Friday, July 6, 2012

The Constant Head Test

After days of working to produce a secure, water tight, and effective constant head column apparatus, testing using the method began today. The difference with this test compared to the falling head test that we have been using up to this point, is that the water level in the column in the sand is meant to stay constant. Water is constantly pouring into the column, and excess water is caught as the water level in the column is kept constant. By knowing the amount of water added to the column and the amount of excess water that was collected we are able to determine the amount of water that went through the sand. This allows us to determine the hydraulic conductivity of the sand. 
A diagram of the setup is displayed below.

Diagram by Susa
We have been working to develop an effective method to add water from a separate cylinder while maintaining a constant water height. Right now the cylinder is prone to add water much faster than our spout exiting the column loses water. To maintain a constant water height we have been starting each trial holding the cylinder low and moving the cylinder up as water drains out to maintain a constant water height. Of course there is always variation in what the water height actually is which adds a level of error to the method. By the end of the day we had developed a method that was working moderately well for us, but we are still considering possible changes that would reduce error.

The reason behind using a more complicated test to determine values we can determine using a simpler test is that when using the computer and MATLAB to perform simulations being able to input a single water height is much simpler. We plan to continue improving our constant head test techniques as we test all the sands we have tested with the falling head test. If we find that both tests produce the same K values then we will continue to use just the falling head test in the lab while simulating constant head tests using MATLAB.


  1. Do you have a diagram of your falling head apparatus?

  2. shows a picture of it from a later year. Its basically just a big glass vase with sand part way up it, that we can stick a clear tube into and then we poor water into the tube and time how long it takes to fall a certain distance. We have attached a waterproof tape to the tube, so we can measure water levels and how far it goes into the sand easily. I have other diagrams of this, if you need more, but I can't post them into the blog comments.