How were French drains constructed prior to geotextiles? My understanding is that geotextiles act as a filter to stop particulates from clogging up the area around the rocks or pipe, so that water can move freely. That said, drains have been built for hundreds of years, so how was this problem solved prior to the use of fabric?

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They used a different aggregate than crushed stone (gravel), which tends to produce a fairly tight matrix that clogs easily from non-organic fines (sand). Draining aggregates of yore include shells, and bones, but rocks from river beds were likely most common in most places. These naturedly rounded rocks leave wide gaps between rocks, allowing fluids to flow through easily.

The jagged and irregular edges produced by crushing rock tend to form a much tighter matrix, the gaps are small and have many particle-catching pinch points. Such material weighs more per cubic volume than pebbles; it's more dense.

Since it's much cheaper to ship giant stone slabs to the crusher than to (unsustainably) harvest pre-rounded rocks, that's what is most commonly used. Sometimes they round off chipped gravel in a tumbler to make pebbles, but that again is more work and expense compared to crushed stone. To mediate the effects of silting on fine matrixes, landscape textiles are used in an attempt to keep those drainage-inhibiting fines out of the matrix.

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