Basement walls vs. Retaining Walls 1

[medeek]

I've spent the last couple of years working on residences with relatively shallow basements, stem walls usually not more than two to four feet in height. I'm now working in an area where uneven terrain seems to be the norm or at least more likely. I've seen some stem walls designed in these situations with unbalanced fill, designed as retaining walls. I'm looking for more examples of this type of situation where you have a stem wall exceeding 4ft in height with unbalanced fill. What is a typical retaining/stem wall design? What factors are at play? And what things should I look out for?

With a typical basement wall the interior slab will provide some lateral restraint or at least that is my understanding. I'm also looking for any good resources on foundation and basement design in general.

Another thing I've noted is that with post and beam foundations usually I see a designer use a 4x8 beam with 4x4 posts. With this new area I'm working in the designer likes to use 2x4 stud walls on a strip footing, rather than post and beams. I'm not saying this is wrong but I'm curious to what the pros and cons are from an engineering perspective.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I'm also looking for any good resources on foundation and basement design in general.

You already have this (Link), right? If so, I'm pretty sure that it covers basement wall design.

What is a typical retaining/stem wall design?

Usually 8" thick. Residential contractors push for unreinforced walls and sometimes they get them per prescriptive code requirements. If your market will tolerate it, try to get one mat of rebar on the inside face of the all. #5 bars are smaller is pretty residential friendly.

What factors are at play? And what things should I look out for?

You may need to deal with:

1) Hydro-static pressure depending on the water table and drainage strategy.
2) At rest earth pressures.
3) Surchage due to vehicular traffic etc.
4) Seismic load of the soil being tossed against the wall.
5) Delivering the reaction from the top of the walls into the floor deck can be challenging. Tough connections and heavily taxed diaphragms. Some situations lead designers into using cantilevered retaining walls rather than propped walls. That can get expensive however.

I'm not saying this is wrong but I'm curious to what the pros and cons are from an engineering perspective.

Not sure I understand this. Where you've got a wall there anyway, efficiency would seem to dictate using that wall for support of gravity loads.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Sometimes we consider compaction load against the walls as well. If you don't have a geotechnical engineer on board, a lot of assumptions need to be made.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Yes, I should note that I have Ambrose's book, which I have used quite extensively for other footing designs and related issues, that book has been a life saver.

Thank-you for your detail response KootK. I've done some reading on retaining walls a few months back and the vehicular traffic was one thing that jumped out at me. I'm wondering though if it is customary to design for long term traffic versus construction traffic, or maybe it is more conservative to consider the worst case scenario (ie. fully loaded concrete truck etc...)

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Yeah, I struggle with what to do with the surcharge too. With deep, commercial basements it tends not to matter so much. With one story basements, however, the impact on design can be significant. And, as with commercial buildings, you always have to wonder if someday there will be a fire truck parked next to the structure attempting so save lives/property (~12 kPa + serious point loads). In my experience, residential walls get designed one of two ways:

1) No surcharge at all.

2) Surcharge equivalent to two more feet of soil. That's roughly on par with densely parked cars or humans.

The attached document has some excellent information on the wall to floor deck connections.

Another thing to watch out for is areas where you don't have a diaphragm to connect the top of the wall too. It occurs quite often where stairwells abut the building perimeter.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

I'm trying out Retain Pro by EnerCalc. The whole retaining wall thing seems more complex as I get into it. Any thoughts on this software or others like it?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I'm happy to use any number of methods or software packages so long as I understand what's going on and the methods are reasonable / defensible. As engineers, I feel that we often spin our wheels in surge of the "right" answer when all we really need is a reasonable answer. If you can stomach the purchase of an additional resource, this is probably my favorite reference for run of the mill residential/commercial retaining wall design: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

I've been playing with the EnerCalc program and also looking at some structural details by other engineers as well as reading Ambrose's book on the subject (chapter 5). I'm starting to get a much better understanding of what is at play but one thing that escapes me is how to balance the toe and heel of the design. I see some details with one larger than the other and vice versa. Is there some rule of thumb I am missing or other practical construction criteria that is driving this?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I usually find that maximizing heel is beneficial as it improves overturning in a way that maximizing toe does not. That said:

1) Having no toe at all causes severe joint detailing issues. Whenever possible, I provide enough toe that I can get all the bars properly developed etc.

2) Some situations, such as zero lot line, require zero/limited heel for project specific reasons.

3) Zero heel retaining walls often do reduce excavation quantities. And it's tough to sort out the economics in a way that applies to all situations. For example, I've had numerous contractors express a preference for using heel-less retaining walls either side of driveways into basements. There, sliding issues can be overcome by connecting the two sides across the drive lane.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Here is what I've come up with so far:

On the pony wall retaining wall the contractor wants the size of the toe and heel reversed since the wall might be near a property line and does not want the heel to project further than necessary.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I would recommend turning your stem bar bends the other way. More info here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

I have them turned into the heel because I didn't think I had enough development into the toe. The large toe, small heel design will have problems with sliding.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I don't want to sound too alarmist but you really must switch the bend direction regardless of the development condition. As it stands, you have an unreinforced toe and a stem to footing moment connection with very little capacity.

For your large toe, small heel wall, can sliding not be resisted by the SOG? You could add a key but you'll likely hear some contractor whining about that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

For the large toe, small heel wall in some situations there will be a basement and a slab but in others there will not be so I'm afraid it is going to cause the retaining wall to get too large.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

With the reinf. in the toe do you mean something like this?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Yessir. Much improved. Just like the other Romans.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Let me know if there are any other flaws with this detail that jump out at you. I'm a newb when it comes to retaining walls so even though I'm a fairly quick learner I'm going to miss a few things, guaranteed. FYI, there is a typo in the header of the table it should read "Horiz. Reinf" in the last column.

I've tried to make a retaining wall work with similar dimensions to those shown with a 18" heel and an approx. 36" toe. No matter what I do the Sliding FS is less than 1.5 , the only way to get this to work is to either add a key or have a basement slab, otherwise it is a "no go". Please let me know if I missing something here, the contractor wants a small heel but I don't think its possible.

With a 6' high retained height and a toe of 3 ft, heel of 2 ft, I get a sliding FS of 1.19 (Soil Friction 0.35), even if I up the friction to 0.4 I'm still at 1.33 on the sliding FS.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Some folks will add passive soil pressure to the toe to improve matters. It's a bit of a dicey thing, however, as there's really no guarantee that soil friction and passive pressure will be activated concurrently during the load history of the wall. Additionally, with building resting on top of the wall, you may not want to allow the kind of lateral movement that passive soil pressure activation implies. As a compromise, I sometimes consider active or at rest soil pressure to be acting against the toe. It doesn't help all that much but it's something.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Doesn't work, getting the heel size down to 18" is the target and modifying the items mentioned above does not drastically change things enough to make it work. I'm sure others have encountered this same problem but I don't see a solution other than introducing a key which is going to cause the contractor even more fits.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

There's gotta be something on the low side of this that you could push against. Some pad footings? A stump? A simpler version of a key might just be footing twice as deep. Depends how the contractor plans to accomplish excavation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.