Flexible Wall Permeameter

[BVTEngineer]

I'm performing a permeability test, ASTM D5084, using a 150mm dia. sample. After saturation and consolidation, the permeability test is performed using a confining stress equal to the previous consolidation. What I'm don't understand is that during the test (constant flow bottom to top), the flow is steady, the gradient is steady but the sample pressures are gradually decreasing together.
Typically, flows are 0.2 cc/sec, gradient = 2, k = 1x10^-4 cm/sec and the sample pressure is usually around 600 kpa with an effective stress of 3 kpa.

Any thoughts on why the sample top and bottom pressures would be able to maintain a constant gradient but be dropping in pressure? thanks

 

[Erdbau]

How are you applying the top and bottom pressures? If both the top and bottom pressures are dropping, you could be consolidating the sample if the cell pressure is constant through the test. How much are the pressures dropping by over the course of the test?

Also, 3 kPa is a VERY low effective stress. Is that the correct value?

 

[BVTEngineer]

Thanks, that should be 3 psi (21 kpa).
The top and bottom pressures are being applied with flow pumps. Since the permeability test is performed at the same effective stress as the consolidation, I don't expect to see the sample still consolidating. The sample is structural backfill with roughly 6% silt. Typical consolidation runs for an hour or more until end of primary consolidation (t100). The pressure drops range between 1 - 15 psi/hr.
Assuming the sample is consolidated and there are no leaks, what other causes could exist?
Thanks

 

[Erdbau]

Well, if you are using flow pumps, you should be able to hold a constant pressure. Hard to say what's going on without looking at your system. Is the cell pressure remaining constant throughout the test? If so, a sample pressure drop of 15 psi/hour means that your effective confining stress goes from 3 psi to 0 psi in 12 minutes.

I would suggest checking the flow pumps to see that they are working properly.

 

[BVTEngineer]

The while the top and bottom pressures are maintained during the permeability test, the cell pressure is also maintained constant with an additional flow pump. Therefore, it seems that if the sample pressure decreases while the cell is constant it is experiencing an increase in effective stress.
I wondering if a small amount of air trapped in the flow pumps could be responsible for this...?
Thanks

 

[Erdbau]

If all the pressures are being maintained, then how are you getting changes in pressure? Pressure is felt by water and air just the same so it's probably not an air bubble, but being sure you've properly deaired the system is always good practice.

 

[molerat2210]

3 psi??? There is no way that the sample is 95 percent saturated. Check your B parameter.

You do not need to worry about consolidation you keep the difference of confining pressure to back pressure less than the effective overburden pressure. Bring up the confining pressure to about 80 psi and saturate the specimen. You can run the test at lower pressure. This may explain the unusual readings since there is air in the sample.

 

[Erdbau]

Molerat, BTV indicated in his OP that 3psi was the effective stress, not the cell or back pressure. You are right though, it needs to be saturated, and the inflow versus the outflow must always be checked that they are relatively close.

 

[molerat2210]

Erdbau,

You are right. No problem. 3 psi effective stress sounds Ok, I guess if there is concern about consoliding the sample.