Lateral pressure from narrow granular zone between two walls

[Brandon]

Does anyone know what the lateral earth pressure would be for the case of a narrow (~2 feet) zone of granular backfill placed between two parallel walls?

The specific case is the basement wall of a condo we were asked to investigate. The masonry block basement wall is buckling inward and we are proposing to construct a separate concrete retaining wall just outside of the basement wall to support the lateral earth pressure and allow the basement wall to continue to support the vertical building loads only. The new wall will be a conventional semi-gravity wall on a spread footing, with the footing mostly on the "heel" side of the wall so as not to interfere with the building footer. The two walls will be just a couple of feet apart, but we need to backfill between them with something. If we use a nice angular stone, would this exert any significant lateral pressure on the basement wall?

 

[GTeng]

You could do a calculation to estimate the earth forces, either by trial wedge, or using "silo" type calculations (I think Bowles covers this). However, it may boil down to things that you can't calculate like:
1) field compaction - if the material is compacted or overcompacted then significant pressure could occur.
2) lateral deflection of the new wall - if the fill is not very compressible,you may still have significant forces.
Finally there are other practical considerations like how the material is placed, is there potential for water to build up between the walls, etc.
Is there a foam material that could be used? Seems like I heard that this has been done in similar circumstancs.

 

[BigH]

Why not use Elastizel - a lightweight foamed cement - or flowable concrete fill (K-crete used to be the name I knew).

 

[MSEMan]

Why not build an MSE wall with a wire face just off (say six inches) the existing wall? The pressure on the heel of the existing structure structure will be no more worse than the current load (may be less if you use open graded backfill such as "3s or 57 Stone). You can then fill the void with 5mm pea gravel which is self compacting and will only impart an element of self weight to the wall.

Another benefit of this type of construction is that you can drill some holes in the existing wall and tie it into the MSE mass to prevent further buckling of the exsting wall.

This type of pressure relief wall for basements is pretty common these days.

 

[GeotechPE]

Sounds like the masonry wall was not reinforced during construction. Building a second wall is expensive.

In similar situations we have excavate the basement wall and found that the masonry wall will return to its 'as contructed' position. Typically, the backfill is unacceptable and no draining system exists (causing the wall to buckle). Open holes in the top course of block and place rebar and concrete in the wall. Backfill with aggregate and install a drainage system.

 

[Panars]

Sometimes the reinforcement can be installed without any excavating. They use carbon fiber straps applied to the inside face of the block wall. One company that does this is here:

http://www.thereinforcer.com/

They give a more detailed explanation of the process.