Drain tile and srw's

[scott C]

How many use drain tile along with #1 stone behind srw walls? Do you install it with any pitch to allow for drainage? I have never read anything or seen pictures of drain tile behind walls laid with any type of pitch Am I missing something?

[Stonehenge]

We install 4" corrugated perforated pipe behind walls all the time. As for pitch - we either keep it level or if there are step-ups, we pitch it toward where it daylights. Pitching it is not a requirement - as a guy who installed sewer pipe for a living told me - "you keep it level, the water will find where it's supposed to go."

I'm not familiar with #1 stone - if that's 3/4" clear, then that's what we use behind walls where we have perf pipe.

[agla]

Perforated pipe takes water in only when the surrounding material is saturated.

When using perforated PVC (2 rows of holes), the idea is as the material saturates from the bottom up, it flows into the holes and into the channel between the two rows. The pipe should be pitched to carry that water away. This is best when you have water that you want to pick up and move a distance away. The pipe will not give up the water untill the end of the pipe or when there is more water in the pipe than that narrow channel can hold and the surrounding material is not already saturated. Sometimes you need to do this, but not always.

When using corrugated perforated pipe you have a much different situation. The surrounding soil must still be saturated in order for water to enter the pipe, but you have no channel. That means that as soon as the water in the pipe comes to an area that is not surrounded by saturated material it drains out of the perforations. The corrugation slows down the speed of the water further facilitating the bleeding out of the water. This piping is effective for evening out where the water is rather than taking it away. It is not very effective at quickly taking water away to an area where it can be managed unless the water is coming in at a fast pace. It more or less balances it out within the area of the pipe. Sometimes that is exactly what you want to do.
One danger is that the pipe can easily get ups and downs in it when simply tucked behind the bottom course of SRW's which renders it only useful as a holding area for water rather than a drainage system.
When laid level behind an SRW it is still largely relying on your base material to ultimately absorb the water. It does create a reservoir to let that happen over a greater time, though.

SRW's don't rely 100% on drainage pipe, but a combination of good drainage material for backfill and base, weeping seams between blocks, and drain pipe. I don't think that you have too much to worry about in how well pitched a perf pipe is behind one under normal circumstances. If it were a solid wall (like poured concrete), I would definitely use a pitched Perforated PVC pipe.

[jwholden]

The perf. pipe is only for a worst case scenario. However, I like to pitch them. What happens if your backfill settles 1/2", the the whole purpose of the pipe is lost and the water may drain away from daylight.

I notice a lot of guys run the perf. pipe through the wall. I have never done this because I don't like the look of the pipe poking through. However, I also realize that doing it that way would be a HUGE time savings.

How do the vets go about running a pipe through a SRW. IE what course do you start on, what size pipe, what do you use to make the outlet 'Blend' with the wall.

Thanks,

JW

[Paul]

We do it on some walls. Outlets are drilled out with a 5" core bit for 4" pipe. Some walls have to have it done, one location sea walls need to have it. We figure them for every 20'. Any time you get a high fluctuation of the water table you need to find ways to relieve water pressure, more correctly the difference in water pressure.

[Stonehenge]

Agla, I think you make some good points about the weaknesses of using corrugated perf pipe. One thing I noted you mention that I know we don't do is rely on the base below the pipe to absorb the water. After we prep the base and lay the first course or two of block, we place backfill (usually clay) behind the wall, to at least the elevation of the finished grade at the face of the wall. I didn't think of this, this is what is recommended by Versa-Lok. The thought is it keeps water from getting to a place behind the wall where it can't easily escape.

Above that compacted backfill we place the pipe.

JW, I guess I've seen projects where the pipe is run out one end of the wall - to me that always looked a little clunky, and there are wall situations where that just wouldn't be possible. Daylighting through the wall is what is recommended in the Versa-Lok install manuals. We don't use a core bit like Paul mentions, but that would be a cleaner solution. The other solution I've seen is to use a nice steel or brass grill plate that is inserted in the square opening you cut in the block to daylight the pipe. Much sexier look.

As for the height of the daylight, I always place it as low as I can on the wall without going below the finished grade at the face of the wall. Where to place it along the length of the wall? Never where it meets a structure, but other than that, I place it wherever it makes the most sense for drainage purposes.

[agla]

I don't think that corrugated pipe use is a weakness as long as you understand what it is doing. It is preferable sometimes and not so at other times. It clearly does something very different than PVC.

Know what you want to happen and use the techniques that will get you there.

Putting a layer of clay down stops water movement out the other side of the clay (down into the ground). That causes saturation in the clay and above it if there is enough water present. That will allow your pipe to pick it up but only because you are causing it to puddle there. If you go to that extreme to put water into the pipe, why would you want to let it work right back out by using corrugated? Why you want to stop it from leaching into the soil and to build up behind the wall under normal circumstances is beyond me, unless you have a steep slope below the wall.

Why create a surcharge? That clay will hold its water longer than anything else behind that wall. Depending on the type of clay it can act like a lubricant between two soil types. what is their reasoning to do this?

Leaching water effectively is ultimately what you want to do. Sometimes circumstances causes it to concentrate, then you move it to where you can manage it. If you have a huge effective leaching zone behind a wall, why interupt the use of it?

[scott C]

Agla good description of the use of pref. pipe behind srw walls. I have built quite a few walls both on my own and previous companies never fully understanding the layout of the pipe. I always assumed it was to be used as a relief valve for water that may get behind the wall. Thanks

[Stonehenge]

Agla, I'm not sure I understand how I'm preventing the water from leaching into the soil. And I guess moreover, I'm not sure why I'd want to let the water leach into the soil anyway, when I would think the preference would be for the water to exit from behind the wall ASAP, and exiting trhough the face or via pipe would seem the quickest. All of the soil around here is clay. So if I were to have total clear stone backfill behind the wall, top to bottom, the water would sit behind the embedded courses, and be subject to all kinds of freeze-thaw movement. The reasoning behind the clay cap over the base prep is to prevent water from getting there. The natural weep seams in the block, coupled with clear stone behind the wall will allow the water to escape the face of the wall; the pipe behind it is an added measure of safety.

Check out the Versa-Lok Tech Specs here: http://www.versa-lok.com/pdf/dni.pdf Page 7 shows a really good graphic of what I'm talking about.

Having clear stone all the way down would be the thing that creates the surcharge, as I can see it.

[Paul]

We need a "cap" to prevent water from entering the totop of the wall, this reduces pressure on the wall wall and keeps water behind the wall to a minimum. The clear stone keeps the movement of the soil directly behind the wall to a minimum.

There are times when we do want water behind the wall to move slowly, this is mostly when you can have water that tends to stay in the soils behind the wall. Here you can do away with the pipe and use the face of the wall as a weep type drainage. This prevents water from pushing out at one spot. Of course these walls should only be designed by a soils engineer.

[Rex Mann]

We cap the clear drain stone with clay to help divert water away the rear of the wall. Putting clay on the base course to help divert or keep out water does not make sense to me. That clay will act as a cap when the frost, which is moisture in the spring, is coming out of the ground. As far as the drain goes, if it is 4-feet or higher we'll put in a drain and only if we can tie it into or create a drain system. I've never cut drains in the face of a wall, that is "old school" engineers and architects not knowing the difference between a solid wall and a SRW. We would use the corrugated perforated 4-inch with a sock when a drain was needed. The walls are segmental for a reason. For more information on SRW's Dr. Bathurst is the most respected in the business.http://www.rmc.ca/academic/civil/personnel/bathurst_e.html

Peace,

Rex

[Stonehenge]

I guess allowing water to get behind the back side of a wall, below the finished grade at the face of the wall, with our clay soils (which will hold that water for a long, long time) does not make sense to me.

I found the install specs for Pisa, and they show what you and agla are speaking of (here - look at page 17: http://www.risistone.com/srw/installation/pisa2/Pisa2_Installation_Guide.pdf ), but they don't show where that drainage pipe is going. Even so, it still does not make much sense to me to allow water to get to that depth.

I certainly don't claim to have all the answers...heck, I don't even have most. But I'm using specs put together by the engineers at Versa-Lok, and having not found a problem or failure for which this method was the cause, it works for me, so I'll continue to use it.

Rex, I'm curious where you daylight your drains, if not through the face of the wall.

I just browsed the report you linked - looks like a great lab he has for running tests. Do you have any links to reports that address the drainage/backfill issue as discussed here, or did I overlook it over there?

[Rex Mann]

Look a page 16 of the document you reference from Pisa 2.

What is the purpose of clear drain material behind a wall? Is it not to allow water to easily drain through it? Therefore, why not put the drain at the bottom of this layer of drainage material. That is where the water is going to end up.

Water follows the path of least resistance and gravity. With frost coming out of the ground coupled with hydrostatic pressure in the spring from the soil directly behind the wall the water is going to end up at the base of the wall. Therefore, if installing only one drain, it goes at this level.

"Not one engineering school in the US offers a specific class on SRW planning, design and specifications"

That is according to Versa-Lok in September 2003 issue of Landscape Contractor.

Agree to disagree?

Peace,

Rex

[Stonehenge]

I believe people of good conscience can do just that, and I know of none with greater quantity than you.

:D

Jeff

[agla]

What I did not realize was that your existing soil was all clay. When I read your post, it sounded like you were introducing clay as a new and separate material that would interupt leaching into the existing soil.

It now makes perfect sense to make a barrier to prevent it sitting in your base gravel since that would be the only place it could go (and stay). I see where you are coming from. You actually do want to force the issue of getting the water into the pipe.

I'm in a glacial outwash plane - spill a drink and there is no puddle. Most of our "soil" is medium to course sand. Here the drain tile is mostly to look like you did what you are supposed to and serves no real function. I swear at this sand as a growing medium, but it has its advantages too.

I would go with the pitched PVC in that clay to get that water out more efficiently. Pick it up and move it, don't redistribute it behind the wall if it won't leach.

Thanks for the explaination. I learn something on this board every day.

PS. Water also moves by capillary movement. Clay has the strongest capillary strength and will move water upward almost as fast as downward untill saturation when gravity completely takes over. When people say that clay sucks, they are right. It just does not do it fast and it won't give it up to a courser material on the other side of it. That is why the base gravel will get no water out of that clay.

[Stonehenge]

I'm wondering about the PVC - the only stuff available close to me has quarter-sized holes in it, not slits like the corr. perf pipe. I'm concerned about material finding it's way into those holes and plugging it up.

I'm accustomed to wrapping the whole works (drain tile and clear stone) in a geotextile envelope, but am wondering if it'd be better to first wrap the pipe (if I switch to PVC instead of corrugated), and then envelope the whole thing. Opinions?

[Rex Mann]

We use a geo-textile sock. They sell them at my local HD stores. They are 4" diameter. Back east we would buy the 4" corrugated perforated with an existing sock. I've never seen it out here.

Peace,

Rex

[Stonehenge]

I've never seen anything other than the 4" pipe you buy with sock already on.

Agla, I learned something new today as well....Didn't know that clay soils could produce capillary action. Through a few engineering courses I'm aware of it (for anyone else, place the tip of a paper towel into some spilled water - the water defying gravity and racing up the paper towel is an example of capillary action), but not related to soils.

Cool stuff! :D

[Bexter]

I believe we do it the same way as you Jeff ... clay up to grade then our pipe and clear stone ... Never went by any specs, just what made sense to me ... doesn't mean it is correct. I also never saw the point of having your clear stone below grade and water sitting there.

[jwholden]

I'm leary of socks around pipe. It silts up quickly and creating an impenetrable barrier to water. I once installed a french drain in silty loam and the darn thing silted up way too quickly. Other friends in the trade have had similar results.

If given the choice between a sock and laying the pipe a few inches high to accomodate for siltation I go with laying the pipe a little high. They will both silt up over time, but I'm going with the pipe with no sock taking longer.

Those of us that don't cut a hole in the front of the wall are forced to run the pipe UNDER the wall to daylight, or create a 'DRY WELL' to run it into. This involves more time, materials, and EXPENSE. Hence, if I could find a better way to get the water out from behind the wall I would go with it.

I have seen pictures of walls with various grates in them to get the water out, not sure if the grates were connected to a pipe or not. I LOVE the simplicity of this solution and enormous time savings. I'm not crazy about looking at a grate in the middle of a gorgeous wall.

[Lanelle]

I prefer to wrap the back side of the clear gravel with the fabric. Also, I want to use a woven fabric with ample spaces between threads so the fabric won't silt closed so quickly.

[agla]

The PVC is going to flush the silt that gets in it far better than corrugated. A smooth bottom channel pitched at 2% is not going to be clogged with clay very easily. It will wind up in your dry well, though.

Clay particles move in water as suspended material. They do not fall out of solution when the water is moving quickly. Corrugation slows down water movement making it much more likely for clay to fall out of solution. That clogs the pipe.

By putting a sock around corrugated, the idea is to keep silt and clay out because it is a major problem once it is in.

The PVC accepts the silt and clay that is in solution when it rises in the drain rock to the point where it enters the holes. The pipe should have drain rock under it (JW). The pipe should be pitched at 2% (~1" per 4') to keep the speed of the water fast enough to carry out the suspended material.

Many walls that I see have corrugated laying on the base material (level) right behind the blocks. A lot of the time they go up and down over obstacles. Maybe they don't really need them to be very effective , so it is not a problem. But it is not a well thought out functional drainage.

What it really comes down to is how much of a problem is the water, how well does it weep, how well does it leach, and is the the piping method adequate to overcome these? There is no reason for over kill. You do want to follow someones minimal spec's in case of failure and litigation.