黄土台塬灌区边坡可靠度及其防治的稳健决策

Irrigation induced landslides in the loess plateau have caused significant retreat of loss plateau and great decrease of agricultural land. This project aims to put forward a scientific and robust program for coordinating the contradiction between agricultural production and slope instability. The irrigation area of Jingyang South Loess Plateau is selected to be the study site. The main work includes: (i) Collect geological data here and collect intact loess and paleosoil samples for obtaining basic physical parameters, soil water characteristic curves (SWCCs), permeability parameters, pore size distribution curves (PSDs) as well as strength parameters. (ii) Determine the relationships between deposit time or soil depth and the permeability function as well as strength parameters. Then a geological model of slope engineering reflecting the depositing history can be established. (iii) Analyze the reliability of the loess slope model under coupling hydro-mechanical conditions. The distributions of related parameters are predicted using the Bayesian methods. Also, the reliability expectations and standard deviations are again estimated by the Bayesian methods. (iv) Establish a multi-objective optimization decision model to provide scientific basis for government decision-making, solving the problems of agricultural development and disaster prevention in such areas. The non-irrigation width between the plateau edge and irrigation boundary is regards as a measure to improve the slope stability. The optimization decision model reflects the relationship between the non-irrigation width (economic aspect) and the slope reliability expectations (slope stability) as well as the reliability standard deviation (robust stability).

黄土台塬地区灌溉诱发的黄土滑坡导致台塬退缩、耕地减少，灾害频发。本项目通过研究提出科学的稳健决策方案，协调黄土台塬灌区农业生产与边坡失稳的矛盾。以泾阳南塬灌区为研究对象，通过收集资料，并现场采取黄土-古土壤地层的原状土样，测试其基本物理指标、渗透系数、抗剪强度参数、土水特征曲线和孔隙分布曲线等，确定渗透系数、强度参数与沉积时间（地层埋深）的关系，建立反映沉积历史的边坡工程地质模型。采用贝叶斯估计确定参数概率分布，将灌溉、降雨和蒸发作为边界条件，分析黄土边坡在渗流场与应力场耦合作用下的可靠度，并用贝叶斯公式估计可靠度的期望值和标准差。以坡顶边缘向塬内预留一定的非灌区作为改善边坡稳定性的措施，建立边坡可靠度的期望值(稳定性)、可靠度的标准差(稳健性)和非灌区宽度(经济性)的多目标优化决策模型，提出能保证边坡稳定的非灌区最优宽度，为政府决策提供依据，解决该类地区农业发展与灾害防治的问题。

利用分级探井人工取样，试验实测了泾阳黄土剖面L1-L9黄土与S1-S8古土壤的强度、渗透、变形参数，并通过扫描电镜和压汞技术获取了黄土和古土壤的微观结构和孔隙分布，采用滤纸法测定土-水特征曲线。建立起采用沉积年代预测黄土干密度、饱和渗透系数、粘聚力、内摩擦角、土-水特征曲线Van Genuchten模型的参数a、m、n和饱和体积含水率、残余体积含水率，共9个参数随着沉积年代变化的随机模型，建立起采用零压力土-水特征曲线和常规直接剪切实验估计非饱和抗剪强度模型，将参数均值和方差都作为随机变量，建立起黄土强度参数c、φ的正态-逆伽马先验分布，基于Bayesian理论，利用共轭先验法推导了参数后验分布和后验概率密度函数的期望值求解公式，开发了基于参数随机模型和强度时间模型的斜坡稳定系数有限元计算程序。预测了灌溉70年(1976-2046年)的地下水位上升过程。结果表明泾阳南塬地下水的补排动态平衡时间预计在未来20年(2042年)以后，即开始灌溉66年后。将边坡失效概率均值作为边坡的安全目标，失效概率的标准差作为防治的稳健目标，禁灌距离与坡高比L/H作为防治的经济目标，进行了泾阳南塬灌区边坡安全性、稳健性和经济性的优化分析。可取失效概率小于5%、失效概率标准差小于5%和L/H>1.5作为安全、稳健和经济的综合平衡点，达到泾阳边坡在坡顶长期灌溉条件下的最优决策方案。