[Tmaxx1]

I've got a couple of questions about installing drainage tile behind a retaining wall:
1. Do I install the drainage tile directly on the ground at the lowest point of my trench below the compacted base. Or do I install the drainage tile on top of the compacted base. My trench is 12” deep. I plan to add 6” of compacted base. My drainage tile is 4”. The height of the wall will be 24".
2. The retaining wall will run 16' and then turn 90 degrees and run up a slope about 9'. I am going to run drain tile the entire length of the 16' wall (which is at the bottom of the slope). I need to pass the drain tile running along the 16' wall underneath the perpendicular wall on the right side to direct water away from the wall. I am not sure how to pass the drain tile underneath the wall. Could someone offer some guidance?

 

[jomama45]

I've got a couple of questions about installing drainage tile behind a retaining wall:
1. Do I install the drainage tile directly on the ground at the lowest point of my trench below the compacted base. Or do I install the drainage tile on top of the compacted base. My trench is 12” deep. I plan to add 6” of compacted base. My drainage tile is 4”. The height of the wall will be 24".

You need to backfill the wall when you get a course or 2 of units in place with impervious soil or gravel (not clear stone) to get the draintile ABOVE the soil on the front side of the wall. If you install the draintile below grade, or install the clear stone that low, the gravel base under your wall will always be saturated with water, which will not work well long term.

2. The retaining wall will run 16' and then turn 90 degrees and run up a slope about 9'. I am going to run drain tile the entire length of the 16' wall (which is at the bottom of the slope). I need to pass the drain tile running along the 16' wall underneath the perpendicular wall on the right side to direct water away from the wall. I am not sure how to pass the drain tile underneath the wall. Could someone offer some guidance?

Inevitably, the draintile will have to exit through the wall at some point. Check out the manufacturers site, as I'm sure they'll have pictures. Also, if the grade behind the 16' wall goes up 9' relatively fast, you have to figure this into the load of the 16' wall. When a wall sits at the "toe" of a large hill, it sees far more pressure than level ground behind. Again, the manu's all have details on these specifics. If your using something along the lines of HD block, I would save my money & labor & look into a larger, stronger system.

 

[Daniel Holzman]

Usually I don't like to correct previous posters, but jomama45 is completely wrong about the installation. First of all, you never want to backfill your wall with "impervious soil", since by definition impervious means that water will not flow through the soil and you would not get any drainage through the backfill, which will totally negate the purpose of putting in drain tile. As for using clear stone, I have no idea what that is. For backfill, you need either clean gravel, 3/4 inch crushed stone, or coarse sand.

The drain tile goes uphill (behind) the wall, at approximately the level of the bottom of the wall. There is no need to prepare a special base for the drain tile itself, of course you need a base for the block that makes up the wall, as you indicated. The simplest type of drain tile is schedule 40 PVC pipe with the perforations already in the pipe. You simply glue it together, wrap it in filter fabric, then install behind the wall at a minimum of 2 percent slope. The pipe exits underneath the perpendicular part of the wall. You will need a T to connect the two runs of pipe. Since your wall is only two feet high, there is no issue about crushing the pipe even if you place it directly under the wall. By the way, the holes face downward.

You backfill above the pipe with 3/4 inch crushed stone, clean gravel, or coarse sand. You only need to backfill about 6 inches wide, and bring the backfill almost to grade. The last few inches you can backfill with topsoil so you can get some grass to grow over it.

The water will drain vertically downward to the pipe, enter the pipe, and run downhill to drain out. This will minimize pressure on the wall, although realistically for a two foot high wall, I would not worry too much about it. When the wall gets to be 4 foot or taller, you need to start worrying about hydrostatic pressure, but still, a drain tile is a good idea, and will minimize the chances of the wall moving. If you get the drain tile set properly, you can use concrete block without weep holes. If you skip the drain tile, you will need weep holes in the block, typically about every five feet.

 

[jomama45]

Usually I don't like to correct previous posters, but jomama45 is completely wrong about the installation. First of all, you never want to backfill your wall with "impervious soil", since by definition impervious means that water will not flow through the soil and you would not get any drainage through the backfill, which will totally negate the purpose of putting in drain tile. As for using clear stone, I have no idea what that is. For backfill, you need either clean gravel, 3/4 inch crushed stone, or coarse sand.

Daniel, I happen to take offense to your mis-statements & correct all of the mistakes you made in your reply, which are numerous. First, I'll admit, I now only use Versa-Lok SRW units. I used to do some work with a Keystone manufacturer about 7 years ago, but no longer use their system. Approximately 9 years ago, I took a training seminar through Versa-Lok & recieved a 200 page installation guide that's full of section illustrations of all typical, & many non-typical applications for the system. Believe it or not, Versa-Lok has done a large amount of there own engineering on their own product, & there process seems to be vastly differnet than your approach. I think I'll stick with there engineering in this case though, as I feel they may be a little better qualified in this field. And I've set somewhere between 12-15,000 of their units in the last 15 years with no issues using their specifications. Well, I dug the manual out & every cross section of every wall section shows the the draintile slightly above the grade on the front of the wall, just like I stated earlier. Under the pipe , it states IMPERVIOUS FILL in every cross section also. That means that impervious fill is added on top of the gravel leveling pad to get the drain pipe above grade, & even pitched slightly towards the wall. At this point, the free draining stone first starts.

The drain tile goes uphill (behind) the wall, at approximately the level of the bottom of the wall. There is no need to prepare a special base for the drain tile itself, of course you need a base for the block that makes up the wall, as you indicated. The simplest type of drain tile is schedule 40 PVC pipe with the perforations already in the pipe. You simply glue it together, wrap it in filter fabric, then install behind the wall at a minimum of 2 percent slope. The pipe exits underneath the perpendicular part of the wall. You will need a T to connect the two runs of pipe. Since your wall is only two feet high, there is no issue about crushing the pipe even if you place it directly under the wall. By the way, the holes face downward.

You backfill above the pipe with 3/4 inch crushed stone, clean gravel, or coarse sand. You only need to backfill about 6 inches wide, and bring the backfill almost to grade. The last few inches you can backfill with topsoil so you can get some grass to grow over it.

Manual also states: Drainage aggregate - 12" deep minimum. Clear gravel is clear of all fines much the same I assume as "clean gravel". I wouldn't waste my time with sand as it not as free draining as stone & IMO has no benefits over stone. The cost of clear stone is minimal in terms of a retaining wall project, so why scimp? Now, assuming the minimum (of all block manufactures that I know of) embedenment of 1 course, how do you get any drainage out of the pipe if you set it directly on the gravel base? If you start with the drainage pipe behind the wall at 4"-6" below grade, it will NOT drain to the face of the wall. That is where the impervious fill comes into play, it's used to raise & pitch the drainage pipe above grade through the front of the wall.

The water will drain vertically downward to the pipe, enter the pipe, and run downhill to drain out. This will minimize pressure on the wall, although realistically for a two foot high wall, I would not worry too much about it. When the wall gets to be 4 foot or taller, you need to start worrying about hydrostatic pressure, but still, a drain tile is a good idea, and will minimize the chances of the wall moving. If you get the drain tile set properly, you can use concrete block without weep holes. If you skip the drain tile, you will need weep holes in the block, typically about every five feet.

Daniel, sorry I couldn't provide links for the above info, but I didn't want to take the time & space to download the entire few hundred pages of install manual & tech info when I have it right in front of me in paper form. Feel free to download it yourself & go over the info carefully before mis-interpretting another professional's advice.

 

[Tmaxx1]

Thanks for the replies. One more question I have is regarding the drain tile. I was planning on using 3" flexible black slotted corregated pipe. I hadn't considered perforated PVC pipe. Are there any advantages of one over the other?

 

[Tmaxx1]

HD Block?

Inevitably, the draintile will have to exit through the wall at some point. Check out the manufacturers site, as I'm sure they'll have pictures. Also, if the grade behind the 16' wall goes up 9' relatively fast, you have to figure this into the load of the 16' wall. When a wall sits at the "toe" of a large hill, it sees far more pressure than level ground behind. Again, the manu's all have details on these specifics. If your using something along the lines of HD block, I would save my money & labor & look into a larger, stronger system.

What is HD block? And what do you mean by a larger and stronger system?
thanks.

 

[jomama45]

Tmaxx, IMO the 3" perferrated is adequate in this situation. As a matter of fact, thats all we normally use for retaining walls.

As for the HD, I was reffering to Home Depot block or the like. Though these block may seem like a value, when you break down the square footage & mass values of the larger systems, they are often less of a value.

 

[Daniel Holzman]

Well, I guess as registered engineer with 30 years of geotechnical experience, and having designed retaining walls using Versalok, Keystone, and dozens of other types of walls, and having testified in court on several occasions about wall failures, and having written numerous papers on the subject, I might know something about retaining walls. Then again, I am just another faceless blogger in the infinite ooze of the Internet, so I might be just another idiot posting crap. I let you decide.

That said, let me explain the theory of why I ALWAYS set the drain pipe on the uphill side of the wall, and approximately level with the base.

The purpose of the drain pipe is to maintain the water table on the UPHILL side of the wall as low as possible. The reason for this is that if the water table rises above the bottom of the wall, the water will exert pressure on the wall. The pressure of the water is equal to the unit weight of the water (62.4 lbs/cubic foot) multiplied by the height of the water table above the base of the wall. So for example, if you have a ten foot high wall, and the water table rises 6 feet above the base, the water will exert approximately 370 lbs per square foot against the wall. The soil also exerts pressure on the wall, and the combination of soil pressure and water pressure, if too great, will either cause the wall to fail by sliding or overturning.

Therefore, the game plan is to keep the water table as low as possible. This is done by placing drain pipe beneath a freely draining layer of gravel, sand or crushed stone, which is place behind the wall. Putting the pipe on the downhill side of the wall forces most of the water to pass through the wall before being intercepted by the pipe, except for the water that flows under the wall. This is not a good arrangement, far better to capture the water on the uphill side, and direct the water away.

I am attaching a pdf file (from Versa Lok) which shows the correct way to design a wall. Note that the drain pipe is on the UPHILL side of the wall. Note also that they are using 4 inch pipe, which is pretty standard, but for a 2 foot high wall, a bit of overkill. Also note that they recommend 12 inches thick of freely draining backfill, again a bit of overkill for a two foot high wall.

Don't believe me? Go to the Versa Lok website, and check out the wall designs. If you see one where the pipe is on the downhill side of the wall, it is for a very special purpose, in my 30 years I have never seen one built that way deliberately by a contractor who knew anything about wall design, but hey, as I said, I am just another faceless blogger on the internet of life.

 

[jomama45]

Daniel, you posted a link that's right in front of me, but mine is much more tattered. I have no argument with the figure at all. But you origanally stated that I was incorrect in saying that the pipe had to be elevated with impervious soil. You stated that no additional substrate was needed under the drainage pipe AND the pipe should be on the upside of the exposure. With the minimum embeddenment of the SRW, & the fact that water runs down hill, how would both of these scenarios be humanly possible WITHOUT additional material under the pipe? If you would post a link to the next cross section (at least in the paper editon) everyone could see where the impervious soil was placed.

BTW, I think you got confused as to thinking I was suggesting to the OP to back fill the wall with impervious soil. I was merely answering his original question, which was what ELEVATION does the drainage pipe get placed. It still stands as I originally answered, ABOVE the grade at the face of the wall, on top of IMPERVIOUS soil.

 

[Tmaxx1]

Tmaxx, IMO the 3" perferrated is adequate in this situation. As a matter of fact, thats all we normally use for retaining walls.

As for the HD, I was reffering to Home Depot block or the like. Though these block may seem like a value, when you break down the square footage & mass values of the larger systems, they are often less of a value.

So you are comparing the smaller blocks (approx 4" tall x 12" long x 6" deep) to the larger blocks (approx 6" tall x 14" long x 12" deep @ 80lbs). Why would the larger blocks be preferred over the smaller blocks?

 

[Shura]

Daniel, your pdf and notes are somewhat off. The pdf shows the draintile starting at about the second course and your personal directions say to basically lay the drain tile on the pad/footer. Which should I do? Also does it matter how close the tile is to the wall. I have a really tight fit so might have to squeeze it right against the back of the wall.
Thanks !!

 

[Daniel Holzman]

I always design to put the tile at the level of the bottom of the first course, because this minimizes the water pressure (the pressure will be essentially zero), but if it is not possible to put the tile that low, one course up is 8 inches or so, which will add about 40 psf pressure to the wall. Not ideal, but unlikely to be a problem with a two foot high wall. The key is to make sure the backfill over the tile is clean, so water can percolate downward easily, and exit via the tile. It makes little difference if the tile is immediately adjacent to the wall, or set back a few inches, so long as the tile is covered to the surface with clean, free draining backfill.