ARSTRACT: Based a upon model tests (simulating sedimentary formation by equivalent material modelling) stable, recurrent failure and collapse zones have been delineated as a function of roof deformation, width of opening, compressive/bending strength of formation and insitu stresses. Equations have been proposed to predict deformations or pressure distribution over the roof span.
RESUME: Base sur des modeles tests (simulation de la formation sedimentaire par modelisation materiel) equivalent), des zones stables, defauts perpetuels et effondres sont delinees comme une fonction de la deformation du toit, largeur de la ouverture, intensite de compression/flexion de la formation et constrainte insitu. Proposon ici des equations pour predir les deformations ou la repartititon de pression sur la largeur de toit.
ZUSAMMENFASSUNG: Basiert auf Modellversuchen, die (die sedimentare Formation mittels Modellen aquivalenten Materials simulieren), sind Stabil-, Periodenbruch-und Einsturzzonen dargestellt als Funktion der Dachdeformation, der Offnungsweite, der Druck-/Biegefestigkeit der formation und der In-situ-Beanspruchungen. Es sind Gleichungen zur Vorausberechnung von Deformationen bzw. Druckverteilung uber die Dachspannweite Vorgeschlagen.
1.0 INTRODUCTION For the design of underground openings, estimation of the roof deformations and the roof pressures are of prime importance. Amongst the various methods equivalent material modelling is more advantageous to get a physical feel of both the qualitative and quantitative understanding of the problem. Therefore, this approach has been preferred in the present work. The magnitudes of the redistributed stresses after the excavation of an opening are governed by a variety of factors such as its shape and size, geological condition, mechanical properties of the overburden, stress state, the method of excavation and the period during which the opening is left unsupported. There are various approaches available for the analysis of rock mass behaviour around the opening. An analytical approach mainly restricted to regular shaped openings requires the properties of the material, the state of the stresses involved and simplification of the mathematics and physics of the problem by making necessary assumptions. The problem, of stress distribution and displacements around a circular tunnel has been solved by Pender (1980). For elastoplastic cases, closed form solutions are difficult to obtain for different shapes and are confined to circular openings only. FEM is more commonly used amongst the available methods. In this method, many complex features which affect the displacements and stresses around the opening could be simulated in the analysis. Rock mass classification systems developed by Barton, Lien and Lunde (1974), Bieniawski (1974), Deere (1964), Lauffer (1958), Terzaghi (1946) etc. relate the experience encountered at previous projects to the conditions prevailing at a proposed site to estimate roof deformations and support pressure. In equivalent material modelling, the protosystem is simulated in the laboratory. The original rock formations are replaced by artificial materials. This approach was used by many researchers. Many modelling materials were developed and used 'by various investigators. These are plaster of paris, paraffin vaseline, portland cement, gypsum and lime. Sand, mica, chalk, talc and clay have been used as filler materials (Singh, 1981). To evolve an appropriate design approach, the initial step is to study the prototype, isolate its constituents parameters and then select parameters which are relevant to the problem (Ghazvinian, 1990). From the various parameters controlling the stability of underground openings, the effect of the size of the opening under various heights of overburden, the roof pressure distribution and also the effect of the height of opening have been studied to enable designing of an optimum support system.