Numerical simulation of landslide dam breaching due to overtopping

The breach of landslide dam often causes significant disaster in the inundated area; the prediction of breach hydrograph is in high demand for the dam breach risk evaluation. In this study, according to the model tests and Tangjiashan landslide dam breach case, the surface erosion accompanied by intermittent mass failure is known as the key breaching mechanism for landslide dam due to overtopping failure. The downstream slope angle would gradually decrease during the dam-breaching process, whereas a planar wedge failure occurs when the breach slopes at the dam crest and downstream breach channel fail. Based on the breach mechanism, a numerical model for landslide dam breach due to overtopping is developed to simulate the coupling process of water and soil. The model focuses on the breach morphology evolution during the breaching for the sake of the improvement of breach hydrograph prediction. Furthermore, the model can handle one- and two-sided breach, as well as incomplete and base erosion at the vertical direction. The case study of Tangjiashan landslide dam-breaching feedback analysis testifies the rationality of the present model with the relative errors less than 10% for peak discharge, final breach widths, and time to peak. The sensitivity analysis indicates that the final breach depth and soil erodibility affect the breach flow prediction of the landslide dam significantly, whereas the one- or two-sided breach mode is less sensitive.     

不同初始含水量条件下的堰塞坝溃决机理

在影响堰塞坝溃决的众多因素中,初始含水量影响堰塞坝的溃决机理仍不清楚。通过开展不同初始含水量条件下的水槽试验,详细探究了初始含水量对溃决过程的影响规律。结果表明：不同初始含水量条件下的溃决过程均具有3个典型阶段,分别是牵引侵蚀过程、溯源侵蚀过程和水沙运动再平衡过程;峰值流量随初始含水量的增大而增大,而溃决历时和残留坝体高度随初始含水量的增大而减小;随初始含水量的增大,溯源侵蚀作用逐渐减弱,牵引侵蚀作用增强;随初始含水量的增大,溃口展宽率降低,侵蚀率增大;初始含水量小于7.8%时,平均侵蚀率增长缓慢,大于7.8%后,平均侵蚀率增长迅速,且10.3%初始含水量对应的平均侵蚀率约为7.8%初始含水量的2倍;溃口宽深比在溃决的前两阶段随初始含水量的增大而减小;溃决结束后的宽深比随含水量的增大呈先趋近于1.00、后远离1.00的演变。     

考虑河床坡度和泄流槽横断面影响的堰塞坝溃决过程试验研究

针对缺乏地形条件和工程处置措施对堰塞坝溃决过程影响研究的现状,采用4种河床坡度（0°、1°、2°、3°）和3种泄流槽横断面型式（三角形、梯形、复合型）,开展了堰塞坝溃决的模型试验。通过分析堰塞坝的溃决流量、溃决历时、溃口发展和坝体纵截面演变过程,研究了不同河床坡度和泄流槽横断面对堰塞坝溃决过程的影响规律。试验结果表明：...     

粘土心墙坝漫顶溃坝过程离心模型试验与数值模拟

利用作者研制成功的溃坝离心模型试验系统,对粘土心墙坝漫顶溃决过程进行了试验研究,结果发现粘土心墙坝与均质坝溃决机理与溃口发展规律明显不同,随着漫坝水流对下游坝壳冲蚀程度的增加,粘土心墙发生剪断破坏,溃口洪水流量迅速增大.基于上述试验结果,提出了一个描述粘土心墙坝漫顶溃坝过程的数学模型,并建议了相应的数值计算方法.该模型...     

考虑不同泄流槽方案的堰塞坝溃决机理分析——以唐家山堰塞坝为例

近年来,频发的地质构造活动和极端气候灾害诱发了大量堰塞坝,严重威胁上下游群众的生命财产安全。开挖泄流槽是最常用降低堰塞坝溃决风险的措施,由于时间非常急迫、交通极度瘫痪,其开挖量非常有限,因此如何利用有限的开挖量将溃坝风险降低至最小是亟待解决的问题。本文基于水土耦合冲刷机理,提出了考虑不同泄流槽方案的堰塞坝溃决机理分析方法,并应用于唐家山堰塞坝。该方法根据水力学参数和坝体抗冲刷性参数动态计算瞬时坝体冲刷率,进而分析泄流槽对溃决全过程的影响,从而自动获取最优的泄流槽设计方案。将此方法应用于唐家山堰塞坝案例发现:唐家山堰塞坝泄流槽最优设计时溃坝洪峰流量为1700m3·s-1,小于实际峰值流量6500m3·s-1,主要是因为增大泄流槽的纵坡率,显著增强溃坝前的冲刷并形成双洪峰,从而有效降低了溃决峰值流量。由于复合槽相对较小的水力半径限制了溃坝前的冲刷,使得临溃时水位较高,因此溃坝峰值流量比单槽大,溃坝风险降低效果不如单槽。     

暗穴环境下非饱和黄土边坡稳定性分析

黄土暗穴在水土流失方面作为一种独特的侵蚀方式,往往会威胁到建设在非饱和黄土层之上的公路边坡稳定性。基于考虑基质吸力的强度折减有限元程序,本文通过开展不同位置暗穴、单双暗穴和基质吸力赋存与丧失等条件下边坡稳定性的对比研究,着重对影响边坡稳定性的暗穴、基质吸力等因素进行了研究。研究结果表明,当暗穴接近地表时由于存在一定的减载效应而利于坡体的稳定,而当暗穴接近边坡坡脚时则成为黄土边坡稳定性的不利因素,在边坡底部存在的双暗穴明显削弱了边坡的稳定性,基质吸力使得非饱和黄土边坡安全系数增大,潜在滑动面变深,反之则使安全系数减小,潜在滑动面上移,基质吸力的丧失或减小以及暗穴的削弱作用使得潜在滑动面位置变化较大。     

基于可靠度方法的全基质吸力段土-水特征模型研究

土-水特征曲线是非饱和土入渗和流-固耦合分析的基础,目前该模型的研究成果由于试验方法和设备的局限,仅能够得到局部基质吸力段(低吸力或高吸力)的土-水特征曲线;而如何根据局部试验数据集推断与试验结果相吻合的全基质吸力段范围的土-水特征曲线是一个难题。对东南沿海广泛分布的原状残积土进行了不同基质吸力段的土-水特征试验。根据残积土低吸力段和高吸力段的试验数据,运用可靠度分析方法,从土壤孔隙微元破裂失稳的角度,建立了土壤全基质吸力段的土-水特征模型。根据UNSODA2.0数据库提取砂土、粉土、黏土等3种类型土样本(52个)的试验数据,讨论了模型参数意义和不同土类的模型适用性,进行了不同土-水特征模型的比较分析。对比验证研究表明,形状参数σ和中值比例参数μ使提出的模型具有很强的适应性,并且具有依据局部试验数据(低或高基质吸力段)快速获得全基质吸力段土壤土-水特征曲线的能力。与其它模型相比,模型计算过程适应性强,模型决定系数R2>0.98。研究结果对快速获取不同类型土壤的全基质吸力段土-水特征曲线具有参考价值。     

喜马拉雅中段冰湖终碛坝体溃决过程实验研究

终碛坝广泛分布于世界各地的高山和极高山区。为了探究终碛坝的溃决过程,了解溃口的演变特征,文章以嘉龙错终碛坝的原位实验,模拟了终碛湖漫顶溃决过程。通过分析实验结果发现:（1）根据观察,将终碛坝溃决过程划分为坝体下游坡面冲刷、“溯源侵蚀”、出水口下切和溃口拓宽四个阶段。（2）上游湖区崩塌体激发的涌浪会造成溃口内的瞬时流量增加数倍,从而使得在有涌浪和无涌浪的条件下,“溯源侵蚀”过程出现陡坎和斜坡两种下切型。（3）通过分析溃口下切侵蚀过程,发现溃口的下切侵蚀发展过程主要受到坝体孔隙比和细粒含量的影响,并且溃口中点侵蚀率与水流剪应力存在一定的线性关系,符合线性侵蚀模型。通过分析发现,嘉龙错终碛坝的侵蚀系数为0.051,临界启动应力为237.64 Pa。与堰塞坝相比,可侵蚀系数比更小,而临界启动应力更大。     

A bounding surface model for saturated and unsaturated soil-structure interfaces

In many geotechnical systems, such as reinforced slopes and embankments, soil-structure interfaces are often unsaturated. Shear behaviour of unsaturated interfaces is strongly dependent on their matric suctions, as revealed by the results of extensive laboratory tests. So far, constitutive models for unsaturated interfaces are very limited in the literature. This paper reports a new bounding surface model for saturated and unsaturated interfaces. New formulations were developed to incorporate suction effects on the flow rule and plastic modulus. To examine the capability of the proposed model, it was applied to simulate suction- and stress-controlled direct shear tests on unsaturated soil–cement, soil–steel and soil–geotextile interfaces. Measured and computed results are well matched, demonstrating that the proposed model can well capture key features of the shear behaviour of unsaturated interfaces, including suction-dependent dilatancy, stress–strain relation and peak and critical state shear strengths.     

Failure of dams arranged in series or in complex

A practical methodology has been developed for predicting flows generated by dam failures or malfunctions in a complex or a series of dams. A twofold approach is followed. First, the waves induced in the downstream reservoirs are computed, as well as hydrodynamic impacts induced on downstream dams and dikes are estimated. Second, the flood wave propagation and the inundation process are simulated in the downstream valley, accounting for possible dam collapse or breaching in cascade. Two complementary flow models are combined: a two-dimensional fully dynamic model and a simplified lumped model. At each stage, the methodology provides guidelines to select the most appropriate model for efficiently computing the induced flows. Both models handle parametric modeling of gradual dam breaching. The procedure also incorporates prediction of breach formation time and final width, as well as sensitivity analysis to compensate for the high uncertainties remaining in the estimation of breach parameters. The applicability of the modeling procedure is demonstrated for a case study involving a 70-m high-gravity concrete dam located upstream of four other dams.     

水位骤降时的非饱和坝坡稳定分析

从非饱和土抗剪强度理论出发,应用极限平衡法和以有限元应力计算为基础的极限平衡法,分别对库水位骤降时的坝坡稳定进行了计算。分析中采用非饱和-非稳定渗流理论,模拟了孔隙水压力的消散过程,并考虑基质吸力对土体抗剪强度的影响,将每个渗流分析结果调入稳定分析模块计算其稳定性。最后将两类极限平衡法稳定分析结果进行了比较,结果表明,库水位的骤降,易引起坝坡的滑动,随着超静孔隙水压力的消散,坝坡稳定性逐渐提高;随着孔压消散,基质吸力对抗剪强度的贡献增大。     

Correlation of unsaturated soil and dielectric property for monitoring of subsurface characteristics: development of unsaturated dielectric mixing models and its application

The behavior due to rainfall infiltrating the ground plays a role in landslides, groundwater recharge and various other ground responses. Most of these geotechnical behaviors have a correlation between soil pore space and soil volumetric water content in the unsaturated and saturated soil porous media. Therefore, the soil porosity associated with soil pores and the distribution of volumetric water content are significantly important hydrological characteristics. In the case of shallow slope failure such as landslide, the infiltration activity due to the connectivity of soil pore spaces in a porous media is induced. Slope failure may be attributed to the effect of a wetting front with the slope due to liquid infiltration, which changes the volumetric water content, soil matric suction and shear strength of the slope. This study was performed with an unsaturated injection test using a frequency domain reflectometry (FDR) dielectric device which measures the dielectric constant of unsaturated soil and the study then proposed the unsaturated dielectric mixing models to calculate soil porosity and effective porosity of unsaturated soils. From the experimental results the ratio of effective porosity to porosity of soils are measured in a range of 70–85%. These experimental results show a decrease of about 5–10% for unsaturated soil compared to the ratio of effective porosity to porosity of saturated soil. The infiltration passages of tracer material are restricted within the pore connectivity in the unsaturated soil which is caused by dead-pores in the soil. Using the FDR device and the unsaturated dielectric mixing models, we can consider the acquisition of physical properties to detect the infiltration activity, the response of the dielectric constant along with the injected tracer and hydrological parameters for the unsaturated soil porous media.     

隧道施工中环境土层工程性质研究

利用非饱和土的三轴仪 ,测定不同吸力状态下的土的强度特性 ,并确立了两者之间的定量关系 ,同时进行了含水量对非饱和土的强度影响试验 ,研究了北京地区的非饱和土的强度特性。试验结果表明 :不同的吸力状态下 ,土的强度明显提高 ;含水量的改变对内聚力有明显影响 ,而对内摩擦角无明显影响 ,并测定本土样对强度改变的含水量拐点值.     

基质吸力对非饱和重塑黄土抗剪强度的贡献

非饱和土抗剪强度随饱和度降低、基质吸力增大而提高.然而,不同类型非饱和土中,基质吸力对抗剪强度的贡献却显著不同.本文通过室内试验,探讨了基质吸力对非饱和重塑黄土抗剪强度贡献的基本特征.研究结果表明,非饱和重塑黄土抗剪强度随着基质吸力的增大,表现为非线性的和明显的4个阶段性增、降和逐渐稳定特征,且分段特征值与土-水特征曲...     

Measurement of debris mass changes and assessment of the dam-break flood potential of earthquake-triggered Hattian landslide dam

The Hattian landslide, which was triggered by the 2005 Kashmir earthquake, formed one of largest landslide dams in the world and it has posed a serious threat of flooding to people living in the lower reach of the Jhelum River. In order to understand deformation occurring in the body of the dam, physical measurements using a Differential Global Positioning System (DGPS) were conducted. Gradual deformation and slowly developing backward erosion initially were observed, leading eventually to a sudden creation of a deep hollow on the downstream slope of the landslide dam. The dimensions of this eroded gully were determined by laser scanning, and the results showed a significant loss of soil volume and a large change in the body of the dam. A breach formation model was used to predict the outflow hydrograph generated by constant downcutting of dam during a breaching event. A run-off analysis of the outflow hydrograph was conducted to evaluate inundation levels of flood waves in case the dam is breached. Hazardous downstream locations were identified near the junction of the Karli and Jhelum Rivers, suggesting a need for early warning system in order to avoid loss of lives.     

Experimental study of steady seepage in unsaturated loess soil

The complexity of unsaturated soil means that its permeability coefficient and seepage law are important topics in soil mechanics and geotechnical engineering. In this study, a new type of equipment for measuring the seepage of unsaturated soil was used to study the steady seepage of unsaturated loess soil collected from northern Shaanxi province in China. The entire process of the steady seepage of unsaturated soil under different seepage hydraulic gradients was obtained, and the permeability coefficient in the steady seepage state was measured. When the soil water content was low, the soil moisture remained only in small pores or at the junction of soil particles, the effective seepage area of the sample was reduced, and the seepage path was extended. Therefore, the lower the water content, the smaller the permeability coefficient of unsaturated soil and the longer the time required for the seepage to reach a steady state. The matric suction of the unsaturated loess soil was measured under different water contents. With decreasing water content, water remained only in the smaller pores, which provided shrink films with a smaller radius of curvature. This allowed the shrink films to withstand a larger pore air pressure and transmit a smaller pore water pressure, so that the matric suction of the loess soil increased with decreasing soil water content. The Gardner model provided an accurate fit of how the matric suction varied with the volumetric water content of the unsaturated soil. The seepage velocity and seepage hydraulic gradient were correlated well with a linear relationship, indicating that the seepage law of unsaturated loess soil accords with Darcy’s law. The experimental results show that the new unsaturated soil permeameter has good performance and controllability, thereby providing support for studying the steady seepage of unsaturated soil.

     

非饱和抗剪强度指标c、φ值与含水率w的关系

非饱和土力学已经确认了基质吸力与非饱和土的强度指标有关,而基质吸力与含水率有着密切的关系。通过理论和三轴剪切试验分析,探讨了非饱和残积土抗剪强度指标c、φ与含水率ω变化的一般规律;同时结合已经推出的土水特征曲线关系,探讨抗剪强度指标c、φ与基质吸力μ的变化的一般规律。结果表明:随着非饱和土含水率ω增加、基质吸力μ的减小,黏聚力c有减少的趋势,内摩擦角φ变化不大。通过线性回归分析,黏聚力与含水率成半对数线性关系,黏聚力与基质吸力成线性关系。现有的非饱和土实用公式精确性较差,非饱和土抗剪强度指标有待进一步的研究与完善。     

非饱和黄土强度参数的试验研究

非饱和土强度理论的研究是非饱和土力学的核心问题。目前公认的非饱和土强度理论是Bishop的单变量理论和Fredlund的双变量理论,二者都引入了基质吸力这一应力参数,只是前者将其归到有效应力中,后者作为独立变量。要得到这两个公式中相关参数需利用控制基质吸力的非饱和土三轴仪或直剪仪测定,控制基质吸力是通过同步增加气压和孔隙水压力保持吸力不变实现的,即所谓的轴平移技术,该项技术一直面临测试周期长的困难和合理性的质疑。鉴于此,本文采用不同含水率的常规三轴CU试验,测定了有效稳态强度参数,得出了吸应力和体积含水率的关系曲线,非饱和强度直接用吸应力函数表达,该强度公式回避了测定基质吸力的问题,便于在工程实际中推广。为了进一步和Bishop及Fredlund的强度公式进行对比,又用张力计测定了同一试样的水土特征曲线,获得了其强度参数。理论上,这3种强度理论对机理的解释不同,但在数学上可以互相转化。从工程应用的角度,基于吸应力的抗剪强度更便于工程应用。     

基于电阻率的非饱和土基质吸力预测

针对非饱和土吸力测量与计算的困难,提出了一种利用土的电阻率结构参数预测基质吸力的方法。通过滤纸法测试重塑膨胀土的基质吸力,分析了非饱和土的基质吸力与体积含水率的关系,即土-水特征曲线的变化规律。在测试非饱和土横向与竖向电阻率的基础上,探讨了非饱和土的电阻率与其体积含水率的关系,并进一步研究了土的电阻率结构性参数 、 和 随体积含水率的变化规律。发现基质吸力与电阻率结构参数 之间存在良好的抛物线关系,利用这种关系可以准确预测非饱和土的基质吸力。利用土的电阻率结构参数预测吸力,避免了吸力量测与计算的困难,简单实用。     

Aspects of Mechanical Behavior and Modeling of a Tropical Unsaturated Soil

The research studies the applicability of two elastoplastic models for the collapse prediction of the lateritic soil profile from Southeastern Brazil. These tropical soils have peculiar geotechnical behavior, due to their mineralogical composition and porous structure coming from intense process of formation. Two elastoplastic models were analyzed: the Barcelona Basic Model (BBM) and another one based on BBM, however developed for tropical soils. Oedometric tests with suction control were performed at three distinct depths of the soil profile. The BBM was not suitable for the upper layer of the soil profile, because BBM considers the compressible behavior of the soil in function of the reduction of the elastoplastic compressibility index with the increase of the matric suction. The model developed for tropical soils showed better suited to the compressible behavior of the soil profile, resulting in good prediction of the collapse potential, mainly by accepting increasing values of the elastoplastic compressibility index of the soil profile with the matric suction rise.