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Discussion Starter · #1 · (Edited)
Hi, I've posted a rough sketch of my plans for this project after doing my homework about retaining walls, CMUs and a bit about masonry:



Color key:
Yellow - existing construction
Green - new construction plans
Orange - new construction I have questions about

In a nutshell, I have an old CMU retaining wall built by a previous owner of my home in the back yard; I want to reinforce it with the multi-wythe construction shown above to handle a moderate surcharge more safely; and I have a few questions about some of the details regarding gaps in my knowledge of masonry.

More background:

Existing CMU is above grade and consists of 3 courses of standard 8x8x16 cinder blocks. There is already terrace there, backfilled with loose, dry soil (not sandy, but no clay either). It's probably been there for decades and shows no signs of failing, mainly owing to luck, as it doesn't have proper drainage or footing. It seems to have been partially grouted on the bottom then packed with dirt and rocks, and reinforced with hollow steel pipes rather than rebar. There is a very slight crack on one side in the lower course that can easily be repaired with a masonry epoxy. The whole terrace is maybe 25' wide by 5' deep. At the back of the terrace is another CMU built by the same previous owner, 2 courses, which I will eventually raise to 4. There is a slope above that leading to a third CMU at the top of our sloped yard. I plan to reinforce the other CMUs in a similar fashion in the future. Just to give you the full picture.

I am going to build a relatively light lean-to/shelter thingy consisting of 2" SS welded tube stock with four legs supporting a leaning roof covered with corrugated roofing to capture rainwater that will feed down to a water tank on a pad at ground level (not on the terrace). It will be about half the width of the terrace. I also want to build a relatively light rocket stove (might weigh about 100-200 lbs. when finished). Finally, I will be welding steel-framed raised bed planters on industrial caster wheels with a wood bed liner my wife plans to garden in. Those will be the heaviest items, weighing maybe 200-300 lbs. a piece when filled with soil, planted and watered, and maybe 4-6 of them. There might also be a bench, some cooking and gardening tools and some firewood. All in all, with say 4 people standing up there, the total surcharge might be something like 3,500 lbs., which comes out to about 28 psf.

I'm planning on using ASTM A706 #4 rebar for reinforcement of the new CMU and the pad footing (not planning on welding, but my understanding is it is structurally stronger than non-weldable) tied with wire. In addition to the bond beam, I also plan to reinforce above the bond beam with 8" lapped wire ladders. The geogrid should be good for 100 psf with a depth of only 3 feet, but I'm just going to run it all the way back the full width of the terrace. I forgot it in the drawing, but there will also be gravel drainage around the French drain. This will terminate in weep outlets with covered drains in the inner CMU that I plan to expose on the outer CMU by drilling simple weep holes in the outer CMU with a hole saw. I plan on using Ash Grove ASTM C476, 80 lb. Core Fill Grout to fully grout the new wall and WINGRID-BX12 Biaxial, Polypropylene Geogrid.

I'm hoping someone with serious masonry experience can weigh in.

Questions:

  1. First, is there anything generally wrong with my plans?
  2. In the drawing above, can I place the concrete footing for the lean-to as shown? Or do I need to join it with additional concrete/expansion joints, et cetera? Or should I space it away from the inner CMU and pad around it with gravel?
  3. Do I need to add any expansion or control joints anywhere to join to the existing CMU?
  4. Is it a good idea to put an additional cylindrical footing beneath my pad footing in line with the footing I will pour for the legs of the lean-to? I've seen masons do this in videos online. Should I reinforce that footing with rebar?
  5. Should I route the French drain around the footings?
  6. Alternatively, could I place the 2"x2" legs of the lean-to inside a cell of the new CMU to double as both legs for the shelter and reinforcement? Is this a bad idea? I could find no examples of something like this anywhere online.
  7. As shown in the drawing, I plan to attach the old and new CMUs with j-bolt anchors. How frequent should I place them? Should I use expansion bolts or something else instead? Is is a bad idea to have them pass on top of and behind the rebar in my bond beam as in the drawing?
  8. How frequent should my weep holes be?
  9. Is it ok to bury the 2nd CMU this way? I think having a double wythe on the surface is unsightly; to handle drainage, I plan to cap them with sloped caps and possibly flashing.
  10. Do I need to peel away the Quiktubes after pouring, or do I leave them in the ground? Strangely Quikrete's own how-to videos never address this. On YouTube, some people do, some don't.

TIA for any expert advice.
 

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Discussion Starter · #2 ·
Image above won't display for some reason so here is an attachment.



Color key:
Yellow - existing construction
Green - new construction plans
Orange - new construction I have questions about
 

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I'm not fond of the bolts on the face, and trying to weep out water coming down the grout space. As long as you extend the footings of your structure down past the retaining wall then the placement of that should not affect your wall as a practical matter.

You have a lot of different detailing and corresponding construction work to do. So backing up a bit, you're talking existing 3 course and 2 course high walls where the soil apparently drains well. To do your design, you have to dig out quite a bit anyway. Wouldn't it just be easier with such short existing walls to start over? Good footing, rebarred into wall, proper drainage/weeps, gravel backfill, etc. Seems much easier and simpler than shoring existing for more surcharge or extending existing higher with all the busy work.
 

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