The presence of middle-lower dip joints is one of the important geological defects in dam foundation.Analysis of development mode and formation mechanism provides an important foundation for the stability of slopes.This paper is based on detailed analysis of the spatial distribution, combined with geological analysis, SEM and numerical simulation. It interprets the formation mechanism of middle-lower dip joints at a large hydropower station dam site. The results are as follows: (1) Intermittent rapid down-cutting of the river is the main reason for the alternative distribution of middle-lower dip joints; (2) Main fracture mechanism is coexist of tension and shear, a small number of samples show the shear fracture; (3) If faults and dykes exist in slopes and their widths are large, relaxation effect along the whole weak belt is obvious and the distribution of shear strain increment belt is notobvious. (4) If the width of weak belt and distance to the slope surface are both small, on the one hand, shear strain increment is concentrated along the dip direction of weak belt, on the other hand, concentrated degree and number of middle-lower dip joints increased. In addition, density of middle-lower dip joints also increased.%中缓倾角裂隙的存在是重要坝肩地质缺陷之一,中缓倾角裂隙发育规律、成因机制分析是研究边坡稳定性的重要基础.通过对我国西南某大型水电站中缓倾角裂隙空间分布规律的深入分析,结合地质分析、微观电镜扫描和数值模拟,阐释了中缓倾角裂隙的成因机制.结果表明:(1)河谷间歇性快速下切是中缓倾角裂隙密集带相间分布的主要形成原因;(2)中缓倾角裂隙的断裂形式以拉裂和拉剪并存为主,少部分试样表现为剪切破裂,力学机制与河谷下切地质演化相对应;(3)如果岸坡内存在断层、岩脉等软弱带,若软弱带宽度较大,则沿软弱带的整体松弛效应明显,而弱化中缓倾角剪应变增量带的分布;(4)若软弱带的宽度较小且距坡表距离较小时,一方面除沿软弱带倾角方向的应变较为集中,另一方面,会使剪应变增量带自坡表至软弱带的中缓顷裂隙的集中程度和数量均有所增加,另外,岩脉间中缓倾角裂隙的密度亦增加.
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