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

• Flexible wall column with fittings (Column, membrane, 2 end plates, 2 support plates, 2 wire screens, 2 bolts, 6 washers, 6 wing nuts, 2 water end connections, 1 vacuum connection.)
• Glass beads or oven dried sand of various sizes.
• Packing rod, ruler, and spoonula,
• Balance
• Burette
• Timer
• Meter stick
• Plastic beaker
• water

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, h_drip.
6. Record burette capacity, V_max and length between the zero and the maximum graduations, L_b.
7. Record height of the burette maximum-volume graduation above counter or floor, h_max.
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.

• Dry bulk density of column packing, r_b given by,

  r_b = Dry weight of soil / CV

where CV is the volume of the column.

• Column pore volume, PV, determined by,

  PV = CV * (1- r_b/r_g )

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

• Calculate the differential head loss, Dh_i for the various time reading with,

  

h_max is the height of the burette's maximum volume, h_drip is the height of the drip point, V_max is the maximum volume of the burette, V_i is burette volume at time t_i, and L_b is the burette length.

• Mean measured conductivity of the material is given by,

  

where K is the hydraulic conductivity, a is the area of the reservoir, A is the area of the column, Dh_o 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 t_i versus Ln(Dh_o/Dh_i). 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

r_b, r_g: gm/cm³

g: 981 cm/s²