ABSTRACT:

Several underground hydroelectric plants in the Niagara Falls region are showing signs of structural distress as a result of preexisting high horizontal stresses and time-dependent rock deformation upon stress relief. Thus, a major emphasis in the feasibility and preliminary design studies toward the construction of a new power plant has been the measurement of the in-situ stress using both overcoring and hydrofracturing. This paper reports the results of the first comprehensive stress measurements in the area, confirming that high horizontal stresses prevail, but also showing that substantial jumps in stress magnitudes exist between rock units, and that stress changes, especially with respect to direction, may occur along the route of the planned power tunnels. These results have been incorporated in the design analysis of the power plant underground components.

RESUME:

Fortes contraintes horizontales à Niagara Falls, leur mesure, et la conception d''une nouvelle generatrice hydro-electrique. Plusieurs génératrices hydro-electriques souterraines dans le région de Niagara Falls montrent des signes de de''tresse de structure résultant de fortes contraintes horizontales et de la déformation de la roche causée par le reláchement de ces contraintes. Par Conséquent, les études préliminaires de faisabilite et de conception pour la construction d''une nouvelle génératrice ont mis l''accent sur la mesure des contraintes en place, par le sur-carottage et la fracturation hydraulique. Cette communication rapporte les resultats des premières mesures d '' ensemble dans la région, lesquels confirment que les contraintes hori zontales sont en général éleveés, mais aussi demontrent qu il existé des variations substantielles dans l''intensité de ces contraintes dans les diverses roches, et que des variations de contraintes peuvent done surveni er le long des tunnels proposés. Ces resul tats furent incorpores dans l''analyse de tous les composants sous-terrains du groupe electrogèe.

ZUSAMMENFASSUNG:

Einige Untergrund - Wasserkraftwerke im Gebiet der Niagara- Fälle zeigen Anzei chen der Baufälligkeit als Resultat der vorgegebenen hohen horizontalen Spannungen und der zeitabhangigen Felsdeformation in Folge des Spannungsabbaus. Daher wird bei vorläufigen Design - Studien bei der Konstruktion eines neuen Kraftwerks der Messung der in-situ Spannung unter Benutzung von "overcoring" und "hydrofracturing" besondere Beachtung geschenkt. In diesem Artikel wird über die ersten vergleichenden Spannungsmessungen berichtet, welche bestätigen, daβ hohe horizontale Spannungen vorherrschen und euβerdem zeigen, daβ groβe Unterschiede in der Höhe der Spannungen verschiedener Felseinheiten bestehen und richtungsabhängige Spannungsanderungen entlang der geplanten Druck-tunnel aufreten können. Diese Ergebnisse wurden in die Designanalyse der Untergrundkomponenten des Kraftwerkse ingebaut.

INTRODUCTION

The Niagara Falls region has long been recognized for its scenic attractions and hydroelectric resources. A number of power plants were completed at the turn of the century. Of those owned by Ontario Hydro - the provincial electric utility - the Toronto Power Generating Station has already been retired, and the Ontario Power Generating Station is coming close to retirement. To make efficient use of the available water, Ontario Hydro is currently evaluating the feasibility of building a new hydraulic plant in the region known as Sir Adam Beck Niagara Generating Station No. 3 (SAB #3). Similar to the 1400 MWSAB #2 plant built in the 1950''s, the proposed SAB #3 project will involve a system of hydraulic tunnel s and canal. The SAB #2 project included twin 9 km long tunnels, each with 15.5 m excavated and 13.7 m finished diameters. As illustrated in the geologic section in Figure 1, the twin tunnels were excavated in horizontally bedded sandstone, shale, dolomite and limestone units of the Silurian age. The new tunnels for the proposed SAB #3 project will likely be excavated in the underlying Queenston shale formation, with the Whirlpool sandstone as the roof. The reference scheme favours a twin tunnel system with a finished diameter of 10.7 m. Alternatively, a single 15 m diameter tunnel could also be contemplated, the limiting factors being excavation cost and machine tunnel technology. Figure 2 shows the 1ayout of the existing SAB #2 as well as the proposed layout schemes for the SAB #3.

2. ROCK ENGINEERING DESIGN CONSIDERATIONS

Rock structure performance in the region over the years clearly suggested that there are two major rock mechanics factors in the design of the underground openings for the SAB #3. The first is the state of high horizontal stresses in the sedimentary units, the second being the time-dependent deformation properties of some shale and shale limestone units. It has been recognized that high horizontal stresses exist in many of the bedrock units in Southern Ontario and adjacent areas (Haimson and Lee 1980; Lee 1981; Lo 1978). Buckling failure may also occur in some quarry floors and rock outcrops (Lee 1981). The origin of this state of stress is not well defined. A number of mechanisms have been postulated, including viscoelastic deformation under glacial loading, changes in the Earth''s curvature due to continental glaciation, erosion, and plate tectonics (Lee 1978b; Lee and Asmis 1979; Asmis and Lee 1980).

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