Cantilevered Retaining Wall: Adding to the Retained Height in an Existing Wall

[1SEngineer]

Hello,

I'm doing some research...

A client wishes to add to the retained height of an existing masonry retaining wall. The wall is approximately 6' high and is built with reinforced solid-grouted 8" cmu. They wish to add about 24" to the top of the wall.

I'm considering different things to do...and was wondering what you all have done. What do you think is the most efficient way to increase the retained height? The following are some of the things I'm considering:

--Add geo-fabric to the wall to reduce the bending span in the wall. Likely no footing change needed here.
--Add masonry buttresses on the inside of the wall to help the masonry. Expand the footing as needed.
--Add a new wall in front of the existing wall to relieve the pressure increase. Expand the footing...or add tie backs.

Thanks in advance for your help!

 

[cvg]

reduce or eliminate the lateral pressure exerted by the new fill - use cement treatment, geogrid or geotextile

 

[1SEngineer]

cvg,

Thanks for the response.

Any particular type of geogrid or geotextile?

Thanks.

 

[cvg]

recommend you discuss with a geotech or at least with the geosynthetic rep, Tensar or TenCate maybe?

http://en.wikipedia.org/wiki/Geosynthetics

 

[1SEngineer]

Ah...OK. I've already designed the wall using a Tensar product. Sounds like we are thinking on the same page.

Thanks for the help.

 

[DaveAtkins]

I think geogrid is the way to go. I assume you intend to lay it horizontally, to act as a tieback, correct?

DaveAtkins

 

[dcarr82775]

Geogrid doesn't eliminate the load from the new soil, it will only help with the bending in the new portion of the wall. The original wall still sees the surcharge of the added soil, and needs to be checked for that higher loading.

http://www.eng-tips.com/viewthread.cfm?qid=374325

Unless of course you plan on removing and re-installing the original backfill with added geogrid in it. That would reduce the load on the original wall.

 

[1SEngineer]

All,

I plan to:

--Remove the retained soil
--Install geogrid at intervals...</=36" o.c...about 12" above the top of the existing footing...and about 24" below the top of the wall.

The wall is reinforced one way...vertically. So the geogrid reduces the bending moment and shear forces in the existing wall. Instead of a 8' cantilever...we have a masonry wall that is supported every 36" o.c. This seems like a good, safe route to take for this type of alteration.

Having thought this through a bit...I'm not sure there's much else that can be done...that is more efficient.?

It's good to hear and learn what others are doing and recommend...thanks!

 

[ATSE]

Gents,
I would proceed with caution.
The mechanism you are describing does not consider strain compatibility.
Your nearly rigid retaining wall is far more sensitive to lateral movement as compared to the mobilized stiffness of your geogrid-soil system.
If you have a flexible wall, like an MSE wall, geogrid works perfectly. Not for a nearly rigid cantilever wall.
Your wall will "feel" the load via lateral active movement before your grid is even 50% mobilized.
You are better off using low strength grout or geofoam (other cautions to be considered) than geogrid unless you can accept large wall rotations.
Good luck.

 

[dcarr82775]

We have the geogrid replacement to deficient walls as described above quite a few times. We provide a small gap 2-4-inches between the backfill and the back face of the wall. This allows the geogrid to mobilize itself without loading the wall.

 

[tsek]

I'm relatively young, so can you guys please tell me why you can't just dowel extra concrete onto the top of the existing wall, and then backfill with lightweight material? Stability analysis would have to be performed, as well as bearing capacity check. If it's a long wall, then the construction savings would probably offset the cost of the subsurface investigation.