How to calculate permeability of soil

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# How to calculate permeability of soil

Water flows from the overhead tank consisting of three tubes — The inlet tube, the over-flow tube and the outlet tube. The constant hydraulic gradient i causing the flow is the head h i.

If the length of the sample is large, the head lost over a length of specimen is measured by inserting piezometric tubes. Here discharge is small. The water level in the stand pipe constantly falls as water flows. Observations are started after steady state of flow has reached. The head at any time instant t is equal to the difference in the water level in the stand pipe and the bottom tank. Let h be head at any intermediate time interval t, and — dh be the change in the head in a smaller time interval dt minus sign has been used since h decreases as t increases.

The laboratory observations consist of measurement of the heads h 1 and h 2 at two chosen time intervals t 1 and t 2. Compared to laboratory tests, field permeability test are more reliable. They give the in situ value of permeability with minimum disturbance. In the pumping out test, drawdowns, corresponding to a steady discharge q, are observed at a number of observation wells. The degree of disturbance and number of samples can affect the reliability of the average value of permeability obtained from laboratory tests, for the large soil mass in the field.

Two types of field tests for determining the coefficient of permeability are:. These tests can be conducted in both unconfined aquifer and confined aquifer.

Pumping in Tests :. Figure 7. If the hole extends below the groundwater level, it should be kept filled with water to minimize squeezing of soil into the bottom of the casing. The test is done by maintaining a constant head by adding clear water through a measuring device. The required data include the amount of head maintained during a constant rate of flow into the hole, diameter of the casing, and elevations of the top and bottom of the casing.

Permeability is computed from the following relation —.Permeability is a measure of the ability of fluid to flow through the pores or interstices of a material. Permeability explains the capacity of materials to store and transmit fluids.

An important application is in the study of the flow characteristics of hydrocarbons in gas and petroleum reservoirs. Permeability depends on density, viscosity of the fluid and also on the properties of the medium. The property described here is strictly the intrinsic permeability which only depends on the properties of the medium. Hydraulic conductivity can also be expressed as permeability. The SI unit for permeability is the square metre m 2although the darcy, the c. Permeability measures are different for different rock materials.

## PERMEABILITY TEST OF SOIL – CONSTANT HEAD METHOD

The main factor which affects the permeability is the size of the grain. The permeability value is high for rocks which have more porous sandstone and whose grain size is coarse. Rocks which have small grain size have small permeability, for example, crystalline limestone, clay and metamorphic rocks.

Rocks with a permeability greater than mD millidarcies - a unit of permeability widely used in petroleum engineering can change into exploitable hydrocarbon reservoirs whereas rocks with a permeability lower than mD tend to form seals. Soil permeability is influenced by many factors including void ratio, temperature, the size and shape of the soil particles, and the degree of saturation. Darcy's law, which was derived in the 19th century by French engineer Henry Darcy, can be applied in finding the permeability of different materials.

It is a result of applying the Navier-Stokes equations, although it was originally deduced by experimentation and the theoretical analysis was arrived at later. Darcy's law gives the flow rate in terms of a pressure difference, fluid viscosity, length and area.

The permeability is the constant of proportionality in Darcy's law, and so by measurement of the other factors the permeability for a material can be derived. Permeability can also be computed, or approximated, by characterization of the pores within the material as a series of equivalent tubes. The permeability can be defined in terms of the product of the square of the effective pore diameter and a dimensionless constant which represents the shape and structure of the flow paths. Bookmark this page in your browser using Ctrl and d or using one of these services: opens in new window Delicious Digg Facebook Reddit What are these?Permeability of coarse-grained soils having high permeability is determined in the laboratory by the constant head permeability test.

Figure 9. The equipment for the test, known as permeameter, consists of a cylindrical mold, a drainage cap with an inlet valve and an air release valve, and a drainage base plate with an outlet pipe. The mold is 10 cm in diameter and The base plate has a recess at the center in which the bottom porous disc is placed.

A filter paper is placed over the bottom porous disc. The soil is compacted into the permeameter mold at the required density and water content. Alternately, an undisturbed soil sample from the soil sampler is cut into the permeameter mold, taking care to avoid leakage through the side walls. A filter paper is placed over the top surface of the soil specimen and the top porous disc is placed on the filter paper.

The porous disc is specially manufactured with sand and cement, with voids to permit the flow of water.

A constant head water tank is connected to the drainage cap of the permeameter. The overflow pipe of the water tank ensures constant water level in the tank so that constant head is maintained for water flowing through the soil specimen. The permeameter mold is connected to a bottom water tank with an overflow pipe.

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The difference between the water levels in the constant head water tank and the bottom tank is the head causing the flow of water through the soil specimen. To ensure complete saturation of the soil specimen, the air in the soil specimen is removed by the following methods:. Allowing water to flow upward by attaching the constant head reservoir to the drainage base for sufficient time and opening the air release valve. Applying a vacuum pressure of about 70 cm of mercury through the drainage cap for about 15 min after closing the drainage valve.

After the soil sample has been completely saturated, the constant head reservoirs are connected to the drainage cap of the permeameter mold. Water is allowed to flow through the soil sample for sufficient time till a steady state is established. When the steady state is established, water is collected from the bottom tank in a graduated measuring jar for a convenient time period, which is measured using a stopwatch.

Substituting these in Eq. The constant head permeability test is suitable only for coarse-grained soils, for which a significant volume of water can be collected in a reasonable time interval.

For soils of low permeability, the quantity of water collected in the graduated jar of the constant head permeability test is very small and cannot be measured accurately. For such soils, the variable head permeability test is used. As per Lambethe variable head permeability test is also more convenient for cohesionless soils than the constant head test because of simpler instrumentation.

The permeameter used in the falling head test is the same as that used for the constant head test, having a cm diameter, A vertical graduated standpipe of known cross-sectional area a is fitted to the top of the permeameter.

The permeameter mold has a drainage base with a recess for a porous disc and a drainage cap with inlet and air release valves, as shown in Fig.

After placing the bottom porous disc, the soil sample is compacted into the permeameter mold and the top porous disc is placed on the compacted soil. The purpose of using the bottom porous disc is to prevent washing and escape of soil particles during downward flow, while the top porous disc helps in the distribution of inlet water over the entire cross-sectional area of the soil sample. The porous disc and water tubes should be de-aired before placing the sample. Before conducting the test, removal of entrapped air and full saturation of soil sample must be done.

Porosity and Permeability Lab

This is achieved by applying vacuum through the drainage cap, after closing the drainage valve in the drainage base and the air release valve in the drainage cap.

The vacuum pressure is slowly increased to 70 cm of mercury and maintained for about 15 min. The soil sample is saturated by allowing the de-aired water to flow upward from the drainage base under vacuum.

### Soil permeability and how to measure it

When the soil sample is saturated, both the top and the bottom outlets are closed. The standpipe is filled with water to the required height. After a steady state of flow has been established, the time required for the water level in the standpipe to fall from height h 1 to height h 2 is noted. The head is measured with reference to the water level in the bottom tank.

If the discharge through the soil sample is q, which is constant, then —.Soil permeability measurements determine how well water flows through soil. Large pores in sand or granular soil allow water to move rapidly, while small pores in silt or clay cause water to seep through slowly. The main tests to measure soil permeability are the constant head, the falling head and the percolation test.

Homeowners may need a permeability test for building, landscape or major gardening projects. You can easily do a percolation test yourself, but first check whether local laws require you to hire a professional. Soil permeability measurements help determine the rate of soil settling, which you need to know before constructing buildings or determining how much water will flow toward an excavation. Good drainage from high soil permeability is needed for installation of a septic system.

Low permeability, found in clay soils, works well for placement of ponds, such as a fish pond. Soil permeability measurements also help determine the stability of slopes and earth dams. Growing vegetables requires good drainage; a permeability test can indicate whether your soil is suitable or needs to be amended before planting.

## Flow of Water Through Soil – Permeability and Factors Affecting Permeability

The constant head test is a laboratory test done on sandy or granular soil samples. Under constant pressure, a piston forces water through a column of water-saturated soil to determine the flow rate of water.

The water in the test is de-aired and kept at constant temperature. The test apparatus has a water reservoir on top and an outlet reservoir on the bottom. The permeability of the soil sample is calculated from the height of the soil sample, the sample's cross section, pressure measurements, the volume of passed water and the time interval. The falling head permeability test is for low permeability soils, such as silts and clays. A relatively small soil sample is used, because water flow will be slow.

After tamping down the sample and saturating it with water, a standpipe is connected to the container holding the soil. The standpipe is filled with water, and the initial water level is measured. The decline in water level in the standpipe is measured again after the water flows through the sample in a specified time.

The permeability of the soil sample is calculated from the size of the soil sample, the cross section of the standpipe, the drop in water level and the time taken.Permeability is one of the important physical properties of soil as some of the major problems of soil mechanics are directly connected with it. Design of highways, airports, earth dams, construction of foundation below water — table, yield from a well, settlement of foundation etc.

Hence to become a good soil engineer the knowledge of permeability is very essential. A material is said to be permeable if it contains continuous voids. Since such voids are contained in all soils including the stiffest clay, all these are permeable. Gravels are highly permeable and stiff clay is the least permeable soil.

Permeability is the property of the soil which allows water to pass through its interconnecting voids. The flow in which all the particles of water move in parallel paths without crossing the path of other particles.

The loos of hydaulic head per unit distance of flow is called hydraulic gradient. The velocity head for flow through soil is negligible. Any elevation can be selected for datum, as the base of elevation heads.

The advantages of choosing the downstream water level as the datum is that the total head at the exits becomes zero and elevation of water in a piezometer at any point in soil measured above the datum line gives directly the hydraulic head.

The loss of head per unit distance of flow or along the length of flow is called hydraulic gradient. In the mid-eighteenth century H. Darcy working in Paris studied experimentally the flow of water through soil.

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It is applied to soil fraction finer than fine gravels. It is an apparent velocity being equal to average rate of flow across a unit gross area in the soil. A be the gross area of soil perpendicular to the direction of flow.

The unit of K is similar to that of velocity i. The empirical relationship between K and D 10 developed by Hazen for loose, clean sand is. Permeability can be obtained from the theoretical equation of Kozeny-Carman for flow through porous medium. From equation 4.

The coefficient of permeability is directly proportional to density of water and inversely proportional to its viscosity.

The value of density of water does not change much with the change in temperature but there is a great variation in viscosity. The viscosity decreases with increase in temperature and therefore permeability increase with increase in temperature.

Permeability of soil is directly proportional to the square of the particle size as shown in equation 4. This is most significant factor affecting permeability of soil as they decide the void ratio, size and shape of pores in a soil mass.

A coarse soil has larger pore sizes and here greater K i. The marked influence of void ratio on permeability of soil as shown in the equation 4.

From the above equation it is clear that K is directly proportional to the void ratio i. A semi-log relationship also exists between K and e. The structural arrangement of the soil particles vary, at the same void ratio, depending upon the method of compaction of soil mass.

The permeability of disturbed sample may be different from that of the undisturbed sample at the same void ratio. The effect of structural disturbance on permeability is much pronounced in fine grained soils. The permeability of soil is observed to vary directly with the cube of the degree of saturation. Thus the more the saturated soil, more will be the permeability.

However the pressure of entrapped air in soil pores obstructs the flow of water. Fine particles of clay are surrounded by films of adsorbed water. Forces of adsorption and development of diffuse ion-layer around the clay particles create immobilized hydrodynamic layers of water, thereby reducing the effective pore space available for seepage.The purpose of this test is to determine the permeability hydraulic conductivity of a sandy soil by the constant head test method.

There are two general types of permeability test methods that are routinely performed in the laboratory:. Permeability or hydraulic conductivity refers to the ease with which water can flow through a soil.

This property is necessary for the calculation of seepage through earth dams or under sheet pile walls, the calculation of the seepage rate from waste storage facilities landfills, ponds, etc.

Measure the inside diameter of upper and lower chambers.

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Calculate the average inside diameter of the permeameter D. Enough water should be added so that the mixture may flow freely see Photo B.

A uniform layer should be formed. Use approximately ten rams of the tamper per layer and provide uniform coverage of the soil surface.

Repeat the compaction procedure until the soil is within 2 cm. Be careful not to disturb the soil that has already been compacted. Continue the placement operation until the level of the soil is about 2 cm below the rim of the upper chamber. Level the top surface of the soil and place a filter paper and then the upper porous stone on it see Photo E.

Secure the cap firmly with the cap nuts see Photo F. Record it as the sample length. Connect the funnel tubing to the top side port see Photo H. Repeat this process three times and compute the average time, average volume, and average temperature. Record the values as tQand Trespectively see Photo I. As temperature increases viscosity decreases and the permeability increases. From Table 1 obtain the viscosities and compute K Geotechnical-Material Engineer.

Save my name, email, and website in this browser for the next time I comment. Notify me of follow-up comments by email. Notify me of new posts by email. Constant Head Permeability Test of Sandy Soil Purpose: The purpose of this test is to determine the permeability hydraulic conductivity of a sandy soil by the constant head test method.

There are two general types of permeability test methods that are routinely performed in the laboratory: The constant head test method, and 2 the falling head test method.

Test Procedure: 1 Measure the initial mass of the pan along with the dry soil M 1. Tags: Soil Test. Ankit Singh September 30, Reply.The permeability of a soil is the ability of water to move through it permeate it. It depends on the physical and chemical properties of the soil, notably particle size distribution the range of particle sizes presentpore space, pore size and the continuity of the spaces. The formal name is hydraulic conductivitywhich refers to the ability of a soil to conduct water.

Hydraulic conductivity is a complex feature of soils, varying with location, soil type, depth, soil moisture content and direction of flow; for example, horizontal conductivity is often greater than vertical on account of soil horizons.

Surface infiltration rate. This is the rate at which a soil surface will take in irrigation or rainfall. It is the number an irrigation farmer or sports field manager wants to know.

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It is influenced by plant cover, the initial moisture content, and the texture and structure of the soil. Surface compaction, slaking, dispersion and crusting impede surface infiltration.

Typically the surface infiltration rate starts off faster in a dry soil and slows down as the soil swells and the cracks close. Saturated hydraulic conductivity K sat. This is the rate at which a soil that is already saturated with water will conduct water away from the source.

It is necessary for predicting such things as whether a purchased soil will have good drainage, whether a dam will hold water, how quickly a subsoil will allow ponded or perched water to drain away, and whether a landfill liner will leak contaminants into the surrounding ground water.

Obtaining accurate measurements of hydraulic conductivity is very difficult, mainly because of the wide variability of soils and the presence or absence of pores, cracks, worm holes etc.

In particular, measurements depend greatly on the size of the sample tested, as hydraulic conductivity usually increases as the sample size decreases.

In addition, disturbance of soil either to collect a sample or even just to measure it in situ will increase the measured value way above the real value. So several methods have been devised to estimate hydraulic conductivity. SESL offers several approaches to both estimating and measuring K.

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Particle size analysis can be used to predict the hydraulic conductivity of unstructured sands and sandy loams. This relies on equations that take values measured in the lab. It is quite a good predictor for loamy sands on sports fields, for example. On the other hand, it is not suitable for well structured soils, especially clays, as structure can make a clay act more like a gravel.

Permeability class can be estimated from the texture and degree of structure. Single infiltration ring. A metal ring at least 30 cm across is driven about 5 cm into the soil surface, and water is poured into it.

The time it takes for the water to soak in is timed.