Falling Head Conductivity Test

Glenn Brown
Oklahoma State University
Stillwater, OK
11/15/00

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Overview

This procedure covers the measuement of saturated hydraulic conductivity by the falling head method. The only special equipment needed is the test column. A burette is used as the supply reservior and the measured data is simply the burette volume at various times.

Reference

Ground-Water Hydrology and Hydraulics, by D. McWhorter and D.K. Sunada. Pages 79-80.

Materials

Procedure for Column Packing

  1. Weigh column and fittings. Include all parts.
  2. Draw vacuum on membrane and measure column inside diameter and length. Measure height of interior support plates. Calculate volume based on inside diameter, and length, less the height of the support plates.
  3. Place end support plate and wire screen.
  4. Note particle size of material used.
  5. If the sand is smaller than #50 place a filter paper on bottom screen.
  6. Pack column with soil in lifts, no greater than 5 cm in height. Tap column to settle sand into place. Carefully remove enough sand to place last support plate.
  7. Carefully assemble column with bolts, washers and wing nuts.
  8. Weigh packed columns with all fittings as before.
  9. Measure diameter of burette and calculate its area.
  10. Measure distance between burette maximum burette volume and calculate change in head with respect to volume.

Test Procedure

  1. Mount burette, fill with test solution, and run water out of line.
  2. Slowly allow solution to fill column.
  3. Disconnect effluent line, mount it over beaker for running, and measure its height.
  4. Run about one pore volume through column to work out air. Turn-off and refill burette.
  5. Record height of drip point above counter or floor depending on your set up, hdrip.
  6. Record burette capacity, Vmax and length between the zero and the maximum graduations, Lb.
  7. Record height of the burette maximum-volume graduation above counter or floor, hmax.
  8. Start column and timer.
  9. Record burette volume and time for a minimum of six readings.
  10. Share results with other lab groups.

Calculations

With the data collected determine the following properties.

where CV is the volume of the column.

where rg is the mineral density and is assumed to equal 2.65 gm/cm3 for sand and glass.

hmax is the height of the burette's maximum volume, hdrip is the height of the drip point, Vmax is the maximum volume of the burette, Vi is burette volume at time ti, and Lb is the burette length.

where K is the hydraulic conductivity, a is the area of the reservoir, A is the area of the column, Dho is the initial head loss at time, t = 0.

Report

Provide Introduction, Procedures, and Results of the experiment. Discuss measured conductivity relative to material and packing density. The results should contain a plot of ti versus Ln(Dho/Dhi). The plot should be a straight line with a slope of aL/AK.

Units

Units are to be reported in the cgs system, (centimeters, grams, and seconds). Variable dimensions are:

L, h: cm

t: seconds

K: cm/s

rb, rg: gm/cm3

g: 981 cm/s2