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B-Value?
- Thread starter Ashayeri
- Start date
Ashayeri:
I would imagine that you could obtain complete saturation by using the back pressure technique. What you do is increase the pore water pressure and the cell pressure in equal increments and use the higher pore pressure to force any air into solution. You check the B-value at every stage and stop when you get sufficiently close to 1.
When the specimen is fully saturated, the B-value should approach 1 if there is not cementation.
Good luck
I would imagine that you could obtain complete saturation by using the back pressure technique. What you do is increase the pore water pressure and the cell pressure in equal increments and use the higher pore pressure to force any air into solution. You check the B-value at every stage and stop when you get sufficiently close to 1.
When the specimen is fully saturated, the B-value should approach 1 if there is not cementation.
Good luck
- Thread starter
- #3
gandersen:
i know about back-pressuring technique but i am dealing with a high plastic clay, it is dense and it took 10 days to show 0.5 cc permeation by applying 150kpa gradient. how such plastic soil can responde to increase in confining pressure and shows up increament in pore pressure.
have you been in such cases yet?
waiting for reply
i know about back-pressuring technique but i am dealing with a high plastic clay, it is dense and it took 10 days to show 0.5 cc permeation by applying 150kpa gradient. how such plastic soil can responde to increase in confining pressure and shows up increament in pore pressure.
have you been in such cases yet?
waiting for reply
Ashayeri:
In order to achieve full saturation, what happens is the air voids in the soil go into solution under the increased pore pressure (during the back pressurization). Have you attempted this in a standard triaxial cell? If you can't get full saturation, you may need to use a high capacity triaxial cell where you can apply a higher pressure.
Unfortunately, I have not had the experience that you speak about with a very high plasticity clay.
If you are having difficulties fully saturating the specimen in the laboratory, will the specimen saturate in the field under your assumed future conditions? Do you need to model the soil as an unsaturated soil?
In order to achieve full saturation, what happens is the air voids in the soil go into solution under the increased pore pressure (during the back pressurization). Have you attempted this in a standard triaxial cell? If you can't get full saturation, you may need to use a high capacity triaxial cell where you can apply a higher pressure.
Unfortunately, I have not had the experience that you speak about with a very high plasticity clay.
If you are having difficulties fully saturating the specimen in the laboratory, will the specimen saturate in the field under your assumed future conditions? Do you need to model the soil as an unsaturated soil?
[blue]Ashayeri[/blue]:
I've dealt with your problem - a few comments.[ol][li]Be sure that your porous stones at the top and bottom of the sample are clean and porous.[li]Use double drainage - top and bottom of the sample.[li]Wrap filter paper around the sample before you put the membrane over it to shorten the drainage path. Cut diagonal slits in it so that it doesn't add to the sample's strength. Make sure the filter paper wrap is in contact with the top and bottom porous stones.[li]Use a much higher back pressure. I would expect that you may need a back pressure of 450 to 600 kPa (roughly 65 to 87 psi) to saturate your sample.[/ol]Have you run any swell or consolidation tests to see what you are getting for t[sub]100[/sub]? (Use aquare root of time, not log time, to evaluate.)
![[pacman]](/data/assets/smilies/pacman.gif "[pacman] [pacman]")
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
I've dealt with your problem - a few comments.[ol][li]Be sure that your porous stones at the top and bottom of the sample are clean and porous.[li]Use double drainage - top and bottom of the sample.[li]Wrap filter paper around the sample before you put the membrane over it to shorten the drainage path. Cut diagonal slits in it so that it doesn't add to the sample's strength. Make sure the filter paper wrap is in contact with the top and bottom porous stones.[li]Use a much higher back pressure. I would expect that you may need a back pressure of 450 to 600 kPa (roughly 65 to 87 psi) to saturate your sample.[/ol]Have you run any swell or consolidation tests to see what you are getting for t[sub]100[/sub]? (Use aquare root of time, not log time, to evaluate.)
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
- Thread starter
- #6
Focht3:
thanks, i followed the steps, the clay has 35% free swell index, (highly expansive, swelling pressure at opt. water cont. was 265 kpa) by the way, do you think how much shall i wait to see the excess pore water pressure when i increased the confining pressure for B-value check? (2 mins or a day!!!)
thanks
thanks, i followed the steps, the clay has 35% free swell index, (highly expansive, swelling pressure at opt. water cont. was 265 kpa) by the way, do you think how much shall i wait to see the excess pore water pressure when i increased the confining pressure for B-value check? (2 mins or a day!!!)
thanks
Check it after a day. It should hit a B value of 0.90 to 0.95 by then - perhaps even 1.00. You want the B value as close to 1.00 as possible before you run the test -
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
- Thread starter
- #8
I've dealt with these clays also, and one way to reduce time needed to reach at least 0.95 is to use as small a sample as possible. When I first started (before I knew better) I was using 3" diameter samples. Now I trim all high clay samples down.
Smaller samples certainly help. After all, you are significantly reducing the drainage path. But they may not be desirable when the soils have significant secondary structure, particularly slickensides. And many times the soil with a "micro" permeability of 10[sup]-9[/sup] cm/sec has enough secondary structure to bring the effective ("macro"
permeability of 10[sup]-7[/sup] cm/sec - or less. But it is clearly an option that should be considered.
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
permeability of 10[sup]-7[/sup] cm/sec - or less. But it is clearly an option that should be considered.Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
davecooper
Geotechnical
Ashayeri
We use a vacuum to presaturate (deair the system and sample) prior to back pressure saturation. Fat clay has never been a problem for us, even though the vacuum is the least effective on a stiff clay. As Focht3 pointed out, filter paper strips have to be used. Many times an overconsolidated clay will not reach a "B" of 0.95. You may notice that the "B" will peak at some point (usually in the 0.85 to 0.9 range) then drop with increasing back pressure. This is also an indication of saturation. We have found that sands are the tough ones, surprisingly. You have to de-air by vacuum, (prevac), prior to back pressure saturation for sands. As an example a sand without prevac, just back pressure saturation, may take well over 100psi to get a good "B". With prevac a sand can be back pressure saturated by 30 to 40psi! You have to be careful not to overconsolidate the sample if you are using confining pressures less than 15 psi.
Best Regards,
David Cooper
Cooper Testing Labs
We use a vacuum to presaturate (deair the system and sample) prior to back pressure saturation. Fat clay has never been a problem for us, even though the vacuum is the least effective on a stiff clay. As Focht3 pointed out, filter paper strips have to be used. Many times an overconsolidated clay will not reach a "B" of 0.95. You may notice that the "B" will peak at some point (usually in the 0.85 to 0.9 range) then drop with increasing back pressure. This is also an indication of saturation. We have found that sands are the tough ones, surprisingly. You have to de-air by vacuum, (prevac), prior to back pressure saturation for sands. As an example a sand without prevac, just back pressure saturation, may take well over 100psi to get a good "B". With prevac a sand can be back pressure saturated by 30 to 40psi! You have to be careful not to overconsolidate the sample if you are using confining pressures less than 15 psi.
Best Regards,
David Cooper
Cooper Testing Labs
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