The foundation bed rocks of the Gezhouba profect on the Yangtze River consist of Cretaceous red soft rocks. About one-third or these bed rocks are of argillaceous type deposiled into 70 layers with many argillized intercalations. The engineering geologic properties of them are very poor, especially for the argillized intercalations that confro1 the stability of foundation of the hydraulic structures. Thuough on-site investigations and systematic studies of argillaceous soft rocks a lot of information such as the mechanical strengths, stability against seepage, quick weathering and its protection measures, rebound and compression effects as well as the tendency of long-form evolution have been accumulated, providing a fully reliable basis for the design and construction.
The Gezhouba Project, 2.3 km. below the outlet of the Three Gorges, is the first dam built on the main stream of the Yangtze River and is now under constrction. The strata in the dam site are gentle, dipping downstream toward the left bank being composed of the lower Cretaceous red, elastic rocks of continental facies. About one third of the thickness of dam foundation rock strata (120 m. total) or 70 layers are composed of avgillaceous soft rocks. These rocks have not only a very large thickness and numerous beddings, but also spread widely. he, engineering geologic properties of these soft rocks are very poor, and above all, the argi11ized intercatlations in these strata critical factors controlling the stability of the hydraulic structures. In view. of the problems mentioned above, a large amount of geological mapping, explorations by adits, core drillings (especia1ly the explorations in large bore holesone meter in diameter), the sample testings in laboratory and in situ, as well as comprehensive analyses and studies had been performed, thus the spatial distribution and engineering geology characteristies of the argillaceous soft rocks were ascertained, providing a fully reliable basis for favorable designs and constructions.
The argillaceous soft rocks in the dam site consist of argillaceous conglomerates, gompholites (nage1fluh), argillaceous siltstones and claystones formed during the sedimentation period, together with the loosely argi11ized inter1ayers and softened materials formed after reformation by geological processes. All these argillaceous soft rocks contain more clay minerals such as illites, montmori110nifes and Kaolinites, thus their mechanical strengths are reduced, and their stabilities against water erosion become poor, as characterized by drying crack due to loss of water and being softened and disintegrated once wetted again. The soft rocks mentioned above can be classified into following four categories (Fig. 1.) according to their differences of engineering geological properties and the compositive features of formation and reformation. Pattern I. Claystons, oontaining three subspecies: IA, the incompetent argillized claystones with widespread strain slips; IB, the sfotened argil1ized claystones with local strain slips and Ic, the claystones. Pattern II. The argillaceous and elayey siltstones, containing two subspecies: IIA, the incompetent argillaceous siltstones with local strain slips and IIB: the clayey siltstones. Pattern III. The gompholite (nagelfluh). Pattern IV. The argillaceous conglomerates.
