Abstract:

This report discusses the proposed design for determining the permeability (hydraulic conductivity) of granular soils by constant head permeameter. We will consult American Society for Testing and Materials (ASTM) Standards for this purpose. This report will discuss various possible design criteria and their issues and then provide the most feasible solution to the given objective.

Standards:

ASTM D2434 - 68(2006) Standard Test Method for Permeability of Granular Soils (Constant Head)

Background:

Before describing how the constant head permeameter works, let us give you an overview about permeability.

Permeability or hydraulic conductivity, symbolically represented as k, is a property of vascular plants, soils and rocks, that describes the ease with which a fluid (usually water) can move through pore spaces or fractures. It has the units of length per unit time.

It is often represented by the permeability coefficient (k) through the Darcy’s equation:

v=ki

Where v is the apparent fluid velocity through the medium i is the hydraulic gradient , and k is the coefficient of permeability (hydraulic conductivity) often expressed in m/s.

Permeability depends on the intrinsic permeability of the material, the degree of saturation, and on the density and viscosity of the fluid

Average permeability for different soil textures in cm/hour

Sand 5.0Sandy loam 2.5Loam 1.3Clay loam 0.8Silty clay 0.25Clay 0.05

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.), and the calculation of the rate of settlement of clayey soil deposits.

Introduction:

Constant head permeameter is used to determine the permeability (hydraulic conductivity) of a sandy soil .

There are two general types of permeability test methods that are routinely performed in the laboratory

constant head test method falling head test method

The constant head test method is used for permeable and fine grained soil whose permeability constant is greater than 10−4 cm/s and the falling head test is mainly used for less permeable and coarse grained soil whose permeability coefficients is greater than 10−4 cm/s.

We will be making a constant head permeameter and we shall use the ASTM D2434 - 68(2006) Standard Test Method for Permeability of Granular Soils (Constant Head) for this task.

Theory:

The constant-head method allows water to move through the soil under a steady state head(constant head) condition while the quantity (volume) 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 Q of water to be discharged, and head h, the permeability(hydraulic conductivity) k can be calculated:

Qt=Av

Where v is the flow velocity.

v=ki

Here I is the hydraulic gradient. Expressing it as:

i=h /L

Where h is the difference of hydraulic head over length L,yields:

Q= AkhtL

Solving for k gives:

k= QLAht

Basis of Test (Darcy’s Law):

Darcy's law describes the flow of a fluid through a porous medium. It is a simple proportional relationship between the instantaneous discharge rate through a porous medium, the viscosity of the fluid and the pressure drop over a given distance at constant elevation.

Q=−kA ( pb−pa )

μL

Where, Q=total discharge(units of volume per time)

k=permeability of medium

A=cross-sectional area to flow

pb−pa=pressure drop across length L

μ=viscosity of fluid

L=total length across which the pressure drop is taking place

General Procedure:

The constant head permeability test involves flow of water through a column of cylindrical soil sample under the constant pressure difference (constant head). The test is carried out in the constant head permeameter, which can vary in size depending on the grain size of the tested material. The testing apparatus is equipped with an adjustable constant head reservoir and an outlet reservoir

which allows maintaining a constant head during the test. The permeameter is also equipped with a spring that is used to apply constant stress to the sample during the test. Before starting the flow measurements, however, the soil sample is saturated. During the test, the amount of water flowing through the soil column is measured for given time intervals.

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 Q of water to be discharged, and head h, the permeability(hydraulic conductivity) k can be calculated by:

k= QLAht