Filter ratio between materials in a rockfill dam

David Yanez Santillan, Juan Jacobo Schmitter Martin del Campo

Dans les comptes rendus d’articles de la conférence: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Embankments and Dams

ABSTRACT: In Dam Engineering, the distribution of materials in a concrete face rockfill dam is well known in what refers to specifications and convention codes for their identification. Material 2 is the continuous support for the concrete face and prevents seepage; at the same time, material 3A protects material 2. In the case studied, grain size and permeability specifications were provided by the owner, but once the permeability tests at the dam under construction were executed, the results were satisfactory for material 2 but not so for material 3A. Analyses of the information were made to determine the impact and, if applicable, to establish the acceptance criterion for placement of materials

RESUMEN: En Ingeniería de Presas, la distribución de materiales en una cortina de enrocamiento con cara de concreto sigue convenciones casi de aceptación mundial en cuanto a su granulometría e identificación. El material 2 constituye un apoyo uniforme de la cara de concreto y protege contra efectos de filtración; a su vez, el material 3A protege al material 2. Para el proyecto analizado se tenían las especificaciones dadas por el propietario para la granulometría y para la permeabilidad; sin embargo, conforme se iniciaron las pruebas de verificación de la permeabilidad en el campo, éstas indicaron el cumplimiento para el material 2 y no así para el material 3A. Se realizó el análisis de la información para determinar cuál era el impacto y en dado caso cuál sería el criterio de aceptación para la colocación de los materiales. 1 BACKGROUND 1.1 Objective of this paper This report has been prepared to serve as the basis for technical discussion in academic terms and not as an analysis of the management performed in the specific situation of a particular project; therefore, personal and institutional information has been deleted, retaining the minimum technical data necessary for its discussion. 1.2 Reference framework September 2002 was the starting date of a bidding process to build a hydroelectric project in Mexico after many years had elapsed without executing in the country one dam of such magnitude and for which the Basic Engineering available was based on the design of a so-called rockfill embankment with concrete facing. Among the specifications provided by The Owner those corresponding to the constituting materials for the embankment were available. The work was awarded in 2003 to The Contractor and detail engineering was included as part of the scope. The first year of the work schedule comprised works related to diversion structures and the retaining work construction started in 2004. After materials started to be placed it became necessary to for The Contractor to propose to The Owner an adjustment to the specifications because in one of the guidelines set forth, specifically the ratio between the permeability of two adjacent materials, was not being accomplished. The technical approach to solve this situation involved the definition of the applicable guidelines so as to reach the objective of the conception provided by the basic engineering in what refers to the behavior of materials in terms of the filter rate. 2 SPECIFICATIONS Figure 1 shows zoning of the materials from the concrete face to the rockfill. Figure 1. Distribution of materials in the dam. Material 2 constitutes a uniform support for the concrete face and protects it against seepage effects; in turn, material 3A protects material 2 and should serve as filter and transition between materials 2 and 3B; it was therefore designed according to filter criteria. The width of zone 2 is of 6 m and that of zone 3A is of 4 m. By specification, the permeability of material 2 should be equal to or smaller than 10-3 cm/s and it was required that after being compacted it would have a void ratio equal to or smaller than 0.22. Material 2

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