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Constant head


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Constant head

  1. 1. ASTM D 2434 Constant Head Permeability Test CIVE 310 Soil Mechanics Civil, architectural, & Environmental Engineering Department Drexel University
  2. 2. Definition <ul><li>  Permeability is a soil property indicating the ease with which water will flow through the soil. Permeability depends on  the following factors: </li></ul><ul><li>the size of soil grains </li></ul><ul><li>2) the properties of pore fluids </li></ul><ul><li>3) the void ratio of the soil </li></ul><ul><li>4) the shapes and arrangement of pores </li></ul><ul><li>5) the degree of saturation </li></ul>
  3. 3. Darcy’s Law <ul><li>The coefficient of permeability, k , is a product of Darcy’s Law. </li></ul><ul><li>In 1856, Darcy established an empirical relationship for the flow of water through porous media . </li></ul><ul><li>Q = kiA </li></ul><ul><li>Where: </li></ul><ul><li>Q = flow rate (volume/time) </li></ul><ul><li>i = hydraulic gradient (unitless) </li></ul><ul><li>A = cross-sectional area of flow (area) </li></ul><ul><li>k = coeff. of permeability (length/time) </li></ul><ul><li>It should be noted that the coefficient of permeability is often referred to as hydraulic conductivity by hydrologists and environmental scientists. In their notation, permeability has a entirely different definition. </li></ul>
  4. 4. Purpose and 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.
  5. 5. Test Method <ul><li>    There are four laboratory methods typically used for measuring the permeability coefficient: </li></ul><ul><li>1) the variable-head (falling-head) test 2) the constant-head test </li></ul><ul><li>3) the capillary method </li></ul><ul><li>4) back calculation from the consolidation test </li></ul><ul><li>Generally, soils which contain 10% or more particles passing the No. 200 sieve are tested using the falling-head  method. The constant-head method is limited to disturbed granular soils containing not more than 10% passing the No.200 sieve. </li></ul>
  6. 6. Typical 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).
  7. 7. Test Apparatus Permeameters Ruler Tamper Balance Watch (or Stopwatch) Thermometer Filter
  8. 9. Test Procedure <ul><li>Using the relative densities given by TA (32,34,36) determine the density of the specimen, γ </li></ul><ul><li>Measure the diameter and length of specimen mold, calculate the volume, V. Then, determine the weight of the sample needed at the particular relative density, W </li></ul><ul><li>Set up the permeameter </li></ul>
  9. 10. Test Procedure (cont’d) <ul><li>a. Loosen the lower hose clamp on the top coupling and remove the reservoir tube. </li></ul><ul><li>b. Place test sample in the mold, level with a straight edge, place in the bucket </li></ul><ul><li>c. Measure the diameter of both the reservoir tube and bubble tube, length of mold, L. </li></ul><ul><li>d. Measure the distance between the top of the mold and top of bucket, H1 </li></ul><ul><li>e. Take the mold out of the bucket, place the reservoir tube back on the mold and tighten the clamps </li></ul><ul><li>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-H1 </li></ul><ul><li>g. Place permeameters in the bucket and fill slowly allowing water to saturate the sample from the bottom up </li></ul><ul><li>h. When water overflows, open the upper and lower ports to allow water in the reservoir tube, keep the water overflowing the bucket </li></ul><ul><li>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 ports with clamps. Note the mark at which it starts </li></ul><ul><li>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. </li></ul>
  10. 11. Calculation <ul><li>Dry density </li></ul><ul><li>γ d = ( γ dmax * γ dmin ) / [D r* ( γ dmax – γ dmin ) – γ dmax </li></ul><ul><li>Where γ dmax =108.5 pcf, γ dmin =90.3 pcf, D r =32, 34, 36, respectively </li></ul><ul><li>Sample Weight </li></ul><ul><li>W= Volume * γ d </li></ul><ul><li>Water Head </li></ul><ul><li>H=H1-H2 </li></ul><ul><li>Gradient </li></ul><ul><li>i=H/L </li></ul><ul><li>Flow </li></ul><ul><li>Q=(H start -H finish ) * A </li></ul><ul><li>Where A=area of reservoir tube - area of bubble tube </li></ul><ul><li>Hydraulic conductivity or permeability </li></ul><ul><li>k=flow/(iAt) </li></ul>
  11. 12. Extended Analysis <ul><li>(1) Take temperature into consideration: </li></ul><ul><li>K T =Q/iA </li></ul><ul><li>(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 K 20 by the following ratio: </li></ul><ul><li>K 20 =K T η T / η 20 </li></ul><ul><li>Where: </li></ul><ul><li>η 20 and η T are the viscosities at the temperature 20 and T, respectively, and can be found from tables; </li></ul>
  12. 13. Graph <ul><li>Permeability vs. relative density (void ratio) </li></ul>Permeability at 20 o C Void ratio
  13. 14. <ul><li>The End </li></ul>