Sliding Resistance Problems

[structeng13]

A client has asked for a retaining wall to be designed in his backyard which has an existing slope. The proposed wall will only be six feet high, CMU, with a concrete footing. The slope that will be behind the wall is at 1:1.

Without a soils report I am assuming a class 4 soil from the CBC which give an allowable friction value of 0.35 and lateral bearing pressure per foot of 150. The only way that I have been able to get the sliding to work (Factor of Safety of 1.5) is to have a very deep key. Does anybody have other suggestions? Thanks!

 

[concretemasonry]

Forget about a rigid wall with a footing. Excessive cost, lack of design flexibility and grading. You are not limited to straight walls, but can use inside and ouside curves in addition to stepping to match the contours.

Segmental retaining walls are great gravity walls up to 4 - 5'. They are great for engineered applications from there up to 45 feet or so. - I saw an installation in a mountainous area of Spain that went from nothing to 40' over a several mile length.

Most of the major licensed wall systems have great engineering support, technical information and many both domestic and international producers. All grid manufactures are very familiar will all major systems also. - Google for Allan Block, Anchor Wall Systems, Keystone and VersaLok.

Dick

 

[DRC1]

When you say lateral bearing pressure, I assume that is the lateral load from the soil. 150 psf is very high. It is more typically around 60 psf plus maybe another 20 or so for surcharge depending on your application. Check the magnatude of your lateral load and see if you didn't accidentlly double it up somewhere. That may solve your problem.

 

[GeoPaveTraffic]

You don't indicate how high the 1:1 slope is above the wall. That could be part of the problem. The short answer is get a local geotech involved who has an idea of the local soil conditions. They should be able to get you a solution.

If you need a key, I would say that a segmental wall is not a good solution.

 

[msucog]

wow...1:1 slope...rather steep. i typically recommend a max 1.5H:1V for temporary slopes...2H:1V permanent. how tall is the slope? besides the stability issue, being at the point that permitting is required, you might have trouble if they see a 1:1 slope. i'd suggest flattening that a little.

and yes, i've seen 50+ high segmental retaining walls...they're absolutely amazing when they blow out...blew the facing blocks right in to the tops of the adjacent trees (~15-20 feet away from the wall). i'm not a fan of them by any stretch of the imagination when they get over about 6 feet tall. up to that height, i've seen vertical slopes stand for years...but you look like a real dumbarse when you say that it's a piece of cake up to that height and then the darn thing falls down.

how did you arrive at 150psf?

 

[structeng13]

Sorry for the confusion, the 150psf has to do with the Table 18-I-A in the CBC which shows the maximum allowable lateral bearing of a footing. In the case of a class 4 soil it happens to be 150 psf/ft below the natural ground surface, as far as the active earth pressure acting on the wall, I have a Ka value of .5 and I am using 125 pcf for the unit weight of the soil, so it comes out to about 62 psf for the lateral earth pressure. Another value that the CBC gives is the frictional coefficient, which is 0.35.

As far as the length of the slope, it extends 10 feet behind the wall before it flattens out. I know that this is a steep slope, but unless I put a segmented wall adjacent to the property line fence, then that is what I have to design for.

After going over different combinations to figure out how to get the sliding resistance, I have seen what a key could do, and it is ridiculous how deep it would have to be. The wall, footing, and key have to be extremely oversized to make this work. Maybe it's just the conservative values that I am using. Thanks for your help!!

 

[msucog]

yipes. 6' wall + 10' tall 1:1 slope above the wall.

Ka = .53 in my part of the world (GA) unless testing indicates otherwise. phi's range from ~25-38 (@ 15% strain) around here due to the mish-mash of geologies. friction coeff. looks reasonable.

do you have any structures or issues with the adjacent property in case the thing falls down? how about on the lower side of the wall (building code requires certain setbacks)? i'd suggest that you stay away from segmental wall in your case. i'd also listen to geopavetraffic and get a geotech involved. this is more complex than i first pictured in my mind and there might be permitting requirements depending on you locale that might require a geotech's input.

how deep of a key is "ridiculous" to you? i'd say 8' is ridiculous...but i have seen some people say 3' is ridiculous. i guess i'm asking what sort of dimensions you're getting. i presume the numbers you're getting are not just for a "6' high wall" but instead are being driven by the "6' high wall with a 10' tall 1:1 slope above".

 

[structeng13]

Well, actually there is only one retaining wall that will be 6.6' high, this wall has a slope behind it of 1:1 for a distance of 10' where the property line screen fence is. At this point, the ground levels out. Setbacks are not an issue I believe because the client previously had a 3' retaining wall located 3' closer to the house.

Now he wants some more yard space and just thought that he could carve out a part of the existing slope behind the wall and put a new wall in.

As far as the key being ridiculous, pretty much the whole thing is ridiculous. Here are the dimensions that I have come up with to combat the sliding forces behind the wall.

Wall Dims h = 6.6' w = 2'
Footing depth = 3' w = 7.33'
Key depth = 7' w = 1.5'

The reason for the large dimensions is the low coefficient of friction of .35. The wall placement has to be such that the heel is minimal. Currently I am using the depth of the footing and the key for passive soil resistance and the factor of safety of 1.5 is not reached until that depth. Thanks again for the help!

 

[Touma59]

I have a different concern. You have mentioned that yur client has an "existing" slope and the final slope behind the wll is 1:1, then you mentioned that your lateral "active" pressure coefficient is 0.5 which is very high and closer to a Ko than a Ka. So, asuming I got the picture right, and before building the wall, I'll be concerned about maintaining the stability of the slope while I am working on the wall. I'll definitely get a geotech engineer involved. A slope stability study might also be warranted.

Tsoft;

http://www.tsoftonline.com

 

[msucog]

CORRECTION TO MY STATEMENT!: Ka=0.36 (talk about a major brain fart on the weekend...had "at rest" Ko coefficent on the brain and wasn't paying attention to my typing)--sorry for the confusion. i see why 2 yr old kids shouldn't be allowed at work.
Ko=.53 & Ka=.36 with phi=28

 

[BigH]

msucog - any pictures of the blow-out you mentioned? Good points by the discussers. I would be concerned about a 1:1 slope behind the wall - this is steep (too steep) and the values given by msucog are okay (but if you use good granular crushed stone - 1.5:1 is okay for permanent). Make sure that you step your fill into the original slope so as to take away concern of a pre-defined movement plane. Is the retaining wall base on level ground? Or is it on a continuing slope (like building the wall part way down a slope). This would be of concern in overall stability.

 

[msucog]

got 'em at my office...i will try to remember to put a few up later in the week.

 

[structeng13]

BigH, The wall and footing will be on level ground, but the slope is already predetermined because it is existing. At this point I believe that a geotech will have to visit the site for an analysis of the existing slope and soil. How much would this cost the client?

 

[GeoPaveTraffic]

structeng13,

First, 0.35 is not a "low" coef. of friction for a soil/wall interface.

Second, due to the slope the effective wall height is more on the order of 10 to 12 feet. A footing width of 8 feet or so would be fairly normal, at least around here. You have listed a width of 2 feet, that is the problem. If you are using geogrid behind the wall, then the width is equal to either the full length of the grid or a major portion of the grid length depening on how you believe that grids function.

If you are not using grid, then your width is way way way to small and the wall would simply turn over before it ever had a chance to slide.

On another point; placing a 6 foot wall with a 10 foot 1:1 slope above it in the back yard of house is a VERY BAD idea. What happens when a kid is playing in the back yard, falls down, rolls down the slope and off the wall? A better plan would be to increase the wall height, decrease the slope height, and flatten the slope. Call is a 14 foot wall with a ~3 foot slope at 3H to 1V. Still not ideal, but better.

 

[structeng13]

Thanks for the recommendations GeoPaveTraffic! That is pretty much the only conclusion that I could come to as well. Increase the wall height and decrease the slope behind it. As for the width that you were worried about. I think we got confused. I was using a 2' width assuming no geogrid behind the wall, this is also assuming the material for both the wall and footing is concrete. Thank you all for your help!

 

[msucog]

pics 6 total

http://groups.msn.com/SIRIUSALTERNA...essage=28472&LastModified=4675626013626267071

 

[structeng13]

For some reason I could only see one pic and it was pretty blurry, it looked like a pretty steep slope. My client found somebody else that would put the six foot wall in for him with the slope of 1:1.

 

[msucog]

you have to scroll right (i don't know why it posted it that way).