Duncan Dam is a B.C. Hydro facility constructed on the Duncan River in southeastern British Columbia, Canada, between 1965 and 1967. The dam was founded on a complex sequence of more than 380 m of glacial drift and glaciofluvial sediments, some of which are pervious and compressible. Some sandy units are potentially liquefiable, in particular a sand layer (unit 3c) up to 23 m thick. Current B.C. Hydro seismic guidelines for dams require that Duncan Dam should be able to withstand the Maximum Credible Earthquake (MCE) without catastrophic release of the reservoir. This paper describes the geologic and seismic setting of the region around the dam and the selection of seismic ground motion parameters. Probabilistic methods were applied to develop MCE ground motions, which were estimated to consist of a firm ground peak horizontal acceleration of 0.12 g , which could be caused by a M 6.5 earthquake at a distance of about 50 km. Several time histories with characteristics similar to this design earthquake were selected for dynamic soil analyses. K~) J ~vorrls: dam safety, embankment dam, liquefaction, sand, seismicity, seismic ground motion. Le barrage Duncan est un ouvrage de B.C. Hydro construit sur la rivibre Duncan dans le sud-est de la ColombieBritannique au Canada entre 1965 et 1967. Le barrage repose sur une succession complexe de plus de 380 m de sCdiments dktritiques et fluvio-glaciaires dont certains sont permkables et compressibles. Quelques unitks sableuses peuvent prCsenter un danger de IiquCfaction, en particulier une couche de sable (unit6 3c) ayant:-jusqu'ii 23 m d'Cpaisseur. Les rbgles sismiques en vigueur actuellement i B.C. Hydro pour les barrages exigent que le barrage Duncan soit capable de rCsister au sCisme maximum envisageable (<< MCE D) sans dkversement catastrdphique du rCservoir. Cet article dCcrit les conditions gCologiques et sismiques dans la rCgion entourant le barrake et le choix des paramgtres pour les mouvements sismiques du sol. Des mCthodes probabilistes ont Ct C appliquCes pour reconstituer les mouvements du terrain correspondant au MCE. On a estimC que l'on pourrait avoir un picdd'accCICration horizontale du terrain de 0,12 g, provenant d'un tremblement de terse d'amplitude M 6,s 4 une-distance d'environ 50 km.Pour les analyses dynamiques des sols, on a choisi plusieurs cas historiques tirCs du pass6 et presentant des caractkristiques semblables ii celles du sCisme prCvisionnel.Mots cle's : sCcuritC des barrages, barrage digue, IiquCfaction, sable, sismicitC, mouvement sismique du sol.[Traduit par la rCdaction]Can. Geotcch. J. 31, 919-926 (1994)
The Canadian geotechnical engineering community has completed a major collaborative 5 year research project entitled the Canadian Liquefaction Experiment (CANLEX). The main objective of the project was to study the phenomenon of soil liquefaction, which can occur in saturated sandy soils and is characterized by a large loss of strength or stiffness resulting in substantial deformations. The intent of this paper is to compare, interpret, and summarize the large amount of field and laboratory data obtained for six sites in Western Canada as part of the CANLEX project. The sites are compared in terms of both flow-liquefaction and cyclic-softening considerations. The paper presents a number of conclusions drawn from the project as a whole, in terms of both fundamental and practical significance.Key words: sand, flow liquefaction, cyclic softening, CANLEX.
Screening-level studies indicated large zones of the foundation soil beneath Duncan Dam could be triggered to liquefy under the design earthquake, resulting in flow slides that could lead to breaching of the dam. In these studies, estimates of the key soil parameters were obtained from an indirect procedure using penetration tests and Seed's chart, which is based on field experience during past earthquakes together with empirical correction factors for high confining stress and static bias. This is state-of-the-practice procedure in North America and is used because of the difficulty in retrieving and testing undisturbed samples of sand. With the high confining stresses present at the site, the indirect approach gave very low values of the key liquefaction properties of the soil, and the q e of such low values in analyses indicated that major and costly remedial measures were required. Thus a more dire6 approach was undertaken. This involved both direct and semidirect determination of the key soil parameters thrqugh an extensive laboratory study based on testing of undisturbed samples obtained by freezing in situ. The results'$howed that both the triggering resistance and the residual strength values of the soil were very much higher than those obtained from the indirect approach. Analyses carried out using direct measurement of the key soil parameters indicated that the dam, in fact, would be stable.Key ~volz1.s: embankment dam, foundation liquefaction, sand, seismic performance, triggering resistance, undisturbed sample.Des Ctudes prCalabies ont indiquC que d'importantes zones de la fondation du barrage Duncan pouvaient &tre amenCe B se liquefier sous l'effet du sCisme prCvisionne1, avec un glissement de terrain pouvant conduire B la rupture du barrage. Dans ces Ctudes les valeurs estimCes des paramtttres fondamentaux des sols ont CtC obtenues par une procedure indirecte qui utilise des essais de pCnCtration et I'abaque de Seed, bask sur 1'expCrience acquise sur place lors de sCismes pas& et sur des coefficients de correction empiriques tenant compte des pressions de confinement ClevCes et du biais statique. C'est la pratique reconnue en AmCrique du Nord et on I'utilise B cause de la difficult& qu'on a B rCcupCrer et B soumettre aux essais des Cchantillons de sable non remaniCs. Avec les contraintes de confinement ClevCs qui prCvalent sur le site, l'approche indirecte a donne des valeurs trtts basses des propriCtCs fondamentales de IiquCfaction des sols et I'utilisation de vaieurs aussi faibles dans les calculs a montrC que des mesures majeures et coDteuses de rCparation Ctaient nkcessaires. On a donc suivi une approche plus directe. Pour cela on a fait appel B la dktermination B la fois directe et semi-indirecte des paramktres clCs du sol au cours d'une Ctt~de complttte de laboratoire b a k e sur des essais faits sur des Cchantillons obtenus par congklation en place. Les rksultats ont montrC que le seuil de rksistance des sols ainsi que la valeur rCsiduelle Ctaient beaucoup plus ClevCs que ceux qu'on avait obt...
The Peace River rises in the Rocky Mountain Trench and flows easterly through the Rocky Mountains before flowing onto the Plains in northeastern British Columbia near the Alberta border. Stress measurements conducted at two of three damsites near the boundary of the Foothills and the Plains reveal high remnant lateral stresses and a K0 of about 2. In proceeding 90 km downriver from the W.A.C. Bennett Dam, past the Peace Canyon Dam to the proposed site C dam near Fort St. John, the stratigraphic section changes from competent, massive sandstones of Lower Cretaceous age to soft, weak shales at the top of the Lower Cretaceous section. Valley downcutting by the Peace River through these highly stressed rocks has led to valley rebound features that have both similarities and differences where observed at each of the three damsites. Differences in rock strength at each site contribute to the variations in features noted. These valley rebound features include vertical relief joints, horizontal bedding plane joints, upward arching of the riverbed, and shear zones that both parallel and cross the bedding. The challenge for the geotechnical engineer is first to find these features and account for them in the design, as they tend to govern the foundation and slope strength even though they may only be a few millimetres in thickness. A second challenge is to appreciate that the remnant stresses that led to the formation of the features are still present and these may cause further response when excavation proceeds at a site. A geologic model for the Peace Valley is used to explain the natural and construction response to deepening of the river valley. Key words: stress measurements, damsite, remnant lateral stress, valley rebound, Lower Cretaceous shale and sandstone, relief joints, bedding plane joints, arching, shear zones, foundation design.
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