Lots of long and large fissures with slight slope and few with medium slope towards the downstream in the bedrock of the foundation of No.1-5 dam units of the left powerhouse of the Yangtze Three Gorges Project constitute the problems of the deep layer slide and stability by the joints of the high slope. The modes of sliding of various monoliths are generalized based on the distribution of the measured long and large gentle inclination structural faces. In addition

two kinds of hypothetical and extreme sliding models were proposed. This paper researches the slide and stability of the No.1-5 units dam from the aspects of engineering structural measures

analysis of deep stability against sliding

analysis for observed data. The rigid limit equilibrium is used for analysis of deep stability against sliding in the No.1-5 monoliths of the left intake dam section

which is accompanied by FEM numerical analyses and geo-mechanical model tests. The results above show that the Kc′ values derived from the rigid limit equilibrium of the No.1-5 monoliths of the left bank intake dam section meet the requirements for stability safety. The values by other analysis methods are: 3.0－4.5 or above for FEM analysis

3.5－4.2 for geo-mechanical model test. The stability analysis of the sliding modes dominated by rigid limit equilibrium and supplemented by various other analysis methods indicates that after engineering structural measures taken for the No.1-5 monoliths of the left bank intake dam section

guarantee has been provided for the stability against sliding of the dam and its foundation rocks. Analysis of observations shows that for the foundations of No.1-5 monoliths of the left bank intake dam section

horizontal deformation is little; vertical deformation is undersized; asymmetric settlement does not appear. The dam foundation is stable.