Drainage question

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.
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jeffc wrote:

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 can't hurt.
R
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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.
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richard wrote:

Plastic blessed by an angel.
I hope you meant in back of the wall, like this:
http://perfectscapes.com/custom/images1/wall_cross_section.jpg
Putting a drain in front of the wall wouldn't accomplish much.
R
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Richard your parents don't have a home, they live in a trailer.
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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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