In order to verify the formation and development of the pressure arches at the specific conditions of a mine, the authors elaborated a numerical model, by computer processing, using the distinct elements method with the UDEC program. The dynamic evolution of the arches as a function of the mining progress was studied. The obtained results are presented in graphs and the conclusions are exposed through qualitative interpretations. Two cases histories relatives to the studies are also presented.
Pour la comprehension du mecanisme de developpement des arcs de pression dans les specifiques conditions d'une mine, les auters avon elabore une modèle matematique, avec la utilisation du le programme UDEC. L'etude avait recherchè Ie caractère dynamique dans l'etablishement de I'arc de pression comme une fonction de la vitesse d'explotation. Les resultats sont presente au milieu des graphiques et les conclusions expose qualitativement. Deux cases historiques sont decrit où rendre evident les conclusions de l'etude.
Um das Verstandnis der Erzeugung von Druckbogen in specifischen Berbau Fallen zu erklaren, benutzten die Autoren ein symboliches Model durch EDV, der die Methode der distinkte Elemente anwendet - das Programm UDEC. Der dynamische Character beim Entstehen eines Druckbogens zeigte sich abhangig von der Gewinnunggeschwindigkeit im Bergbau. Die Ergebnisse sind als Abbildungen und die Folgerungen durch qualitative Schatzungen dargestelt. Zwei fruehere Falle in erlauternden Form sind auch dargestellt und beschieben.
The pressure arch concept is very important for the mining purposes of ore bodies in stratified rock masses. Its adoption allows safe mining procedures, by the use of sufficiently wide yielding and barrier pillars according to the geomechanical conditions of the roof strata, with protections pillars being loaded with only a fraction of the load estimated by tributary area criteria.
The pressure arch concept is illustrated by Figures 1 and 2, showing stress trajectories and stress peaks over the abutments as defined by Herget. The vertical load directly above the openings shifts toward the ribs, leaving an unstressed zone in the roof, as demonstrated by observation of the reaction of rock strata to mining operations. The arching process results from a stress redistribution with an increase of compression over pillars and abutments (and a consequent stress reduction in strata immediately above the rock opening). The compression zone is assumed to arch or branch out from pillars and abutments, closing in the overlying strata high above the excavation.
Figure 3 corresponds to an illustration given by Fairhurst, showing the effect of deformability of roof, floor, and pillar on pillar loading. In the figure a pillar is represented by a hydraulic jack. If the jack is slowly lowered, roof and floor will converge, increasing the load in the abutments (F) and reducing pillar load (Fi). When the roof is sufficiently strong to support the load between abutments, the jack can be removed, and no collapse will occur, which can be compared to the situation of a failed pillar. The remaining strength of the pillar is small but the opening will remain stable when the roof strata are strong enough to carry roof loads to other pillars or abutments. As stated by Herget, "having sufficient thickness of roof strata, load transfer might occur, although the immediate roof is unstable. Mine design has made use of this process because pillars within the span of the ground arch do not have to carry the full overburden load. This allows mining at a depth where rock strata in pillars are not strong enough to carry overburden load.