The equilibrium porosity of a porous material composed of a random packing of spherical grains isdependent upon the stability given to the rock by frictional and cohesive forces operating betweenindividual grains. These forces are proportional to the exposed surface area of the grains. The specificsurface area (exposed grain surface area per unit solid volume) is inversely proportional to grain size.This indicates that, when all other factors are equal, a given weight of coarse grains will be stabilizedat a lower porosity than the same weight of finer grains. For a sedimentary rock composed of a givensingle grain size this general rule is borne out in Figure 2.3. It can be seen that the increase in porosityonly becomes significant at grain sizes lower than 100 mm, and for some recent sediments porositiesup to 0.8 have been measured. As grain size increases past 100 mm, the frictional forces decrease andthe porosity decreases until a limit is reached that represents random frictionless packing, whichoccurs at 0.399 porosity, and is independent of grain size. No further loss of porosity is possible forrandomly packed spheres, unless the grains undergo irreversible deformation due to dissolution-recrystallisation, fracture, or plastic flow, and all such decreases in porosity are termed compaction.
Full Answer
Write an outline of 7 or se Permeability is the fluid transmissivity of soil, sedimento steps you would take in quantitatively measuring the rock-e, the case with which water moves through such porosity in a sample of, say, the course spheres. material Hint: For this mental exercise, you will need all of the items in the figure except the fine spheres, the fine Q14.6 (A) Write an outline of …
Q14.7 In conclusion, (A) how do grain sizes of coarser spheres and finer spheres affect porosities? (B) How do grain sizes affect permeabilities? Clear plastic tube Graduated glass cylinder 800 Figure 14.5 This is equipment that can be used to quantitatively measure porosity and qualitatively measure permeability in samples of spheres.
up to 0.8 have been measured. As grain size increases past 100 µm, the frictional forces decrease and the porosity decreases until a limit is reached that represents random frictionless packing, which occurs at 0.399 porosity, and is independent of grain size. No further loss of porosity is possible for
Grain size effect on properties Grain size has a measurable effect on most mechanical properties. For exam-ple, at room temperature, hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size. Machinability is also affected; rough machining favors coarse grain size while finish machining
The porosity of samples is inversely related to the grain size and decreases linearly as grain size increases. While a direct relationship was observed between grain size and dry bulk density, as bulk density increased with increasing median grain size.
Well-sorted materials have grains of the same size, while poorly sorted mate- rials have grains of many sizes. Permeability decreases as the degree of sort- ing varies from good to poor because small grains can fill the spaces between large grains. Permeability is also influenced by grain shape.
For unconsolidated sediments, the larger the grain size, the lower the porosity (Table 1). b. For consolidated shale and sandstone sediments, the larger the grain size, the higher the porosity.
In poorly sorted sediments, those with a larger range of grain sizes, the finer grains tend to fill the spaces between the larger grains, resulting in lower porosity. Primary porosity can range from less than one percent in crystalline rocks like granite to over 55% in some soils.
Because, in general, larger particles cannot pack together as tightly as smaller particles can, a rock made out of larger particles will usually be more porous than a rock made out of smaller particles.Mar 1, 2012
The porosity of a soil depends on several factors, including (1) packing density, (2) the breadth of the particle size distribution (polydisperse vs. monodisperse), (3) the shape of particles, and (4) cementing.
Grain size, shape, and packing are key parameters affecting porosity and permeability in unconsolidated clastic sediment.
How does grain shape affect porosity? Rocks containing rounded grains have a higher porosity than rocks containing angular grains that fit together.
Porosity varies depending on particle size and aggregation. It is greater in clayey and organic soils than in sandy soils. A large number of small particles in a volume of soil produces a large number of soil pores. Fewer large particles can occupy the same volume of soil so there are fewer pores and less porosity.
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials.
Primary porosity in clastic and some carbonate rocks (such as oolites) is a function of grain size, packing, shape, sorting, and amount of intergranular matrix and cement. In theory, porosity is independent of grain size. Changes in grain size, however, affect grain shape and sorting.Jan 20, 2022
major importance of a soil’s pore size distribution is that it relates to other soil prop-erties in a complex and useful way. It indicates complexity of structure in far more detail than porosity alone. The spatial variation of pore size is an important characteristic of the me-dium. The pore size distribution of different parts of soil is the fundamental basis for the concept of aggregates, for example. By some definitions, pore size can permit essential dis-tinctions between micropores and macropores (and mesopores, where that term is used for intermediate-size pores).
The characterization of pore space is a vital and fruitful aspect of soil investigation. Liquid, solid and gas constituents of the soil govern the form and development of pores, whose character in turn profoundly influences the na-ture and behavior of the soil.
Another significance is that within the pore space, the complement to θ is gas content . That is, volumetric gas content equals the dif-ference between θ and porosity.
Because pores are fluid conduits, their size distribution is useful for predicting hydraulic conductivity K, as well as for water retention as described above. Gas and other types of fluid transport can be treated, though water flow is the most common application. By analogy to laminar flow in tubes as quantified by Poiseuille’s law, the conductance of a single pore can be inferred to be propor-tional to the fourth power of its effective ra-dius. This makes its hydraulic conductivity proportional to the square of its effective ra-dius. An estimated f(r) distribution indicates the relative abundance of each conduit size, thus providing the information needed to pre-dict K.
The smallest pores measured by water retention or mercury intrusion are typically 50 or 100 times larger than this, though with greater cost and effort, greater magnitudes of pressure and hence smaller measured r can be achieved. As the imputed pore size approaches zero, in reality the water is likely to be held in thin films that coat parti-cle surfaces. The equivalent-capillary ap-proach associates this water not with film thicknesses but with effective radii of hypo-thetical filled pores. Because r relates in-versely to P, the whole dry portion of the re-tention curve is contained in a very small re-gion near r=0, where the apparent number of pores becomes large. In the extreme case, when θ is held to remain artificially finite at an assumed residual water content as P goes to negative infinity, f(r) at r=0 is a delta function. There is no real upper limit to pore size, though instability will cause the capillary hy-pothesis to break down for r of a few mm. The experimental limit is typically about 0.5 mm. At either pore size extreme, where capillary phenomena lose dominance, Equation (2) no longer applies, though it still can give r values corresponding to P. These values may be use-ful for translating one property to another (dis-cussed below) even though they are invalid in terms of the capillary analog.
The initial (pre-diagenesis) porosity is affected by three major microstructural parameters.These are grain size , grain packing, particle shape, and the distribution of grain sizes.However , the initial porosity is rarely that found in real rocks, as these have subsequently beenaffected by secondary controls on porosity such as compaction and geochemical diageneticprocesses. This section briefly reviews these controls.
Porosity is also controlled by a huge range of secondary processes that result in compactionand dilatation. These can be categorised into (i) mechanical processes, such as stresscompaction, plastic deformation, brittle deformation, fracture evolution etc., and (ii)geochemical processes, such as dissolution, repreciptation, volume reductions concomitantupon mineralogical changes etc. The effect of stress mediated compaction on porosity will bediscussed in section 5.4. The effect of chemical diagenesis is more complex, and is betterassessed for any given rock by examination of SEM or optical photomicrographs.
Total porosity is defined as the fraction of the bulk rock volume V that is not occupied bysolid matter. If the volume of solids is denoted by Vs, and the pore volume as Vp = V - Vs, wecan write the porosity as:
Real rocks contain a distribution of grain sizes, and often the grain size distribution is multi-modal. The best way of understanding the effect is to consider the variable admixture ofgrains of two sizes (Figure 5.3).