retaining wall design question 3

[jay156]

I'm designing a retaining wall made of those 2'x2'x6' concrete bin blocks. The client needs it to be 8' tall. You can see in my attached calculation below, I'm coming up with an overturning moment that's way higher than the resisting moment. I've been told by the block supplier and the contractor, "We've been stacking these things four high for 30 years and never had one tip over". I'm sure they're telling the truth, and I have seen walls built that high around town, so I feel like this should work, but my numbers are not backing that up. Did I make a mistake in my analysis?

I was able to make it work if I assume a cohesive soil, (depending on the c value I use), but I've seen these around supporting bins of gravel too.

 

[Manit_R]

Lateral earth pressure should be (Ka.γ.h)
The Ka factor seems to be missing in the calculation here.

I'm still new to engineering though so it will be nice if someone can back this up

 

[jay156]

You're right, I didn't include that because I assumed it was cohesive soil and the Ka value would be one. I forgot about it in the bins of gravel case though and that would reduce the lateral pressure by a lot if the internal friction angle were around 45 deg.

Still not sure how these are standing up and holding soil back, unless the soils are really cohesive. We don't have a geotech report but the soils in the area are typically some type of silt loam. c can vary a lot for those.

 

[DaveAtkins]

Yes, the 880 lb/ft needs to be multiplied by Ka. Ka should be around 0.33, depending on the backfill.

DaveAtkins

 

[BridgeSmith]

Backfilling a retaining wall with cohesive soil is usually the worst thing you can do. A well-graded rock mixture (crusher run base) is the best. It will give you a Ka value < 0.3, but most granular soils will give you, as DaveAtkins stated, a Ka around 0.33.

 

[jay156]

Ah okay, thanks for pointing out my mistake. My assumption of Ka=1 was wrong then. Ka of .33 helps a lot, but that only reduces my overturning moment to 3101 ft lbs, still greater than the resisting moment. Forget about any kind of factor of safety. How are these things not tipping all over the city?

Also, the reason for the natural soil behind the wall is the client wants to put a pool in and the contractor says they can't put it in gravel.

 

[DaveAtkins]

I believe an 8' high retaining wall would normally have geogrid extending back into the backfill.

DaveAtkins

 

[strucbells]

Yeah if it is a fill wall, adding geogrid tie backs helps a lot. Cut wall scenario can make that less economical. Usually when you get to around 8'-10' tall with these block walls overturning can become a challenge.

Typical practice is to backfill with granular fill and provide a foundation drain. You can put a 1-2' clay cap at the top to limit water infiltration. You definitely want to provide a relief mechanism for hydrostatic pressure during rainfall events. I would not recommend backfilling with clay to try to rely on cohesion to limit your pressure on the wall, that seems backwards to me.

A couple other things that provide some improvements:
1. Turn the bottom course of block 90 degrees and get a larger "footing" for your wall. Can use the soil sitting on top of this to help resist OT.
2. More for aesthetics..it's common to batter these walls backwards at a 1 horizontal to 6 vertical slope to prevent them from looking like they are leaning outwards.

 

[LOTE]

I have designed a lot of these. They only need a factor of safety of 1.0 to "not tip over", but you aren't going to be putting your seal on that.

The bin blocks (or World Block V-Interlock) can only be built 3 units high (6-ft tall) max without geogrid unless: 1) You have fairly stable rock in the retained zone in which case I would still only go 4 units high including embedment, or 2) You slope the leveling pad back toward the fill and create a batter in the wall. The interlocking piece in the block is designed for a vertical face, but you can create a batter by sloping the leveling pad.

Generally, for gravity walls with a vertical face with good soil and no slopes, the base length (L) generally needs to be at least 35% of the height (H), and at 3 units high you are already at L/H = 33%.

You will want a minimum of 12" of clean crushed stone (#57) behind the blocks to collect water. The higher friction angle of the crushed stone layer is neglected unless you fill the entire active wedge zone with it. The maximum friction angle for #57 typically used in design is 40-degrees, which will give you a Ka = 0.22.

Geogrid is cheap, especially if the onsite soils are adequate to use in the geogrid zone. I would suggest using geogrid for anything above 3 units even if you can get the math to somehow work for taller.

 

[DaveAtkins]

More for aesthetics..it's common to batter these walls backwards at a 1 horizontal to 6 vertical slope to prevent them from looking like they are leaning outwards

And it will actually help with overturning. It is a more rigorous calc - each block has a different moment arm as you batter them backwards.

DaveAtkins

 

[strucbells]

@Dave, yeah, I usually just ignore that & design it conservatively as if it were vertical, but it's a good thing to have in your hip pocket if you're really close

 

[PEinc]

jay156, I have seen many of these block walls standing without obvious problems. I have designed a number of them and they rarely figure on paper unless the wall is very low or the blocks have their long direction perpendicular to the face of the wall. Even then, the walls will be much thicker than the 2' that your sketch shows. The walls have too little weight and too small of a moment arm to resist overturning. They are too light to resist sliding. If you design the wall to have proper safety factors for overturning and sliding, your client or the client's supplier will think you are crazy. Your design needs to show the proper safety factors.
As Manit R said, you need to multiply your overturning and sliding forces by an active earth pressure coefficient of about 0.33. That will improve your safety factor, but not enough. Also, you probably need to add a live load surcharge which will reduce your safety factors significantly.
I recommend building a different type of wall, such as a small segmental block MSE wall with geogrids or a cantilevered soldier beam wall with lagging and possibly a more permanent finished facing.

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