Residential CMU Wall - What to assume?

[strcengr]

Hey all, structural engineer for industrial facilities here, need some help from folks familiar with residential construction:

Currently leveling out a backyard which is stepped (2 foot elevation drop halfway across the yard). After leveling I am adding an additional 2 feet of soil up against one of the perimeter CMU walls. See below image:

I haven't got any details about the walls and not even sure if these are typically reinforced (1960s Southern California construction). I'd like to confirm the additional lateral load imposed by my 2 feet of soil won't cause any failure or bowing of the wall.

Anyone have typical details for these sorts of walls so I can run calcs?

If not, what are some conservative assumptions I can make about the wall construction (reinforced, unreinforced, min footing size) to check against additional load?

 

[JoelTXCive]

Unfortunately, I don't think you can make any assumptions about the existing construction. Even if it was new construction; it's a crapshoot.

Do you get along well with your neighbor? If so, you could cut a 3ft wide opening in the wall and install a gate. That region can also serve as your destructive testing zone to determine the wall's construction setup.

 

[strcengr]

Oof. That's tough. I wonder if I might just dig to determine size of footing then check:
- wall for bending/shear based on unreinforced condition & mortar beds @ 50%
- footing for overturning + sliding
If utilization < 0.5 for both these conditions, am sure I will be fine.

Your thoughts?

 

[XR250]

I imagine the bond between the CMU base course and the footing will fail before the footing has an issue.
You cannot count on the top of the footing being cleaned before it was layed.
Regardless of the outcome of your calcs, it is still a crapshoot. Don't forget to add the wind pressure.
If it is your wall, you can cut vertical slots in it and install rebar epoxied into the footing.

 

[strcengr]

I calculated the stresses based on an ungrouted 6" wall with mortar just at the face shell and the tension stresses from the lateral load due to soil are so low the wall still remains well in net compression due to self weight.

Does this match your intuition?

Open question to anyone reading...

 

[JLNJ]

I'm guessing that your ungrouted wall is questionable at best under seismic loading as it is now. Adding the soil only makes it worse.

I suppose the most justifiable route is to see if the addition of the soil meets the 10% rule. If the increase is under 10% you have some justification for leaving it. No point in just guessing and making up calculations when there are so many unknowns.

 

[XR250]

I thought it was a 5% rule. No way that wall passes on self weight with wind and soil.

 

[SE2607]

To answer you question directly, even back then, although my experience doesn't go back to the 60s, is that there is probably a #4 vertical bar every 24".

To me, the real question is are you looking at this for your own peace of mind or to justify it to a building official? If the latter, the biggest thing you have on your side is that the current design loads are lower than they were back then. Back then, the wind load was 20 PSF. Today, it's more like 10-15 PSF. With regards to seismic loads, I still remember ZIKCSW=0.186 or possibly 0.3 for elements like this. I recommend finding a copy of an old code and compare loads. For structures like this, very few of them were designed with engineering backup. Most jurisdictions here in SoCal have prescriptive design sheets showing reinforcement, etc. so a homeowner would not have to engage an engineer. The problem with this is that I was never able to get the footings to work for overturning. That's why you probably saw so many of these walls on their side after Northridge. Something else to consider is that most jurisdictions will not require you to provide engineering if the retained height was two feet or less, so you can put a dimension on your sketch showing the difference in grade to be two feet or less (<2'-0"). If it were me, I would be concerned about litigation. Since the low side is your neighbor's property, failure would occur on his/her property, regardless of the retention. If/when there was just a moderate earthquake, he/she could claim the wall failure was due to the retained earth when, in reality, it would have failed anyway. Unless your jurisdiction has a "standard drawing" showing a property line block wall detail showing footing width, reinforcing AND a retained height of two feet or less, I would build a new wall with an adequate footing.

Just my two cents.

 

[Greenalleycat]

Something else to consider is waterproofing
I have a similar situation where my neighbours property is ~400mm (16") higher than mine
I built a 2-block CMU wall along my boundary when I did some landscaping
For 8-10 months of the year water pisses through it as it was practically impossible for me to waterproof the wall in any way
Not a major problem but it is unsightly, requires maintenance every 1-2 years, and will impact the wall over time
I'd consider waterproofing your side in some way to avoid pissing your neighbour off if you do this

In terms of the structural aspect, I have zero expertise relevant to your jurisdiction and historic construction practices.
However, in my area that wall could be anything from completely ungrouted/freestanding to fully grouted and reinforced over a proper footing
Gut feel says that the vast majority of such walls lean towards the former not the latter so I'd be careful adding too much more load to it
There's an argument that adding the soil will minorly reduce the wind load in the critical direction (as no wind will affect the bottom region) so you probably don't make it much worse in practice
Retaining loads of 2ft of soil or whatever will be pretty tiny

Can you access the top of the wall to have a better look at it?

Alternatively, you have a nice footing there.
Can you just build something up on the inside and load up the footing and leave the wall alone to enjoy a happy and fruitful life as a landscaping feature

 

[JLNJ]

I thought it was a 5% rule.

I was thinking seismic would be the worst of it and the 10% would apply. Gravity might fail the 5%, as you point out.