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Posted by Jonny on September 19, 2006, 9:33 am
>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 before overflowing.
>
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.
--
Jonny
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