Engineering geological conditions in the construction of water projects China is a country with vast territory and a wide variety of complex geological conditions. As view from lithological characters, breccia, conglomerate, sandstone and siltstone of detrital rocks; mudstone, shale of clay rocks; limeatone, dolomite, marlite of biochemical rocks and detritus soil, sandy soil and clayey soil of friable and loose deposits with different geneses are distributed extensively. The composition of all kinds of metamorphic rocks is also very complicated. Intrusion of igneous rock in various periods and volcanic activities occurred, particularly along the coastal region and southwestern region. From the view point of regional structure and evolutional history of geology, not only had the tectonic movements in the folded areas happened violently, but the intensity of tectonic movement within the Platform were also rare in the world, hence the complexity of geological structures. Crust movements also occurred frequently in recent times, e.g. the eruption of volcano was experienced in Xinjiang Autonomous Region in 1950. According to historical seismic data, all the ten seismic regions in China had encountered macroseisms with magnitude of M≥6 (Richter soale). Apart from the two seismic regions of Northeast China and the South China sea, the other eight regions had experienced macroseism with magnitude of M≥8. These complicated natural conditions inevitably bring forth a series of engineering geological problems to the construction of water projects, of which weak rocks is just one of them. According to the statistics conducted on concrete dams and masonry dams of large and medium water project8,three quarters of them have encountered problems raised by weak rocks or intercalated layers therefore the problem regarding this engineering geological aspect is of universal nature.
Primary engineering characteristics of weak rocks
Just as the term suggests, weak rocks are rock(or stratum) that are relatively incompetent in nature. It is of common consideration that weak rocks have four features outlined as follows:
Low strength. This implies compression strength and shear strength. Poor lithogenesis or changes in latter stage(weathered, solubilization, structural failure) lead to the decreasing of strength as well as low loading capacity. Constitution materials are usually rich in argillaceous substances, which have a low shear strength and liable to slides.
Low modulus of deformation. Liable to deformation and larger magnitude of settlement, displacement, and nonuniform deformation, thus leads to the loss of stability of structures.
Poor physical properties when affected by water. Liable to be soften and disintegrated when contacted with water. If water content increases or decreases, "expansion or contraction will occur significantly. Under high water pressure of long duration, piping and other troubles may probably occur which cause failure to foundations.
Distinct rheologic effect. The strength of rock would decrease significantly when subjected to long duration loading.
In the construction of Water projects, the weak rocks commonly encountered are: Mesozoic red rock and some of the other era such as clay rock, shale, siltstone, marlite of the sedimentary rocks; most of the phyllite and schist of the metamorphic rocks;