Measuring dry density of tailings as it desaturates

[Odeere]

I am looking for a novel way to measure the dry density of tailings as it desaturates due to an evaporation potential.

I am trying to establish the relationship between moisture content and dry density. As a tailings settles out from the slurry it remains saturated and so working out the dry density and moisture content is simple. (It can be assumed the relationship follows the zero air voids line - any comments?)

However at a specific moisture content the sample starts to desaturate. At this point it becomes difficult to determine the dry density as one needs to measure both volume and mass. Further complicating the matter is that the sample cracks which makes determining the volume harder.

Any suggestions as to test methods or devices to measure the dry density post saturation?

 

[fattdad]

at the liquid limit it's normally consolidated. Not sure dry density would change after the tailings' moisture content reduced below the liquid limit. Then again, this is an unusual topic and I'm not really sure my initial thoughts are fully thought out. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[aeoliantexan]

It is common in geotechnical sampling to push thin-walled tubes to obtain samples, cut specimens square-ended, measure the length and diameter, weigh the specimen, dry part of it to get the water content, and calculate the dry density. I'm not sure what is special about the tailings. You can probably deal with the cracks by measuring the length in the tube before extruding.

Weak unsaturated soil is easily compressed during sampling and again during extruding due to the side friction between the sample and the tube. This can lead to measured densities higher than what is in place. The disturbance is less for larger-diameter tubes - say 4 to 6 inches. Slough in the bottom of the hole or a zone compacted below the drill bit can increase the side friction and make the problem worse. For sampling low-density loess, I used to use a 4-1/2 inch diameter Shelby tube. We would push it about 3 inches and bring it out to get rid of the slough and compacted zone, clean the tube, and go back into the hole and push it just 12 inches. If we could not extrude the sample by hand, it was probably compressed too much.

If you want the density between the cracks, carve out a potato-shaped chunk between cracks and run a density by the wax-dip method. Let me know if you need details.

If you are wanting to monitor the change in density with time, there are down-hole geophysical probes that measure density by nuclear methods. You may be able to run one repeatedly down a hole cased with plastic pipe.

 

[Mercycity]

Dear Odeere,

I agree with Aeoliantexan but to add, you have to oiling the inner part of the tube you are using for easy pushing and extraction of a sample from the tube and this will reduce the friction of sample and the tube, hence eliminate some arrors. To get dry density and moisture content at different depths such as at 0m 1.0m,2.0m you can dug a pit and push the tube at that depth. As Aeolina said, there is possible of sample disturbance during sampling especially during pushing of the tube into the ground but this can be eliminated by pushing slowly. I have using this method for measuring dry densities of fly ash materials and was very successful.If you can have many samples and find an average it will better if the budget can allow.

 

[dgillette]

Not familiar with the material you are working with, so I have to ask: when it first begins to desaturate (by evaporation at the exposed surface), how does the water content compare with the shrinkage limit? If you don't have montmorillonites or other swelling clays that absorb water into the particle, how much volume change could occur after desaturation? Up to a point, capillarity would increase eff stress with decreasing moisture content, which would appear to influence the value we measure for shrinkage limit, but how much difference could there be between e at beginning of desaturation, and e at shrinkage limit?

Is the ultimate goal to predict pond capacity (with filling resumed after desiccation) or final settlement after filling is completed, or what? In either case, to calculate the thickness of the desiccated layer, you would somehow have to allow for the difference between actual shrinkage limit (at the centimeter scale) and the "effective" shrinkage limit that accounts for the shrinkage cracks (at the meter or 100-meter scale)? In highly plastic Florida phosphate slimes, the shrinkage cracks can get deep enough and wide enough to stick your foot in all the way to the knee (and snakes lurk in them).