This study examines the subsidence due to underground mining works in a coal mine in France. Coal is mined at a depth reaching 1000 m, according to longwall face method with caving. The seam ranges between 2 and 3.4 m in thickness. The dip strata does not exceed 10°. As predicted by the empirical curves, the numerical and measured maximum subsidence does not exceed 70% of seam thickness. Numerical modelling and empirical methods overestimate the subsidence for extracted longwall width greater than 700 m.
Cette etude des affaissements induits par l'exploitation de charbon a ete faite pour une mine situee a une profondeur de 1000 m en France. La puissance de la couche exploitee par longue taille varie de 2 à 3,4 m et Ie pendage des strates ne depasse pas 10°. Comme Ie predisent les courbes empiriques, I'affaissement maximal mesure ou calcule numeriquement n'excède pas les 70% de l'ouverture. Pour des largeurs exploitees superieures à 700 m, les valeurs mesurees sont nettement inferieures à celles calculees.
Diese Untersuchung der durch Kohlegewinnung induzierten Bodensenkung ist in dem Rahm eine 1000 m tief französische Zeche durchgefuehrt. Die Machtigkeit des durch Strebenmethod abgebauten Flöz variiert zwischen 2 und 3,4 m. Die Langsneigung der Schichten ist nicht grösser als 10°. Wie Von empirischen Kurven vorausgesehen ueberschreitet die abgemessene oder berechnete Höchstsenkung 70% der Machtigkeit nicht. Fuer ueber 700 m abgebauten Breiten sind die abgemessenen Werte entschiedend geringer als die berechneten Werte.
Knowledge of subsidence engineering is of considerable importance to the planning and development of surface. Subsidence is generally associated with the extraction of minerals and natural resources such as oil, gas and water. It is vital for population to be aware of the risk of possible surface collapses above the underground openings. In such situations, knowledge should be established. Prediction methods exist for determining the likely magnitude and extent of subsidence development at the surface. These methods include empirical, numerical and analytical approaches. Empirical methods were essentially based on experience and field measurements. There are intended to similar other sites. Also, surface subsidence has been predicted using a number of numerical models based predominately on simplified elastic and isotropic assumptions. Many of these methods appear to make assumptions that are invalid and, consequently, give results which oversimplify subsidence prediction. The main objective of this study is to compare analytical, empirical and numerical methods for a specific site (Provence Colliery) in which the subsidence data were available, with intent to valid the methods of modelling and to evaluate their estimated character in relation to new sites. Empirical or analytical methods cannot take into account some local phenomena well known as flight of stairs near faults. The numerical modelling, especially the one based on the Distinct Element concepts, allows for the presence of discontinuities, therefore permits to display prominently the relative movement of discontinuities and to analyse more accurately this movement. The special feature of the numerical modelling presented here is that we have taken into account the tectonic history affecting the site during the loading sequences. Numerical analyses described in this paper were conducted using the two dimensional UDEC code.
The coalfield under investigation belongs to the Gardanne basin, which forms the eastern part of the Arc basin, and is an E-W oriented geological unit located 10 km north of Marseille. The geological structure is simple: the asymmetric syncline is strongly affected by a dome-shaped anticline (Fig. I). In the presently mined area, the structure is overridden by a major thrust sheet overthrusting Northward (Etoile unit), due to the North-South shortening and the major tectonic event of the geological history of the region. The same episode is responsible for strike-slip faulting all over the area (Gaviglio et at. 1990). The lignite seams belong to the Fuvelian (upper part of the Campanian), sequence which is made up of limestone. This occurrence of coal amid calcareous beds is major geological particularity of the coalfield, and is responsible for the great difference in mechanical stiffness between the coal seam and the calcareous beds forming the floor and roof. The Fuvelian limestones are 250 m thick in the mining zone.
