I see in many cases it's recommended to put in gravel for drainage. For one
example, you are installing a retaining wall. It's usually recommended to
put in some gravel behind the wall for drainage. How does this help
exactly? You haven't really done anything here except provide a bigger
space for water to collect into. But it's not really big enough to hold all
the water anyway. It seems that it would be more important to have a place
for the water to go to, rather than have a bit of extra room to sit in
There should be drain tile at the bottom of the gravel to allow water a
convenient exit. Gravel alone is okay for low retaining walls of
interlocking block or dry laid stone as the water can exit through the
face of the wall. Filter fabric should be used to prevent fine
aggregate from filling the voids.
The gravel is meant to promote swift drainage. It is not meant to act
as a catch basin, so the amount of water it would hold is not a
meaningful measurement. The gravel also allows a bit of expansion
space - not really sure how much that actually accomplishes, but it
Learn more about "french drains".
At my parents house we had to rebuild a retaining wall that fell. The
contractors dug a line in front of the wall, layed in holy pvc pipe then
covered it with gravel. The pipe was extended out farther into the main yard
for better drainage. They also built into the wall drainage holes which
helped the water be guided into the drain.
That's how the levees failed around New Orleans. The flow over the top of
the levee backwashed the foundation, ate it up at its base. Over time, a
retaining wall can "see" the same thing. Gravel acts as a splash
preventative basin. That's surface erosion.
If the native soil incline is not conducive to drainage, does not promote
water away from the retaining wall, another method is needed to get water
away from it. This may be an unseen french drain. This does 2 things.
Absorbs alot of water, and spreads its water wealth. Backfills the
foundation of the retaining wall on the drainage side, lots of gravel won't
give way like wet soggy soil will. The typical french drain uses 1" or
better stone gravel. Depth may vary.
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