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Siegesmund S.,University of Gottingen | Friedel C.-H.,Landesamt fur Geologie und Bergwesen Sachsen Anhalt LAGB | Vogel J.,HAWK Fachhochschule Hildesheim | Mosch S.,University of Gottingen | And 3 more authors.
Environmental Earth Sciences | Year: 2011

The St. Servatius Church in Quedlinburg (UNESCO's World Heritage Site, Germany) is characterised by long-standing stability problems and structural damages, which have been known over the last several centuries. The monotonous Cretaceous sandstone with its poor lithification is considered to be the main factor. The sandstone is characterised by a high porosity of around 30 Vol.% (max. ca. 35 Vol.%) and a corresponding high w-value. The porosity and the degree of cementation are responsible for the very low compressive strength of around 8 MPa at maximum, whereas under moisture these values are significantly reduced up to 40%. The freeze-thaw tests indicate a very poor resistance to frost weathering, which may explain the near-surface softening of the sandstone. Direct shear experiments with an approximate 60° angle of friction on the sandstone clearly demonstrate the safety margins. Initial geotechnical modelling does not favour the hypothesis that a landslide of the hill parallel to the southward dipping bedding planes (i. e. shear failure along weak zones) can occur. A prominent clay layer (also with a southward dip) below the entire castle hill is monitored to determine the possible amount of movement with respect to the geological discontinuities (e. g. joints, fractures). At present, a combination of foundation problems are being considered for the stability situation at the church. These include construction deficiencies due to deformation or softening of the foundation of the forerunner churches and missing or faulty connections from building additions. Geological factors responsible for the stability problems include the softening of the sandstone by the influence of weathering and penetrating water as well as the presence of possible shear planes and joints. © 2010 The Author(s).

Gunther R.-M.,Institute fur Gebirgsmechanik GmbH | Salzer K.,Institute fur Gebirgsmechanik GmbH | Minkley W.,Institute fur Gebirgsmechanik GmbH | Pudewills A.,Karlsruhe Institute of Technology | And 7 more authors.
Mechanical Behavior of Salt VIII - Proceedings of the Conference on Mechanical Behavior of Salt, SALTMECH VIII | Year: 2015

In this joint project series, fundamental features of the authors’ advanced constitutive models are investigated in detail and compared with one another. The study includes procedures for the determination of salt-type-specific model parameter values and for the performance of numerical calculations of rock salt around underground openings. In the current third project, selected benchmark calculations are carried out in order to check and compare the abilities of the involved models to describe correctly the damage and dilatancy reduction and healing of rock salt. At first, a unique set of model parameter values was determined with back-calculations of an extensive and systematic series of laboratory tests. Then, several simulations of a real bulkhead structure in the Asse II salt mine in Germany were performed. In the contribution, a brief general overview of the project series, a short comparison of different physical formulations of the partners for modeling damage reduction and healing, a summary of calculations, a comparison of results, and conclusions for the modeling are given. © 2015 Taylor & Francis Group, London.

Minkley W.,Institute fur Gebirgsmechanik GmbH | Popp T.,Institute fur Gebirgsmechanik GmbH | Salzer K.,Institute fur Gebirgsmechanik GmbH | Gruner M.,TU Bergakademie Freiberg | Bottge V.,Landesamt fur Geologie und Bergwesen
Physics and Chemistry of the Earth | Year: 2013

The so-called Red Salt Clay (T4) is deposited as clay-rich clastic sediment at the base of the Aller-series forming a persistent lateral layer of up to 20. m thickness above the lower Zechstein-series. The clay layers may act as a protective shield in the hanging wall of gas storages or underground repositories in salt formations, thus resulting in a multi-barrier system. As a proof of its reliability comprehensive hydro-mechanical investigations were performed on clay samples recovered at different sites in Germany. Most important, rock tightness against various fluids was confirmed in the lab and field-scale. Remarkably, only if the fluid pressure equalises the acting minimal stress (i.e. violence of the "minimum stress criterion") a significant increase of permeability is observed ("pathway dilatation") but no macro-frac. However, the material properties from different locations showed a significant variability according to different burial depths. Thus the Red Salt Clay may act as natural analogue, representing the material variability of various indurated clays. In addition, the existing knowledge gained from practical mining activities can be used to evaluate extreme in situ loading conditions. © 2013 Elsevier Ltd.

Popp T.,Institute fur Gebirgsmechanik GmbH | Salzer K.,Institute fur Gebirgsmechanik GmbH
Underground Storage of CO2 and Energy - Proceedings of the Sino-German Conference, and the Sino-German Workshop "Eor and New Drilling Technology | Year: 2010

In the context of a research program on radioactive waste disposal the hydro-mechanical properties of Opalinus clay from the Mont Terri site (CH) were investigated as analogue for a potential argillite host rock. The clay rock has a low natural water content (∼6%) corresponding to a porosity of ∼16%. Despite some sample disturbances due to core extraction and sample preparation the gas permeability and ultrasonic wave velocities were measured at hydrostatic and deformational stress conditions. During the gas flow measurements 2-phase phenomena, e.g. a capillary threshold, and the dependence on the flow direction related to the bedding are obvious. Increase of confinement established, at least partial, the initial tight rock conditions, whereby the compaction was most efficient perpendicular to the bedding. Triaxial deformation tests in a Kármán-cell delivered relevant information about the stress-dependent development of dilatancy by measuring simultaneously Vp, Vs and permeability, in addition to the volumetric strain during deformation.

Minkley W.,Institute fur Gebirgsmechanik GmbH | Popp T.,Institute fur Gebirgsmechanik GmbH
44th US Rock Mechanics Symposium - 5th US/Canada Rock Mechanics Symposium | Year: 2010

Salt formations are favoured for storage of radioactive waste due to their unique isolation capacity. Because their water content is extremely low only solid state behaviour predominates with grain boundary diffusion. From this follows impermeability of salt rocks under undisturbed conditions. Linked flow paths inside the salt barrier may be created only under deviatoric stresses (1) if the acting stresses exceed the dilatancy boundary (= dilatancy criterion) or (2) at increased fluid pressure conditions if the acting normal stresses at the grain boundaries are lowered (= minimum stress criterion). Implementation of this advantage, i.e. inherent tightness, in repository concepts of storage of radioactive waste in salt requires as a prerequisite that the barriers are dimensioned with a sufficient thickness in that way that an entire inclusion succeeds. For long term warrantee of this state it has to be ensured that in the geological salt barrier under all acting stresses, i.e. convergence- or thermo-mechanical-induced, neither the dilatancy criterion nor the minimum stress criterion is violated. Copyright 2010 ARMA, American Rock Mechanics Association.

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