【作者】 张芳枝；

【导师】 陈晓平；

【作者基本信息】 暨南大学 ， 工程力学， 2010， 博士

【摘要】 江河堤防是我国防洪工程体系的重要组成部分,我国堤防一般傍河而建,堤线选择受到河势条件制约,地基多为第四纪松散沉积物,堤身填筑土的组成及密实程度都具有非均质性,在季节性或周期性发挥作用的过程中造成堤岸冲刷现象十分普遍和严重,对堤防安全构成威胁。因此,河流冲刷作用下的堤岸渗流、变形、稳定性研究对堤防的安全运行和管理具有十分重要的意义。河流冲刷作用下堤岸稳定性研究涉及两大方面问题,一是关于堤身填筑土体及堤基土的特性研究,如土体的非饱和特性、反复干湿特性、固结特性、强度特性等；二是关于冲刷力与渗透力、自重应力共同作用下堤防及河床的冲刷稳定性状研究。本文针对广东北江大堤某典型堤段工程对此两方面内容进行了系统研究,主要研究内容与获得的主要成果如下：1.利用双压力室非饱和三轴仪进行了非饱和黏土脱湿、等吸力固结和排水剪切试验,研究了非饱和黏土的体变、屈服和强度特性。试验结果表明：(1)在低吸力范围内,非饱和黏土在脱湿过程中的体积收缩呈弹塑性体变性状,验证了非饱和土本构模型中SI(suction increase)屈服曲线的存在；(2)在等吸力固结试验中,黏土固结屈服应力随着吸力增加而增加,LC (loading-collapse)屈服曲线呈二次抛物线变化,固结压力的增长比基质吸力的增长对体变性状的影响效应要大；(3)在剪切试验中,非饱和黏土应力应变关系依赖于净围压和基质吸力的不同组合,基质吸力增强土体应力应变关系的硬化特性；(4)在不同的吸力条件下,固结排水剪的黏聚力与吸力基本呈线性关系,有效内摩擦角基本不变。2.通过非饱和土三轴仪测试黏土试样经过多次吸湿-脱湿循环路径后的力学特性,研究了反复干湿循环对非饱和土变形和强度特性的影响效应。试验结果表明：(1)干湿循环使SWCC(土水特征曲线)产生变化,相同含水率所对应的基质吸力减小,收缩特性亦发生改变,干湿循环后试样在基质吸力增加的开始阶段(收缩屈服前)收缩性增强,屈服后收缩特性基本与循环前一致；(2)非饱和土固结屈服后的压缩指数增高,但增高的幅度随基质吸力增大而降低；(3)有些试样经过干湿循环后,剪切时产生了滑裂破坏面,其应力-应变关系表现有明显峰值和应变软化特征；(4)吸湿-脱湿循环过程不仅使非饱和土有效内摩擦角φ′降低,而且对吸力内摩擦角φb值产生一定影响。说明非饱和土经过反复干湿循环的应力路径后,其力学特性将产生不可逆转的变化。3.通过理论分析和有限元技术,建立了综合考虑河流冲刷力、渗透力、自重应力耦合力系下的堤岸(水上、水下)与河床的整体分析模型,以直接分析河流冲刷作用对堤岸渗流和变形的影响。数值计算结果表明：(1)渗流流速的最大值出现在堤脚,在不同水位下堤脚都是最易受到渗流影响和出现破坏的地方,冲刷作用使堤脚处的渗流流速有所提高；(2)堤岸坡脚沿外江方向的水平位移明显增加,愈靠近坡脚,河流冲刷作用使外江方向的水平位移增加的幅度愈大；(3)通过比较变形的数值分析结果与实测结果,两者较为吻合；(4)冲刷作用进一步加大了堤岸和河床塑性区范围,对堤岸安全将产生非常不利的影响；(5)河流冲刷对堤岸渗流和变形产生的影响随着河水位的上升而加剧。4.采用强度折减有限元法,以分析河流冲刷作用对堤岸边坡整体稳定性的影响,数值计算结果表明：(1)把强度折减有限元法应用到非均质土层渗流-应力耦合的边坡稳定分析,在理论上、数值模拟实现上都是可行的；(2)土坡破坏是一个局部破坏逐渐扩展形成滑裂面的渐进破坏过程,考虑河流冲刷作用能更真实地反映坡脚和河床的变形与破坏情况；(3)堤岸边坡失稳时,在土层的分界面处滑动圆弧塑性区出现不连续现象,等效塑性应变非零区域不仅分布于边坡坡面的滑动圆弧范围,且分布于一定范围的土体内部,依此无法确切地评判和描述边坡失稳状态,只可将塑性区(或者等效塑性应变)从坡脚到坡顶贯通作为边坡失稳的参考依据；(4)河流冲刷使堤岸的安全系数降低,河水位越高,冲刷作用使堤岸的稳定性降低的幅度越大。5.通过理论分析,建立了饱和-非饱和渗流应力耦合模型,以分析非饱和土堤岸非稳定渗流场特征,数值计算结果表明：(1)堤岸非饱和区与饱和区之间存在连续的水流,部分水流在基质吸力的作用下向上流入非饱和区；(2)在饱和-非饱和渗流场中,堤脚区域仍是最易发生局部渗透破坏的区域；(3)水位快速上升时,靠近上游坡面土体首先达到饱和,堤脚渗透流速下降后又逐步上升并趋向稳定；(4)河水位骤降时,在渗透性较大的砂土层饱和度下降较快,但在渗透性较小的黏性土层饱和度变化相对滞后,河水位的骤降加大了堤岸的水力梯度,使渗流速度加大,从而降低了堤脚的渗透稳定性。6.基于饱和-非饱和渗流应力耦合模型和前述试验结果对非饱和土堤岸的整体稳定性进行了研究,以分析河水位反复升降对非饱和堤岸稳定性的影响。数值计算结果表明：(1)河水位快速上升使堤岸边坡的稳定性降低,此后随着渗流场孔隙水压力的调整,堤岸边坡的安全系数亦有所调整并趋向稳定；(2)河水位骤降使堤岸边坡的稳定性降低,水位下降后120h是较危险的时刻；(3)考虑非饱和土的基质吸力提高了堤岸边坡的安全系数,河水位反复升降后堤岸边坡稳定性降低。更多还原

【Abstract】 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.更多还原