[Jeremy Hillary Boob PhD]

I need to build a retaining wall under an existing deck, where there's not much space to maneuver. The wall will be about 4' high (maybe a bit higher on one end, and a bit lower on the other) and about 12 to 15 feet long. Around here, a lot of people build retaining walls by digging deep holes (like about 2x as deep as the wall height) and putting in metal beams. Then they fill the holes with concrete to hold the beams in place, and pressure treated lumber (2" thick, or 4" thick) is used for the wall itself. The beams are generally placed about 6' OC.

In my case, the limited space would make it virtually impossible to get long steel beams in place, so I was thinking that concrete poured into sonotubes could work in place of the steel beams. Any thoughts on this? Any specifics, like how deep to go, beam spacing, suggested diameter of sonotubes, etc.? Thanks.

 

[Gary in WA]

You would want what they have. Ask your building department as to frost depth for your local area.

To prevent frost heave: http://books.google.com/books?id=1g...=X&oi=book_result&ct=result&resnum=3#PPA31,M1

Be safe, G

 

[Jeremy Hillary Boob PhD]

You would want what they have. Ask your building department as to frost depth for your local area.

To prevent frost heave: http://books.google.com/books?id=1g...=X&oi=book_result&ct=result&resnum=3#PPA31,M1

Be safe, G

I was hoping that someone on this forum might have seen something similar to what I'm considering and have some useful thoughts on the matter, but, apparently, that's not the case. Anyways, there are no frost issues here, just earthquakes.

Yesterday I did notice someone in the neighborhood has a retaining wall that is exactly the sort of thing I'm considering doing. So, I guess I can have "what they have" and what I want too. Hopefully, the current owner put in the wall and can provide me with some of the details.

 

[jogr]

You'ld probably have to go quite deep for the sonotubes and put a lot of rebar in them. The soil has a tremendous amount of pressure and can easily tip over retaining walls that aren't designed right. The soil pressures will depend greatly on the type of soil you have. Almost everyplace requires engineering for retaining walls 4' high.

But, have you considered a modular block retaining wall? They would be easy to move under your deck. They don't require a concrete base and can be layed with minimal excavating (usually placed on some compacted gravel). I'm pretty sure the manufacturers have engineering data to support using their product at appropriate heights.

Another thing to consider is breaking that 4' rise into two 2' rises spaced 3 or 4 feet apart. I would think modular blocks would easily handle 2' of nearly any soil conditions.

 

[PaliBob]

.... Anyways, there are no frost issues here, just earthquakes.....

If you are hinting at California, then here there is also just sliding.

There are super strict rules in California on retaining systems. If you are close to LA PM me and I can give you some first hand info.

Here is a pdf on general requirements:
http://www.documents.dgs.ca.gov/dsa/pubs/IR_16-3_rev_09-18-07.pdf


The idea of using concrete blocks is good, but at least in CA, a foundation is required. The height to grade is also measured from the bottom of the foundation.

Here is a PDF on a CA city requirements for Retaining Walls, which is very similar to those in LA.
http://www.atascadero.org/files/CD/Retaining%20Wall%20Handout.2.11.pdf


I would advise that you visit your local Building Department and get their handouts on the Rules and Regulations.

 

[Jeremy Hillary Boob PhD]

If you are hinting at California, then here there is also just sliding.

I'm in northern California.

Unfortunately, the local building department makes it virtually impossible to do anything, so, predictably, almost everything is done without permits. My plan is to overbuild by a large degree.

Thanks for the suggestions of a segmental retaining wall, and I have thought about that. However, I still like the sonotube idea, since I can use them as piers to support the deck, and also as a retaining wall---making the retaining wall just high enough to keep dirt away from my exterior walls (less than 4' might actually do it).

Anyways, I have found a neighbor who did something like this (actually, much more aggressive than what I've got in mind), so I'm going to check and see how he managed it.

 

[PaliBob]

....where there's not much space to maneuver. The wall will be about 4' high ...and about 12 to 15 feet long. ... so I was thinking that concrete poured into sonotubes could work in place of the steel beams.

There is a huge range of sonotube Sizes:
http://www.sonotube.com/products/sonovoid_sizechart.html

But installing them under an existing deck presents some problems

1. How are the deck joists supported now?
2. How much space is there now between ground and the bottom of the joists
3. Are the deck boards nailed or screwed?

There are too many unknowns, Can you post some pics?