Abstract: Weak intercalations, which are composed of clastic rocks, are widely distributed in Upper Jurassic red strata in Sichuan Basin. Due to the high clay content and low shear strength, these weak intercalations are prone to cause shear failure which leads to instability of the dam foundation of Xiaonanhai Project. Markov chain analysis is applied to the stratigraphic succession in order to point out any depositional regularity of the weak intercalations. In consideration of the grain size, mudding degree and structure characteristics, the weak intercalations are divided into three types (Ⅰ) cracked rock; (Ⅱ) cracked rock with mud interbedded; (Ⅲ) mudstone with cracked rock interbedded, or mudstone. Type Ⅰ mainly exists in the thick mudstone, which is attributed to the flood basin sub-facies, or develops at the junction of thick mudstone and siltstone. Such weak intercalations cannot extend long distance. Type Ⅱ generally lies in the soft rock and suffers more shear force owing to tectonic activities, a sheet of mud with scratches is common on the top and bottom. Type Ⅲ frequently appears in the second semi-rhythmic structure. Slow shear test reveals that the lowest strength parameter of weak intercalations is 0.223, and the value of strength parameter according to the quantified index of granularity composition is 0.72 to 0.86 times of that obtained from the slow shear test. In addition, recommended values of typical weak intercalations were given by considering other similar engineering experience.

Key words: Xiaonanhai Project, Markov chain, weak intercalation, depositional regularity, strength parameter