and do not change approximately during the post-yield stage of shrinkage; (2) Secondly, the compression index increases during the post-yield stage of isotropic compression, and the influence on compression behavior decreases with the increasing suction; (3) And thirdly, a few samples subjected to repeated drying and wetting cycles exhibit a slide plane in triaxal shear tests. Consistently, the stress exhibits an obvious peak value and then decreases sharply with the increasing strain. (4) In addition, repeated drying and wetting cycles not only decrease the triaxal shear coefficientφbut also have an effect on coefficientφb with respect to matric suction to a certain extent. It indicates that the change of mechanical properties is irreversible after the unsaturated soil is subjected to repeated drying and wetting cycles.3. Considered tractive force of river, gravity stress and seepage force comprehensively, an whole analysis model about the bank slope (above water and underwater) and the riverbed subjected to coupling force system was established to study directly the influence of river scour effect on seepage flow and deformation in embankment by using theory analysis and finite element technique. The results of numeric simulation indicated that:(1) Maximum value of seepage velocity distributes in the bottom of embankment bank slope where suffers from the influence of seepage flow and seepage failure occurs most easily for different water levels. Besides, river scours improves seepage velocity in embankment bank slope to some extent. (2) Horizontal displacement increases greatly at embankment bank slope, and the closer to the bottom of embankment bank slope, the more increase of horizontal displacement appears. (3) Through the comparison between numerical results and measured results of deformation, there is very good inosculation between them. (4) plastic-strain range expands further in embankment and riverbed when the effect of river scour is considered, and it will have negative effect on the safety of embankment. (5) Influence of river scour on seepage flow and deformation in embankment increases with the risetime of water level of river.4. The influence of river scour on whole stability of embankment bank slope was analyzed by strength reduction FEM. The results of numeric calculation indicated that:(1) It is feasible in the theory and numeric simulation that strength reduction FEM is applied to seepage-stress coupling slope stability analysis for heterogeneous soils. (2) The process of unstable failure of soil slope is a progressive precess that partial failure extends gradually to whole failure, and the results considering river scour can reflect reallier the deformation and failure of embankment and riverbed. (3) The contours of plastic strain in layered position of sliding circular Arc appears discontinuous phenomenon during unstable failure of the embankment bank slope, and the zero-free region of the equivalent plastic strain distributes not only in sliding circular Arc on slope, but also within a certain range of internal soil layers. It thus is impossible to judge and describe accurately unstable state of the slope according to plastic strain area, and it only is a reference basis for unstable state of the slope that the plastic strain (the equivalent plastic strain) area is connected from slope top to slope bottom. (4) The safety factor of embankment reduces when the effect of river scouring is considered, and this influence on safety factor of embankment increases with the risetime of water level of river.5. A saturated-unsaturated seepage-stress coupling model was established to analyze unsteady seepage flow in unsaturated embankment by using theory analysis. The results of numeric simulation indicated that:(1) There is continuous water flow between saturated zone and unsaturated zone in embankment, and some water flow enters upward to unsaturated zone under the action of the matric suction. (2) The bottom of embankment bank slope is still the region where the local seepage failure appears most easily in saturated-unsaturated seepage flow field. (3) The soils on upstream bank slope reach saturation firstly after rapid risetime of river water level, and the seepage velocity in the bottom of embankment bank slope decreases firstly, and then increases gradually and tends to be stable. (4) The degree of saturation in sandy layer of greater permeability decreases fastly and that in clayey soil of lower permeability changes slowly after rapid drawdown of river water level. In addition, rapid drawdown of river water level increases the hydraulic gradient and the seepage velocity of embankment bank slope, therefore decreases seepage stability at the bottom of embankment bank slope.6. Whole stability of unsaturated embankment was studied to analyze the influence of repeated risetime and drawdown of river water level on the stability of unsaturated embankment based to the saturated-unsaturated seepage-stress coupling model and the test results obtain above. The results of numeric simulation indicated that:(1) Rapid risetime of river water level decreases whole stability of embankment, and then the safety factor of embankment bank slope is somewhat adjusted and tends to be stable with adjustment of pore water pressure in seepage flow field. (2) Rapid drawdown of river water level decreases whole stability of embankment, and 120h after river level drawdown is a dangerous time raletively. (3) Considering the matric suction of unsaturated soils increases the safety factor of embankment bank slope and the repeated risetime and drawdown of river water level decreases the stability of embankment bank slope. |
