山岳型古冰水堆积体非线性渗透特性及其与沉积特征的相关性研究

In the midstream valley of Dadu river, Southwest, China, there are many ancient mountain outwash accumulations, which have deposited in Pleistocene and have complicated sedimentary properties, some of those are located in the seasonal water table fluctuation zone of hydropower stations. During the period of reservoir water level fluctuation and rainfall, the complicated saturated-unsaturated seepage of groundwater in those accumulations is one of the most important factors for those accumulations' stability. According to pre-research, those outwash accumulations, deposited in Pleistocene, have complicated sedimentary properties, and have clear nonlinear permeability. In this project, those outwash accumulations' sedimentary properties, such as grain size distribution, porosity, compactness, roundness, sorting and cement characteristic, will be investigated and tested carefully, and the key parameters representing sedimetary properties would be obtained. The saturated-unsaturated permeability of those ancient mountain outwash accumulations will be explored carefully according to the improved in-situ and laboratory tests, and the nonlinear permeability equation, fitting those ancient mountain outwash accumulations, will be suggested. Based on those works, the relationships between the saturated-unsaturated permeability and such factors as sedimentary properties and matric suction will be analyzed. At last, the research results will be applied to analyze the saturated-unsaturated seepage field characteristics and stability of those accumulations during the period of water level fluctuation and rainfall.

在我国西南河谷广泛分布一套形成于更新世的山岳型古冰水堆积体，这些冰水堆积体很多位于水电站库区水位变动带范围内。库水位上升或下降以及降雨期间,地下水复杂的饱和-非饱和渗透作用是影响其稳定性的关键因素之一。申请者的前期研究表明，这些山岳型冰水堆积体大多具有复杂的沉积特征，其渗透特性往往表现出明显的非线性特点。本项研究拟从现场调查入手，结合室内外试验，详细分析堆积体包括颗粒组成、孔隙特点、密实度、磨圆性、分选性以及胶结特点等在内的沉积特征，获取反映冰水堆积体沉积特征的关键性指标。然后采用改进的现场渗透试验和室内渗透试验方法，探索山岳型古冰水堆积体饱和-非饱和非线性渗透规律，建立适合该类堆积体的非线性渗透本构方程，探索山岳型古冰水堆积体饱和-非饱和渗透特性与沉积特征、基质吸力等因数之间的关系。最后结合工程,将成果应用于分析库水位变动以及降雨期间堆积体饱和-非饱和渗流场特征和规律及对稳定性的影响。

通过对大渡河、澜沧江和金沙江等西南河谷典型冰水堆积沉积特征以及渗透性特性调查研究，揭示了西南河谷冰水堆积体包括颗粒组成、结构特征在内的沉积特征，探索了西南河谷典型冰水堆积体的形成演化过程，研究分析了不同水力梯度下的渗透本构方程以及渗透特性与沉积特征之间的关系。研究表明：西南河谷典型冰水堆积体主要由具有较好磨圆和分选，成层性好的粗颗粒土和轻度磨圆、无分选的巨颗粒土组成，曾接受至少三期冰水沉积作用，期间经历过两次沉积间断，该地区古气候至少经历过三次较明显升温（280 ka B.P.-120 ka B.P.，110 ka B.P.-80 ka B.P.，70 ka B.P.-25ka B.P.）和两次较明显降温过程（120 ka B.P.，77 ka B.P.）。冰水堆积体具有密度高、天然含水率低、孔隙率低，主要由sand、gravel、cobble and boulder 构成，fines 含量一般小于5％等沉积特点。冰水堆积体在低水力梯度下表现出Darcy 渗流特点，当水力梯度超过0.5-0.7后，表现出非线性渗流特点，渗透破坏坡降约1.1-1.5，渗透系数约0.003 cm/s-0.009 cm/s。通过对nondarcy渗流试验数据采用Forchheimer equation and Izbash equation拟合，认为冰水堆积体nonDarcy渗流符合Izbash 幂函数渗流方程，指数m约0.6-0.7，基于Poiseuille 渗流模型得出反应冰水堆积体permeablity特点的特征粒径约0.024 mm-0.031 mm。