Retaining wall drainage system 5

[BFstr]

Hi Everyone,

I am trying to understand the use of the perforated drainpipe located along the backside of the retaining wall. I don't understand how it collects water behind the wall? Do they have any holes that water go to them? Is there any reason that they need to be perforated? Is there any requirement on the material of the perforated drainpipe? Can be plastic? Steel?

How do we size them? What is the base for sizing them?
Again my main question is how they function and how they help?
Are they needed even after having the wipeholes?

I really thank anyone helps me to understand this.

Sincerely,
BF str

 

[fattdad]

If water accumulates in the backfill behind a retaining wall, the water will apply additional "hydrostatic" pressure laterally on the retaining wall. If this pressure is not accounted for in design, the wall may become overstressed and fail. The use of perforated (yes this means there are holes) at the lowest depth (or near the lowest depth) of the wall backfill minimizes the development of saturated soils in the backfill. Proper design is necessary, which depends on the nature of the wall backfill and the opportunities to daylight the pipe.

You can also use weepholes.

f-d



¡papá gordo ain’t no madre flaca!

 

[Dirtguy4587]

If I can add to f-d's post, depending on the wall geometry and soil backfill conditions, the drain alone may not be sufficient. Often, either granular backfill against the wall, and/or some sort of wall drain system is connected to the drain pipe to ensure no hydrostatic buildup of pore pressure against the wall.

 

[BFstr]

Dear fattdad (Geotechnical), thank you so much for your clear explanation. It was really clear and well explained. Thank you so much.

I was wondering when I use this type of drainage (the perforated) do I need still use weep holes? At what elevations and how often I need to use the weep holes? Can I neglect the usage of weep hole when I am using the perforated drain? Any code requirements?

Thank you so much again
Sincerely,
BFstr

 

[fattdad]

Can't comment on code requirements, don't know where you are located. You wouldn't need weepholes if you are using a perforated pipe to collect and discharge the water.

Regarding the elevation of the drain pipe or for the case of weep holes the weep outlets, you need to located these items as low in the backfill as practical. If for some reason, you can only position them 2 ft higher than the toe of the wall, then a prudent designer would assume that the lower 2 ft of the wall backfill could (would?) become saturated and account for hydrostatic pressures in this lower 2-ft of wall backfill.

Don't overthink these things, once you understand the basics, it all makes sense.

I do want to add one thing: If you are using perforated pipe, you need to provide more than one outfall location. That way, if one location gets squished or otherwise blocked, you have another route to take.

Regarding "dirtguy's" post, sure wall backfill and connectivity between the soil and the drainage media adjacent to the pipe are critical design elements (maybe the drainage media is more critical for controlling hydrostatic pressure?). Whomever is working with you on the earth pressures for this wall should take into account the specified wall backfill material and provide you with either at-rest or active earth pressures.

Let's think through this for a minute: If you have a soil with an effective friction angle of 30 degrees, your Ka value would be 1/3. So if you are dealing with a soil with a unit weight of 120 pcf the horizontal pressure acting on the wall would be 40H depending on where you are behind the wall face (i.e., if the wall is 10 ft tall, the pressure would vary from 0 psf at the top and 400 psf at the base). If you allow saturation, the coefficient of active earth pressure would still be 1/3, but you'd base the accumulation of earth pressure on the bouyant unit weight (i.e., moist unit weight minus the unit weight of water - 120-62.4=57.6 pcf). So, if you take 1/3rd of that value you'd get 19.2H. Wow, so much lower, eh? Not so quick, you then have to add back the unit weight of water, which would run you up to 81.6H, which is more then twice the unsaturated value. Wall drainage saves alot in structural design.

The benefit of a granular media behind the wall is that you typically can use a higher friction angle, which in turn will lower the coefficieint of active earth pressure. That said, you still have to worry about hydrostatic pressures.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[GabionGuy]

If I may add something to Fattdad's comments, your choice of materials to construct the retaining wall will also affect your design. Concrete is impereable, therefore you need to design in the drainage. Other materials may behave differently, thus making your life much easier or that much more complicated.

A gabion is a free draining stucture. If a perforated drain is used, it is usually for other purposes. I have been involved in the designs of concrete structures that intentional had a gabion wall at the lowest part of the structure so that the gabion acted as the drain for the entire structure.

 

[hokie66]

Fattdad's explanation of how drains work to reduce the load on retaining walls is impeccable except for one thing: drains sometimes clog. They have to be constructed with appropriate filter media to prevent this from happening, but even if all precautions are taken, sometimes Murphy intervenes. So I never assume that a drain will be totally effective.

 

[fattdad]

Just out of curiosity, how do you account for partial effectiveness? I mean what is the engineering solution for partial effectiveness?

Recognizing the presence of Murphy, I would also provide a proper filter design for the interface between the backfill and the drainage media (as well as the perforations), and would provide multiple gravity discharge points. Redundancy being my Murphy safeguard.

One more point to make on this whole topic: You really need to understand the geologic setting and the conceptual ground water model. I mean, if you are on a sandy site and the water table is 20 ft below the toe of the retaining wall, who's to say you even need drainage provisions at the lowest depth of the wall backfill. In this case you may provide weeps.

If you are designing a wall on stiff clay and the specificaitons call for granular backfill, you may "invite" the development of a perched water table and wall drains will be critical.

If you are designing a pump station in the lowground and the basement wall extends below the water table, who cares about wall drainage? You need to design the wall for full hydrostatic pressure as there is no point for gravity discharge and you don't want failure during a power outage (or if the generator fails).

f-d

¡papá gordo ain’t no madre flaca!

 

[BFstr]

fattdad (Geotechnical), may I ask this question since you have a good knowledge in this subject.
The other day some ran to me and asked me that he noticed there are no drainage system behind the culvert walls and wing walls. May ask do you think this true? Is this statement only depend on the culvert walls and wing walls that have a certain size of pipe going trough them? But if this statement is true why these type of walls do require a perforated pipe behind them? I am talking about those small culvert walls, say a wall 10 foot high that has a 14" pipe going through and has two wings at each side? wings are 10 and 15 feet long.

Thanks in advance for your response.

Sincerely,
BFstr

 

[fattdad]

BFStr: Doubt I could provide a proper reply to your question. Sometimes you can see signs of drainage and sometimes you may not. Sometimes things are designed with drainage and sometimes they are not. There are just to many possible options to imagine why or whether a wing wall or culvert may or may not need drainage.

f-d

¡papá gordo ain’t no madre flaca!

 

[BFstr]

fattdad (Geotechnical), I appreciate your reply. Thank you so much.

Regards,
BFstr

 

[lorekat]

Fattdad,
I am building a series of 3 tierred retaining walls - each is 3 feet tall. I laid perforated pipe wrapped in a mesh sock (to prevent clogging) on 3 or 4 inches of gravel and then wrapped all of this in landscape fabric at the base of each wall. (I just finished the second wall.) I did not provide any weepholes and there is only one outlet for the pipe to drain. The walls are about 40 feet long. Do you think I should cut drainage holes into the wall now? The bottom wall has not failed so far. It was built and dirt backfilled in about a year ago. We have had a lot of rain in that time. But I am concerned about the weight each of the subsequent walls and their dirt will place on the bottom wall. Can you give me any advice?

 

[fattdad]

lorekat,

My advice is to follow the engineered design and build what the professional engineer designed. I trust that drainage was considered by the designer. I also trust that the whole wall was designed as one unit (i.e., that the stresses acting on the lowest wall include the stresses from the upper two tiers. My grestest concern would be that this is a segmental retaining wall that has little 4-ft layers of geogrid, when in actuality it's more like a 9 ft wall that may need 7 ft layers.

Hard to help when I know no details.

f-d

¡papá gordo ain’t no madre flaca!

 

[lorekat]

Fattdad,
Thank you for your response. I appreciate it.
Lorekat

 

[concretemasonry]

The horizontal spacing and the soil type will have an effect of the loads on the lower tier of a terraced design.

If it is 3 different 4' walls that are spaced properly, they may be no additional geo-grid required. - The the situation is a global problem.

Rigid concrete wall or SRW?

Dick

 

[lorekat]

It is 3 different 3 foot stone masonry walls that are spaced 7 feet apart. The lowest wall is built on a concrete footer about 6 inches thick with 1/2" rebar in it. the 2nd wall is built on a 6 inch footer, which is poured on top of 3 foot tall concrete piers, which are 8 inches in diameter and spaced 6 feet apart. The third wall is still in contruction and I intend to pour its footer on top of 6 foot tall piers. I really can't tell you the soil type. How can I find that out? Could my county extension agent answer that? I know I seem ignorant - because I am. This is not my field of expertise. This project is being done in the bottom of an old rock quarry and the quarry itself is very porous and does not hold water for any length of time. That is why I did not pour piers for the 1st wall. Any comments or advice anyone can give me is appreciated.