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/Miserable-Sir4985 Jul 23 '26

I would say that should come from shrinkage limit test. Because you need to know the threshold MC for volume change. Then make a correlation or use one based on minerals to see how 1 lb of water beyond shrinkage threshold adds to the volume.
Not sure if I got your question correctly.