The formation and mechanism of rock cavern roof arches are studied and effects of rockbolts reinforcement are evaluated. It is concluded that a self-supported roof arch may be formed by the excavation. To stabilize such a roof arch, fully grouted rockbolts are effective. Tensioned cable anchors may have a negative influence on the arch stability.


Die Formation und der Mechanismus von Deckenbögen in Bergkavernen wird studiert und der Effekt von Verstarkung mit Bergbolzen wird bewertet. Es wird festgestellt, dass die Exkavation einen selbst-stuetzenden / freitragenden Deckenbogen formen kann. Eingegossene Bergbolzen sind effektiv um einen solchen Deckenbogen zu stabilisieren. Spannkabel können unter Umstanden eine negative Einwirkung auf die Stabilitat des Bogens haben.


Le mecanisme et la formation de voûtes dans les cavernes rocheures sont etudies et l'effet du renforcement par boulonnage est evalue. Nous arrivons à la conclusion qu'une voûte autosupportêe peut être formee par excavation. Des boulons entièrement scelles par coulis de bêton s'avèrent efficaces pour stabiliser de telles voûtes. Les tirants d'ancrage peuvent quant à eux nuire à la stabilite de la voûte.


When a rock cavern roof is excavated, a self-supported roof arch structure may be formed. The characteristics of the roof arch may inherit from the voussoir arch, in which the voussoir has 'unlimited' compressive strength. The arch shape has advantage to make use of the high compressive strength in order to transfer loads, meanwhile, to compact the arch itself and thus forms a stable structure. To ensure the stability of the arch, conditions that induce tensile stress or/and sliding along masonry joints should be avoided. In stabilization of rock masses, rockbolting is a routine practice as the bolts are inexpensive, flexible to apply in changing ground conditions of underground excavation and easy and fast to install. Rockbolting contributes to the modern rock reinforcement system that is based on assisting the rock mass to form a self-supporting structure such as the cavern roof arch. Rockbolts mobilize the inherent strength of the rock mass by offering confining pressure, increase the stiffness of the rock, provide shear resistance to the rock joints or discontinuities and suspend loosed rock blocks. However, it is important to distinguish the effects of full grouted rockbolts and the tensioned cable anchors in reinforcing the roof arches.

Formation of roof arch and reinforcement
Formation of voussoir arch and cavern roof arch

Voussoir arch (Figure 1) action arises directly from the properties of the material - masonry blocks are laid in weak mortar or placed one on another to form the arch. This method of construction ensured that the structure would sustain no tension and sliding between blocks. Stability of the whole arch is assured by the compaction under gravity of the various elements and high capacity of compressive strength of masonry blocks. Coulomb, in his memoir on statics in 1773, examined the behavior of the masonry structure in the light of three assumptions as (Westergaard, 1964):

  • voussoir has no tensile strength,

  • voussoir has an unlimited compressive strength

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