parallel retaining walls

huangyhg

parallel retaining walls
a co-worker came across this perplexing condition.  he recieved prototype drawings from an architect showing a building with a full basement 14'-0" below grade.  the drawings show an exterior counterfort retaining about 3'-0" away from the buildings basement wall.  the space between the walls is filled with gravel.  the intended purpose of the exterior retaining wall is to relieve earth pressures against the basement wall so that this wall can be built out of cmu.  is this logical?  i have always understood that the lateral earth pressure is a function of ka/kp, density, and depth.  does having only a 3'-0" width of soil behind the basement wall effect the pressure on this wall.  
i wouldnt think it would help much. the external wall will give a little and therefore compress the gravel between exerting loads on the internal wall.
couls do it as a cavity wall with a 2" air gap between.
or a battered slope with shotcrete and a supended slab over the top.
can the internal wall be done as 10 or 12" cmu?
i'd leave the gravel out and build a suspended slab over the top...after the retaining wall has been backfilled.
two walls system is more expensive than one wall.  use one wall and reinforce to match earth pressure required.
i have come across a formula someplace for lateral pressure of granular material when two walls are relatively "close" together. i, however, would be reluntant to apply this to the problem descibed. i agree with civilperson.......use one wall!
i have never seen this concept used before, go for gravel and good drainage and spend the extra money designing the basement wall correctly. you can also post in the foundations forum and see if they have come across this idea before.
i agree with others.  why design one wall so that you don't have to design the other?  he wants to design one "retaining" wall so that the basement wall can be cmu?
i have to imagine it is cheaper to just pour once concrete wall for the basement wall and be done with it.
if for some reason you did proceed with the twin walls tieing the walls together with durable ties may be an option to minimise load on the inner wall.
thanks for your help.  i'm not exactly sure why the two wall system was suggested, but i'm fairly sure it will be used as its been used before.  others in the office believe that the basement wall pressure diagram would linearly increase to a depth of 3'-0" then remain constant for the remaining length of wall.  does this make any sense?  it doesn't to me.
this sort of makes sense for the two wall system.  i am sure this depends on the distance betweent the (2) walls and the friction angle of the soil.  that depth is probably the depth of the failure zone of the soil for the active pressure.  below that depth, the pressure will not increase any further because no more soil is mobilized.  it makes sense that the traditional "equivalent fluid pressure" wouldn't be used here.  using efp makes sense when there is an infinite amount of soil against the wall.  the suggestion you mention takes advantage of the frictional nature of soil that "fluids" do not have.
all of that being said, i'm not sure i would use it.
bbookz,
the "outer" wall will bear the brunt of the retained soil loads. the "inner" wall will see a reduced lateral pressure.
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