A case history of ground support in a large block caving mine located in altered andesite is presented. Rock support traditionally used in the underground openings consists of steel frames and yielding steel arches with wood lagging, sometimes complemented with grouted bolts and locally in heavily loaded areas, with reinforced concrete. Based on field observations of ground conditions, general recommendations from rock mechanics classification systems and on the results of simple analytical models, a new support system has been adopted for certain areas of EI Salvador mine in Chile, a flexible lining obtained with rock bolts and mesh-reinforced shotcrete applied in stages. The satisfactory behavior of this support in the LHO zone of the mine is described and the associate savings are commented.
Presentation d'un cas historique de soutèmement de roche dans une grande mine de "block caving". Le soutènement de roche utilise traditionnellement dans le travaux souterrains consiste de cadres d'acier et de cintres d'acier glissants avec du bois entre le cadre et la roche, completes parfois de boulons injectes, aussi bien que de beton arme dans des localisations très sollicitees. D'accord aux conditions du terrain, aux recommendations generales issues des systèmes de classification geotechnique des roches et aux resultats de modèles analytiques simples, on a adopte un nouveau système de soutènement pour certaines zones de la mine El Salvador au Chili. Ce système consiste d'un revêtement flexible obtenu au moyen de boulons d'ancrage et de beton projete, renforces de grillages d'acier et appliques par etapes. On decrit le comportement satisfaisant de ce soutènement dans la zone LHD de la mine, en ajoutant un commentaire sur les economies associees.
Vorgestellt wird ein geschichtlicher Fall ueber einen Felsstuetzsystem innerhalb eines grossen Bergwerkes im ungeschischtetem Andesit. Das ueblich benutzte Felsstuetzsystem in diesen unterirdischen Bauwerk besteht aus Stahlrahmen, Stahlboegen die mit Holzkeile angepasst werden sowie auch mit Felsbloecke und injekzierte Schrauben. Je nach den Felsanforderungen wird auch armierter Beton benutzt. Aufgrund von Beobachtungen der Gelaendebedingungen, allgemeine Empfehlungen die aus den geotechnischen Felsklassifizierungen gegeben sind, und aus den Ergebnissen der einfachen analytischen Modellen, ist ein neues Stuetzsystem angenommen worden fuer einige Zonen des Bergwerkes in El Salvador, Chile. Dieses biegsame Stuetzsystem wird aus de Verbindung von Stahlgitterverkleidung mit Ankerschrauben und Spritzzement erreicht. Das ausgezeichnete Verhalten dieser Stuetzart in der LHD-Zone des Bergwerkes wird hier beschrieben sowie die verbundenen materiel len Vorteilen.
El Salvador copper mine is exploited by block-caving in an isolated mountain such that the average depth from the ground surface to the multiple excavations of the undercut and haulage levels ranges from 260 to 700 m. The main portion of the underground workings covers an approximately rectangular area 1300 m long and 500 to 600 m wide. Figure 1 shows a simplified View of the underground openings. Several undercut galleries 2.2 m-wide and 2.5 m-high, are excavated at 14.3 m on centers; parallel to such galleries, and 14.6 m below, the 4.0 m-wide by 3.4 m-high production galleries are excavated. The broken ore descends through the ore passes and is removed with mechanical equipment (LHD) at the lateral draw points, connected to the production drifts.
The exploitation system described above results in multiple underground openings at different orientations, leaving rock pillars of complicated geometry, usually an undesirable situation from the rock mechanics point of a view. Moreover, as the mining proceeds, both the areal extent of the broken ore and the boundaries between broken and intact rock change with time. Thus, an extremely complicated stress distribution develops on the rock pillars and around the underground openings. Field observations of ground and lining behavior carried out at the mine are in agreement with the results observed elsewhere under similar Circumstances where large stress concentrations occur as the limit between broken and intact ore approaches and passes over a particular observation point (Bolmer 1965,Legast 1981,Bjornfot & Stephansson 1984).
(Figure in full paper)
The steel lining support traditionally used in the mine has been designed based on precedent; in this paper emphazis is placed on describing the selection and behavior of a flexible lining consisting of rock bolts and mesh-reinforced gunite. Such support system, very common in civil engineering projects, have not been so widely used in mining engineering, particularly when the block caving method is used, where dramatic changes in ground stresses are induced. Thus, the re is a need to document cases where a flexible support, mainly based on shotcrete and rock bolts, has been used, whether successfully or not, under the difficult environment found in caving mines.
