Bentonite/soil rate lab test question

[caddyr2]

Ok, here's a fairly simple one. I have a contractor looking for us to run a proctor, perm, hydrometer, atterberg limits test on a sample of silt with a mix of bentonite for a liner material. The contractor wants to add 1.5 lbs of bentonite per square foot of surface area (6" lift) in the field and i need to duplicate that in the lab. The silt is at 103 PCF at 18% optimum and he wants 95% compaction at optimum moisture content. What I am trying to figure out is the amount of bentonite i need to add to a 2500g sample of the silt to get a equivalent sample. I got a number and so did another guy and he didnt match up with me so I want to check. Thanks for any help. This should be a simple one but I fear I am looking way too far into it.

 

[fattdad]

That's right about 3 percent by dry weight. Take your bulk sample, get a representative moisture content of the bulk sample and then add 3 percent bentonite after you figure out the dry weight of the bulk sample. For example:

40 pounds of moist soil
12 percent dry weight (as received)
35.7 pounds of dry soil
35.7*0.03=1.071 pounds (this is how much bentonite you'd add)

Now I can rant about the problem with the overall approach. What is "optimum" moisture content for 95 percent compaction? The correct answer is something greater than the optimum moisture content for 100 percent compaction. What you'll find is that the permeability of the sample prepared at 95 percent of the 100 percent optimum moisture content will likely be lower (maybe even much lower) than the permeability you'd acheive if you actually knew the optimum moisture content for 95 percent compaction. Jim Mitchell published quite an interesting paper on this - JSMF ASCE Vol 91, SM-4, July 1965, pp. 41-65. Please note that a soil compacted over optimum will often acheive a 10 fold decrease in permeability. This usually relates to compaction at about 90 percent saturation.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[DMcGrath]

Looks like about 3% bentonite by (dry) weight. You've got to also figure the contractor is going to have some variance in the field when applying bentonite. If permeability is the key result, I would work backwards from a required permeability specification for the material and figure how much bentonite you need, rather than seeing what perm you get from a specific amount of bentonite.

Don't forget that these samples will take a little longer to run as the bentonite needs time to fully hydrate.

 

[caddyr2]

All right, I had 3 percent as well. My contact had 1.5 percent but I'm unsure how he got that and wanted to check since he has been in the business for much longer. Fattdad, when you talk about optimum moisture at 95% compaction I am not sure how this applies to this exact test. The contractor is going to moisture condition each lift to 18% and then incorperate the bentonite and compact that to 95% of optimum and then water a slight bit more so that there is succeptible moisture for the bentonite to hydrate. So to replicate what he is doing inthe field shouldn't I go at it the same way? If I were to adjust my moisture down to say 16% that would not line up with what is planned in the field. I may be wrong but that is just my thinking. I am not designing the perfect liner here, we were just hired to replicate what will happen in the field and check that it will work. Backwards yes but thats just how this one goes. DMcGrath: This is just the amount that we are told that the contractor is using and that is all that we are to test. I agree with you both but as far as I can understand this it is basically a check on what is already planned.

 

[fattdad]

Then push a shelby tube and measure the actually saturated permeability. My post may have confused you. If somebody asked me to measure the permeability of a soil-bentonite mixture, I'd first make the mixture, then I'd do a proctor then I'd compact to 95 percent using a moisture content that represented >90 percent saturation. During construction, I'd do what's required to ascertain that the contrator is taking the appropriate steps to properly blend the materials (Bomag makes a nice piece of equipment for soil mixing) and I do a few drive tubes for permeability testing. Maybe even install a few permeameters for in-situ testing.

There are other methods; however. . .

f-d

¡papá gordo ain’t no madre flaca!