SUMMARY: The technical feasibility of hydroelectric development on the Peace River at Dunvegan, Alberta has been investigated. The Cretaceous sandstones and shales of the region are heavily over consolidated and may be classified from an engineering viewpoint as soft rocks. Determination of their physical characteristics required the use of site and laboratory procedures of both soil and rock mechanics, while data interpretation and parameter selection posed several unique problems. Both concrete gravity and embankment construction were considered for three alternate dam heights, though concrete construction proved suitable only for the lowest height because of the limited competence of the bedrock.
RESUME: Des investigations de la praticabilite technique d'un projet hydroelectrique sur la Peace River à Dunvegan en Alberta ont ete complètees. Les grès et schistes du Cretace couvrant la region sont très surconsolides et peuvent être classifies, d'un point de vue ingenierie, comme roches tendres. Pour la determination des caracteristiques physiques, on a eu recours à des methodes en laboratoire et in-situ provenant à la fois de la mecanique des sois et des roches, alors que l'interpretation des donnees et la selection des paramètres ont presente plusieurs problèmes particuliers. Pour trois hauteurs de barrage, des ouvrages en beton et remblai furent consideres bien que, dû au bedrock peu competent, une construction en beton n'ait ete prouvee appropriee que pour la plus faible hauteur.
ZUSAMMENFASSUNG: Eine Projektsstudie wurde an der Stelle des geplanten Kraftwerkes bei Dunvegan, Alberta am Peace River durchgefuhrt. Das Grundgebirge besteht aus ueberverdichteten Sandstein und Schiefer aus Kreide, und diese konnten als weiche Gesteine bezeichnet werden. Fuer die Bestimmung der felsmechanischen Kennziffern wurden bodenmechanische und felsmechanische Versuche durchgefuehrt. Die Auswertung von Versuchsergebnisse und die Wahl der Felskennwerte stell einige besondere Plobleme dar. Von Ausfuehrungen von Gewichtsstaumauern und Steinschuttdamme welche in drei verschiedenen Höhen studiert wurden, konnte auf Grund der niedrigen Felsfestigkeit nur die niedrigste Höhe der Gewichtsstaumauer empfohlen werden.
1. INTRODUCTION The Peace River, one of Canada's principal rivers, flows eastwards from the Rocky Mountains in British Columbia and then turns north within Alberta to eventually discharge into the Arctic Ocean. Hydroelectric development of the Peace River was initiated by the completion in 1968 of the Portage Mountain Development which embraces the W.A.C. Bennett Dam, a zoned earth fill structure of 163 m maximum height. Construction is currently underway some 22 kms downstream at Site 1 (see Figure 1), and will include a 50 m high concrete gravity dam and an embankment saddle dam. Studies for further development of the Peace River downstream of Site 1 have centred on Sites C and E in British Columbia and the Dunvegan site in Alberta. In the studies performed at Dunvegan, which form the subject of this paper, consideration was given to three alternate heights of dam, the lowest creating a reservoir wholly within Alberta and the highest a reservoir extending to Site 1. The stretch of the Peace River embracing all the above developments is contained within a plain formed in marine and non-marine sediments of primarily Cretaceous age which were heavily-overconsolidated by Pleistocene glaciation. The continuing rebound of these sediments is evidenced by lateral swelling and gradually degrading slopes within the valley downcut by the river.
2. DUNVEGAN SITE The site selected for the proposed development at Dunvegan comprises a locally narrow, relatively steep sided stretch of the valley just upstream of the Dunvegan bridge. The river here has down cut to a depth of some 250 m below the plain and has a width of from 250 to 400 m. The river runoff averages 1500 m/s with a design flood of 29,000 m/s. The dam designs developed in the study entail maximum heights above foundation grade of 76,104 and 155 m. The profile shown in Figure 2 corresponds to the common axis for the intermediate and high dams which are of earth rockfill construction, while the low dam of concrete gravity section is located 1.1 km upstream where the depths of river alluvium are less. The data presented in this paper are common to both locations. Round-the-clock working was adopted to counteract the low winter temperatures, which fell to below -40°C, but water lines still frequently froze. The adoption of the Christensen core barrel and wireline techniques were successful in counteracting low productivity and poor core recovery, though at a 125% cost increase per meter.
3. GEOLOGY The sediments of the region are composed of varying proportions of clay, silt and sand sizes. For classification purposes, the convention was adopted that those rocks containing 75% or more, by weight, of sand size particles be terms sandstone, while the designation shale was likewise applied to rocks containing 75% or more, by weight, of silt and clay sizes. In a further division, shale was classified as siltstone and claystone depending upon the predominance of silt and clay sizes respectively.