The concrete retaining structures of the Cheboksarskaya Hydropower Plant on the Volga comprise a spillway dam, a run-of-river power house and a navigable lock.

The construction of these structures is conditioned by complex engineering geological characteristics of the site. The foundation of the structures is composed primarily of rock (weak limestone, silts and relatively durable marl) weakened by marl and clay interlayers horizontal and continuous along the strike.

At the design stage (1976) the structural stability assessments included the laboratory testing of the soil specimens sampled by boring from various foundation zones (prior to stripping of the foundation trench). Assuming that the difference in the properties of the foundation soils is negligible, the unified design shear resistances tg 4I =0.3m and Cr=0,3 kgf/cm2 obtained from the test results were adopted for all the structures. During the foundation stripping a detailed geologic study of the underlying rock was performed.

Immediately under the structural base of the power house, in a weak rock bundle, some 4–40 cm thick continuous interlayers of very weak clay and marl were disclosed. The most critical design shear surface pases along these interlayers.

The detailed analysis of the foundation geology at the foundation stripping stage involved additional studies of the strength properties of the soils (primarily of the weak interlayers). These studies were carried out both in-situ and in the laboratory. The in-situ tests dealt with the most critical design shear surface underlying the power house. During the in-situ tests a 5.5–7m thick mass of weak rock including weak interlayers was investigated. Its lithology is given in Fig. 1.

(Figure in full paper)

The 75×11m site for the in-situ tests was selected in a close vicinity to the cut-off of the impervious blanket of the power house. The tests were conducted on large-scale pillars containing zones of weak interlayers and the adjoining areas of natural weak rock. The linear dimensions of the pillars (2×2m in plan) were selected so that the ratio between these dimensions and the thickness of the weak interlayers should be no less than 10. With such a ratio being obtained, the experimental deforrrability conditions of weak interlayers were practically adequate to those in the structural foundation. The tested pillar was enclosed into two reinforced concrete casings placed one over another (Fig. 2). Between the casings along the entire section of the pillar a gap was provided within which the weak soil of the interlayer was placed. The height of the gap amounted to the maximum thickness of the weak interlayer within a given pillar, thus facilitating unconfined deformation of the weak interlayer particularly in the limiting equilibrium state (when the shear, surface passes along this interlayer).

(Figure in full paper)

The lower casing is designed for preventing a possible. destruction of pillars below the studied interlayer when the limiting state is reached.

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