Retaining wall for Bioretention Area

[T_Bat]

Hey everyone,

I have a 12' concrete wall I'm designing around a bio retention area. I'm assuming that there will additional pressure on the wall from the water in the bio-retention media. I'm trying to work a back door calc on this is RetainPro. My thought was to set the water table height to the top of the retention area. The problem is I have no idea what the active EFP for this media would be. I've run the calc using our standard assumed backfill material (110 pcf and 35 psf/ft active) but I'm afraid this will be overly conservative with the added water.

Anybody have an idea what this stuff is made of? Or if assuming hydrostatic pressure behind the wall is even appropriate?

 

[SlideRuleEra]

Anybody have an idea what this stuff (bioretension media) is made of?

A mix of sand, topsoil, and mulch. Percentages of each vary, 50% sand, 25% topsoil, 25% mulch is a typical mix.

http://www.SlideRuleEra.net

 

[oldestguy]

For us that don't do these bio retention things every day a cross section drawing would help. I'd be reluctant to design for active pressure if saturation is in the picture. More likely the pressures will be equal to density.

 

[T_Bat]

Sorry. I definitely one of those guys who doesn't do this (bioretention) everyday. This is the section from the civil drawings...

 

[T_Bat]

and this...

 

[SlideRuleEra]

T Bat - Your questions are answered on the documents.

Here is media mix:

Yes, you need to consider hydrostatic pressure (up to the spillway elevation)... plus "submerged earth" pressure:

Strongly advise you to get help on this project.

http://www.SlideRuleEra.net

 

[T_Bat]

Thanks - I agree. I am basically designing it as your diagram shows. I was really hoping to see if the drainage layer provided would allow me to exclude the hydrostatic pressure (seems unlikely). I didn't want to be too conservative but it sounds like this is where we are at. I totally overlooked the media mix info for giving me the weight of the backfill.

Sounds like I'm not too far off. It's interesting that in a few places the toe side soil will be about 7'-0" deep. There is a possible case where the retention media is removed and I'm left retaining from the toe side...

 

[SlideRuleEra]

1) I'm trying to work a back door calc on this is RetainPro.
2) My thought was to set the water table height to the top of the retention area.
3) The problem is I have no idea what the active EFP for this media would be.
4) I've run the calc using our standard assumed backfill material (110 pcf and 35 psf/ft active) but I'm afraid this will be overly conservative with the added water.
5) I didn't want to be too conservative but it sounds like this is where we are at.
6) ...in a few places the toe side soil will be about 7'-0" deep. There is a possible case where the retention media is removed and I'm left retaining from the toe side...
7) Sounds like I'm not too far off.

1) I read this to mean that you are relying on software, not an understanding of pressure from "submerged earth", to "solve" the problem.
2) Hope that you have raised that water level now.
3) You will have to find out... otherwise garbage-in-garbage-out from the software package.
4) That is not how pressure from "submerged earth" works. Hydrostatic pressure + submerged earth pressure is "worse", not "better", than "dry" earth.
5) Not conservative at all.
6) Yes, check that... but the media is only 3' thick. There is quite a lot of "rock" underneath it to provide passive pressure.
7) Don't fool yourself. Get help on this project, nothing wrong with that. What is "bad" is not to recognize when help is important.

http://www.SlideRuleEra.net

 

[T_Bat]

No doubt SRE. I appreciate you comments and will have someone in out office review this - I'm no expert. My goal for this exercise (beyond the fun of figuring stuff out) is just to get do some preliminary sizing in retain pro for speed's sake. I'm trying to get something out for rough pricing. I definitely understand that the submerged soil is much worse for my wall. RetainPro allows you to set a water table height to load the wall with the soil+hydro-static. Although it will not add the water above the top of the soil (ponded). I think I would need to add some additonal loading to get the equivalent moment on my stem and the right loads for global stability. My question was more about what I could reasonably assume for the soil weight and, from that, the EFP. I'm told, by the civil guys, that this stuff is much light than the backfill soil we usually spec (110 pcf). I don't want to throw something much heavier than the actual media so that my wall is grossly oversized.

That being said, without solid ground to stand out I'm going to run with what believe to be heavier loads then discuss with some folks in the office.

 

[oldestguy]

By the time you really get into this with a consulting engineer, do not be surprised to find your concepts have to be significantly changed. Possibly you may abandon this project, it is that serous a thing.

 

[T_Bat]

OG - Do you feel I'm completely missing something here? I'm designing a bunch of PEMB foundations on this job and a few standard retaining walls. This just happens to be the first time I've encountered a wall with bio retention media behind it. I don't assume I've got this thing totally figured out yet but if it sounds like I'm grossly mistaken in my approach I'd like to know.

 

[oldestguy]

Look at the pressures from the water. It is quite unlikely a wall as you indicate is no where near what is required. In addition it does not appear that the stone zones are properly filtered.

 

[BUGGAR]

From on old dam builder: Height of flow over an emergency spillway? Put in a little extra height for driving head. There is provision for an emergency spillway, I trust. Planned spillways plug up. And don't get Fed Dam Safety involved! Or maybe you should.

 

[T_Bat]

OG, is there some confusion? I haven’t indicated an actual wall design. The wall shown in the graphic is just what the civil people show generically in their bioret detail. My wall is thicker and my footing is much, much larger.

 

[SlideRuleEra]

I'm told, by the civil guys, that this stuff is much lighter than the backfill soil we usually spec (110 pcf).

It usually is lighter, with lower sand content and higher organics. However,the media mix is site-specific and the mix depends on permeability of underlying soil. The site for this project must require the media to pass water fairly quickly (causing the media to have high, heavy sand content).

Basins are sized to handle small and medium size rain events. Heavy rain will overflow the basin, so the spillway will be used fairly often. (BTW, good advice from BUGGAR.)

The goal is to have most of the media filtered storm water migrate down to recharge groundwater. The underdrain pipe should transport as little water as possible. A common design criteria is to have water level drop 0.5 inches per hour after the basin is filled. The basin should be empty most of the time.

A consequence of groundwater recharge is that everything in the basin (media, stone, soil, concrete foundation, etc.) needs to be considered as "submerged"... making loading on the wall higher than usually assumed.

Many structural loads are statistical probabilities (eg. wind, seismic, even live load to some extent). Unlike those, hydrostatic loading is "real" (exactly what can be calculated) and is being applied continuously when water is present, it has to be carefully considered.

Edit: There is another small, but unaccounted for load. As the media does its' job, particulate matter, silt, heavy metals, etc. will be deposited in the media. This could be considered a submerged surcharge load.

http://www.SlideRuleEra.net

 

[T_Bat]

SRE - thanks for the comments. That’s what I thought. If the media is light it will reduce my load on the wall. The hydrostatic obviously remains. It sounds like my worst case will be when the bioret media is old an full of heavier material. That being said, maybe I don’t get any benefit from lighter material if it’s going to fill up with heavy stuff. BUGGAR - I had not thought about additional head. Unfortunately the hydraulic design is by the civil folks.