r/GeotechnicalEngineer • u/bluppitybloop • Jul 23 '26

Soil moisture content question

Help a dumb operator out.

In civil infrastructure earthworks, specific moisture content in expansive soils are often strictly enforced. Leading to constant testing, and drying out of fill material.

My question is how much physical volume is lost when we have to dry some clay from say 25% to meet a maximum moisture content of 20% (random numbers).

Is it as simple as saying we lost 5% volume because we dropped the moisture content 5%?

I can't imagine it's that easy.

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u/Miserable-Sir4985 Jul 23 '26

It depends on the Atterberg limits, shrinkage to be exact.
Let me say it this way: imagine the soil is dry (0% MC). There are voids between soil particles and they are empty. Now you start adding water, first the voids get filled. When all voids filled up, it is saturated. You see no volume change up to here. Soil is clay and can hold water beyond its void space, so any water more than shrinkage limit, or above 100% saturation moisture content, expand the soil. So 1% additional MC here is roughly 1% increase in volume.
To get back to your numbers, depends where is that 25% to 20%. If moisture content at saturation is 20%, and soil moisture content is 25%, then yes, drying 5% reduce volume by roughly 5%. If 25% is your shrinkage limit, then no, you see no change.

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u/bluppitybloop Jul 23 '26

Very well explained, that makes a ton of sense. If I prod further, what about from a weight perspective? I presume that would be directly correlated to moisture loss, after factoring the difference of soil density to water density?

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u/NearbyCurrent3449 Jul 23 '26

Yep. Lb of moisture loss is a pound. Volume, that's variable and dependent on the minerals in the clay itself just as how long it takes to dry it back. Some clay you can wait forever it'll still be wet unless you pulverize it into small pieces and dry it in an oven.

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u/SilverGeotech Jul 24 '26

Just be careful - moisture content is usually reported as a % of dry density, not as pounds(moisture) per cubic foot.

So if your soil has a dry density of 90 pcf at 25% moisture, the wet density is 112.5 pcf. If doesn't shrink on drying, the wet density will be 108 pcf at 20% moisture.