|Publisher||International Society for Rock Mechanics||Language||English|
|Content Type||Conference Paper|
|Title||Contact Effects At the Interface Between Rock Foundations And Concrete Dams With Power Plants At Their Toes|
|Authors||A.N.MARCHUK, The B.E.Vedeneev All-Union Research Institute of Hydraulic Engineering (VNIIG); A.A.KHRAPKOV, The B.E.Vedeneev All-Union Research Institute of Hydraulic Engineering (VNIIG); YA.N.ZUKERMAN, The B.E.Vedeneev All-Union Research Institute of Hydraulic Engineering (VNIIG); M.A.MARCHUK, The V.V.KuibyshevMoscow Institute of Civil Engineering|
|Source||6th ISRM Congress, August 30 - September 3, 1987 , Montreal, Canada|
|Copyright||1987. A.A. Balkema. Permission to Distribute - International Society for Rock Mechanics.|
Analysis of field observation data revealed an opening of concrete-rock contact beneath the upstream face of some large Siberian dams. The calculations performed enable one to establish possible causes of this phenomenon.
Les études in situ ont mis en évidence l'ouverture du joint de contact beton-rocher sous les parements amont de grands barrages en Sibérie. Les calculs ont perrrue de définir la cause possible de ce phénomène.
Die Naturbeobachtungen haben das Offnen der Kontaktfuge zwischen dem Beton und der Felsgründung unter den Wasserseiten von hohen Talsperren in Sibirien gezeigt. Die Berechnungen erlaubten eine mögliche Ursache dieser Erscheinung zu ermitteln.
In the USSR layouts with power plants located at dam toes are most commonly used and believed to be highly economical. However, analysis of field observation data on stress-strain behaviour of the largest Siberian dams revealed certain limitations of this structure arrangement. In particular, it was found that beneath the upstream face of some dams an opening of concrete-rock contact occurs which results in subsequent increase of uplift pressure over an extended area of the dam toe. This phenomenon is more distinct under the power house sections. Apart from the well-established causes of disruption of contact between rock and concrete (Eidelman, Durcheva 1981; Marchuk 1983) one must mention the effect which is specific for power plant sections: the increase in deformability of foundation below the downstream wedge of the dam. Since the downstream wedge transfers maximum compressive stresses to the rock foundation, its pliability contributes to the total displacements of the dam profile. The stress-taking rock step cut by the foundation pit for the power house is always more deformable than the rest of the rock mass underlying the dam toe. Decrease of the step rock deformation modulus can be explained by stress-relieving of the rock mass during excavation as well as by rock loosening due to blasting and frost weathering at the construction stage. The relevant investigations (Marchuk, Khrapkov, Zukerman, Marchuk 1985; Rubtsov 1965) confirm that rock excavation is responsible for stress-relieving of the rock mass overlying zone and cracking due to, reduction of the vertical component of the natural stress tensor. Analysis of the geophysical data on rock foundations of the Ust-Ilymskaya, Boguchanskaya and some other hydro power plants showed· that zones of technogeneous stress-relief extend to a depth of 15-20m and the deformation modulus E of the overlying subzone of the intensive stress-relief is 6-12 times less than that of the underlying subzone of weak stress-relief. This stress-relief of the power house section is further facilitated by blasting and frost weathering effects. Investigations of frost weathering action at the Zeiskaya dam revealed that in 4.5 years cracking of unweathered diorite showed 38% increase for a depth of up to 4 m. Cement consumption in the consolidation grouting beneath the downstream monoliths appears to be 1.3-1.7 times higher than that under the first upstream monolith, which is indicative of intensive foundation cracking under the downstream dam wedge due to excavation of the pit for the power house foundation. It can be stated that by the time of the reservoir filling the deformation modulus of the rock foundation along the dam toe has an irregular distribution. This irregularity results from varied degree of stress-relief throughout the rock mass and different pattern of concrete distribution in the structure. The maximum value of the deformation modulus is observed under the upstream toe. The subsequent concrete placement in the downstream dam wedge and power house section permits neither to restore the initial rigidity characteristics of rock within the stress-relieved zone of the foundation, nor to obtain the values of deformation modulus equal to those under the other sections of the structure. During reservoir filling the stress and settlement curves of the contact cross-section undergo transformation due to decrease in vertical compressive stresses under the upstream toe and (in certain conditions) due to development of corresponding tensile stresses. The analysis of this problem from the viewpoint of the elasticity theory shows that the necessary condition of the tensile stress development is the increased rigidity of the rock foundation under the upstream face at rather economical cross-section of the dam. This condition is met in the construction of all sections of the Bratskaya and Ust-llymskaya dams; however, field observations reveal opening of contact under the power house sections. At that much lesser opening is detected below the commissioning sections which take the water load when their construction is nearly completed (Fig. 1).
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