Sliding Resistance Problems
Sliding Resistance Problems
(OP)
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!
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!





RE: Sliding Resistance Problems
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
RE: Sliding Resistance Problems
RE: Sliding Resistance Problems
If you need a key, I would say that a segmental wall is not a good solution.
RE: Sliding Resistance Problems
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?
RE: Sliding Resistance Problems
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!!
RE: Sliding Resistance Problems
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".
RE: Sliding Resistance Problems
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!
RE: Sliding Resistance Problems
Tsoft; www.tsoftonline.com
RE: Sliding Resistance Problems
Ko=.53 & Ka=.36 with phi=28
RE: Sliding Resistance Problems
RE: Sliding Resistance Problems
RE: Sliding Resistance Problems
RE: Sliding Resistance Problems
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.
RE: Sliding Resistance Problems
RE: Sliding Resistance Problems
h
RE: Sliding Resistance Problems
RE: Sliding Resistance Problems