中文摘要：

覆盖层深厚且为强弱透水互层的闸坝基础在我国西南部河流中很常见，防渗墙通常作为坝基渗流控制设施。渗流有限元计算中，该种坝基上下游覆盖层截断边界面位置及其边界条件的设置往往是影响结果的重要因素，对其确定方法的研究具有重要的实用意义。提出了渗流计算中采用缩尺单元方法分析坝基覆盖层截断面边界合理位置的方法。该方法只需要一个计算网格就可实现合理截断面边界位置的确定和正式的渗流计算分析。硬梁包水电站坝基为典型的强弱透水互层，其渗透系数差异达到了3个数量级。初步设计方案稳定渗流计算工况对渗流量基本不影响的上下游不透水截断边界面的位置离坝轴线远达40—80kin。尽管河谷宽度与水位差之比高达16，该坝基相对不透水土层的局部缺失使渗流场有显著的三维特征。计算结果中防渗墙毗邻区域弱透水土层内的渗透坡降数倍于允许坡降，然而这些部位在防渗墙完整的前提下渗透破坏的风险却很小。渗透坡降的方向特征及渗透变形发生后的演化特征对渗透稳定风险评价是很重要的。

英文摘要：

Dam foundations with deep deposit composed of strong and weak permeable layers alternatively are common in rivers in the south-west China. Cutoff wall is usually used to control the seepage of such a foundation. The upstream and downstream truncated boundary positions and conditions are important factors which affect the numerical simulation results of the seepage of the dam site. Research on the methodology for determining the truncated boundary position and condition is of importance for application. A scale re- duced element method is proposed to analysis the reasonable upstream and downstream boundary position. Only one mesh is needed for the analysis of reasonable boundary positions and the seepage analysis. The foundation of Ying-liang-bao dam consists of 5 layers of sand or gravel soils which are permeable strong and weak alternatively with a total depth up to 126.5 m, the difference of the coefficient of permeability reaches 3 orders of magnitude. The seepage field and the internal erosion risk of the preliminary design scheme of Ying-liang-bao dam are analyzed. A reasonable distance of the upstream or downstream hound- ary to the dam axis is up to 40 to 80 kin. Although the ratio of the width of the valley to the difference of the upstream and downstream water level is as high as 16 where usually a two dimensional flow will take place, the partial loss of weak permeable layers in the foundation causes remarkable features of three dimensional flow. The hydraulic gradient in soils adjacent to the concrete anti-seepage wall is several times of the allowable values, the risk of internal erosion is still low in condition of an undamaged wall. The character of the seepage gradient where the hydraulic gradient is greater than the allowable value and the evolution of the internal erosion while it occurs should be further investigated for a seepage stability risk as- sessment.