土体抗冲蚀特性及土石坝渗透破坏和漫顶溃决机理研究

Worldwide, China has the largest number of earth-fill dams, and the failure ratio of the Chinese dams is much higher than that of the worldwide dams. The majority of the dam failures are caused by seepage failure and overtopping. At present, the numerical method is popularly employed to study the dam failure process. However, the existing numerical methods commonly have the disadvantages of results disperse and computing instability. The erosion law of geo-materials is the key factor to successfully predict the dam failure process by the numerical methods. This project attempts to study deeply the seepage failure and overtopping mechanism of earth-fill dam from the three aspects: (1) Study on internal erosion and surface erosion characteristics by conducting erosion function apparatus tests and seepage failure triaxial tests. Consequently, propose the functions of critical stress, erosion rate and the expansion pattern of the seepage failure channel for the cohesive soil and non-cohesive soil by combining with the principles of continuum mechanics and micro-scale analysis methods. (2) Study on breach cross-section collapse mechanism by considering the characteristics of compacted soils. Develop a new simplified physically-based breach model based on soil erosion and cross-section collapse mechanism study. (3) Centrifuge modelling of homogenous and geotextile-reinforced earth-fill dams breaching process with self-design circulating system for water supply. Study the earth-fill dam failure process, improve the centrifuge test mode and verify the established model against centrifuge tests data. This project can provide the theory foundation and test data reference for the emergency treatment and risk assessment of earth-fill dam failure.

我国的土石坝数量居世界首位，土石坝溃坝率远高于世界平均溃坝率，且多数为渗透破坏和漫顶溃决。已有的土石坝溃坝模拟软件普遍存在结果分散、计算不稳定等问题，而筑坝材料抗冲流速和冲蚀速率计算是影响模拟结果的关键因素。本项目拟针对土石坝的渗透破坏和漫顶溃决问题，开展三方面的研究工作：1）应用冲蚀函数测定仪和渗透破坏三轴仪研究土体的表面冲蚀和内部冲蚀特性，结合连续介质力学和细观力学分析提出无黏性土和黏性土抗冲流速、冲蚀速率的函数式和渗透破坏通道的扩展模式；2）考虑土体的压实特性提出土石坝溃口侧壁坍塌分析方法，结合土体抗冲蚀特性研究成果，建立土石坝溃坝机理模型，并编制计算软件；3）应用自行研制的封闭式循环供水离心机溃坝模拟系统开展均质土坝和加筋土坝离心模型试验，研究土石坝溃坝过程，完善离心机溃坝模拟技术，并验证所建立的土石坝溃坝机理模型。研究成果将为土石坝溃决应急处置和风险调控提供理论支撑和实验依据。

本项目结合单元体试验、模型试验和数值模拟研究了土体抗冲刷及土石坝渗透破坏和漫顶溃决机理。研究了推移质起动条件，提出了水流有效切应力区的概念，得到了理论更为完善的随机性推移质输沙率公式；研究了不同类型土体的起动切应力和土体物理力学性质的关系，总结了不同性质土体的起动切应力的变化规律。提出了重塑黏性土-砂砾混合土体的起动应力表达式，可较好地对黏性土-砂砾混合体的起动应力进行预测。提出了冲刷黏性度的概念，给出了冲刷黏性度表达式，为衡量混合土体的抗冲刷行为提供了度量方式。提出了过剪应力模型和 Wilson模型冲刷参数的取值范围以及其和常规土性参数的统计关系，给出了冲刷参数的快速确定方法，并以白格堰塞体为例，对其冲刷参数和冲刷速率进行了分析。研制并改进了一套用于土石坝漫顶溃坝试验的循环供水系统，分析了加筋土石坝的漫顶溃决机理和溃决过程，推导了溃口下切速率关于时间的关系式。提出了一个土石坝漫顶溃决物理模型，该模型应用双曲线型冲蚀速率方程计算坝体材料的冲蚀速率，通过极限平衡理论计算溃口的侧向扩展。从溃口流量平衡方程、筑坝材料抗冲剪应力和冲蚀速率、溃口扩展模式三方面分析总结了当前较常用的17种土石坝漫顶溃决物理模型；以唐家山堰塞坝溃坝事故为工程背景，选取BREACH模型、DB模型和IWHR模型对其溃坝过程进行了数值模拟，研究了上述三种模型计算得到的筑坝材料冲蚀速率、流量过程线和水位变化曲线与实测值的差异；应用BREACH模型研究了筑坝材料的黏聚力、内摩擦角、不均匀系数和孔隙率对流量过程线的影响。