Changes In Pore Space Research Articles

Model for erosion-deposition patterns

We investigate through computational simulations with a pore network model the formation of patterns caused by erosion-deposition mechanisms. In this model, the geometry of the pore space changes dynamically as a consequence of the coupling between the fluid flow and the movement of particles due to local drag forces. Our results for this irreversible process show that the model is able to reproduce typical natural patterns caused by well-known erosion processes. Moreover, we observe that, within a certain range of porosity values, the grains form clusters that are tilted with respect to the horizontal with a characteristic angle. We compare our results to recent experiments for granular material in flowing water and show that they present a satisfactory agreement.

A Time Domain Reflectometry Coaxial Cell for Manipulation and Monitoring of Water Content and Electrical Conductivity in Variably Saturated Porous Media

Time domain reflectometry (TDR) is an effective and accurate method for determination of porous media dielectric permittivity (i.e., for soil water content) and electrical conductivity (EC). Characterization of these properties using controlled wetting or drying of the porous media is of interest for studying and quantifying porous media physical and chemical properties. Our objectives were to develop a TDR measurement cell providing sample water content adjustment and solution exchange capability without repacking or disturbing the sample. A coaxial cell provides a well‐defined sample packing volume in which radial distribution of the electromagnetic field averages the transmission line measurements over the entire sample. Using concentric porous stainless steel tubes as electrical conductors, complete or partial saturation of the porous medium (via matric potential control) was possible with the additional key advantage of soil solution exchange without changes in pore space or sample porosity. Solution exchange and suction adjustment were controlled through the center tube with influent solution (ECw) entering the sample through the outer wall and for unsaturated measurements an air inlet port facilitates air exchange. The cell was temperature controlled using an external PVC jacket connected to a circulating water bath. Both saturated and unsaturated measurements of electrical conductivity resulted in excellent agreement between measurements and model predictions. Permittivity measurements were dependent on cell orientation, likely due to differences in the water distribution within the cell. The utility of this new coaxial TDR pressure cell design for porous media research is the ability to simultaneously measure dielectric and ECb in a fixed pore space where water content and solution electrical conductivity are adjustable.

Mass transfer controlled by fracturing in micritic carbonate rocks

The fractured Coniacian chalk from the Omey area (Paris Basin, France) displays strong evidence of modifications controlled by brittle deformation. Fracturing is associated with important changes in pore space (decrease in total porosity and pore interconnection, change in distribution of pore access diameters and capillary characteristics), nannofacies (gradual evolution from a point-contact fabric to a welded, interlocked or coalescent fabric) and chemical composition (Sr concentration decrease). These modifications result from fluid–rock interaction that control significant mass transfer (percentage of secondary calcite >50%). Sr is a remarkable indicator of these mass transfers. Sr analyses allowed us to prove that the deformed zone (26.7 m) is wider than the fractured zone (11.3 m). They also indicate that the footwall block is less affected than the hanging wall block. A physicochemical model of the deformation mechanism is proposed. It shows that a cyclic process of fracturing controls the temporal evolution of the fluid saturation and fluid pressure and, consequently, the mass transfer.

Dynamics and characteristics of pore space changes during the crumbling on drying of structured agricultural soils

An investigation was carried out to examine the characteristics and dynamics of pore space changes, during drying of a structured agricultural soil under capillary bonding stresses. The procedure involved the preparation, in batches, of several identical cylindrical soil samples. Having subjected each batch to selected sample treatments, the samples were dried to different moisture contents and a tensile strength test performed on each one. The antecedence of freezing was found to have considerable effects on pore sizes, swelling and shrinkage characteristics and moisture retention in inter-aggregate pores. Pore volume relationships are proposed. Using these relationships, a preliminary attempt has been made to gain further insight into the dynamics of pore space changes in the soil water suction range 0–100 mbar.

Pore pressure in heated concrete walls: theoretical prediction

Synopsis Pore-water pressures in concrete can be calculated by a previously developed theory which is based on thermodynamic properties of water and takes into account the huge changes in permeability and sorption isotherm with temperature, as well as the changes of pore space due to temperature and pressure. After reviewing the theory, finite-element solutions are com-pared with weight-loss tests of Chapman and England, and theoretical predictions are made for rapid heating of thick walls, either sealed or unsealed. A two-dimensional axisymmetric finite-element solution is developed to analyse theffect of a hot spot on the wall. The pore-pressure peaks are found to be much higher than for slow heating (25 atm versus 8 atm), and still about 50% higher when the heating is confined to a hot spot. The moisture movement in regions where the pressure gradient is opposite to the temperature gradient is found to be rather irregular and to exhibit oscillations. The theory predicts the phenomenon of ‘moisture clog’ suggested by Harmathy on the basis of tests.

The effects of changes in soil compaction and porosity on germination, establishment and yield of barley and globe beet

Two degrees of compaction, 'heavy' with a flattyred vehicle wheel, and 'light' with a ring roller, were given to the seed beds after sowing barley and globe beet on three contrasted soils. Changes in pore space of the soils and the responses of the crop to the changed physical properties were measured. The sites used were: Barnfield at Rothamsted, a heavy clay loam long under arable cultivation and having little organic matter; Pastures Field at Rothamsted, a field of silty clay loam ploughed after an 8-year ley; Stackyard field at Woburn, a light sandy loam long in arable cultivation and containing little organic matter.