Math type question about retaining wall

[biggcheese]

I'm building a round retaining wall on a slope at the corner of my house. I'm using stackable bricks that look like this:

http://images.lowes.com/product/000001/000001015348.jpg?wid=158&cvt=jpeg

The dimensions of the brick are 12" W (at the widest point) x 4" H x 8" D. The backside of the brick is 9" W and it's 8" along the side from front to back.

Since I'm building on a slope, I have to step the bricks up 4 bricks high from the lowest point to ground level at the top, kind of like this:

http://www.wieserconcrete.com/images/evergreen_3.jpg

I want the top row to be 10' ft. in diameter and since the bricks stack with a 1" lip in the back, the bottom row will be 10'3" in diameter.

Since the bricks are conforming to a circle of my own dimensions, they will only touch at the widest point, leaving a wedge shaped gap between each brick. Given the size and shape of the bricks, how can I determine the space between the back of each brick to insure that it theoretically forms a 10'3" circle?

[Paul]

and your worried because????????? just cut the last block so it fits your 10' 3" circle

[Stonehenge]

Mark a center point in your driveway and tape one end of a piece of string there. Measure out 10'. Mark a portion of the circle with chalk. Lay block with the fronts at the chalk line. If they fit there, they'll fit in your wall.

Or, you could just call the supplier and ask what the minimum inside radius is for those block. I'm betting it's about 3'.

[biggcheese]

The problem with not setting the bricks with an exact gap, is that when you stagger the next row on top, the bricks become offset a tiny bit each time as you work from the middle out so that instead of being centered over the two bricks below they become offset like so.

_________________________________
______|______|______|______|______|
___|_______|_______|______|_______|

I hope this helps to explain....

[Pelican]

The exact gap has nothing to do with it. As you add each course the radius of the curve tightens due to the set back. This is what causes the joints to misalign. This will occur no matter how careful you are with your gaps, it's a law of math.

[Stonehenge]

Big chz - That happens with every wall that is curved. Start building each wall at the point that is most visible, and set the seam stagger there. As the seams get skewed, it'll be in places that are less noticeable.

As for gaps - there shouldn't be any. Simple as that. For that style block there may be a pie-shaped void between pieces, but every block should have block touching it on the right and the left.

[Pelican]

I think he was referring to the gaps on the back side, at least that's what I understood.

[GLAN]

Originally posted by biggcheese
The problem with not setting the bricks with an exact gap, is that when you stagger the next row on top, the bricks become offset a tiny bit each time as you work from the middle out so that instead of being centered over the two bricks below they become offset like so.

_________________________________
______|______|______|______|______|
___|_______|_______|______|_______|

I hope this helps to explain....


Ok this is not so much my thing.......we mostly do stone stacking.....but I would think that with his diagram......that if the joints line up....What would be wrong with placing a half block at some point so that the seams are always offset?

[biggcheese]

Thanks for all the tips. I think the idea of setting them out on the driveway will help, that way I can create the circle and measure the gap. I did try to lay them out on the site, but since it was on a slope it didn't help much.

Just for the sake of clarity, I was referring to the gap at the back of each brick. The front side will always touch brick-to-brick.

Top view:

Gap___Gap____
\ / . . .\ ./. . . .\
\/ . . . .\/ . . . . \
/\_____/\_____ /
Front

Thanks again for all the help. I may end up cutting the bricks as well. I really didn't think this would be so complicated!

-John (budding artist and landscaper)

[Stonehenge]

Don't worry about the gap at the back of the wall - for better or worse, those block are designed to have that gap. To keep the gap in back uniform, just make sure the curve is uniform (i.e. based on a radius). If the look really bugs you, use some other cap block and cut them to fit. But don't cut those block - with that 3-sided face, they'll look pretty goofy cutting some of that off to make the sides butt against each other.

[mark allen]

be sure to put a clear stone ( pea gravel) behind the wall, cause if you put dirt you just built a dam, and it will blow out ...... Just got thru redoing...completley... a retaining wall built by another landscaper that was not done right... be sure to gle the top couple of courses for stability and it is also a good idea that was said to put a top cap on and cut to fit so you don not see the wedge shaped gaps behind the wall.
I know it's kinda late response ,,,but somebody said better late than..... well you know....