SUMMARY:

In the Norwegian mining industry very large, stable caverns have been and are excavated. This paper gives case histories from mines present in operation. Examples of seemingly stable spans up to 80 meters are given. The use of rock stress measurements and other rock mechanical investigations connected to the chambers is described.

ZUSAMMENFASSUNG:

In den Norwegischen Bergbauindustrie wird sehr grosse stabile Kavernen abgebaut. In dieser PUblikation wird ''Case histories'' von Gruben heute in Betrieb beschreiben. Felsmechanische Untersuchungen in Zusammenhang mit den Abbau wird vorgestellt. RESUME: Dans le mine norwegienne des cavernes de tres grand postees sont creuse. presente des etudes de cas de mines a present a operation. Des measures de la Contrainte et des deformation sont exposees.

INTRODUCTION

In Norway, as in many other countries, there is a strongly increasing interest for underground caverns in connection with non-mining activities. This may be due to several factors among which energy considerations, lack of building grounds in urban areas and safety reasons (bomb shelters) may be the most important. Traditionally, Underground situation of hydroelectric power plants, defence constructions and storage plants for oil and other products has a strong position in Norway. The span of the chambers used for those purposes has usually been in the range of 10–20 m. During the recent years, however, demand for larger spans has arisen. One of the reasons for that is favourable government support to the construction of municipal bomb shelters. This has led to the idea of combining the Construction underground in rock. Until now, at least 5 sport halls or swimming halls has seen constructed with spans up to 27 m, and several others are planned.

While 27 m is regarded as a very large span in civil inegineering practice, much larger spans are known from mining engineering. In many Norwegian mines very large caverns have been and are excavated as s part of the mining methods. The majority of these chambers are completely without support or lining, but have neverthless in some cases been stable for centuries. Many of the chambers are flat-roofed as the ore excavations has followed the bedding or foliation planes of the rock.

FACTORS DETERMINING THE POSSIBILITY OF CONSTRUCTING STABLE LARGE SPANS

Several factors will influence the possibility of constructing stable large spans. The most important may perhaps be:

  • The geological conditions i.e. rock type and composition, strike and dip, orientation and properties of joints, cracks and faults, mechanical properties of the rock and rock mass.

  • The geometry of the opening i.e. span width, height/width ratio, radius of curvature.

  • The virgin stress conditions. At shallow depth the existence of horizontal stresses in excess of the gravimetric stresses is particularly important.

In several Norwegian mines it has been observed that mine openings with surprisingly large spans have been stable for very long periods of time. Spans of 50 m and more with almost completely flat roofs have been reported in areas with not too favourable geological conditions. As stress measurements became usual, it seemed evident that these openings were usually situated in areas with favourable stress conditions. A usual picture is that the major principal stress is approximately parallell to the roof, giving a ''prestress'' effect of the immediate roof. Direct measurements of this effect will consequently give valuable informations about the general stability of a roof. The Rock Mechanics Laboratory of the Mining Division, the Norwegian Institute of Technology, has for many years performed such measurements. In this case the doorstopper is the common method, as the stress acting parallell to the measuring hole at or near the surface of the roof is zero or very small.

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