Gravity retaining walls - with saturated soil 3

[LR11]

I'm trying to figure out if this would be a real case, if anyone has designed for it?
A gravity retaining wall holding saturated soil?

What if you have a wall without mortar, would you take into account the pore pressures?

 

[r13]

Note, if there is tail water at down stream, the pressure at down stream will be r_w*h_TW. You shall also consider water pressures in the vertical seams (not shown in the sketch below), although the pressures will cancel each other out globally, it may cause instability of the down stream blocks.

 

[PEinc]

Essentially, you are trying to design a dam with a build-up of soil behind the dam. I agree with r13. You probably need to design each block level of the "dam" considering there is upward water pressure beneath each level of blocks. Start designing at the top level and proceed downward.

http://www.PeirceEngineering.com

 

[oldestguy]

Nice. So don't have to retain any saturated soil in additio9n to water.

 

[r13]

The focus is how to apply hydrostatic pressure on an unbonded stone/block retaining wall.

 

[oldestguy]

Great. Unbonded wall that doesn't leak. Why such nonsense here?

 

[r13]

My garden retaining wall was not bonded. It hasn't been fully saturated though.

 

[PEinc]

Oh, as OG said, it will leak like a sieve, or squirt like a fire hose if the wall is tall enough, until the joints clog (if they ever do). If you are able to stack block segments, you probably can also apply a waterproofing membrane vertically behind the blocks and below the bottom blocks. However, a segmental block "dam" would not be how I would want to retain the unbalanced hydrostatic head.

http://www.PeirceEngineering.com

 

[LR11]

Thanks for the responses.

The events I had in mind were a rainfall event lasting 1-2 hours, a burst water pipe, or daily watering for a garden. I don't think a perched water table would be relevant near the wall.
Is this an instance where the soil would be saturated? I have used the term "saturated" vaguely in the first post, and should have put more thought into it.
What if the wall was bonded with mortar? Both methods are common.

 

[r13]

For enclose flower bed in such events, the soil can be saturated (soaked), but there is little hydrostatic pressure will build up, because there are too many paths for water to escape. If you bond the blocks, or use liner, some head can build up behind the wall, but no pressure in the inter block seam though.

 

[LR11]

OK thanks for the response.

 

[Settingsun]

NAVFAC manual DM7.2 has a graph for increase in wall pressure due to rainfall behind a drained wall, ie slow flowing water rather than full hydrostatic. It's around 30% increase compared to dry.

 

[MIStructE_IRE]

You’re talking about rain saturated soil rather than this holding back the Pacific Ocean. I’m much less concerned now than when i read the initial post!

Do a strip of free draining material behind the wall with a land drain at its base.

 

[LR11]

steveh49, this is somewhat what I was looking for. Thanks very much.
I'll look into the effects of the granular material at the wall, to see the reduction. Cheers.

 

[Settingsun]

To clarify, the NAVFAC graph is for a soil-retaining wall with granular backfill. It gives the increase in overturning/sliding force for backfill when water (eg rain) is draining through the backfill compared with a simple dry backfill analysis. Most other documents take the easy way out and say to draw a flow net (who would bother?); NAVFAC gives a close-enough answer.

 

[PEinc]

I guess the wall is not very high. Why not just backfill the wall with clean stone wrapped in a filter fabric? Then, use either weep holes or a porous drain pipe or drain board to get the water out at the end(s) of the wall.

http://www.PeirceEngineering.com

 

[r13]

I would go with PEinc's suggestion, use liner and weep holes to positively control the hydrostatic pressure. The garden/top soil tends to clog the seams quite easily, so the amount of retained water is uncertain.

 

[LR11]

OK thanks for the additional comments.