Slope Modifications - New Wall to Replace Slope

[RFreund]

Overview -
There is an existing slope that is too steep to grow trees and the curb atop the slope is starting to separate and slide down the slope. The client would like to put in a retaining wall to level the slope. At the bottom of the slope there is an existing sheet pile wall which we know little to nothing about. The goal is to not impact the existing wall. See the attached sketch.

So basically we would be taking soil from directly behind the existing wall and placing it above the new wall. It seems as though this would be inherently stable, no? We could drop the base of the new wall down and make sure that we have at least a 1:1 or 1.5H:1V distribution however I'm curious as to what would be the best way to analyze this to prove that the new wall is not adversely effecting the existing.



EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

Where to start...

I understand your concept, and generally I would agree that what you are propsing should have little impact on the existing lower wall. However, you indicate a couple of things that cause me concern.

1. You know little to nothing about the lower wall.
2. The curb is separating and moving down slope.

Movement of the curb could be related to slope creep, an active slope instability, or an indication that the lower wall is already over loaded and failing.

While I'm willing to bet that your client does not want to get involed with hte lower wall, I wouldn't touch this without dealing with the slope and wall(s) as a unit. Too many ways for modifications to the slope above the lower wall to change pressures, drainage patterns, etc. in small and unexpected ways. Be assured (at least if you are in the US) that if you touch the slope and the lower wall moves or fails, that you will be responsible in the eyes of the owner of the low wall and anyone else who has a loss.

One option would be to move the new upper wall to the curb line or further with the goal to move it outside the possible acitve zone for the lower wall. If you did this and the new wall was well outside (several feet) of the active zone for the lower wall, then you could reasonable claim that you only improved the situation for the lower wall. Probably wouldn't keep you out of court, but might let you win. The limits of the active zone will depend on the soil types and specifics of the site geometry.

Good luck.

Mike Lambert

 

[RFreund]

Mike - thanks for the response, I have similar thoughts as well.
I think the goal will be to move the new wall back far enough and drop the bottom (some sort of SRW wall) low enough so that "in theory" we are beyond some sort of established failure plane. Unless there is a better "more accurate" (there is a scary term in world of geotechnics) way of dealing with this situation.
I will take a site visit to see the existing slope and wall. From what I have seen/heard it appears that the existing wall is in relatively good shape. However it seems as thought the existing slope is failing in some localized areas. Some of this soil will be removed and replaced when the new wall would be constructed. Although we would not be changing the drainage on a 'global' scheme I do think some thought needs to be given to this as well.

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

Very detailed and high quality soils information would allow the affected area of the lower wall to be acurately determined. At a minimum, I would look at pressures with a method of wedges as well as slope stability methods.

Any area of failing soil is big red flag. Soil strengths and strain incompatabilities can abound in situations like that.


Mike Lambert