Falling-Head Perm Setup Question

[caddyr2]

Can anyone help me with a problem I have been having with my falling-head perm test that I am trying to set up? I am using a compaction mold which uses a 3/8" O.D. 1/4" I.D. clear pastic hose to create the head. What I am trying to do is keep the air out of my system. It seems as if the plastic tubing sidewall friction will not allow the air to pass up the tube by the permeant water so I get air bubbles in the line. I tried tapping on the line but it does not always do the trick, plus if it breaks up the bubbles too much it defeats the purpose of the de-aired water. I'm sure my problem is the hose size but that is the size that it came with so I do not want to increase this. The supplier that I bought the molds from has never heard of this problem. Can anyone offer some help?

 

[singingk]

What kind of soil did you try to test?
If sand samples are percolated from the upper side, it is neccessary to flood them from the bottom (with de-aired water) before testing. Therefor we put the sample in a bucket with an overflow above the upper side of the sample.
Clean the hose carefully and fill it also from the bottom before testing, adhesion of the bubbles on die hose sidewalls is caused by particles (or scratches).

 

[caddyr2]

It does seem that after some use the problem with the entrapped air in the line does decrease, probably due to the flushing of the lines. I learned that you need to carefully add the water so that you minimize the chance that you entrap air. It took a little thinking over the weekend and it seems that my problem has been solved. Thanks

 

[oldestguy]

What's wrong with a larger diameter tube? The stuff is cheap.

I used 1/2 inch ID tube with heights of about 13 feet. with no problems with air.

 

[fattdad]

Just to agree with oldestguy - use a larger diameter tube.

On the matter of soil type, this is the bigger issue. If you are trying to test anything with a permeability under 10-5 cm/sec your bigger problem is getting the air out of the sample. Just because you submerge a sample in a mold in water does not mean that you have acheived saturation. As such, you may really have entrapped air, which acts to retard the flow of water. This in turn will give you lower results then realized if the sample were truely saturated.

How big of a factor is this? I don't know. It is just another factor as is compaction moisture content and compacted dry density. Compaction moisture content can be a 10 to 100-fold factor in saturated permeability.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[soiltech]

I have always used a small vacuum to suck the air out before adding the water. Then usually taping the hose as you add water will help as well.
Soil type can play a big factor as well

 

[caddyr2]

I actually created a vacuum using a standard air hose and sucked the water up the hose which worked well. I'm not sure if I want to use a larger hose because my compaction mold only has a fitting on it the size that fits the hose (1/8") and that is what the manufacturer supplied. I also have 13 feet of head. As long as the person adding the water is careful I havent had a problem with air trapped for some time.
The problem that I have with tapping the line is that it adds unneeded air to the water so minimizing tapping helps.
Typically we deal with lagoon liners so we are using 2 percent over optimum at 95 percent compaction. After we got a trial perm completed the remainder have run smoothly.

 

[fattdad]

using 2 percent over optimum at 95 percent compaction

This may be hard to fully understand in text only, but 2 percent over the optimum for 100 percent compaction is not over optimum for 95 percent compaction. The work by Mitchell, Hooper and Campanella (asce 1965) shows that the affect of compaction moisture content on hydraulic conductivity is affect by which side of the LOO (line of optimums) you are on. The LOO is not a vertical line drawn from the optimum moisture content for 100 percent Standard Proctor.

Just as the optimum moisture content for Modified Proctor is less than the optimum moisture content for Standard Proctor, the optimum moisture content for 95 percent compaction is greater than the optimum moisture content for 100 percent compaction. This has an affect on the hydraulic conductivity.

It's better to specify percent saturation (typically 90 percent) as percent saturation relates to both the ZAV and the LOO.

Hope I didn't confuse. . . .

f-d

p.s., if you are applying 13 ft of head to a 6 in sample you are using a gradient of 23, which is much greater than any typical field condition.

¡papá gordo ain’t no madre flaca!

 

[DocDirt]

How about using a capillary (glass) standpipe with a plastic hose at the bottom with a pinchcock?
DocDirt