The foundation rock of the first stage construction of Gezhouba project belongs to the lower Gretaceous series strata of land facies and rocks are sedimentary siltstones, argillaceaus siltstones with medium-fine grained sandstones. The rock strata have strikes of N20–40 degrees E, forming an angle of 70 degree with the axis of the dam, inclining slightly downstream and to the left bank, dip of SE and dip angles of 4–8 degrees. In the strata numerous weak intercalations parallel to beds are distributed, which are the control factor of the dam stability. Among them, the dislocated mudded intercalations (shear zone) resulting from tectongenesis are of lowest mechanical strength. Ever since the early 70s., a great deal of scientific researches have been conducted by Yangtze Water Conservancy and Hydro-electric Power Research Institute, with both rock and soil mechanical methods in various sample sizes in labs and in situ. Numerous scientific data for engineering design and construction have been prowided. In this paper, test results of a typical mudded intercalation No. 202 are analized comprehensively.
Mudded intercalation No. 202 is distributed very widely over the foundation rock mass composed of monocline in structure. The intercalation located at 0–14m beneath the barrage spillway at the second channel of the river is about 0.1–3.2m thick and is composed of brown4red argillacious siltstones with silt claystone; The intercalation located at 7–13m beneath the second channel powerhouse is about 0.05–2.6m thick and is composed of purple-red argillaceous siltstone with claystone. The occurrence of the intercalation is the same as the rock strata and have great effect on the stability of the structures. According to the geological analysis, the rock strata was subjected to the action of geotectonic movement three times. At the first, the azimuth of the maximum principal stress is 125 degrees; at the second, 225 degrees and at the third, 240 degrees. Under the action of these geotectonic stresses, dislocation along intercalation No. 202 took place. There are two continuous dislocated plane. On the bedding plane, scratches having strikes of 120–130 degrees, are seen clearly and are in the same direction with the first time geotectonic stress. Along the plane of dislocation, the intercalation has been mudded and has formed a layer of plastic mud 0.1–0.5cm thick. Above and below the mudded zone there are typically a fracture cleavage zone 2–5cm thick and a joint zone 10–20cm thick.
Physical properties and mineral composition of the intercalation and micro structures of mudded zone Physical properties of various parts of the intercalation are apparently different from each other. Those of the mudded zone are the worst; those of the argillaceous siltstone above and below the intercalation are the best and those of the weakened zone are between them (tab. 1). Minerals of the clay are illetes, chlorites koalinites and montmorillonites.
(Table in full paper)
The microstructures of the mudded intercalation dislocated plane were observed through polarimicroscope and scanning electron microscope (Fig. 1).