Equivalent Pore Diameter Research Articles

The influence of pore-water tension on the strength of clay

Substantial pore-water tensions are set up by the removal, under undrained conditions, of the confining pressure from saturated clay samples from which the pore water has been free to drain during consolidation. These pore-water tensions control the mechanical behaviour of the unconfined samples, and in particular their strength and brittleness. In the present paper the magnitude of the pore-water tensions and the change in strength on stress release are estimated theoretically for the range of pressures within which the sample remains fully saturated. Tests on samples of two clays consolidated in the triaxial apparatus under a wide range of pressure indicate that the limiting porewater tension above which the sample ceases to remain fully saturated is related to the equivalent pore diameter. Above this limit the loss in strength on stress release is very marked The experimental results also show the dramatic change in brittleness resulting from high pore-water tensions. A large reduction in total stress without a change in water content is sufficient to change the failure mechanism from a plastic failure with some strain-softening to a brittle failure in which the sample shatters. The significance of the results in relation to sampling and testing of soils for engineering purposes is briefly considered.

Pore Size Distributions in Clays

AbstractA knowledge of the distribution of pore sizes in clay and soil bodies is a useful element in the microstructural characterization of such materials. Pore-size distributions and total porosity of a number of reference clays, naturally-occurring subsoils, and commercial clay samples prepared in various ways were determined by mercury porosimetry. The range of equivalent pore diameter explored covered almost five orders of magnitude, from several hundred microns down to approximately 150 A. The method and its assumptions are critically evaluated, and measurements of the contact angle of mercury on clays yield values of 139° for montmorillonite and 147° for kaolinite and illite clays. The extent of shrinkage on oven-drying prior to mercury intrusion is assessed in each case and found to vary from insignificant to as much as 30 per cent of the pore space, depending on microstructural state and degree of initial saturation. The development of techniques for water removal which do not involve change in pore structure is explored. Some preliminary results for structurally weak saturated clays suggest that critical-region drying and perhaps freeze-drying procedures may be practical.

Flow of gases through channels with reference to porous materials

Knowledge of the gas flow in pumping lines, in internal channels, and in porous materials undergoing vacuum processes is basic to the design and operation of a vacuum system. The gas flow equations for viscous and molecular flow in cylindrical channels are ideally simple, in slip flow they are not so easy, but in porous materials in all the flow regions they are more complicated. The permeability of a porous material has been obtained directly by measuring with a flowmeter the amount of gas flowing through it under an absolute pressure gradient, and the same has been done with model materials. Correlation of the permeability and mean absolute pressure provides basic information on structural factors of the material such as porosity, tortuosity factor, and equivalent pore diameter which could, for example, be applied to vacuum and freeze-drying processes.

Flow of Adsorbed Molecules

JUST as Knudsen and others1 have shown for single capillaries, flow of a gas through a porous medium also undergoes a transition from viscous flow to molecular flow when pore diameters become comparable in size with the mean free path of the gaseous molecules. The position has been reviewed recently2, but more data in the molecular flow region are desirable. I have therefore been studying the flow of various gases through plugs of 'Linde silica', a finely divided, amorphous silica stated by the makers to have a maximum particle size of 0.05 µ. The experiments cited here were carried out on a single plug compressed to a porosity (e) of 0.678. The specific surface, S0, calculated from the permeability equation of Carman and Arnell2, was 2.35 × 106 cm.2/cm.3. This gives an equivalent pore diameter which was usually well below the mean free path of the various gases studied. Application of the Brunauer—Emmett—Teller calculation3 to gas adsorption isotherms gave S0 = 2.8 × 106 to 4.1 × 106 cm.2/cm.3, indicating that particles possibly possess an internal surface which is not accessible to flow. To cover a wide range of molecular weights, flow-rates were measured at 22° C. for hydrogen, air, carbon dioxide and 'Freon-12', CF2Cl2, all of which gave consistent values for S0.