Cantilevered Retaining Wall Footing Sliding Keyway Location

[marinaman]

I have designed a series of cantilevered concrete retaining walls that not only resist soil lateral forces but also have surcharge upon their retained earth. By calculation, a flat bottomed footing will not provide me with the sliding resistance I need to meet the code factor of safety versus sliding. I will need keyways in the footing extending down into the soil below.

My question is.....where is the best place, along the length of footing, to place the keyway?

CRSI shows a keyway located slightly set back from the open face of retaining wall so that the tensile retaining wall r/f can extend into the keyway and act as tensile r/f in the keyway also. There's something in my gut that doesn't like the idea of extending the critical tensile reinforcing into the keyway.

Some part of me likes the keyway directly beneath the wall above, same thickness, with the bars for the keyway and the wall above independant of one another and each terminating in a 90 degree hook.....but, I don't like the keyway being built so close to the foundation toe pressure.

Some part of me likes the idea of putting the keyway at the end of the heel, but I've never seen it done that way....and since I haven't seen that in 20 years, that makes me gunshy of that idea.

Any comments here?

 

[marinaman]

Here is some sketches of what I'm talking about.

 

[Archie264]

I wouldn't be quick to bet against CRSI. But, looking at your sketches I'm not sure you represented their sketches correctly. In their sketches I do NOT see the primary flexural reinforcement extending into the keyway. I see it extending towards the toe, where it belongs. Indeed, for a cantilever retaining wall to have the primary flexural reinforcement extending any direction other than that is to invite problems - it can create a hinge.

Bottom line: go with CRSI's recommendations but make sure you're representing them accurately. The version I have is the tenth edition (2008), by the way, and the sketches I'm referring to are on pages 14-2 and 14-3.

 

[marinaman]

My fault.

The CRSI I have (1996...old, I know) shows the key directly under the stem wall with the compression wall reinforcing extended into the tension face of the key.

Joseph Bowles book Foundation Analysis and Design, 4th ed, shows the key set back with the stem wall tensile reinforcing extended into the tensile face of key (I mixed up CRSI's location and Bowles location).

Bowles aslo says he recommmends the heel location for the key....at the very end of the heel.

Kenneth Leet's book "Reinforced Concrete Design" 2nd ed shows the key set-back from the front face of stemwall and has a "Z" shaped bar into the stem (which to me does not look right at all).

I'm thinking I like the key on the end of the heel. I don't like the idea of excavating a deep trench for a key, right beside where my maximum toe pressure will be. Doesn't make sense to me. I don't want to excavate right beside where I know I will be bearing!

Anybody else have experience with the key near the end of the heel?

 

[Archie264]

I doubt much changed between the '96 and '08 versions of CRSI so we're probably in sync in that regard.

As for the location of the keyway, I might be missing something but it seems to me that from the point of view of resisting sliding it wouldn't matter where along the base it was located. However, I suspect there might be a practical reason related to field-placement of the rebar. There usually is and it's usually something I didn't think of. It will be interesting to see if Hokie66 or Ron chimes in as their knowledge of such issues is second to none. (As well as others on this site too, I'm sure, but I'm still learning the players.)

 

[BAretired]

My copy of CRSI Design Handbook was revised in 1961. It shows the tensile reinforcement of the wall extended into the tensile face of the key. I'm not sure why that should cause concern. It puts the key a little behind the center of footing (toward the heel) which seems reasonable to me. The face of key, in order to be effective must generate passive pressure against the soil. This is better accomplished where there is a reasonable amount of vertical pressure on the soil. At the end of the heel, the pressure is very low and the available passive pressure is correspondingly low.

I can't find anything wrong with the 1961 CRSI detail.

BA

 

[Archie264]

Well, I've got the '57 version as well and sure enough, it shows half of the tension bars extending into the key. I still believe that extending it into the toe is the better way to go, though, and I think that the fact that the more recent versions don't show it going into the key might be worth noting.

As for the location of the key, I suspect you're right; I hadn't thought of that. I have a spreadsheet based off of CRSI's example calculation so I'll play with that a bit. It's usually very interesting to see how the forces resolve themselves.

 

[marinaman]

The more I think about this, the better I like the keyway located at the end of the heel.

Joseph Bowles eludes to this being the best location in his book....and the more I think about it and think about the failure planes, the more I like the heel idea.

I also called the geotechnical engineer on this project and told him my concerns. He too likes locating the keyway on the end of the heel.

Other than having to work with the reinforcing at the end of the heel to get it reinforced, I think its better located at the end of the heel in all regards.

That's the way I'm leaning.

 

[msquared48]

If the key is deep, I will do as BA suggests, but if it is about 2' or less, I go directly under the stem wall.

I do not like placing the key at the end of the heel, because that makes the cut into the embankment even deeper at the most critical point. I like to stay away from that scenario. Just my thing.

Mike McCann
MMC Engineering

 

[hokie66]

Archie264, you are too gracious...to be compared to Ron...my head swells!

Now as to the location of a key, I want it to be well back under the footing. At the toe is not good, as the soil in front of the footing may be subject to "boiling up". In addition, the possibility always exists that some fool will dig a trench beside the footing. I don't much like the idea of trying to coordinate the toe location with the wall itself, as that just gives the contractor another chance to screw up. Back at the heel mobilizes the most soil. If I am building on flat ground with fill to follow, I would generally place a toe, if required, at the heel. If building into an enbankment, I would move it away from the excavation a bit, for the reason Mike gave.

 

[hokie66]

I said "toe" twice when I meant "key". And I can't even spell "embankment". Ron wouldn't make mistakes like those.

 

[Archie264]

...with a sense of humor to boot.

 

[FixedEarth]

If you have cohesive soils in the base soils, you get more resistance to sliding the further back (near heel) you place your keyway. If you have descending slope at the toe side or you think future excavation can happen, best to place the keyway back towards the backfill. Also if you want to reduce the length of your heel bar, you can take the keyway tension bars and bend them to act as heel bars as well. So it seems alligning the front face of the keyway to where the development length of the heel bar ends is economical.