The visualization of flow paths in experimental studies of clay-rich materials

One of the most challenging aspects of understanding the flow of gas and water during testing in clay-rich low-permeability materials is the difficulty in visualizing localized flow. Whilst understanding has been increased using X-ray Computed-tomography (CT) scanning, synchrotron X-ray imaging and Nuclear Magnetic Resonance (NMR) imaging, real-time testing is problematic under realistic in situ conditions confining pressures, which require steel pressure vessels. These methods tend not to have the nano-metre scale resolution necessary for clay mineral visualization, and are generally not compatible with the long duration necessary to investigate flow in such materials. Therefore other methods are necessary to visualize flow paths during post-mortem analysis of test samples. Several methodologies have been established at the British Geological Survey (BGS), in order to visualize flow paths both directly and indirectly. These include: (1) the injection of fluorescein-stained water or deuterium oxide; (2) the introduction of nano-particles that are transported by carrier gas; (3) the use of radiologically tagged gas; and (4) the development of apparatus for the direct visualization of clay. These methodologies have greatly increased our understanding of the transport of water and gas through intact and fractured clay-rich materials. The body of evidence for gas transport through the formation of dilatant pathways is now considerable. This study presents observations using a new apparatus to directly visualize the flow of gas in a kaolinite paste. The results presented provide an insight into the flow of gas in clay-rich rocks. The flow of gas through dilatant pathways has been shown in a number of argillaceous materials (Angeli et al., 2009; Autio et al., 2006; Cuss et al., 2014; Harrington et al., 2012). These pathways are pressure induced and an increase in gas pressure leads to the dilation of pathways. Once the gas breakthrough occurs, pressure decreases and pathways begin to close. This new approach is providing a unique insight into the complex processes involved during the onset, development and closure of these dilatant gas pathways

Pore-pressure cycling experiments on Mx80 bentonite

The Swedish concept for geological disposal of radioactive waste involves the use of bentonite as part of an engineered barrier system. A primary function of the bentonite is its ability to swell when hydrated by its surroundings. One particular uncertainty is the impact on this function, resulting from deviations in pore-water pressure, pw, from expected in situ hydrostatic conditions. We present results from a series of laboratory experiments designed to investigate the form of the relationship between swelling pressure and pw, for compacted Mx80 bentonite, from low to elevated applied water pressure conditions. The experiments were conducted using constant volume cells, designed to allow the total stresses acting on the surrounding vessel to be monitored (at five locations) during clay swelling. The results demonstrate that swelling pressure reduces nonlinearly with increasing pw, becoming less sensitive to changes at elevated pressures. After cyclic loading a marked hysteresis was also observed, with swelling pressure remaining elevated after a subsequent reduction in applied water pressure. Such behaviour may impact the mechanical and transport properties of the bentonite and its resulting performance. However, such hysteric behaviour was not always observed. Further testing is required to better understand the causes of this phenomenon and the controls on such behaviour

Final report of FORGE WP3.1.1: the large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory

This report summarises the set-up, operation and observations from the first 2890 days (7.9 years) of the large scale gas injection test (Lasgit) experiment conducted at the Äspö Hard Rock Laboratory. During this time the bentonite buffer has been artificially hydrated and has given new insight into the evolution of the buffer. Three gas injection tests have been conducted during the duration of Lasgit. The first two tests were conducted in the lower array of injection filters at FL903. Both of these tests showed similar behaviour with a well-defined pressure peak; spontaneous negative transient; evidence of dynamic behaviour and unstable gas pathways; asymptote close to stress. The results were remarkably qualitatively similar to the laboratory test results. However, the high gas entry pressures seen in the laboratory were not seen in Lasgit as stress state is much lower due to non-complete hydration of the buffer and the expansion of the buffer to fill construction voids. The third gas test was conducted in an upper array filter (FU910). The response at the time of gas peak pressure was subtly dissimilar to that seen at FL903 with two peak pressures. Lasgit has confirmed the coupling between gas, stress and pore-water pressure for flow before and after major gas entry at the field scale. All observations suggest mechanisms of pathway propagation and dilatancy predominate. In all three gas tests the propagation was through localised features and the general movement direction was towards the bottom of the deposition hole in the direction of the prevailing stress gradient. The injection tests have shown that the interface between barriers is a key part of the system. Gas appears to have exited the deposition hole in Gas test 2, but failed to find a way out during Gas test 3; where gas continued to migrate along the canister/buffer interface. Throughout the history of Lasgit parts of the system have been artificially and naturally hydrated. Hydraulic results, from controlled and uncontrolled events, show that the buffer continues to mature and has yet to reach full maturation. Hydration of the clay is progressing well but sections of bentonite remain in suction and in hydraulic disequilibrium

A study into marine landscapes applied to habitat mapping

In recent years there have been many new seabed-mapping programmes carried out around the world using the latest data acquisition techniques. The need for these maps is driven by the recognition that an ecosystem-based approach to the management of national Exclusive Economic Zones (EEZs), as required by the Convention on Biological Diversity (CBD) adopted at the Rio Earth Summit in 1992, requires both detailed bathymetry and maps of the physical properties of the sea floor. In Europe, the implementation of the CBD is through the Habitats and Birds Directives, which require the identification of Special Areas of Conservation (SACs) and Special Protected Areas (SPAs). A network of SACs and SPAs will be set up across Europe known as Natura 2000. In addition, the International Council for the Exploration of the Seas has developed the concept of the ecosystem-based approach in the context of fisheries management, subsequently adopted by the European Union in its review of the Common Fisheries Policy; the World Wildlife Fund for Natures’ marine policy has developed the concept of Marine Protected Areas (MPAs); the OSPAR Convention for the Protection of the Marine Environment of the North East Atlantic is also working to promote networks of MPAs and Ecological Quality Objectives (EcoQOs). In the UK, the Department of Environment, Fisheries and Rural Affairs (DEFRA) major reports on Marine Stewardship,’ Safeguarding our Seas: A Strategy for the Conservation and Sustainable Development of our Marine Environment’ published in 2002 and followed by a consultation paper ‘ Seas of Change’, form the basis for developing a practical application of the ecosystem-based approach. The DEFRA Review of Marine Nature Conservation (RMNC) produced an interim report in 2001, which recommended that a pilot scheme at a regional scale to test a proposed framework for nature conservation. This has led to the Irish Sea Pilot Study managed by the Joint Nature Conservation Committee (JNCC), which has adopted the concept of ‘marine landscapes’, first developed in Canadian waters, based on geophysical features recognising that their importance in determining the nature of biological communities. As a result of these national and international initiatives, a number of habitat classification schemes have been introduced in different parts of the world. Since 2001, a group of geologists with interests in the application of geological data to habitat mapping have met each year to present their views and mapping programmes and to discuss their ideas with scientists from other disciplines, mainly biologists and oceanographers. The GeoHab (Geological Mapping of Habitats for Marine Resources and Management) group have provided the impetus for this review of BGS geological data in the context of habitat mapping classification schemes, and proposes ways in which our BGS data may be applied automatically, within a Geographical Information System, to the selection of relevant sources of information

The Shallow Geological Model: mapping and monitoring the marine landscape

Managing our seas without a sound knowledge of the sea-bed environment and the processes that shape the sea-bed characteristics makes it difficult to conserve key areas, maintain biodiversity and economically develop our marine resources in an efficient and sustainable way. Recent developments in marine landscape mapping, visualisation techniques and instant data access provide timely, proven technologies that are now in widespread use

Magnetic switching modes for exchange spring systems ErFe2/YFe2/DyFe2/YFe2 with competing anisotropies

The magnetization reversal processes of ½10nm ErFe2=nYFe2=4nm DyFe2=nYFe2" multilayer films with a (110) growth axis and a variable YFe2 layer thickness n are investigated. The magnetically soft YFe2 compound acts as a separator between the hard rare earth (RE) ErFe2 and DyFe2 compounds, each of them bearing different temperature dependent magnetic anisotropy properties. Magnetic measurements of a system with n ¼ 20nm reveal the existence of three switching modes: an independent switching mode at low temperatures, an ErFe2 spin flop switching mode at medium high temperatures, and an YFe2 dominated switching mode at high temperatures. The measurements are in qualitative agreement with the findings of micromagnetic simulations which are used to illustrate the switching modes. Further simulations for a varied YFe2 layer thickness n ranging from 2 to 40nm are carried out. Quantitative criteria are defined to classify the reversal behavior, and the resultant switching modes are laid out in a map with regard to n and the temperature T. A new coupled switching mode emerges above a threshold temperature for samples with thin YFe2 separation layers as a consequence of the exchange coupling between the magnetically hard ErFe2 and DyFe2 layers. It reflects the increasing competition of the two conflicting anisotropies to dominate the magnetic switching states of both RE compounds under decreasing n

Evidence of localised gas propagation pathways in a field-scale bentonite engineered barrier system: results from three gas injection tests in the large scale gas injection test (Lasgit)

Three gas injection tests have been conducted during a large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory, Sweden. Lasgit is a full-scale experiment based on the Swedish KBS-3 repository concept, examining the processes controlling gas and water flow in highly water-saturated compact buffer bentonite. Three preliminary gas injection tests have been performed. The first two tests were conducted in the lower array of injection filters (FL903). Both of these tests showed similar behaviour that corresponded with laboratory observations. The third gas test was conducted in an upper array filter (FU910), which gave a subtly dissimilar response at major gas entry with an initial pressure drop followed by a secondary gas peak pressure. Lasgit has confirmed the coupling between gas, stress and pore-water pressure for flow before and after major gas entry at the field scale. All observations suggest mechanisms of pathway propagation and dilatancy predominate. In all three gas tests the propagation was through localised features that tended to exploit the interface between the copper canister and the bentonite buffer. Considerable evidence exists for the development of a highly-dynamic, tortuous network of pressure induced pathways which evolves both temporally and geospatially within the clay, opening and closing probably due to local changes in gas pressure and or effective stress

Specht modules and semisimplicity criteria for Brauer and Birman--Murakami--Wenzl Algebras

A construction of bases for cell modules of the Birman--Murakami--Wenzl (or B--M--W) algebra $B_n(q,r)$ by lifting bases for cell modules of $B_{n-1}(q,r)$ is given. By iterating this procedure, we produce cellular bases for B--M--W algebras on which a large abelian subalgebra, generated by elements which generalise the Jucys--Murphy elements from the representation theory of the Iwahori--Hecke algebra of the symmetric group, acts triangularly. The triangular action of this abelian subalgebra is used to provide explicit criteria, in terms of the defining parameters $q$ and $r$, for B--M--W algebras to be semisimple. The aforementioned constructions provide generalisations, to the algebras under consideration here, of certain results from the Specht module theory of the Iwahori--Hecke algebra of the symmetric group