ALBERT

Description: We provide detailed observations on the shape of dikes from well-exposed field locations in the Isle of Rum, Scotland, and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the Helam kimberlite dikes and have a smaller length to thickness ratio (∼100:1 Isle of Rum, ∼1000:1 Helam Mine). We compare the dike thickness field measurements with the geometry predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic theory, being too thick at the dike ends and too narrow in the middle. Even for dikes where the model fit is acceptable, the calculated overpressures and stress gradients are very large and much larger than independent estimates based on rock strength, particularly for the small-scale basaltic dikes on Rum, where calculated overpressures average 687 MPa, and calculated stress gradients average 622 MPa m−1. The Swartruggens dikes have calculated overpressures of between 4 and 40 MPa and calculated stress gradients in the range of 15–87 kPa m−1. Dike shape can be explained by a combination of host-rock inelastic deformation prior to and coeval with magma emplacement, and by magma chilling at the dike's tapering edges, which prevented its closure as magma pressure declined during emplacement; this sequence provides the most complete explanation for the mismatches between the data and the model. The permanent wedging of the dike edges due to chilling has implications for crustal magma transport and strain response in the crust due to dike emplacement.

Description: This paper presents fundamental analysis of the injection and release of fluid into porous media or geological reservoirs saturated by a different fluid undergoing a background flow, and tests the predictions using analogue laboratory experiments. The study reveals new results important for an understanding of the transport of hazardous contaminants through aquifers and the long-term fate of carbon dioxide in geological sequestration. Using numerical and asymptotic analysis, we describe a variety of flow regimes that arise, and demonstrate an almost instantaneous control of injected fluid by the far field conditions in geological reservoirs. For a continuous input, the flow develops a horizontal interface between the injected and ambient fluids. The background flow thereby effectively caps the height of the injected fluid into a shallower region of vertical confinement. For a released parcel of fluid, gravitational spreading is found to become negligible after a short time. A dominant control of the interface by the background pressure gradient arises, and stems from the different velocities at which it drives the injected and ambient fluids individually. Similarity solutions describing these dynamics show that the parcel approaches a slender triangular profile that grows horizontally as , where is time, a rate faster than relaxation under gravity. Shock layers develop at the front or back of the parcel, depending on whether it is more or less viscous than the ambient fluid. New analytical results describing the long-term effects of residual trapping due to capillary retention are developed, which yield explicit predictions for the time and length scales on which a parcel of becomes retained. We end by applying our results to geological contexts, concluding that even slight background motion can have considerable implications for long-term transport through the subsurface. © 2017 Cambridge University Press.

Description: The limb kinematics used for stepping or leaping over an obstacle are determined primarily by visual sensing of obstacle position and geometry. In this study, we demonstrate that changes are induced in limb kinematics even when obstacle geometry is manipulated in a way that does not introduce a mechanical requirement for a change of limb trajectory nor increase risk of collision. Human participants performed a running leap over a single raised obstacle bar. Kinematic changes were measured when an identical second bar was introduced at a ground level underneath the obstacle and displaced by a functionally insignificant distance along the axis of travel. The presence or absence of a baseline directly beneath the highest extremity had no significant effect on limb kinematics. However, displacing the baseline horizontally induced a horizontal translation of limb trajectory in the direction of the displacement. These results show that systematic changes to limb trajectories can occur in the absence of a change in sensed mechanical constraints or optimization. The nature of visuomotor control of human leaping may involve a continuous mapping of sensory input to kinematic output rather than one responsive only to information perceived to be mechanically relevant.

Description: The removal of potentially harmful radioactive waste from the anthroposphere will require disposal in geological repositories, the designs of which often favour the inclusion of a clay backfill or engineered barrier around the waste. Bentonite is often proposed as this engineered barrier and understanding its long-term performance and behaviour is vital in establishing the safety case for its usage. There are many different compositions of bentonite that exist and much research has focussed on the properties and behaviour of both sodium (Na) and calcium (Ca) bentonites. This study focusses on the results of a swelling test on Bulgarian Ca bentonite that showed an unusual gel formation at the expanding front, unobserved in previous tests of this type using the sodium bentonite MX80. The Bulgarian Ca bentonite was able to swell to completely fill an internal void space over the duration of the test, with a thin gel layer present on one end of the sample. The properties of the gel, along with the rest of the bulk sample, have been investigated using ESEM, EXDA and XRD analyses and the formation mechanism has been attributed to the migration of nanoparticulate smectite through a more silica-rich matrix of the bentonite substrate. The migration of smectite clay out of the bulk of the sample has important implications for bentonite erosion where this engineered barrier interacts with flowing groundwater in repository host rocks.