Construction & Contractors/soil question

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Question
I am building a stock tank to water my cattle but I am in very sandy soil. There are several options, all expensive, but my question pertains to a couple of choices.
I have been told, with no proof, that if limestone fines are wet and compacted it can be made nearly waterproof.
The other option is that they are doing a lot of road construction near me and they are crushing all the old concrete. I have also been told that this crushed concrete can be compacted and made water proof.
I also found a company that builds ponds and they mix a lot of tripolyphosfat with the existing soil to make the pond waterproof.
Do you know if any of these methods have been tried. Any direction you can give me is appreciated.  Charlie

Answer
The way to make soil waterproof (or nearly so) is to formulate it so that the interconnected voids in the soil structure, after it is compacted, are completely filled.  Since voids in soils typically occur in a range of sizes, materials used in filling those voids will have to represent a similar size range.  And since water molecules are small it can travel through some pretty small voids.  In reality, waterproof behavior does not occur unless the material forms a membrane whose flow pathways are so tortuous and small that the water will not pass through it in noticeable quantity, such as with plastic liners, or cemented natural materials where the material grows together to seal voids.  With real construction materials that don't self-cement, the permeability becomes dependent on the grain size distribution (is there a range of sizes including the very fine fraction) and the degree of compaction (which diminishes overall void volume).

I haven't personally used limestone fines or crushed concrete for treating soils, but I have used hydrated lime and can vouch for its abilities to fill in voids and make soil stronger and much less permeable.

Crushing will produce a range of material sizes, the fine component being the material that flies off the fracture faces during the crushing process.  If there are natively small grain sizes in the source material then the proportion of small size material will be higher in the crushed product, but crushed material typically will not contain enough fines to be much of a water flow retardant unless it is ground to dust after crushing.  

Therefore I would be very suspisious of crushed concrete.  If the concrete is crushed to, say, 100% minus-1/2 inch there may be enough fines generated from that to be a decent membrane, but there is no reason a road contractor would be doing that just to demo pavement.  I doubt they are crushing it fine enough to make an improvement over the soils you already have.  Crushing concrete will produce a range of fines but since concrete contains aggregate that is selected for its durability I would guess the proportion of fines will not be as high as necessary to seal voids.

Limestone fines could be much better since they are generally a byproduct of aggregate production where it is undesirable to have too many fines.  There is also the possibility of secondary cementation from the oxidation of the limestone fines.  On the other hand, limestone is a carbonate and carbonates are soluble in acidic conditions, even rainwater and fresh water because there is always a slight acidity due to dissolved CO2.  

None of this is as important as the proportioning, mixing, and compaction that you will do.  It makes relatively little difference what you add to the soil if you don't mix it in uniformly and get it well compacted.  Many soils will be 95% compacted at a dry density of 118 lb/cu ft, but have a dry density of 125 lb/cu ft at 98% compaction.  That's a difference of 7 lb worth of air voids in each cubic ft of soil, and those air voids are available to transmit water.  

You asked about tripolyphosphate.  I have not used it but I understand that it is applicable, along with soda ash and other chemicals, in soils with appreciable clay content to disperse the clay out of clumps, and make it more amenable to compaction.  Since you say you have sandy soils it sounds like tripolyphosphate is exactly the opposite of what you want to use, unless your sandy soils have appreciable clay.

You could probably get a major improvement by the addition of sodium bentonite to your sandy soils or the addition of hydrated lime or limestone fines if you get the proportion right.  Before spending the money on the material large-scale you might seriously consider taking some soil samples and have a soil lab mix up various proportions, compact them in a mold, and see how much permeabilility improvement you get.

One more thing -- I frequently see sands being compacted with sheepsfoot rollers.  Sands are not cohesive so kneading them under a sheepsfoot will not be as beneficial as it would be in a clay soil.  If the soil is a mixture of sand and clay, OK, but in real sandy soil the trick is to get the moisture content right and use a roller -- sheepsfoot or smooth -- that vibrates.  The vibration is more appropriate in sands to get the grains to rearrange themselves.

Here's a pretty good article on the subject of pond sealing, in case you haven't seen it.  http://extension.missouri.edu/p/g1555

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Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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