Retaining Wall Drainage

[GreenMonster]

I'm quoting a wall rebuild:

175 x 4. Middle section (50') has failed. Surcharge behind this Allan Block wall. No grid, and backfilled with sand/gravel. No drain field or tile.

So, on a wall of this length, how would you guys vent the drain? Do you think it would be beneficial to vent the drain at more locations than just the ends? Only probably being, if I were to vent it in the middle somewhere, base course/drain is below grade?

Just kind of thinking out loud. Any thoughts?

[D Felix]

Is it planted in front, or bare?

Could you install weep holes?

If you can take it out both ends, I'd probably go that route, otherwise I'd probably bump up to a 6" perforated pipe behind the wall. Once the water is inside the pipe, it'll find it's way out. You just need somewhere for it to go.....

Are you rebuilding the entire wall, or just the section that failed?


Dan

[GreenMonster]

Wall runs along sidewalk.

Weep holes by drilling into bottom course with hammer drill?

Original bid called for repair of failed section only. I talked to them (City bid) and told them that it's only a matter of time before the rest of the wall comes down too.

So, of the 175', they want a bid to correct the worst 120', then a separate $$$ amount to rebuild the entire wall.

The wall has to be totally disassembled, excavated for crushed stone base and drain, geogrid install (due to surcharge). Rebuild and backfill.

Wall is 5 courses high, with cap. I'm working out manhours in my mind right now.

What say you, Dan?

[D Felix]

For the weep holes, yes, that's what I had in mind if you couldn't do any thing else...

Ideally, as you know, re-building the entire wall would be the way to go, and do it right when you do.....

As far as man hours go.......... Hmmmmm............ Roughly 7 hours for excavation....

Around 130 or so for re-build? That's quick and dirty time, but close to our production numbers...... Doesn't include any material procurement, etc, etc. And that's two of us. Add a third, and it may drop to 110 or so, I dunno.

If you want the way I figured excavation times, let me know, I'll email you the spreadsheet I've got set up.....

HTH.


Dan

[GreenMonster]

Originally posted by D Felix
For the weep holes, yes, that's what I had in mind if you couldn't do any thing else...

Ideally, as you know, re-building the entire wall would be the way to go, and do it right when you do.....

As far as man hours go.......... Hmmmmm............ Roughly 7 hours for excavation....

Around 130 or so for re-build? That's quick and dirty time, but close to our production numbers...... Doesn't include any material procurement, etc, etc. And that's two of us. Add a third, and it may drop to 110 or so, I dunno.

If you want the way I figured excavation times, let me know, I'll email you the spreadsheet I've got set up.....

HTH.


Dan

Spreadsheet? Yeah, cool. send it over. I hope to work on a bunch of stuff like that over the winter:rolleyes:


so 137 manhours? Did you figure that at 120' of rebuild, or 175'?

[D Felix]

120 feet.

Based on a wall we did last fall....

I'll try to send the spreadsheet and an explaination sometime this week.


Dan

[Randy Scott]

Sorry to sway from your question, but I've discussed this with a few people and don't think I have a good answer yet with drainage behind walls. As far as perforated pipe goes. My question is, what good does a perforated pipe behind a wall do? I understand it collects the water as it seeps through the fractured stone behind the block, and goes into the pipe. The problem I have is I think it's a joke. The pipe is perforated all the way around. The water that as easily goes in the top, will just as easily run out the bottom. In my mind, the only way water would drain through the pipe, would be a rush of water. There's no way you will get a rush of water 3 4 5 or whatever feet the wall is tall. I personally think it's useless.

If someone could explain that it truly does something, I'm all ears. One guy told me at least it's a piece of mind. No it's not. It doesn't make sense that this could even drain anything. It's a fricken' perforated pipe that lets water in as easily as out, right to your base buried course. Please indulge me with a good answer. Thanks!

[GreenMonster]

Originally posted by D Felix
120 feet.

Based on a wall we did last fall....

I'll try to send the spreadsheet and an explaination sometime this week.


Dan

:)

Well, that makes me feel good. I came up with 144 manhours!

[D Felix]

Plain and simple, the perf pipe is a hollow area for water to collect.

As the water travels down through the clean crushed backfill, it will find it's way into the pipe. From there, as the water level inside the pipe rises, it will find its way out. That's the reason to "outlet to daylight".

The perf pipe does not need to have slope; it's the trench that it's in that does. Make sense? Basically if the trench is level, and the pipe has slope, the water will have to rise in the voids of the gravel until it hits the pipe in order to drain very fast.

No, you won't have a "gush" of water in most cases. And it's more than just "peace of mind". It's a necessity behind almost every SRW.

The reason I mentioned bumping up to a 6" pipe for the length of the 175' of this particular wall, is that it is possible that a 4" pipe could be filled to capacity fairly easily. There's less than .15 gallons of capacity (I think I did that math right) per foot of 4" pipe. It doesn't take a lot of rain once everything is saturated to reach that capacity.....

Not bustin' ya, Randy, just 'splaining.:D


Dan

[Randy Scott]

I understand the pipe will be sloped or pitched by the crushed stone backfill. My question again is, how can a perforated pipe drain water to the low end when the pipe is perforated? It takes water in through the perforations on top, it can just as easily have it run out the bottom perforations. How, with all the perforations, does it flow out the end. It magically comes through the top perforation's, but doesn't exit the bottom perforations, rather instead it flows out the end? Do you understand what I mean?

[MarcusLndscp]

Randy the perf pipe we typically use does not have holes all the way around it although some perf pipe does. We use a solid perf pipe which has 2 lines of holes that are probably a half inch or so in diameter. To install it properly you lay the pipe down with the writing UP. When you do this it leaves the 2 lines of holes oh...about a quarter or a fifth of the way up the side of the pipe. The water level behind your wall has to raise to the grade of these holes in order for the pipe to work. Depending on the type of wall.......your water can percolate through the face of the wall or simply disperse itself to the path of least resistance. Perf pipe does work in instances of extreme water and I have seen walls without proper drainage fail. For the amount of money a client puts into the stone materials and labor to build a wall, the pipe is a minor cost to assure their wall will last forever.

[Randy Scott]

Now I can see how THAT would work. I guess I need to find suppliers with that type of perforated pipe. Most I've been able to find is either solid or completely perforated. I agree, $30 or $40 for 100 feet of pipe is nothing at this point, but when it actually does nothing (full perforation) I thinks it's stupid to even kid yourself that it is actually helping.

Thanks

[bottlefed89]

I think we've all seen it work though, doesn't dan's explanation help??

[D Felix]

Yes, I understand what you mean.

It's simple, really. All the pipe is, is a void for the water to flow through un-impeded.

I suppose if you backfilled with pea gravel, you'd accomplish the same thing. The water would drain through the pea gravel and eventually find it's way out, if there was a way out.

However, since usually you use clean, angular, fill, the water won't flow as fast. Also, most walls don't just come to an abrupt end, most usually turn a corner or curve back into a hillside. This is where the pipe is needed the most; to direct the water to daylight.

Think of the water behind the wall as a miniature water table. As the water level rises, it will fill the pipe from the bottom up. Once the water gets inside the pipe, it should start to flow out rather easily, finding its way to daylight.

It's kinda like an overflow drain on a bathtub, only the water shouldn't have to get so high to drain out!

Pipe that is perforated all the way around works, I gaurantee it.

The water really doesn't enter through the top slits very much. Most of the water entry is through the bottom. I have a feeling the reason the corrogated pipe is perforated all the way around is to save a lot of headaches and confusion for installers.

As long as the pipe doesn't fill with silt, the pipe will do it's job, whether it's slotted all the way around, or has holes that go on the bottom.....

Again, the water doesn't "magically come through the top perforations, but doesn't exit the bottom perforations". You're thinking about it wrong. Think more along the lines of the bathtub. That's the best analogy I can come up with right now!

HTH.


Dan

[kootoomootoo]

I typically use half perf. ie top half is perforated..bottom isnt. Water takes the path of less resistance.

[D Felix]

If you are putting it in with the top perforated, it's upside down.....

[GreenMonster]

Originally posted by D Felix
Yes, I understand what you mean.

It's simple, really. All the pipe is, is a void for the water to flow through un-impeded.

I suppose if you backfilled with pea gravel, you'd accomplish the same thing. The water would drain through the pea gravel and eventually find it's way out, if there was a way out.

However, since usually you use clean, angular, fill, the water won't flow as fast. Also, most walls don't just come to an abrupt end, most usually turn a corner or curve back into a hillside. This is where the pipe is needed the most; to direct the water to daylight.

Think of the water behind the wall as a miniature water table. As the water level rises, it will fill the pipe from the bottom up. Once the water gets inside the pipe, it should start to flow out rather easily, finding its way to daylight.

It's kinda like an overflow drain on a bathtub, only the water shouldn't have to get so high to drain out!

Pipe that is perforated all the way around works, I gaurantee it.

The water really doesn't enter through the top slits very much. Most of the water entry is through the bottom. I have a feeling the reason the corrogated pipe is perforated all the way around is to save a lot of headaches and confusion for installers.

As long as the pipe doesn't fill with silt, the pipe will do it's job, whether it's slotted all the way around, or has holes that go on the bottom.....

Again, the water doesn't "magically come through the top perforations, but doesn't exit the bottom perforations". You're thinking about it wrong. Think more along the lines of the bathtub. That's the best analogy I can come up with right now!

HTH.


Dan

Dan,

The sink/bathtub overflow is a GREAT analogy. I started a reply to Randy back on page one, but didn't finish it because I was having trouble explaining it.

This post says it all.

[kootoomootoo]

Originally posted by D Felix
If you are putting it in with the top perforated, it's upside down.....


Didnt say I installed it that way ........half full or half empty......

[MarcusLndscp]

Randy,
Dan is correct though, the flexible perf pipe with holes all the way around does work too. The flex pipe is usually easier to snake around in trenches or around corners where the solid pipe you obviously need to have plenty of fittings and a saw to make various cuts. The pros and cons I know of and hear people complain about are that the flex pipe crushes easily and the more solid type depending on the kind you buy can fracture or break in freezing temperatures. The kind we currently use has a bit more flex to it and I have yet to see it break. If we are crossing a driveway or road with a pipe to a catch basin or a swale we typically over kill it and use SDR pipe instead which is much stronger (also available in perf)

[landscapingpoolguy]

typically when installing drainage pipe behind a wall you would need to run the pipe behind the bottom course of the wall, Below that should be a qp base or compacted stone base, I prefer qp. OK now the reason for putting 3/4 behind a wall is simply drainage. For instance its dec 11 and it was freezing rain all day and now snow.....Any water that has traveled below the frost line will continue to run...the 3/4 will prevent a mass frezing behind the wall and thus a heeve of the wall.....the 3/4 will also allow the water to travel all the way down to your pipe without pooling. Now even if you have used perf pipe with perfs all around the water will still run to daylight ..most french drains are perf pip all the way around and they work great....the pipe will only assit the flow of the water. Advantage to perf pipe all the way around verses half perf pipe, is that the full perf pipe will not allow water to pool until it hits the perferations. Rather it will follow gravity and seek daylight. Trust me watch a vent pipe from behind a wall during a spring time thaw and tell me that it doesnt work......4' high allan block wall should not fail because of no grid.....grid was invented after the allen block as a secondary measure of strenth. I beileve most allen block is rated to 36 " and i know some are rated to 56" inches. reguardlessplease instll pipe behind teh wall and vent it at the lowest possible area. i would cut curb and vent to street if right along a sidewalk. Wheepholes will simply clog and will freeze in the winter

hope this helps

Chuck

[Randy Scott]

Originally posted by D Felix
Think of the water behind the wall as a miniature water table. As the water level rises, it will fill the pipe from the bottom up. Once the water gets inside the pipe, it should start to flow out rather easily, finding its way to daylight.
Dan

Now that answers my question and makes sense. I can totally see how that explains it. Now at least when I talk with someone, I'll actually have a good answer and explanation for it. Thanks.

[gvandora]

Ask your block supplier. They probably have catalogs from the suppliers. Also, check their website. Most likely you will be able to find specs for drainage.

[D Felix]

I'm surprised no one caught me on my math in one of my previous posts...

I believe I said foot long section of 4" pipe will hold .15 gallons.... When I posted that I thought it sounded *really* low, but couldn't figure out why....

I figured out why a couple of minutes after I posted that statement, but forgot to correct myself until now. A foot long section of 4" pipe will hold .15 cubic feet of water, not gallons. Translate that into gallons (IIRC, there's ~7.8 gallons/cubic foot), and a foot long section of 4" pipe will hold roughly a gallon of water.

I doubt it created confusion, but hope this may clear any of it up.

And Mark- one thing I didn't consider in that rough estimate of 130 hours is picking up the block that failed. Between excavation and construction, that 137 hours should be close, but adding in picking up the failed block will probably add some time...


Dan