Why should the falling head permeability test be used for clay? 1

[HDGolden]

I'm purchasing equipment to do some permeability testing on sand, but in the future we might need to do some testing on fine grained soils. I'm planning on purchasing a constant/falling head permeater so I can use it for either. Can someone explain why a falling head test is recommended for fine-grained soils and a constant head is recommended for coarse-grained soils?

 

[dgillette]

It's because of the practical aspect of the test and measuring the water, not for the fact of it being constant or falling.

For clay, the falling test is more common, because water volume gets measured very easily as the head in the burette or calibrated tube drops over hours or days. A constant-head test on clay would require you to collect and precisely measure water in drops per hour.

For sand, there are orders of magnitude more water coming through, so you can just collect it and measure it easily.

 

[HDGolden]

Thanks so much!