Parking Area Retaining Wall

[XR250]

I have a retaining wall on two sides of a residential parking area. Due to property line issues, the heel of my footing is very small. As such I have a sliding problem. I tried a keyway but it was not very effective. How do y'all feel about me using the parking slab for the additional sliding resistance? Ok to use expansion joint material and just let it slide a tiny bit? This is the architect's drawing, not mine.

 

[BridgeSmith]

As long as the parking slab is permanent, and it's noted in the design and the plans that it is an essential part of the retaining wall's resistance, I would say you could use it. You could also use a fairly high value for passive resistance of the soil in front of the footing, since it's also permanent and confined.

 

[phamENG]

I don't like it. That slab will get torn up and eventually need to be replaced. Do they have to shore the wall? What are the odds the drawings will survive, and even if they do, that they'll be looked at?

Are you detailing the slab? Can it handle the compression if only braced by its deadweight? Can a plain concrete slab be used as a compression element?

 

[XR250]

I don't like it. That slab will get torn up and eventually need to be replaced. Do they have to shore the wall? What are the odds the drawings will survive, and even if they do, that they'll be looked at?

Not my problem. I will be long retired by then (just kidding). Has anyone actually ever seen and old retaining wall slide? - I haven't. This one is actually L-sheped about 20 ft. x 20 ft. which will likely help matters.

Are you detailing the slab? Can it handle the compression if only braced by its deadweight? Can a plain concrete slab be used as a compression element?

Did not think about it. Basement slabs are used for this all day long without detailing and I have yet to see one buckle. It's only 6'-8" of backfill so only about 1000 plf of sliding force if I ignore all friction. Maybe 500plf if friction is involved. Can't imagine that being an issue.

 

[LOTE]

Are you including the planter weight for resistance?

I would only include passive resistance below the parking slab since the footing would need to slide to engage the parking slab.

How deep of a shear key did you try?

 

[XR250]

Are you including the planter weight for resistance?

I would only include passive resistance below the parking slab since the footing would need to slide to engage the parking slab.

How deep of a shear key did you try?

I did not include the planter weight as I can't guarantee that dirt will actually ever be put in there or maybe it'll be filled with foam. I went back and tried to 24 inch deep key and that basically took care of the issue.
Most residential contractors around here have never even seen the key so I was trying to avoid using it.

 

[bones206]

I wouldn't consider the slab at all, just not in my comfort zone. Soem stray thoughts:

Is an MSE wall possible here?

What coefficient of friction did you use? Did you consider base friction + passive with the shear key option?

 

[gte447f]

I would not consider the parking slab for resistance to retaining wall sliding. Add a key or increase the footing size as needed to get the sliding resistance. It is what is. Owners/developers/contractors have to accept that property line foundations are unique and different compared to typical foundations.

 

[DoubleStud]

Why do you say keyway is not effective?

 

[Greenalleycat]

Utilising driveway slabs is never my Plan A but is usually my Plan B
I don't see a problem with it, on the proviso that it is reinforced
I would not rely on an unreinforced slab for a structural function

I'm in the camp of being dubious as to how much 'retaining wall sliding' actually exists (within reason of course)
If this is a decent sized reinforced slab then I can't see how your wall will ever really move anywhere

I would just note it clearly on the drawings that there is to be a connection
In my experience, that doesn't actually guarantee that people will pick it up BEFORE they cut it out later, but most contractors are smart enough to realise that there is a reason if things are connected
If someone cuts it out later and sees the bars they will most likely reinstate a connection or get an engineer's advice (if they didn't pick it up from plans beforehand)
And if they don't...well, shitty contractors gonna shitty contractor, they can get the bill if it slides a bit

 

[gte447f]

Greenalleycat... reinforced residential driveway slab?... residential driveway slab connected to a retaining wall foundation via dowels?... these things would be more unusual than a key on a retaining wall footing in my experience in my area.

 

[Greenalleycat]

As I said, it's never my Plan A but it is a useful Plan B
I prefer to keep them separate when I can but sometimes it becomes the most cost effective solution to connect them
It is then a collaboration between engineer, client, builder, and architect typically to make sure everyone is aware of the implications
I am in New Zealand not the USA also so let's call it regional differences

I personally take the view that we should also mesh reinforce slabs, I think unreinforced slabs are stupid
Particularly in our country that has earthquakes and commonly challenging soil conditions
Luckily too, around here retaining wall = hill = expensive = owner has high standards and money = willing to spend money on some mesh in a driveway = not much of an extra cost or effort to drill and epoxy bars at 400-800mm crs over a critical section of wall

 

[EireChch]

I know it’s done regularly in USA, but I think it is mind blowing not to put a sheet of mesh in a slab

 

[gte447f]

Mesh is often put in common, non-engineered slabs in the USA. It is intended as temperature and shrinkage reinforcing to reduce the likelihood of shrinkage cracks. However the workmanship is complete crap. The mesh is usually not chaired properly and is trampled during concrete placement resulting in the mesh being located against the soil at the bottom of the concrete cross section. These slabs should not be considered reinforced for structural loading. In the southeast USA, outdoor driveway slabs are often poured without mesh, and these slabs rely on tooled crack control joints to reduce the likelihood of shrinkage cracks.

 

[Greenalleycat]

It was common practice here before the quakes for houses...then all the unreinforced slabs performed like shit and the legislators realised how bad of an idea it is
So now we have to reinforce houses

I extend that to all slabs where I can anyway... it adds like 5-10% to the concreting bill to get mesh chucked in your patio or driveway
In return you get way better shrinkage/thermal performance and much better loading/soft soil resilience
It's an absolute no brainer in my mind

Edit: Also Eire, been wondering for a while - is your name a reference to you being from NZ?

 

[JoelTXCive]

This is more money, but what if you drop the footing another 1.5 to 2ft? Then, you can pick up some more passive pressure?

 

[phamENG]

and sometimes, instead of tooled crack control joints, they'll just put a 1x or 2x in there. Some think it looks nice to have the wood pattern...and then the owner of the house 40 years later has to deal with big gaps where the wood rotted away and weeds grow like there's no tomorrow. Ask me how I know...

 

[Greenalleycat]

Please Pham, tell us how you became privvy to the wisdom of filling gaps with random bits of 1x

 

[phamENG]

Just a little after hours gripe (I realize you're starting your day in NZ...our day is over here on the east coast of the US). It's bad practice to leave form work in place for flat work like that. And yes, the walks around my house all have these annoying gaps that I'm constantly fighting with. They used dyed concrete and colorful river rock as aggregate, so I have no hope of matching the mix to fill them in. But thanks for making me explain it.

 

[Aesur]

I would not use a driveway slab as a restraint ever. A few things to consider is, is the slab concrete or asphalt, asphalt has no capacity for compression of this magnitude, hence why asphalt is not to be used as a restraint in the pole foundation formulas in the IBC. Concrete could maybe work, but is there enough weight. For 1000 plf sliding force, once you account for 0.6 DL, Friction coefficient and a 1.5 safety factor, you are going to need quite a bit of weight to resist sliding. Assuming friction coefficient = 0.3, that is around 8333 lbs weight per linear foot. That weight is a 6" concrete slab that is 111' wide per linear foot of retaining wall to resist sliding.. good luck... I honestly rarely even use restrained retaining walls now due to the actual calculation of restraint force needed at the top unless there is a load path to dump that loading into side walls or retaining on opposite like a basement. (if anyone has any good information for why this isn't correct, please share because I rarely see failures for such a case and know people will use even a 4' wide sidwalk to say restrained, and I just don't see how it works).

I would try burying the footing deeper and using a key, for something like this, there has to be a way to make that work. As others have said, the contractor and owner have to just live with it and understand that the nature of such a retaining wall with property line footing.