ABSTRACT
The Stress Control Method of mine design is highly effective in stabilizing underground openings which cannot economically be stabilized by any conventional method. The stabilizing mechanism of the Stress Control Method is demonstrated by analyzing the behavior of a 4-room entry made using this method and comparing it with that of a single room entry designed according to the conventional method. To facilitate direct comparison, both entries are made in the same salt mine under the same geomechanical conditions. The analysis is conducted by finite element simulations of the two entries using the REM computer code. Close comparison of the simulation analysis to field observation demonstrates the effectiveness of the Stress Control Method.
1 INTRODUCTION
The Stress Control Method was introduced in the early development of Saskatchewan potash mining in the 1960s by Serata (Serata 1982). The method eliminates a high concentration of stresses in the vulnerable roof strata by redistributing them into the more stable configuration of a protected stress envelope, as illustrated in Fig. 1. This stress re-distribution is accomplished by excavating three or more entries grouped together, partitioned only by narrow yielding pillars (according to a particular time sequence of excavation), also depicted in the figure. All rooms created in this multiple-entry configuration were found to be more stable over a long time period than the conventional single isolated openings. This method of mine excavation not only eliminates serious roof failure problems, but also increases the productivity of the mining. The permanently protected center rooms of the stress controlled entry have proven to be stable without any roof bolting for more than 20 years, showing no sign of deterioration in the ground, where conventional openings with saturation roof bolting used to fail extensively within a few months. The Stress Control Method has spread throughout the potash industry in Canada and Europe. The method is now spreading rapidly into salt and coal mining, achieving dramatic improvement of working conditions (Serata & Gardner, 1984; Serata, Carr, Martin, 1984; Carr,' Martin, Gardner, 1985; Hamilton, Dickie, Serata, 1985). Despite its success in mining applications, the principle of the Stress Control Method has not been well-received by the academic community. The reason for the academic controversy appears to be the method's radical deviation from conventionality in its basic concept of stabilizing underground openings. This controversy may be put to rest by comparing the behavior of a stress-controlled multiple-room entry with that of a single-room entry made by the conventional method, where the latter is suffering severe roof failure which cannot be economically cured by any conventional means of roof support. Two test entries were made in the Sifto Salt mine, which is excavating a 13.4 m (55 ft) room in the 24.4 m (80 ft) thick A-2 salt bed in the Michigan basin at Goderich, Ontario, Canada. The mining began in 1959 with the conventional room-and-pillar method. Shortly after the initial development, roof failure started with slabbing separation of the immediate roof.
