Hydraulic conductivity, symbolically represented as K, is a property of vascular plants and rocks, that describes the ease with which a fluid can move through pore spaces or fractures. It depends on the intrinsic permeability of the material, the degree of saturation, on the density and viscosity of the fluid. Saturated hydraulic conductivity, describes water movement through saturated media. By definition, hydraulic conductivity is the ratio of velocity to hydraulic gradient indicating permeability of porous media. There are two broad categories of determining hydraulic conductivity: Empirical approach by which the hydraulic conductivity is correlated to soil properties like pore size and particle size distributions, soil texture Experimental approach by which the hydraulic conductivity is determined from hydraulic experiments using Darcy's lawThe experimental approach is broadly classified into: Laboratory tests using soil samples subjected to hydraulic experiments Field tests that are differentiated into: small scale field tests, using observations of the water level in cavities in the soil large scale field tests, like pump tests in wells or by observing the functioning of existing horizontal drainage systems.
The small scale field tests are further subdivided into: infiltration tests in cavities above the water table slug tests in cavities below the water tableThe methods of determination of hydraulic conductivity and other related issues are investigated by several researchers. Allen Hazen derived an empirical formula for approximating hydraulic conductivity from grain size analyses: K = C 2 where C Hazen's empirical coefficient, which takes a value between 0.0 and 1.5, with an average value of 1.0. A. F. Salarashayeri & M. Siosemarde give C as taken between 1.0 and 1.5, with D in mm and K in cm/s. D 10 is the diameter of the 10 percentile grain size of the material A pedotransfer function is a specialized empirical estimation method, used in the soil sciences, however has increasing use in hydrogeology. There are many different PTF methods, they all attempt to determine soil properties, such as hydraulic conductivity, given several measured soil properties, such as soil particle size, bulk density.
There are simple and inexpensive laboratory tests that may be run to determine the hydraulic conductivity of a soil: constant-head method and falling-head method. The constant-head method is used on granular soil; this procedure allows water to move through the soil under a steady state head condition while the quantity of water flowing through the soil specimen is measured over a period of time. By knowing the quantity Q of water measured, length L of specimen, cross-sectional area A of the specimen, time t required for the quantity of water Q to be discharged, head h, the hydraulic conductivity can be calculated: Q t = A v where v is the flow velocity. Using Darcy's law: v = K i and expressing the hydraulic gradient i as: i = h L where h is the difference of hydraulic head over distance L, yields: Q t = A K h L Solving for K gives: K = Q L A h t In the falling-head method, the soil sample is first saturated under a specific head condition; the water is allowed to flow through the soil without adding any water, so the pressure head declines as water passes through the specimen.
The advantage to the falling-head method is that it can be used for both fine-grained and coarse-grained soils. Calculating the hydraulic conductivity is more complicated because of the changing pressure head, requires solving a differential equation. In compare to laboratory method, field methods gives the most reliable information about the permeability of soil with minimum disturbances. In laboratory methods, the degree of disturbances affect the reliability of value of permeability of the soil. Pumping test is the most reliable method to calculate the coefficient of permeability of a soil; this test is further classified into pumping out test. There are in-situ methods for measuring the hydraulic conductivity in the field; when the water table is shallow, the augerhole method, a slug test, can be used for determini
Permeability (spatial and transport planning)
Permeability or connectivity describes the extent to which urban forms permit movement of people or vehicles in different directions. The terms are used interchangeably, although differentiated definitions exist. Permeability is considered a positive attribute of an urban design, as it permits ease of movement and avoids severing neighbourhoods. Urban forms which lack permeability, e.g. those severed by arterial roads, or with many long culs-de-sac, are considered to discourage movement on foot and encourage longer journeys by car. There is some empirical research evidence to support this view. Permeability is a central principle of New Urbanism, which favours urban designs based upon the ‘traditional’ street grid. New Urbanist thinking has influenced Government policy in the United Kingdom, where the Department for Transport Guidance Manual for Streets says: Street networks should in general be connected. Connected or ‘permeable’ networks encourage walking and cycling and make places easier to navigate through.
There are two principal reservations concerning permeability. The first relates to property crime. Although the issue is contested, there is some research evidence to suggest that permeability may be positively correlated with crimes such as burglary. New research has expanded the discussion on this disputed issue. A recent study did extensive spatial analysis and correlated several building, site plan and social factors with crime frequencies and identified nuances to the contrasting positions; the study looked at, among others, a) dwelling types, b) unit density c) movement on the street, d) culs–de-sac or grids and e) the permeability of a residential area. Among its conclusions are that a) flats are always safer than houses and the wealth of inhabitants matters, it re-established that simple, linear culs-de-sac with good numbers of dwellings that are joined to through streets tend to be safe. As for permeability, it suggests that residential areas should be permeable enough to allow movement in all directions but no more.
The over-provision of poorly used permeability is a crime hazard. The second reservation concerns the effects of permeability for private motor vehicles. Melia proposed the terms "unfiltered permeability" and "filtered permeability" to distinguish between the two approaches. Unfiltered permeability is the view supported by the New Urbanists that urban designs should follow "traditional" or mixed use streets, where pedestrians and motor vehicles follow the same routes; the principal advantage claimed for this approach is that it "leads to a more spread of motor traffic throughout the area and so avoids the need for distributor roads". There are a range of arguments advanced by the proponents of Shared space that where speeds are low, road users should be mixed rather than segregated. Filtered permeability is the concept, supported by organisations such as Sustrans, that networks for walking and cycling should be more permeable than the road network for motor vehicles. This, it is argued will encourage walking and cycling by giving them a more attractive environment free from traffic and a time and convenience advantage over car driving.
Evidence for this view comes from European cities such as Freiburg, its rail suburb Vauban, Groningen which have achieved high levels of walking and cycling by following similar principles, sometimes described as: "a coarse grain for cars and a fine grain for cyclists and pedestrians". Filtered permeability requires cyclists, pedestrians to be separated from private motor vehicles in some places, although it can be combined with shared space solutions, elsewhere in the same town or city; this is the case in Dutch towns such as Drachten. The principle of filtered permeability was endorsed for the first time in British Government guidance for the eco-towns programme in 2008 and that year by an alliance of 70 organisations concerned with public health and transport in their policy declaration: Take Action on Active Travel. A parallel debate has been occurring in North America, where researchers have proposed and applied the Fused Grid, an urban street network pattern which follows the principles of filtered permeability, to address perceived shortcomings of both the'traditional' grid and more recent suburban street layouts.
A study conducted in Washington State found that the fused grid was associated with higher levels of walking than the other two alternatives. A recent comparison of seven neighbourhood layouts found a 43 and 32 percent increase in walking with respect to a conventional suburban and the traditional grid in a Fused Grid layout, which has greater permeability for pedestrians than for cars due to its inclusion of pedestrian-only paths, it showed a 7 to 10 percent range of reduction in driving with respect to the remainder six neighbourhood layouts in the set. Stephen Marshall has sought to differentiate the concepts of "connectivity" and "permeability"; as defined by Marshall, "connectivity" refers to the number of connections to and from a particular place, whereas "permeability" refers to the capacity of those connections to carry people or vehicles. Traffic studies on the influence of street patterns on travel overlook this distinction and two metrics are instead used to characterize a street pattern for trip purposes: connectivity and intersection density, both of which
Permeability (foundry sand)
Permeability is a property of foundry sand with respect to how well the sand can vent, i.e. how well gases pass through the sand. And in other words, permeability is the property by which we can know the ability of material to transmit fluid/gases; the permeability is tested to see if it is correct for the casting conditions. The grain size and distribution of the foundry sand, the type and quantity of bonding materials, the density to which the sand is rammed, the percentage of moisture used for tempering the sand are important factors in regulating the degree of permeability. An increase in permeability indicates a more open structure in the rammed sand, if the increase continues, it will lead to penetration-type defects and rough castings. A decrease in permeability could lead to blows and pinholes. On a prepared mould surface as a sample, permeability can be checked with use of a mould permeability attachment to permeability meter, readings such obtained are of relative permeability, not absolute permeability.
The relative permeability reading on a mould surface is only used to gauge sample-to-sample variation. On standard specimen as a sample For sands that can be compressed, e.g.: bentonite-bonded sand known as green sand, a compressed or rammed sample is used to check permeability. For sand that cannot be compressed, e.g.: Resin-coated sands, a filled sample is used. To check such a sample, user may have to use an attachment to the permeability meter called a core permeability tube; the absolute permeability number, which has no units, is determined by the rate of flow of air, under standard pressure, through a rammed cylindrical specimen. DIN standards define the specimen dimensions to be 50 mm in diameter and 50 mm tall, while the American Foundry Society defines it to be two inches in diameter and two inches tall. Rammed cylindrical specimen. Formula is PN = /PxAxT where V = volume of air in ml passing through the specimen H = Height of the specimen in cm A = Cross sectional area of specimen in cm2 P = Pressure of air in cm of water T = Time in minutesAmerican Foundry Society has released a chart where back pressure from a rammed specimen placed on a permeability meter is correlated with a Permeability number.
The Permeability number so measured is used in foundries for recording permeability value