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Washout through retaining wall

44K views 21 replies 12 participants last post by  nac 
#1 ·
So here is the problem:
About five years ago (on an account I have had for the last year), the previous LCO built 3 tiers of large stone retaining walls. He then back filled them with fill. They look pretty good and have not moved, but because he only used very large rocks there are sizable holes in between. During a heavy rainstorm the water runs behind the wall and washes fill through the wall and onto the turf.
The customer is definitely interested in mitigating this in some way, but short of tearing down the walls and rebuilding them with smaller rocks, drainage and landscape fabric, I cannot come up with a quick fix. So my question is has anyone else ever run into a situation like this, and what have you tried to fix it?
 
#2 ·
Whether you build a retaining wall from Segmental Block, or natural boulders, there needs to be a drain field behind the wall to capture water and drain it away. The only way to fix your problem would be to excavate behind the wall down to grade (equal to, but NOT below grade) and install a drain pipe. You can use perforated ADS pipe, or regular perforated PVC pipe. PVC pipe has hole only on one side so those holes MUST face down. Since your using boulders, you'll need to cover the backside with geotextile fabric and back fill with clean stone. You do not need geotextile between the stone and your retained earth, but should cap off the drain field with geotextile just before reaching your final grade. This prevents organic soil from washing down through the top and clogging your drain field. The drain pipe must pitch away from either side of the wall and exit to daylight.

When building a wall using SRW blocks, never use clean stone for the base. This must be solid non-impervious stone right up until exposed grade at the base level. You install your drainpipe at this level, and continue with clean stone from that point up. You do not want to allow water to saturate below grade (below the drain pipe too) and wash out under your wall.
 
#4 ·
Thinking outside the box a little...not that it really 100% solves the problem. But..it is a quick fix to the washout problem.

Get a bunch of plants that grow well between rocks and are drought/heat tolerant. Some chicks n' hens, rock jasmine, stonecrop, creeping juniper, etc. This should help a lot with the washout and help stabilize the soil. Water will still get through but the washout won't be nearly as bad. You'll basically be fixing the problem by softening the walls and adding some interest and color.

good luck:waving:
 
#9 ·
When building a wall using SRW blocks, never use clean stone for the base. This must be solid non-impervious stone right up until exposed grade at the base level. You install your drainpipe at this level, and continue with clean stone from that point up. You do not want to allow water to saturate below grade (below the drain pipe too) and wash out under your wall.
There is nothing wrong with using 3/4 clean stone as retaining wall base.
 
#10 ·
There is nothing wrong with using 3/4 clean stone as retaining wall base.
The only time this would be permitted would be in a shoreline retention project, and a job like that is totally different than any inland design. When building a wall, your base, back fill behind, and inside any hollow units must be soild 2A Modified. You want non-pervious material there right up until the grade of the property in front. At the same time, this is at the point you place your drain pipe and clean stone for drainage.

IF you were to use clean stone for the base layer of any wall project, water will fill up BELOW the drainpipe and wash out under the base course of block.

Here's a PERFECT example of water undermining this retaining wall, because the contractor used clean stone instead of non-pervious base material. :cry:

It won't be long before this wall settles and becomes unstable. For those of you who have stood at the beach when a wave washes over your feet..same thing...the sand washes out and you sink.

Bedrock Grass Building material Flooring Landscape
 
#12 ·
The best thing to do I see is to pull the wall down put the proper drainage behind the wall and landscape fabric behind the wall. Then if there is a water issue above the wall address that as well. Fix the job right the first time your account might like the cost in the beginning but they will be happier in the long run that there will be no more problems.
 
#13 ·
Sorry I have been away from the site for a while, things got a little crazy here. (i have been taking care of my sick grandma). But i really appreciate everyone's response. I am a huge believer in doing it right the first time so I agree with you that recommend tearing it down and building it the right way in the first place with drainage and geotex backing it. I also would build the wall a lot tighter, mixing some smaller rocks in to fill the gaps between the boulders better.

That being said I know this customer has figured on spending a certain amount of money of contracting projects this year and I know that fixing these walls is not going to be a priority for her. I could try and sell her on the danger that these walls pose (they are on a big hill in her back yard) if one of them let go the washout would hit the house. However the rocks are so big and the run off (even from a 100 year storm) would never be enough to take down an entire tier or enough material to damage the house. And I really do not like to hype things to customers to get a sale out of it.

My main concern is running my mower close to the edge and having a huge sinkhole open up. Also the dirt that washes onto the lower tier of turf after big storms is annoying to clean up.

Seems like the options I can give my customer are:
1. Fix it right, rip them down rebuild them properly
2. Patch them up and have her pay me to clean them up after each storm
3. (possibly in addition to #2) plant something in and above the wall with a strong root system

Either way unless she fixes it right I am going to mow next to them with a pushmower so I don't loos a machine over the cliff. I told her that if she fixed it right I could knock a few bucks of my mow price... we will just have to see which way she leans

As a side note: Does anyone know what quality of geotex I would use behind a drystack natural stone wall to hold back my back fill? Its got to be a stronger type than what they sell to wrap french drains in right? Any geotex gurus or salesmen out there?
 
#14 ·
Acctually just thinking about it.... the walls are only 4-5 feet tall, what if I just excavated 1 foot from behind the wall then put down geotex, backfill with gravel and pipe it off to either side into the woods (put raprock on pipe exits). Use perforated pipe with holes installed down, gravel over the top, geotex on top of that and then put my topsoil back.

This way I do not have to rebuild the walls (digging down a foot will not disrupt the integrity of the wall). It should capture the surface run off and keep it from getting behind the wall.

What do we think about this?
 
#17 ·
Champion, the wall shown in your pic was not burried enough below grade. Is that a shoreline application? Looks sandy. I wouldn't say failure was because of clean stone used as base. I have used it for years as base and no probs. Even used it after approved by engineer on state highway project.
 
#18 ·
I did not read all the post. But unless its a total rebuild i will not touch anyone else work for any reason. Lets say you attempt to fix the proablem. Then 1 year later something goes wrong with the wall that is not your fault, there is too much of a chance of the customer placing blame on you.

Just something to ponder.
 
#19 ·
EARTHWORKER:

This was not my job... it caught my eye while driving thought a bank's parking lot. (near new construction) I don't know whats behind it, all I know is it rained like hell the day before and that washout was evident.

You can build a wall on clean stone, ONLY if you can drain water away to a LOWER grade. If your at grade, you must use solid fill up until grade, then place your pipe. (Otherwise you leave the base saturated, and eventually causing settling and/or failure)

I personally don't like using clean stone for any base preparation. I want my base to be solid and less susceptible to any movement.
 
#20 ·
I have built 50,000 + sf of keystone wall all with 3/4" clean stone for the base as per my engineered shop drawings from keystone with out any problems. Where is the stone going to wash out to when it is buried and there is fill placed in front of it (toe).

Plant Water Nature Road surface Natural environment


Sky Road surface Asphalt Tree Grass
 
#21 ·
It may not wash out, but your creating a troth of saturation. Clean stone will act as a sponge, storing the water. What happens when you have saturation?- your subgrade gets soft and can settle. This can cause a wavy wall over time. Older specifications said clean stone was adequate. If you look into any of the manufacturers spec's now, almost all of them are on revisions. Solid material for your base, up until grade. Then add your pipe, and drainage aggregate. Those pictures you posted are Keystone. They appear to be 15 yrs. old. Am I right?
 
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