SUMMARY:

At the beginning of this paper the authors give a brief description of the main geological conditions of the foundation rock under the second-channel spillway of the Gezhouba Project built across the Yangtze. They subsequently describe a comparative study of measures taken to improve the stability against sliding along the shear zones which lie several metres below the structure of the spillway gate. Finally, the degree of safety against sliding is evaluated by means of mathematical and physical models.

RESUME:

Les auteurs commencent par presenter brièvement l'environnement geologique general des roches de fondation des vannes de decharge de la seconde voie d'eau du complexe hydraulique de Gezhouba construit sur le Yangtse. On decrit, ensuite, les etudes comparatives sur diverses mesures de stabilisation destinees à prevenir le glissement des intercalations argileuses. Enfin, on evalue la limite de securite de resistance au glissement au moyen de modèles mathematiques et physiques.

ZUSAMMENFASSUNG:

Die Autoren geben am Anfang des Beitrages eine kurzgefaßte Darstellung der hauptsachlichen geologischen Verhaltnisse im Gruendungsfelsen des zwelten Hochwasserentlastungskanals im Projekt Gezhouba, das quer ueber den Yangtze gebaut wird. Dann beschreiben sie die vergleichenden Studien der getroffenen Maßnahmen zur Erhöhung der Stabilitat gegen die Abgleitung entlang der Scherzonen, die einige Meter tief unter dem Überlauf liegen. Schließlich wird der Sicherheitsgrad gegen Abgleiten mit Hilfe von mathematischen und physikalischen Modellen beurteilt.

The Gezhouba Project is the first multiple purpose project built on the main stem of the Yangtze. Its first-stage construction, comprising a 27-bay Spillway with a maximum flood discharging capacity of 84,000 cu.m. per sec, a Second-channel Power plant housing 7-turbogenerator units with an aggregate capacity of 965 MW, a 27m-lift large ship lock with chamber dimension of 34m wide, 280m long, 27m-lift medium sized ship lock with chamber dimension of 18m wide, 120m long, and a 6-bay silt-scouring sluice, has been in operation since Jan.1981; and its second-stage construction is progressing successfully. The foundation of the project site is a series of cyclothem, terrestrial and clastic rock of the lower Cretaceous period. The lower part of the formation is conglomerate, and interbeds of sandstone, siltstone and clayey siltstone dominate the middle and upper parts with intercalated layers of claystone or silty claystone. The thickness of the clay beds varies greatly, from a few to scores of centimeters. Some of the clay beds spread out extensively, and others exist with varying sedimentary facies, sometimes as lenses in thincker strata. With the exception or conglomerate, the bedding planes are poorly cemented, the strength of the rocks is low and deformability high. After the deposition of the red series in the Cretaceous period, the region of the project had experienced for several times the influences of tectonic, movements, with large scale faults, striking NW and NNW, flanking the east and west side of-the site, about 20 to 40 kilometers away. But only slight tectonic features can be, traced at the site and its vicinities, such as small syclines and anticlines. The beds of the foundation rock strike N 20°to 40° E, dip 4° to 8° SE. For the purpose of building safe economic, lasting hydraulic structures on such a rock mass, a series of comprehensive and in depth studies have been initiated. The focus of the study has been placed on the following two aspects:

  1. Study of major geotechnical problems, both at field and in laboratory, and

  2. Study of alternative measures for enhancing the stability against sliding along the shear zone several meters underneath the spillway gate structure, by physical and mathematical models.

Study of Major Geotechnical Problems as aforementioned, shear zones are the main stuctural feature of the foundation rock mass. The low shear strength of the shear zone makes it a key to the settlement of the question of stability against sliding. Upon the following points the studies are thus centred. (1) The trend of change of the shear zones during long-term operation of the project after impounding, (2) The proper shear strength of the shear zone, and (3).The failure mechanism of the rock mass, comprising thin and weak beds and shear zones, in the immediate vicinity of the downstream toe, acting as resistant block, of the Spillway gate structure. The conclusions drawn from these studies were the contents of a written discussion contributed to the 4th Congress, ISRM, 1979, and published in volume 3 of the Proceedings of that Congress. Alternative Measures for Enhancing Stability Against Sliding It was found out by preliminary analysis, with conventional limit equilibrium method as stated below, of stability against sliding of the spill way gate structure along the shear zone directly underneath, that the factor of safety of the left most five bays of the gate structure did not meet the criterion, 1,30 for normal operation condition and 1,10 for extreme cases, set by the Design Code.

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