Anchorage of power line tower foundation on rock; Classification of rock and design rules

T. Valstad

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: FUNDAMENTALS - III Soil & Rock Mechanics

ABSTRACT: After a failure in one of Statnett´s power line foundations in Norway 2011, a new procedure for design of tower foundations on rock has been developed. The procedure is based on classification of the rock mass below each leg of the tower and corresponding rules for design of the rock anchorage. The classification procedure involves identification and description of the rock mass at each tower leg based on the main parameters; weathering, compressive strength, and rock jointing, i.e. number of sets, orientation, spacing, roughness, aperture and filling. The rock is then classified into one of five classes (0-4). Each class is associated to potential failure models for tower legs under tension. Rock class 0 to 3 is given a set of defined parameters needed for design of tower foundations. The design rules are based on the general rules for overhead electrical lines and focus on two prime issues: the bearing capacity on, and the pull-out resistance of the rock ground. Norwegian practice is to construct cylindrical chimney foundations where the vertical reinforcement is extended through boreholes into the rock mass so that they act as anchoring tendons. The paper describes principles of rock classification, the associated rock mass strength and the design of foundations based on classification and different failure models.

RÉSUMÉ: Suite à la rupture de l'une des fondations de pylônes de ligne électrique du réseau Statnett, une nouvelle procédure pour la conception des fondations sur roche a été mise en place. Cette dernière est basée sur la classification de la roche sur laquelle repose chaque pied du pylône, ainsi que les règles pour la conception de l'ancrage dans la roche. La procédure de classification consiste à identifier et décrire la roche à chaque pied de pylône à partir des paramètres principaux: altération, résistance à la compression, et le jointement de la roche; i.e., quantité, orientation, espacement, rugosité, ouverture et remplissage. La roche est alors rangé dans l'une des cinq classes (0 - 4), qui correspondent aux différents modèles de rupture sous tension des pieds du pylône. Un jeu de paramètres nécessaire à la conception des fondations de pylônes est alors donné aux roches classées de 0 à 3. Les règles de conception sont basées sur les règles générales pour les lignes électriques aériennes et se concentrent sur deux principaux aspects: la capacité de charge et la résistance à l'arrachement de la roche. La pratique Norvégienne est de construire des fondations cylindriques renforcées vertica-lement par des puits de forage qui se comportent comme des tendons d'ancrage. L'article décrit les principes de la classi-fication des roches, la résistance de la roche associée, ainsi que la conception des fondations basé sur la classification et les différents modèles de rupture. 1 BACKGROUND 1.1 Norwegian overhead powerlines The prime power grid in Norway is presently undergoing a major upgrade. Existing 132 and 300 kV lines are being replaced or supplemented by new 420 kV lines. The most commonly used powerline tower is a self-sup-ported steel lattice structure resting on four independent, concrete chimneys (Fig. 1). Towers with guy lines anchored in buried foundations are only rarely used. When founded on soil ground, the foundations for the tower legs are usually a vertical concrete pier supported on a buried, square concrete pad. When founded on rock ground, the chimneys are usually erected directly on the rock surface (Fig. 2). The focus of this paper is on chimneys on rock ground only. Figure 1. Sketch of standard Norwegian power line with self-supported steel lattice tower.

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