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Lekube B.M.,Fritsch GmbH | Hermann W.,IFG GmbH | Burgstaller C.,Fritsch GmbH
Annual Technical Conference - ANTEC, Conference Proceedings | Year: 2016

The aim of this work was to investigate the influence of the formulation and the porosity on the properties of partially compacted composites made of commingled polypropylene and glass fibers. Furthermore, we wanted to develop a model to predict the properties of such composites to aid materials development. We found, that porosity is indeed a decisive factor influencing this type of composites. The most important parameter affecting the porosity was the number of layers used in the compaction process, reaching an optimum porosity of about 20-30% by stacking 3 to 4 layers of the fleeces. Furthermore, the use of MAH-PP as additive incorporated in the PP fibers improved the mechanical properties of the composites. The modeling of the elastic modulus was found satisfactory; however the influence of glass fiber orientation and length should be evaluated more in detail.

Minkley W.,IFG GmbH | Ludeling C.,IFG GmbH
50th US Rock Mechanics / Geomechanics Symposium 2016 | Year: 2016

Longwall caving is rather rarely applied or even considered in potash mining, mainly due to the major risks of flooding and rock bursts. To analyse the loads induced by longwall caving, we employ a discontinuous modelling technique which describes rock mass properties and fracture development with constitutive models for visco-elasto-plastic and adhesive shear behaviour. As a result, the characteristic geomechanical phenomena in longwall caving can be described convincingly. We present results on the triggering of rock bursts at the longwall face, discussing related concepts in coal mining as well as pillar collapse experiments from the 1980s intended for the development of new excavation methods with yielding carnallite pillars. The hydraulic barrier is impaired by (shear and tensile) fractures reaching upward from the mine and by pressure-driven percolation downward from the lowest aquifer in the overburden. The latter process is initiated by the stress redistribution following the excavations, in particular by the lowering of the minimal principal stress due to extensional strains induced by subsidence over mining edges. Once the minimal stress falls below the groundwater pressure, hydraulic integrity is lost and percolation proceeds in the direction of the major principal stress. If the fluid reaches the fracture zone around the cavities, hydraulic pathways open up which lead to a flooding of the mine. We present coupled hydromechanical calculations which can resolve fractures and fluid percolation through the barrier strata. Copyright 2016 ARMA, American Rock Mechanics Association.

Minkley W.,IFG GmbH | Ludeling C.,IFG GmbH
50th US Rock Mechanics / Geomechanics Symposium 2016 | Year: 2016

Rock salt provides the world's best barrier, and has thus long been considered as a host rock for nuclear waste repositories. Salt domes have vertical extensions of several kilometres and are consequently well-suited for deep borehole disposal, as an alternative to a mined repository. The isolation capacity of undisturbed salt rocks is based on the creep behaviour which tends to close any access paths. A number of natural analogues where fluids have been contained in cavities under high pressure for millions of years support this picture. For waste disposal, one has to show that the tightness of the geological barrier is not compromised by the repository excavation and the thermal loading due to the waste, and that suitable geotechnical barriers can be constructed. We discuss the option of deep borehole disposal in rock salt. In contrast to the mined repositories which are usually considered, this option employs boreholes of several kilometres, drilled into a salt dome. The waste canisters are emplaced in the lower part of the borehole. As backfill material, we propose molten salt mixtures, similar to the ones used in solarthermal plants as heat exchange and storage fluids. The waste-generated heat will keep the salt liquid for a long time, ensuring complete containment without the possibility of ground water reaching the waste. The upper part of the filled boreholes, still inside the salt dome, converges under lithostatic pressure due to salt creep, and the top part can be sealed with asphalt, bentonite and concrete. We present some experimental results on the closure of boreholes under pressure and the properties of liquid salt to support our proposal. Copyright 2015 ARMA, American Rock Mechanics Association.

Jandard F.,Aix - Marseille University | Fauquet C.,Aix - Marseille University | Dehlinger M.,Aix - Marseille University | Ranguis A.,Aix - Marseille University | And 5 more authors.
Applied Surface Science | Year: 2013

In this work we show the feasibility of simultaneous topographic and luminescence mapping with an home-built Shear-Force Microscope under X-ray irradiation with a tabletop microfocused X-ray source (maximum electric power of 30 W). A commercial fluorescent screen, containing europium uranyl compounds, is used as test sample. Simultaneous topography and luminescence maps on the fluorescent screen are first obtained with the apparatus. The two images totally overlap, however the luminescence is not homogeneous on the whole scanned area. Moreover, a photoluminescence spectrum is presented with a good signal-to-noise ratio under X-ray irradiation on a grain of the fluorescent screen and shows peaks in agreement with Europium uranyl compounds. A ZnO/ZnS powder mixture embedded in PMMA is then studied with the same equipment. A 20 μm-wide grain is clearly visible on the topographic image. With the instrument, the grain could be unambiguously identified as containing mainly ZnO. © 2012 Elsevier B.V. All rights reserved.

Popp T.,IFG GmbH | Minkley W.,IFG GmbH | Salzer K.,IFG GmbH | Schulze O.,Federal Institute for Geosciences and Natural Resources
Mechanical Behavior of Salt VII - Proceedings of the 7th Conference on the Mechanical Behavior of Salt | Year: 2012

Due to their unique isolation capacity, rock salt formations are favored for the long-term storage of radioactive waste. Most relevant for performance assessment is demonstration of integrity of the geological barrier during the various stages of a repository, i.e., construction, operation and in the post-closure phase. In this context, the impacts of disturbance induced by the excavation of the underground facilities and long-term effects during re-compaction of the EDZ are important items. In addition, because the formation of gases by processes occurring in many radioactive waste packages is unavoidable, the impact of increasing gas pressures on the salt integrity has to be assessed, too. The objective of this paper is to review current understanding of the gas transport properties of rock salt in a rock salt environment associated with a radioactive waste repository. This knowledge has been mainly developed by laboratory and field work during the last two decades.

Minkley W.,IFG GmbH | Muhlbauer J.,IFG GmbH | Ludeling C.,IFG GmbH
Rock Mechanics and Rock Engineering | Year: 2016

The paper describes the principal geomechanical approaches to mine dimensioning in salt and potash mining, focusing on stability of the mining system and integrity of the hydraulic barrier. Several common dimensioning are subjected to a comparative analysis. We identify geomechanical discontinuum models as essential physical ingredients for examining the collapse of working fields in potash mining. The basic mechanisms rely on the softening behaviour of salt rocks and the interfaces. A visco-elasto-plastic material model with strain softening, dilatancy and creep describes the time-dependent softening behaviour of the salt pillars, while a shear model with velocity-dependent adhesive friction with shear displacement-dependent softening is used for bedding planes and discontinuities. Pillar stability critically depends on the shear conditions of the bedding planes to the overlying and underlying beds, which provide the necessary confining pressure for the pillar core, but can fail dynamically, leading to large-scale field collapses. We further discuss the integrity conditions for the hydraulic barrier, most notably the minimal stress criterion, the violation of which leads to pressure-driven percolation as the mechanism of fluid transport and hence barrier failure. We present a number of examples where violation of the minimal stress criterion has led to mine floodings. © 2016 Springer-Verlag Wien

Minkley W.,IFG GmbH | Knauth M.,IFG GmbH | Wuste U.,IFG GmbH
Mechanical Behavior of Salt VII - Proceedings of the 7th Conference on the Mechanical Behavior of Salt | Year: 2012

Discontinuum-mechanical aspects are of importance for the integrity of salinar barriers, not only in the macro- but also in the micro-scale. In the grain scale, polycristalline salts represents a discontinuum constituted of intergrown crystal grains. Grain boundaries are micro-mechanical weakness planes that preferably fail under loading. For the mechanical description we therefore employ an elastovisco-plastic model for the salt crystals as well as an adhesive frictional model for the grain boundaries. Based on this discontinuum-mechanical approach, hydro-mechanical coupling can be analyzed like the pressure-driven percolation of fluids, intergranular microcracking and hydraulic fracturing. The results demonstrate how THM-processes determine the integrity of salinar barriers.

Gunther R.-M.,IFG GmbH | Salzer K.,IFG GmbH
Mechanical Behavior of Salt VII - Proceedings of the 7th Conference on the Mechanical Behavior of Salt | Year: 2012

A powerful constitutive model is presented which describes the complex mechanical behavior of rock salt in a good approximation. The model contains a state variable to characterize the effective material hardening which is growing with ongoing creeping. However, hardening is reduced again by recovery and damage. The viscosity of the creep model depends on the hardening state variable so that the three well known creep phases develop. The measured dilatancy is formulated as a function of a special deformation work and the minimum stress. The identification of dilatancy parameters for the constitutive model is shown. Additionally a new concept for the handling of tensile stresses within a creep model and also a term for damage healing are presented. This model has been verified based on the results of numerous strength and creep tests. The capability of the approach will be demonstrated by the back calculation of special laboratory tests and two field applications.

Gunther R.-M.,IFG GmbH | Salzer K.,IFG GmbH | Popp T.,IFG GmbH | Ludeling C.,IFG GmbH
Rock Mechanics and Rock Engineering | Year: 2015

Actual problems in geotechnical design, e.g., of underground openings for radioactive waste repositories or high-pressure gas storages, require sophisticated constitutive models and consistent parameters for rock salt that facilitate reliable prognosis of stress-dependent deformation and associated damage. Predictions have to comprise the active mining phase with open excavations as well as the long-term development of the backfilled mine or repository. While convergence-induced damage occurs mostly in the vicinity of openings, the long-term behaviour of the backfilled system is dominated by the damage-free steady-state creep. However, because in experiments the time necessary to reach truly stationary creep rates can range from few days to years, depending mainly on temperature and stress, an innovative but simple creep testing approach is suggested to obtain more reliable results: A series of multi-step tests with loading and unloading cycles allows a more reliable estimate of stationary creep rate in a reasonable time. For modelling, we use the advanced strain-hardening approach of Günther–Salzer, which comprehensively describes all relevant deformation properties of rock salt such as creep and damage-induced rock failure within the scope of an unified creep ansatz. The capability of the combination of improved creep testing procedures and accompanied modelling is demonstrated by recalculating multi-step creep tests at different loading and temperature conditions. Thus reliable extrapolations relevant to in-situ creep rates ($$10^{-9}$$10-9 to $$10^{-13}$$10-13 s$$^{-1}$$-1) become possible. © 2015, Springer-Verlag Wien.

Popp T.,IFG GmbH | Rolke C.,IFG GmbH | Salzer K.,IFG GmbH
Geological Society Special Publication | Year: 2015

Deep underground repositories for radioactive waste generally rely on a multibarrier system to isolate the waste from the biosphere. It consists of the natural geological barrier provided by the repository host rock and its surroundings, the waste container and an engineered barrier system (EBS): that is, the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. Bentonite emplaced in compacted block form is the preferred option for the clay buffer for most waste management organizations. In assessing the performance of bentonite block masonries, conductive discrete interfaces inside the sealing elements (i.e. contacts between blocks) and to the host rock may act not only as mechanical weakness planes but also as preferential fluid pathways. We performed hydraulic tests on prefabricated bentonite-sand block assemblies (60:40). The results document that despite existing interfaces, the investigated bentonite block assembly behaves no different to that of the homogenous matrix during the saturation of the buffer. This has been confirmed by gas-injection tests on the former interface, as well by shear tests. The outstanding observation is that our results convincingly demonstrate that interfaces between bentonite bricks may 'heal' (not only seal), as was physically verified by confirmation of cohesion after presaturation. © The Geological Society of London 2015.

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