Bauxite in Greece is usually mined by the room and pillar method. Stability of the hanging wall is of paramount importance for the safe exploitation of the deposits. Various types of failures have been observed during the operation of the mines and have been grouped according to their mode. The knowledge of the involved mechanisms will undoubtedly lead to the mitigation of the risk of failure.
L' exploitation des mines de bauxite en Grece est d' ordinaire effectuee en suivant les methodes de depilage par chambres et piliers. La stabilite du toit est un objectif fondamental pour la securite d' exploitation des gisements. Divers types des ruptures sont observes pendant I' operation des mines et sont groupes conformement a leur mode. La connaissance des mechanismes participants va nous guider a la mitigation du risque des ruptures.
Der Bauxit in Griechenland wird meistens nach den Verfahren der Raume und Seulen abgebaut. Die Stabilitat des Hangendes ist die grundlegende Bedingung fur den sicheren Abbau der Erzschicht. Verschiedene Bruchtypen sind wah rend des Betriebes des Bergwerks beobachtet und sind auf Grund ihrer Art gruppiert. Die Kenntnis der mitwirkenden Mechanismen wird uns anleiten das Risiko der Erfolglosigkeiten zu vermindern.
Greece is the only EEC bauxite producer and holds approximately 2.75% of world's total proven reserves. Mining activities expand within the geosyncline of Parnassos - Giona - Helicon - Oiti mountains, the main axis of which has a northwest-southeastorientation. Rock formations Overlying the bauxite deposits consist of white, usually non crystalline Maestrichtian limestones. They are usually thin-bedded with the exception of the upper strata, which are thick-bedded, with occurrences of hornfels nodules. Three main bauxite stratigraphic horizons may be distinguished in the region, the deepest called the first, the intermediate the second and the top the third. The mineral composition of the bauxite deposits varies substantially with respect to their content of boehmite and diaspore. The two upper horizons and especially the uppermost third one are nowadays intensively mined. The rock of the immediate roof of the deposits is dark-coloured and consists of Turonian, roudists bearing, bituminous limestones. The footwall of the deposits of the third bauxite horizon consists of lower Cretaceous white limestones which establish a characteristic angular unconformity with the limestones of the hangingwall. Due to the impermeability of the bauxite orebodies, groundwater, percolating through the limestone of the hangingwall, lays down clayey muddy sediments, particularly in areas of high joint density. These sediments have formed, at the contact of the bauxite deposits with the limestone hanging wall, a relatively thin yellow clayey marl layer, which is usually within the range of 20 to 50 cm thick and is completely distinguished from the deposit and the hanging wall. Mechanized room and pillar mining is used as the main underground exploitation method. The task of roof control depends on the unmined mineral which forms the pillars and to the self supporting ability of the rock mass of the roof, which is reinforced by intense, mechanized rock bolting. Pillars are designed on the assumption that acting stresses on any member should not exceed pillars' strength. This should hold till the completion of the gradual reduction of pillars' dimensions during the depillaring stage.
Extensive laboratory tests evaluated a uniaxial compressive strength of the limestone rock mass of about 100 MPa, a Young's modulus between 9 and 14 GPa, a Poisson's ratio of 0.25, a unit weight of 26.5 kN/m3 and a Point Load Index of about 4. The limestone has an RQD value within 50 and 75 and an average spacing between the joints of the main sets between 0.3 and 1.0 m.