Lab 8: Constant Head Permeability Test This presentation covers the measurement of
hydraulic conductivity using a constant head permeameter
This figure illustrates the key parts of any constant head permeameter
The tests consists of a cylindrical specimen of soil with a cross sectional area of A and
length of l.
The top of the specimen is hydraulically connected to an upper reservoir and the bottom of the
specimen is hydraulically connected to a lower reservoir.
The reservoirs are designed such that the elevation of the water in each is constant.
The change in head across the sample, delta H, is then just the difference in the elevation
of the water surface of the upper and lower reservoir.
If we maintain a constant delta H, the flow out of the lower reservoir, q, will be equal
to the flow of water through the sample.
We can measure q by measure the time it takes for the flow to fill up a container of known
volume.
To determine the hydraulic conductivity, we first compute the hydraulic gradient, i, as
delta H over l.
We will then use Darcy's law, q equals k, i, A, to compute the hydraulic conductivity,
k.
To do this we rearrange the equation to get k on the left side of the equation and we
have k equals Q divided i, A. This photo shows the permeameter we will be
using in your lab.
The test section containing the soil specimen is located near the bottom of the device.
Below the test section is the drain section which is simply used to hydraulically connect
the bottom of the sample to the lower reservoir.
Above the test section is a Mariotte bottle which will be used to both control the elevation
of the reservoir above the specimen and allow us to compute the flowrate, q.
The entire device will be placed into a bucket which will be used to keep the sample saturated
and control the elevation of the lower reservoir.
The Mariotte bottle contains a lower fill line and an upper vent line.
These are used to fill the bottle with water before the test, but are both kept closed
during the test.
The top of the Mariotte bottle is sealed and an adjustable standpipe sticks down the center
of the bottle.
This standpipe is moved up and down to control head of water above the specimen.
Finally, there is a scale along the side of the Mariotte bottle.
This will be used to measure the elevation of the bottom of the standpipe and to measure
the drop in water level in the bottle.
We will measure the drop in the water level over time and use this measurement to compute
the flow rate through the soil.
This picture is a closeup of the soil test section.
The test section has a diameter of 15.25 centimeters and a height of 11.66 centimeters.
The bottom of the test section is made of a perforated stainless steel sheet.
A number 100 wire mesh screen is placed on top of the stainless steel sheet to retain
all the soil in the test section.
The specimen is prepared by filling the test section with soil and compacting that soil
to the desired density.
Vibratory compaction is used to densify the soil.
Once the test section is filled and compacted another number 100 mesh screen is placed on
top of the soil and another perforated stainless plate on top of the mesh.
This keeps the soil in the test section.
The Mariotte bottle is then connected to the top of the test section using the neoprene
band and hose clamps as shown.
This image shows a close up of the soil specimen in the test section with the Mariotte bottle
connected above and the drain section connected below.
When the entire apparatus is assembled, it's placed in a bucket to saturate the sample.
The bucket has a standpipe attached which will control the elevation of the water level
in the bucket thereby creating a fixed elevation for the lower reservoir.
This schematic diagram will be used to describe how the test is performed.
First with the upper vent line open, the Mariotte bottle is filled through the lower fill line.
Once bottle is filled, both valves are closed.
They will remain closed throughout the test.
The top of the standpipe in the Mariotte bottle is open to the atmosphere.
Therefore, the both points A and B are subject to atmospheric pressure.
Since the Mariotte bottle is sealed at the top, there is a vacuum formed at the top of
the bottle.
This vacuum is what holds up the column of water in the Mariotte bottle.
Since A and B are both at atmospheric pressure, they have the same pressure head.
Therefore, the change in head across the specimen, delta H, is simply the difference in elevation
between the top of the water in the bucket and the bottom of the standpipe in the Mariotte
bottle.
Since we know the length of the specimen, we can compute the hydraulic gradient as delta
H over l.
This animation will illustrate how the test is performed.
Once the Mariotte bottle is filled and the standpipe height set, water will start flowing
through the specimen, out the bottom of the permeameter and into the bucket.
At the same time air bubbles will enter the bottle through the standpipe and travel up
to the top of the bottle and the water level in the Mariotte bottle will drop.
All this time water will be flowing out of the standpipe attached to the bucket.
During the test, you will measure: The initial elevation of the water in the bottle, the
final elevation of the water in the bottle, and the time required for the water to drop
this distance.
The change in elevation can then be determined.
You now have all the data needed to compute the hydraulic gradient.
The computations are: first, compute the hydraulic gradient as delta H over l.
The volume, V, flowing through the sample is the inside diameter of the Mariotte bottle
squared, minus the outside diameter of the standpipe squared, time pi, divided by 4,
all times the change in elevation.
Once you know the volume, you can calculate the flowrate, Q, as V over delta t.
And, finally, you can compute the hydraulic gradient, k, as Q over the quantity i times
A. You should make measurements at 3 or four
different gradients by adjusting the elevation of the standpipe in the Mariotte bottle.
This will give you multiple measurements which you can average to the get the average hydraulic
conductivity.
That's all there is to it.
It's really a simple test.
Have fun with it.
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