ASTM D 2434ASTM D 2434

Constant Head Permeability TestConstant Head Permeability Test

CIVE 310 Soil MechanicsCivil, architectural, & Environmental Engineering DepartmentDrexel University

  Permeability is a soil property indicating the ease with which water will flow through the soil. Permeability depends on  the following factors:

1) the size of soil grains

2) the properties of pore fluids

3) the void ratio of the soil

4) the shapes and arrangement of pores

5) the degree of saturation

DefinitionDefinition

Darcy’s LawDarcy’s Law

The coefficient of permeability, The coefficient of permeability, kk, is a product of Darcy’s Law. , is a product of Darcy’s Law. In 1856, Darcy established an empirical relationship for the flow of In 1856, Darcy established an empirical relationship for the flow of water through porous mediawater through porous media. .

Q = kiAQ = kiAWhere:Where:QQ = flow rate (volume/time) = flow rate (volume/time)ii = hydraulic gradient (unitless) = hydraulic gradient (unitless)A A = cross-sectional area of flow (area)= cross-sectional area of flow (area)kk = coeff. of permeability (length/time) = coeff. of permeability (length/time)

It should be noted that the coefficient of It should be noted that the coefficient of permeabilitypermeability is often referred to as is often referred to as hydraulic hydraulic conductivityconductivity by hydrologists and environmental scientists. In their notation, permeability has by hydrologists and environmental scientists. In their notation, permeability has a entirely different definition.a entirely different definition.

PurposePurpose and Significanceand Significance

Purpose:

Determine the permeability, k, (hydraulic conductivity) of soils by appropriate test method.

Significance:

Permeability 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.

Test MethodTest Method

      There are four laboratory methods typically used for measuring the There are four laboratory methods typically used for measuring the permeability coefficient:permeability coefficient:

1) the variable-head (falling-head) test1) the variable-head (falling-head) test2)2) the constant-head testthe constant-head test

3) the capillary method3) the capillary method

4) back calculation from the consolidation test 4) back calculation from the consolidation test

Generally, soils which contain 10% or more particles passing the No. 200 Generally, soils which contain 10% or more particles passing the No. 200 sieve are tested using the falling-head  method. sieve are tested using the falling-head  method. The constant-head The constant-head method is limited to disturbed granular soils containing not more than method is limited to disturbed granular soils containing not more than 10% passing the No.200 sieve.10% passing the No.200 sieve.

Typical Soil PermeabilityTypical Soil Permeability

The constant head test method is used for permeable soils (k>10-4 cm/s), and the falling head test is mainly used for less permeable soils (k<10-4 cm/s).

Test ApparatusTest ApparatusPermeametersPermeameters

RulerRuler

TamperTamper

BalanceBalance

Watch (or Stopwatch)Watch (or Stopwatch)

ThermometerThermometer

FilterFilter

Test ProcedureTest Procedure

1.1. Using the relative densities given by TA (32,34,36) Using the relative densities given by TA (32,34,36) determine the density of the specimen, determine the density of the specimen, γγ

2.2. Measure the diameter and length of specimen mold, Measure the diameter and length of specimen mold, calculate the volume, V. Then, determine the weight of the calculate the volume, V. Then, determine the weight of the sample needed at the particular relative density, Wsample needed at the particular relative density, W

3.3. Set up the permeameterSet up the permeameter

Test Procedure (cont’d)Test Procedure (cont’d)a. Loosen the lower hose clamp on the top coupling and remove the reservoir tube. a. Loosen the lower hose clamp on the top coupling and remove the reservoir tube. b. Place test sample in the mold, level with a straight edge, place in the bucketb. Place test sample in the mold, level with a straight edge, place in the bucketc. Measure the diameter of both the reservoir tube and bubble tube, length of mold, L.c. Measure the diameter of both the reservoir tube and bubble tube, length of mold, L.d. Measure the distance between the top of the mold and top of bucket, H1d. Measure the distance between the top of the mold and top of bucket, H1e. Take the mold out of the bucket, place the reservoir tube back on the mold and tighten e. Take the mold out of the bucket, place the reservoir tube back on the mold and tighten

the clampsthe clampsf. Measure the distance from the bottom of the bubble tube to the top of the mold, H2; the f. Measure the distance from the bottom of the bubble tube to the top of the mold, H2; the

water head difference will be H2-H1water head difference will be H2-H1g. Place permeameters in the bucket and fill slowly allowing water to saturate the sample g. Place permeameters in the bucket and fill slowly allowing water to saturate the sample

from the bottom upfrom the bottom uph. When water overflows, open the upper and lower ports to allow water in the reservoir h. When water overflows, open the upper and lower ports to allow water in the reservoir

tube, keep the water overflowing the buckettube, keep the water overflowing the bucketi. Seal the top of the bubble tube, use vacuum, draw the water into the bubble tube so that i. Seal the top of the bubble tube, use vacuum, draw the water into the bubble tube so that

the water level is between 20 and 25cm high as marked on the reservoir tube. Close the water level is between 20 and 25cm high as marked on the reservoir tube. Close the ports with clamps. Note the mark at which it startsthe ports with clamps. Note the mark at which it starts

j. Open the bubble tube and start the timer, end test when the water level drops to the j. Open the bubble tube and start the timer, end test when the water level drops to the bottom of the bubble tube, or stop after between 15 and 30 minutes.bottom of the bubble tube, or stop after between 15 and 30 minutes.

CalculationCalculation Dry densityDry density

γγdd= (= (γγdmax *dmax *γγdmin dmin )) // [D[Dr*r*((γγdmax dmax ––γγdmindmin) –) –γγdmaxdmax

Where Where γγdmaxdmax=108.5 pcf, =108.5 pcf, γγdmindmin=90.3 pcf, D=90.3 pcf, Drr=32, 34, 36,=32, 34, 36, respectively respectively

Sample WeightSample Weight

W= Volume W= Volume ** γγdd

Water HeadWater Head

H=H1-H2H=H1-H2

GradientGradient

i=H/Li=H/L

FlowFlow

Q=(HQ=(Hstartstart-H-Hfinishfinish))**AA

Where A=area of reservoir tube - area of bubble tubeWhere A=area of reservoir tube - area of bubble tube

Hydraulic conductivity or permeabilityHydraulic conductivity or permeability

k=flow/(iAt)k=flow/(iAt)

Extended AnalysisExtended Analysis(1) Take temperature into consideration:

KT =Q/iA

(2) The viscosity of the water changes with temperature. As temperature increases viscosity decreases and the permeability increases. The coefficient of permeability is standardized at 20°C, and the permeability at any temperature T is related to K20 by the following ratio:

K20=KTηT/ η20

Where: η20 and ηT are the viscosities at the temperature 20 and T, respectively, and

can be found from tables;

GraphGraph

Permeability vs. relative density (void ratio)Permeability vs. relative density (void ratio)

Permeability at 20oC

Void ratio

The EndThe EndThe EndThe End