Parallel Retaining Walls

[bbookz]

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.

 

[csd72]

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?

 

[IceNine]

I'd leave the gravel out and build a suspended slab over the top...after the retaining wall has been backfilled.

 

[civilperson]

Two walls system is more expensive than one wall. Use one wall and reinforce to match earth pressure required.

 

[jike]

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!

 

[haynewp]

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.

 

[Lion06]

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.

 

[apsix]

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.

 

[bbookz]

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.

 

[Lion06]

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.

 

[jdonville]

bbookz,

The "outer" wall will bear the brunt of the retained soil loads. The "inner" wall will see a reduced lateral pressure.

See:

http://www.ce.utexas.edu/prof/zornberg/khyang_files/Yang's Student Competition Paper.pdf

http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_abst_e?cgj_t01-063_38_ns_nf_cgj

Frydman, Sam and Keissar, Israel (1987), "Earth pressure on retaining walls near rock faces," Journal of Geotechnical Engineering, ASCE, Vol. 113, No. 6, June, pp. 586-599.

Jeff

 

[lkjh345]

I have used this system once before. It does work.

We used a segmental retaining wall as the outer wall. We backfilled behind the retaining wall and allowed the wall to 'creep' for a while before backfilling the space between the retaining wall and the basement wall with granular fill.

The geotech engineer helped us out extensively with the design parameters. My recollection is that on an approximately 14' deep basement wall, with the retaining wall approximately 2' away at the base, we designed the basement wall for something like 200 psf for its entire height. With a normal basement wall, this would have been approximately 700 psf at the base using a 50 PCF equivalent fluid.

We used this system on an addition to an existing building where the floor levels did not line up, so on the side towards the existing building there was not place for the floor level at the ground floor to 'push' into, and the side walls were too short to get the full lateral loads out in.

 

[lkjh345]

P.S. StructuralEIT has the concept right in his post above. Once the soils failure line intersects the retaining wall, no additional soil is activated for the pressure on the basement wall.

 

[csd72]

I agree with the points above regarding the theoretical soil pressure becoming constant. The reason why you normally have a triangular distribution is because you have a triangular wedge of soil above the failure plane, when you cut off the back end of this wedge you reduce the soil load on the wall.

This theory assumes that the external wall is not going to move which is not strictly correct. You can minimise this movement though by allowing some time before you backfill the space between but it still would not be perfectly as stated above. Additional surcharge loads will also cause movement.

This is one of these hair brained ideas that architects come up with that have no benefit at all and actually end up costing the client money.

I would definately expect more cost involved in this due to:
1. a higher volume of excavation required.
2. The cost of building 2 walls instead of one.
3. The placement of such a large volume of gravel between as well as the additional drainage for this.

I would also be concerned about unconventional problems such as:
1. the possibility of frost heave in the porous gravel causing horizontal loads on the internal wall.
2. If the drains block then there will be a pressure on the internal wall far in excess to what it is designed for.
3. Waterproofing issues.

As professional engineers our job is not only to make the architects ideas work, but also to steer them away from costly options that have no benefit.

csd

 

[msucog]

are both retaining wall footings at the same level? if not, i'm picturing that the stress distribution from the outside wall footing would need to be added to the soil pressure against the basement wall.

 

[jdonville]

csd72,

You shouldn't get frost heave in the gravel for two reasons - there is always room for the water to expand in the gravel pore spaces during freezing, and the frost won't penetrate to the drainage elevation.

Jeff

 

[csd72]

Jeff, what if the drains freeze over and the water rises behind the wall?

We had floods near my work because of frozen storm drains.

csd

 

[concretemasonry]

Will a reinforced 16" CMU wall work for the soil conditions if there is restraint at the top (either pinned or a propped catilever)? 16" block are available in different strengths in most of the civilized world.

Thay way you get the appearance the poster desires. - Probably to match interior walls.

If you want en exercise in calculations/sophitication, try prestressed.

Dick

 

[csd72]

I agree with concretemasonry,

I was trying to infer this above. One wall will be much cheaper than two.

This is the reason why you are involved in the first place, not just to ensure life safety but also to ensure that it is done with a reasonable level of economy.

In my opinion, just going ahead and designing the double walls without trying to steer the architect away from it is a definate case of client neglect.

csd

 

[lkjh345]

In very specific instances, this solution is the economical one. If you have no where for the lateral force to go one it gets into your single basement wall, then the idea of the double wall may be the economical solution.