A design approach is used at Noranda underground mines which incorporates both numerical and empirical modelling techniques. It requires the collection of a comprehensive rock mechanics data base and is backed up by a rational monitoring program. Two case histories from mines with widely differing stability concerns are outlined in this paper.
L'approche pour l'ingenièrie de conception des mines souterraines de Noranda comporte des techniques de modelisation numerique et empirique. Elle necessite la creation d'une base de donnees en mecanique des roches et est supportße par un programme rationnel d'instrumentation des massifs rocheux. Deux histoires de cas de mines ayant des problemes de stabilite passablement differents sont mis en relief dans cet article.
Eine Entwurfsmethode fuer Noranda's unterirdische Bergwerke wurde entwickelt, welche numerische als auch empirische Modelliertechniken anwendet. Diese Methode erfordert die Zusammenfassung einer umfangreichen felsmechanischen Datenbasis. Das Verhalten der so entworfenen Struktur wird dann durch ein rationelles Überwachungsprogramm bewertet. Zwei Beispiele werden gegben wo die Methode in zwe1 Bergwerken angewendet wurde, welche grundlegend verschiedene Stabilitatsprobleme hatten.
In recent years rock mechanics has finally begun to take its place as a practical tool for underground hard rock mines. The increased acceptance of rock mechanics techniques in mining is largely due to rapid advancement in numerical modelling techniques. Using two case histories, (Worita and Chadbourne Mines, Quebec, Fig. 1), we hope to show how these techniques can be used in design to augment the experience and empirically based methods which will always remain important mine design tools. The Norita Mine will be discussed in detail. Chadbourne will then be covered briefly to show that widely different rock mechanics questions can be addressed using similar techniques.
The Norita deposit is located on the north limb of a regional anticline. The deposit, a stratiform massive sulphide, is a vertical, tabular ore body located in a thick volcanic sequence (Fig. 2). In the area presently being worked, the ore body is about 25 m wide, has a strike length of about 200 m and extends to a depth of 670 m. The mine has experienced' stability problems in the past using sub-level caving. Present mining is by transverse stopping as shown on the longitudinal section, Fig. 3.
In order to evaluate stability and optimize the mining sequence, a rock mechanics study was initiated. A thorough database was developed. To estimate virgin stresses, seven overcoring tests in two HX diamond drill holes were done using CSIR strain cells. The tests were done at depths of 443 m and 714 m below surface in areas removed from major mine excavations. Results were difficult to interpret but suggest vertical stresses about equal to overburden weight and horizontal stress about 2 1/2 times the vertical.
Problems were reported at the end of 1985 in the form of ground movement and slabby, unstable back conditions in the 8–8 drift. This drift is located in the 25 m thick sill pillar between the "A" zone and the transverse mining block below (Figs. 3 and 4). Initially problems appeared local and related to weaker stringers included in the massive sulphide. Monitoring in the form of drift closure stations was recommended. Continued deterioration of ground conditions in the new year included:.
Continuing vertical and horizontal closure of up to 2 cm in the 8–8 sill drift after only one month of readings.
Poor ground conditions in the level 9–9 draw point pillars (Fig. 4). These pillars, located between the 9/10 and 10/11 vertical stopes, required frequent reconditioning.
At this time questions concerning longer term mine planning were also raised, resulting in a more comprehensive analysis. A complete modelling program was then started. Also, a comprehensive monitoring package was recommended to assist in interpretation of model results and assessment of overall mine behaviour.
Two numerical programs, a two-dimensional boundary element program (BEA) and a displacement discontinuity program (Mintab) were used. BEA was used to model transverse and plan sections. Principal stress and stress directions are calculated while failure is estimated based on the Hoek and Brown failure criteria (Hoek and Brown 1980). The Mintab program was used to simulate longitudinal sections and to evaluate the effect of stope sequences on minewide stress distribution.
Before predictive modelling can be done it is essential to back analyse known failure conditions. At Norita, failure in relatively unconfined rock occurs at stress levels predicted by Mintab at about 1/3 to 1/2 of the unconfined compressive strength (UCS) of the intact material.
Following the initial calibration, the lower "A" zone and transverse stoping block (Fig. 3) was modelled to simulate present mining conditions (May 1986). Fig. 5 shows the resulting normal stresses acting through the pillars.
