Tiered walls

[JLSE]

I have a slightly different approach for this issue, than anything I have seen on this site.
There is an existing, and unknown, cantilevered retaining wall, which varies from about 3ft-6ft, supporting a fairly steep slope, within a couple feet of a property line.
I have a client who is the neighbor, and wants to build another retaining wall adjacent to the existing.

I dont know anything about the existing wall, and want to avoid loading it at all... no passive, no thrust, no surcharge.

My clients' wall will be about 5ft from the existing wall. From everything I have read, in order to avoid loading the existing wall, I would have to place the new wall's footing at 1/2 that distance above the existing footings wall... or 2.5ft. I am taking this to mean, the top of footing of the new wall can be no higher than 2.5ft above the top of the existing footings' wall.

Thoughts?

 

[Lomarandil]

I agree, except for two items:

Technically, it should be that the bottom of the new footing should be no more than 2.5 feet above the bottom of the existing footing.

You would want to check that building a new retaining wall (and any associated earthwork) does not affect the global slope stability and create a failure plane that could take both walls for a ride. Im not clearly understanding the desire for or the geometry of the new wall, so this may or may not apply.

----
just call me Lo.

 

[JLSE]

Lomarandil,
Thank you. Great addition.
The client wants to add fill behind the new wall, and make his yard flat. The new wall, will likely be about an 8ft tall, and cantilevered. Does that clarify?

 

[EireChch]

Upload a sketch.

Use passive and active wedges to determine if the walls are interfering with each other.

Lo is right regarding the global stability, this needs to be assessed.

 

[MotorCity]

Since you are adding fill, there will be an additional surcharge load. If it were me, I would have to sketch out the existing wall, the new wall, and overlay their pressure diagrams.

 

[JLNJ]

I would think the greatest risk is failing the existing wall when you excavate next to it.

 

[JLSE]

sketch uploaded
Ok, I dont have any experience with the passive and active wedge thing. Ive never done a tiered wall analysis before... and had no other reason to. Any references?, maybe something online? We have no geotech report for this, any thoughts on that? From what I remember about these wedges, they are largely dependent on soil type... and soils have not been evaluated. So, in that case, in the case where I am using code default criteria, how dependable are they?

Wouldnt it be conservative to take the approach I have suggested? What I am looking for is a conservative and dependable solution, with minimal engineering.

I also dont have any experience considering global stability. The walls I design are simple site walls on the small side, or a geotech report is provided.

Thank you all.

 

[kipfoot]

The things I notice about your sketch:
1) I think that influence line is better when it springs from the bottom of footing to bottom of footing (as stated by Lo);
2) 1:2 would be a shallower line;
3) the key makes me think that you're relying on passive pressure, but can you realize passive pressure from that mass of soil when it is behind the lower wall?

I think you'd really want a geotechnical report here. You need an appropriate lateral earth pressure to use and they may also help you understand the global stability issues at the site.