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When dealing with tunnels in weak rock mass and with high overburden, the high displacements imposed on the lining dictate the application of ductile yielding elements with controllable stiffness and yield load. These properties are chosen with two goals in mind: the time-dependent strength of the shotcrete shell must not be exceeded; however the support pressure must be kept reasonably high and controllable. The attainable load-displacement lines of the ductile support elements are almost arbitrary. There are almost countless possible combinations of their stiffness and yield load, thus enabling the development of custom-tailored support systems and leaving considerable room for adapting to the encountered ground conditions. Tunneling in weak ground should be accompanied by increased efforts on monitoring the system behavior, best by a dense pattern of absolute displacement measurements. A simple technique for calculating the shotcrete utilization ratio has been developed. It applies a Newton- Raphson root-finding algorithm to determine the interpolation parameters while obeying the requirements of force equilibrium and fitting the measured displacements. The influence of non- symmetrical displacement behavior caused by heterogeneity and anisotropy of the rock mass, on the lining loading can be quantified and used for support system optimization. 1. INTRODUCTION High primary stresses associated with tectonic faulting frequently create problems during construction of Alpine base tunnels. Keeping the displacements in a range which could be sustained by the support would lead to economically unfeasible lining thickness. Ductile lining systems using in mining cannot be be transferred to traffic tunnels with their requirement of long term stability. First concepts of yielding supports for tunnels date back to the nineteen fifties (Rabcewicz 1950). The technical requirements posed on a ductile support system are quite clear:The load-displacement characteristics should be "steerable" within a broad range, allowing the avoidance of overstressing the shotcrete shell, while enabling easy modifications in order to cope with the ground heterogeneity and usually long-lasting displacement increments. The support resistance has to be reasonably high, allowing a certain amount of control over the displacement magnitude. Various types of yielding elements integrated in the lining have been developed over time, mostly based on steel and sometimes on porous cement-based materials (Schubert 2008). One of them has been developed at the Graz University of Technology (Moritz 1999), featuring an enclosed steel tube subject to controlled buckling (Figure 1), called Lining Stress Controller (LSC). Its advantage lies in an excellent general load-displacement characteristic, being highly ductile and thus dissipating the major amount of the external work, and a broadly variable yield force level and initial loading stiffness. (Figure in full paper) 2. MOTIVATION When tunneling in conditions leading to application of integrated yielding elements, great attention should be paid to comprehensive monitoring of the system behaviour. Due to the unreliability of methods measuring the kinetic quantities (e.g. forces or stresses) the measuring of the absolute displacements currently yields the most direct insight on system behaviour. Usually 5 measurement points are being used.
- Management (0.75)
- Reservoir Description and Dynamics (0.48)
ABSTRACT: In weak rock or under high overburden, considerable displacements occur during excavation of tunnels and galleries. The strains developing in many cases exceed the deformability of standard linings, frequently leading to severe damages and the necessity of costly repairs. To allow for a safe and economical tunnel construction, strategies have to be used, which guarantee support characteristics compatible with the strains, and at the same time utilize the supports as much as possible. After a review of traditional methods, mainly used in mining in the past recent developments to deal with high displacements in combination with modern standard supports, such as shotcrete and rock bolts are shown. The different systems currently available are critically reviewed. For the design of such supports the development of the expected displacements must be predicted and the time dependent properties of shotcrete considered. Special tools have been developed to predict displacements. A relatively simple analysis method to design shotcrete linings with integrated steel elements, based on predicted displacements and the transient lining properties is used to demonstrate the effectiveness and practical applicability of the various systems available. 1. INTRODUCTION Large displacements during excavation of tunnels due to poor rock and high stresses are a challenge for designers and contractors. Displacements can reach several tens of centimeters, in some cases displacements of one meter and more have been reported [1,2]. Associated with those large displacements are difficulties in predicting their magnitude and development, as well as problems of the limited deformability of standard supports. Tunnel supports on the one hand should provide as much resistance against deformation as possible, on the other hand should be able to sustain the large imposed strains. Various methods have been developed over the decades to cope with the difficulties. This paper addresses some aspects of consequences of large displacements in relation to the lining design. This includes a review of support techniques used in the past and recent developments of yielding supports, and the experience made with their application on site. 2. DEVELOPMENT OF SUPPORTS FOR LARGE DISPLACEMENTS A traditional method in mining when experiencing large displacements was to use U-shaped steel sets with sliding couplings in combination with wire mesh or lagging. Figure 1. Destroyed steel set support with sliding couplings. Photo: DMT(available in full paper) Figure 1 illustrates the deficiencies of such supports. In particular in cases of anisotropic deformation the steel sets buckle, and costly and dangerous repairs are required. Timber supports had to be replaced many times until stabilization was reached. 2.1. Timber elements With the introduction of concrete and shotcrete linings in the late nineteen fifties, the previous problem of excessive loosening diminished, but the comparatively low deformability of concrete lead to destruction of the lining in case of larger displacements. Rabcewicz proposed timber elements integrated into a concrete lining as early as 1950 to provide sufficient deformation capacity of the system [3]. Depending on the required ductility and resistance different types of timber can be used. Figure 2. Concrete support with integrated timber element, as proposed by Rabcewicz [3](available in full paper)
ABSTRACT: This paper aims to provide insights into the effectiveness and failure mechanism of shotcrete lining in mine developments. For this purpose, a two-dimensional numerical program based on the Finite-Discrete Element Method (FDEM) was utilized to simulate an instrumented section of a 10-m diameter mine shaft at a depth of 1.2 km in an average quality rock mass. The shaft wall was supported by shotcrete and concrete liners installed at 3 m and 12 m behind the face, respectively. However, only shotcrete support was considered in the numerical simulations. The shotcrete liner was modelled as a material with a thickness of 50 mm and calibrated based on the mechanical properties of early-age fiber reinforced shotcrete obtained from available laboratory test results and empirical equations. The rock material near the modelled shaft was calibrated against the extensometer measurement data. It was found that the shotcrete liner fails due to bending caused by the shaft wall convergence and local rock mass bulking. Therefore, for the rock mass and in situ stress conditions representative of the mine shaft, it is concluded that the addition of a permanent support element, such as a concrete liner, is necessary for the long-term stability of the shaft. 1. INTRODUCTION Underground mines aim to reach deep orebodies by enhancing the advance rate of access developments while maintaining safe workplaces. Underground lateral (drifts) and vertical (shafts) developments are excavated using two main methods: drill and blast and mechanized excavations. The application of mechanized excavation machinery leads to an increase in the overall performance and thus the net present value of the projects. While mechanized excavation is ideal for moderately strong to weak rock masses, the drill and blast method has been used in a wide range of rock mass conditions [1]. Although the mining industry has been investigating mechanized excavation as an alternative method in hard rocks, the conventional drill and blast excavation method is still the norm in modern-day developments in underground mines [2].
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- Materials > Metals & Mining (1.00)
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ABSTRACT When constructing long base tunnels passing various geological formations and tectonic faults, advance through weak ground under high overburden frequently occurs. Loads unsustainable by a stiff support concept, support damage and very costly re-profiling works are a frequent consequence. Various approaches for a ductile support system appropriate for such geotechnical conditions have been proposed, developed and applied. The ductility of a shotcrete lining is usually ensured by integrating yielding elements. In order to prevent overloading of the young shotcrete, the elements have to fulfil the requirement of low initial stiffness, preventing mobilization of excessive thrust in the initial deformation stages. On the other hand, the capacity of the shotcrete lining should be maximally utilized, after it has developed sufficient strength. No objective comparison between currently available yielding element types has been conducted. In order to examine the system behaviour with different yielding element types, a numerical parameter study in FLAC3D has been conducted. The separate mobilization of cohesion and friction has been captured by the strain softening constitutive law. The parameter calibration has been performed on the basis of direct shear and triaxial laboratory tests. The rheological behaviour of shotcrete is simulated by using Norton's power law creep model and continuous time and strain-history-dependent updating of shotcrete properties via FISH routines. The numerical calculation procedure thus switches hence and forth between mechanical calculation (initial equilibrium after round excavation) and viscous calculation, modelling the creep and shrinkage processes in the young shotcrete. The comparison is conducted with regard to displacement development and shotcrete utilization.
SYNOPSIS: Experimental investigations of the strength and deformability of steel fibre reinforced shotcrete linings are presented. The tests show that there are several advantages associated with the use of high strength steel fibres with hooks in shotcrete linings:These fibres slip under a certain resistance in the matrix and the failure becomes very plastic-both in bending and in shear. Advanced support systems utilizing the interaction of rock bolts and shotcrete are simplified in the design when steel fibre reinforcement is used. The fibres prevent the breaking up of the shotcrete, and the risk of downfall is decreased. RESUME: La resistance et les deformations de beton projete à fibres d'acier ontfait l'objet de recherches experimentales. Les essais mettent en evidence plusieurs avantages que presentent l'emploi, dans les revêtements de beton projete, les fibres en acier de haute resistance terminees par des crochets:Ces fibres glissent dans la matrice en developpant une certaine resistance et la rupture devient très plastique aussi bien en flexion qu'en cisaillement. L'emploi de revêtements armes de fibres d'acier simplifie la conception de systèmes de soutènement bases sur l'action reciproque d'ancrages et de beton projete. Les fibres empêchent la desagregation du beton projete et diminuent le risque d'effondrement. ZUSAMMENFASSUNG: Experimentelle Untersuchungen ueber die Starke und Biegsamkeit des Stahlfaserspritzbetons liegen vor. Bei Tunnelausklei- dungen aus Spritzbeton unter Verwendung von Fasern aus hochfestem Stahl mit gebogenen Enden, erweisen die Experimente mehrere Vorteile:Diese Fasern gleiten im Material unter gewissem Widerstand und resultieren in ausgesprochen plastischen Biegeund Schubbruechen. Der Entwurf schwieriger Konstruktionen aus Spritzbeton und Felsankern wird vereinfacht bei Verwendung von stahlfaserbewehrtem Spritzbeton. Die Fasern verhindern das Zerbrechen des Betons und reduzieren das Einsturzrisiko. 1. INTRODUCTION In 1973 a joint project between the Swedish Rock Engineering Research Foundation and the Royal Swedish Fortifications Administration started. Its aim was to get better knowledge of the strengthening function of a shotcrete lining when applied to a hard rock. The investigation started with large scale tests on the failure mechanism and strength of shotcreted jointed rock (Holmgren, 1979). It was then continued with laboratory tests on the tensile adhesion of shotcrete to different rock types (Hahn, 1979). It was found that the adhesion between the rock and the shotcrete plays a determining role for the strength of the lining unless other support is arranged e.g. systematic rock bolting. In Holmgren, 1979 there are described tests on bolt supported reinforced shotcrete linings. These results served as a basis for a standardized design for bolt supported shotcrete linings in military caverns. This solution is also used in an underground storage for radioactive waste. There are many advantages with this design:The strength is independent of the obtained adhesion strength. The strength is possible to calculate according to principles for concrete plates on columns. A systematic rock bolting may be required - for other reasons, too. End anchored rock bolts with rolled threads interacting with reinforced shotcrete seems to be the most promising support in squeezing rock. The disadvantages are:The installation of mesh reinforcement is very costly and time consuming. The irregularities of the rock surface make it difficult to shotcrete a reinforced lining. The probability that the reinforcement fits the distribution of the bending moments in the lining is very small. It is impossible to reinforce a shotcrete lining against shear failure using conventional reinforcement. By using fibre reinforcement instead of a conventional one most of the above mentioned difficulties disappear. The most important remaining one is the cost because of fibres being an expensive material. 2. TEST PROGRAM 2.1 General remarks The punching of a single block through a shotcrete lining which is supported by rock bolts was also studied by Holmgren, 1979. The tests were performed in a rig consisting of three large granite blocks on a steel stand. The shotcrete was produced according to the wet mix method and the fibres were added at the nozzle by means of a so called Fibre Feeder which is patented by the Swedish company Besab. The fibres were hooked steel fibres Bekært Lz 35x.35 and Lz 45x.35 with a failure strength of about 1200 MPa. Standard cement and 0 - 4 mm sand were used for the concrete. Most test specimens were cured for eight days. 2.2 Details about the test specimens The test program consisted of four parts:Large scale tests on possible designs of a fibre reinforced shotcrete lining supported by rock bolts. There were tested stiff washers of two sizes at the end of the bolt, flexible washers and also bolts without washers. Large scale tests for the comparison of mesh reinforced shotcrete and fibre rein- forced shotcrete. There were tested meshes of cold tensioned steel and meshes of mild steel.