Need to build a short concrete block retaining wall on sloping ground

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  #1  
Old 08-04-09, 01:05 AM
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Need to build a short concrete block retaining wall on sloping ground

Hi,

I'm in Southern California and need to make a small concrete block retaining wall. The tricky part is that it's along ground that slopes. The wall will be 41 feet long. At the shortest, the wall will be about 6" tall. At the highest, it's will be about 35" tall. The slope between the two ends is 29 inches.

I assume that the foundation will have to "step". I plan on using 8" X 16" X 6" wide blocks. I worked out that it's 8 1/2 blocks wide per 8" step up. The questions I have are:

1 - How would I build a form and pour this foundation? All in one piece? In layers?

2 - Does the foundation need rebar to strengthen it?

3 - Do I need to embed rebar vertically for the wall as well?

4 - Assuming vertical rebar, do I need to fill the entire wall with concrete or can I just fill it in the channels with vertical rebar?

5 - Do I need any sort of gravel backfill or is it okay to simply have the dirt behind it?

6 - Do I need to waterproof the back?

7 - What sort of drainage will I need?

I know it's a lot of questions, but I want to get this right.

Thanks,
PJMonty
 
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Old 08-05-09, 05:53 PM
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I do these for a living. There are two ways to pour the footer the USA way or what I call the Puerto Rico way. Down there they don't step footers they pour it at an angel. Lay the block ( no head joints) and pour the cap to level or slope it. Then parge it to hide all the rough work.

So my question to use is are you going to step the cap or slant/slope it? I suggest slope it simply because even as a mason this needs to be laid out perfectly to get it looking right.
 
  #3  
Old 08-06-09, 11:01 AM
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21Boat,

The retaining wall is retaining a small area of lawn. The lawn is level so the retaining wall needs to be level.

Given that, any chance you can help with the rest of my question? The forum seems oddly quiet for what I assumed to be a basic masonry question.

Thanks,
PJMonty
 
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Old 08-06-09, 11:03 PM
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At the shortest, the wall will be about 6" tall. At the highest, it's will be about 35" tall. The slope between the two ends is 29 inches
It could be the question you asked. Not many real masons left and on top of that many really aren't aware of "how too" on a retaining wall. Its one of the biggest failures in the trades on holding up. Then add the decorative stone for the DIYer/contractor. Which thats an option here depending on your look.

The basic on Any Block/concrete retaining wall is the water build up gets behind the wall and or freezing in cold Geo's and they lean in time or fall over. Small low retaining walls aren't taken seriously enough.

That said,

1 - How would I build a form and pour this foundation? All in one piece? In layers?
No need to Form any footer. Dig down below frost line if you have one. Either or get it at least 8 to 12" below grade... Footer should be min 18"" wide

2 - Does the foundation need rebar to strengthen it?
Not necessarily but 2 parallel runs of 3/8 bar is good and cost effective. Footer should be Min 6" thick. I pour 8" 3500 psi. To tweak that a yard of Crete is 46.656 cubic inches

3 - Do I need to embed rebar vertically for the wall as well?
Yes every 6 to 8 feet.

4 - Assuming vertical rebar, do I need to fill the entire wall with concrete or can I just fill it in the channels with vertical rebar?
One of the secrets to a retaining wall is mass to mass being equal in force. Dead weight plays in here depending on the block wall size. Fill in the rebared cores with Crete.

5 - Do I need any sort of gravel backfill or is it okay to simply have the dirt behind it?
yes not only do you need 2b clean stone behind it you need to slope the back bank 45 degrees and stone fill in. Then a good top grade landscape cloth over that top of the stone. Fill the last 8" to a 1' of dirt. What the angeled dirt does is transfer the push from the dirt towards the bottom of the wall and at the same time acting as a "dirt brace" against the bank. A natural bank with no wall. The wall is essentially holding the crushed lime stone. ( Good trick and natural science applied ) This especially works when the ground behind the wall freezes and has the crushed stone to cushion that force and even transfer it up vertically to dissipate it lessing the solid horizontally force that pushes the walls over

Do I need to waterproof the back?
Yes even concrete block can rot over time. It get parged/coated with mortar. Let dry and then foundation pitch.

7 - What sort of drainage will I need?
Yes. when you backfill the dirt you have some choices here. 1. is back fill dirt behind wall to the level the lower finish grade is. tamp dirt. Here is where is where the stone starts. Since you have such a small wall I would use 2" Pvc pipes and drill holes in the pipe to let water in. Then T off of them. This can be downsized to 3/4" for the penetration through the wall. Slope penetration pipe. Run them about 3" or so off of lower finish grade every 4 feet.

One question you forgot to ask

8. What size block should I use
Min 10" 2 core cement block. This gives you enough room for the rebar and a wider filled core for strength. Also lay the wall up slightly out of plumb towards the bank. Transfers stress towards footer.
 
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Old 08-07-09, 07:28 AM
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Around here (Minnesota) the most common wall for a wall like that is a segmental retaining wall made with interlocking, colored architectural block. They are routinely used by the state and municipalities on civil projects. Most municpalities have the standard design/plates in their file.

The good thin is the appearance and the fact that mortar and a concrete footing is not required or permitted, so you can handle slope changes as long ar you keep the the bottom of the bottom layer deep enough (no need to go below frost). They handle slopes well and the and can easily be curved (inside and outside curves) to match your needs.

In your area, I know the major block producers (Orco and Angelus around LA and RCP(name?) in San Diego) all make a few different types or brands.

They are popular here because of the design and construction flexibility and the resistance to the drastic weather (-30F to +95F) and soil differences. I have some 35' high walls within a mile of two of me.

Dick
 
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Old 08-07-09, 12:47 PM
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21Boat,

Thanks for the detailed reply.

I should clarify the height issue a bit more. At the short end, the wall, while 6" tall, is actually 6" above the ground it is holding back. In other words, at the short end the upper and lower finish levels are flush and the wall is actually above both. At the 35" high area, the wall will again be 6" above the ground it is holding back, hence the wall is really holding 29 inches of soil.

Furthermore, even though the wall is 41 feet long, by the 20 foot mark, the wall is only holding back 16 inches of soil. A large percentage of wall is very short and holding very little.

I have a couple more questions...

1 - I asked if I needed to fill every core or just the ones with vertical rebar in them. You replied that the key to a retaining wall is mass (which implies every core), but then said to fill the rebar'ed cores. Sorry to belabor this, but which is it? Do I need lots of mass (all cores) or simply the rebar'ed cores?

2 - I had indicated that I planned on using 8" X 16" X 6" wide block. Your suggestion of 10" wide block seems like overkill. My reason for the 6" block is to tie in with the rest of the house visually. What about having the lower courses made from 8" block and the upper course being the 6" block?

3 - The tall end of the wall will butt up against the house. The house is a 1933 Spanish style with stucco exterior. The area that retaining wall will connect to is a car port wall. The wall is not foundation, and is open space on the back side (the car port area). Do I need to tie my wall in other than cosmetically?

These images will hopefully show the situation better than I can explain it. As you can see, the area of soil being held back is pretty modest in size:








Thanks,
PJMonty
 
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Old 08-07-09, 12:49 PM
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Concretemasonry,

I am familiar with the mortarless systems, but they don't fit in cosmetically with my house. While a good fit for a contemporary home, they will not work visually with a 1933 Spanish style house.

Thanks,
PJMonty
 
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Old 08-07-09, 01:04 PM
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I agree with your choice of the more costly rigid wall for your situation. You can then stucco/plaster even and make it blend in better.

Dick
 
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Old 08-07-09, 02:12 PM
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I think that with the limited amount of dirt that you are trying to "retain", you really should investigate the segmental wall block that Concretemasonry refereed to. These block were made for your project. No concrete to pour, just a stone leveling base. Google it and see if you think it will work for you.
Good luck!
 
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Old 08-07-09, 11:13 PM
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Hi Monty. I use 10" block for low retaining walls. You can lay them below grade and up to grade and then step back with 8 or 6" block so it gives the "Appearance" of a smaller wall to scaled surroundings. I would stick with 8" min after looking at the 12" + wall at the steps. Aesthetically this may change depending on how you want to Cap the wall. Personally I would run the rebar up through the some cores and above them. Form up the cap with 2x4s flush to top of wall a 2x6" over that to duplicate the capped wall at the steps. This in essence will be a post and beam of concrete and all tied in.

At times I will build a pilaster at or towards the end of the wall for corner strength. This is basically a buried Pier up to grade level Or a short block return corner. Use some Rebar and tie into that carport for a solid end. I use a short piece of PVC through the pilaster so not to trap water.

Being were its at did you ever think to running some lights in the wall for the driveway? Adds a lot of richness to it
 
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Old 08-11-09, 12:04 PM
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21Boat,

Thanks for the suggestion about the lights. No, I hadn't thought about it, but you're absolutely correct about lights making the finished product much better looking.

Thanks,
PJMonty
 
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Old 08-11-09, 05:57 PM
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Monty Some years back I poured some concrete caps on some low walls like yours.

As I formed up the 1 1/2 lip out from the wall I added a 3/4 by 3/4 piece of lumber on top of the 1 1/2" lip. This creates a small channel for inexpensive rope lighting to go up in to hide the light and light up the wall from the lip down.
 
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