During the last few years a large number of pillars in different mines were investigated. Pillar conditions were evaluated using subjective, empirical, geophysical and numerical methods. A critical comparison of the different methods showed some interesting coherence. This paper concentrates on experiences using the p-wave velocities as an indicator of pillar conditions.
Wahrend den letzten Jahren wurde eine große Anzahl von Bergfesten in verschiedenen Bergwerken auf ihren Zustand hin untersucht. Hierzu wurden subjektive, empirische, geophysikalische sowie numerische Methoden angewandt. Dieser Vortrag beschaftigt sich mit den Erfahrungen beim Einsatz der p-Wellengeschwindigkeit zur Beschreibung des Festenzustandes.
Le condition geotechnique d'une grand nombre des piliers des mines souterraines differentes sont examinees ces dernières annees. On a applique des methodes subjectives, empiriques, geophysiques et numeriques. Ce rapport occupe aux experiences avec l'application de la velocite des ondes de compression pour tracer le condition des piliers.
The main objective of this research project was the development of a methodology for determining pillar conditions. For this purpose a considerable amount of data on pillar conditions in three mines had been collected.
In situ p-wave measurements showed to be very useful for a qualitative description of the pillar conditions. Additionally empirical methods were used to supplement these measurements.
The studies reported in this paper were conducted in one magnesite and two gypsum underground mines. In each mine at least three areas were investigated in detail. In the first two mines the areas were selected on the following basis:
Area No.1: - favourable stress conditions - pillars with no visible signs of overloading
Area No.2: - high stress conditions - pillars with typical signs of overloading
Area No.3: - high stress conditions - pillars with no visible signs of overloading
The purpose of selection of these areas was to investigate the sensitivity of the different methods under different conditions.
The third mine was selected because of the mining method, which allowed an investigation under a constant load but with changing height to width ratios and changing geology.
Mine No.1 Mine No.1 is a magnesite mine in the Austrian Alps. The massive deposit dips at ± 25°. The dip direction is into the mountain slope. The overburden varies between 0 m and 1,000 m. The mining method employed is post pillar mining with uncemented backfill. The pillars are rectangular in cross-section with a width of ~5 m and a length of 15 m. In a first step a 7 m high opening at the deepest point of a mining area is excavated. Backfill is then placed to a height of 3.5 m. The backfill is used as a working level for the next 3.5 m mining slice. Depending on the geometry of the deposit up to 26 slices have been mined resulting in pillar heights ranging from 7 m to more than 90 m.
Mine No.2 Mine No.2 is situated about 30 km south of the city of Salzburg. It extracts a massive Gypsum/Anhydrite mineral body.
