×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Parallel Retaining Walls

Parallel Retaining Walls

Parallel Retaining Walls

(OP)
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.  

RE: Parallel Retaining Walls

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?

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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!

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

(OP)
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.

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

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%20Student%20Competition%20Paper.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

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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

RE: Parallel Retaining Walls

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.

RE: Parallel Retaining Walls

lkjh345,

I agree this may be an appropriate solution in certain circumstances, but I dont think this is one of them.

Why cant the basement block wall be designed as cantilevered?

csd

RE: Parallel Retaining Walls

bbookz -

Since these were prototype drawings, I suspect this was an architects idea of how to solve a soils/structural design challenge.

All that is laid out is the height of the walls and the requiremnt that the interior appearance of the wall be CMUs for some reason (possibly appearance compatibility with interior walls - a guess).

Ultimately, the best solution would meet these requirements for the site/soil conditions and the loads. Any structural design that meets the requirements and should be viable. If the cost is lower and the construction poses no problems, it would enhance the acceptibility.

Suggest an alternate since these are only "prototype" drawings and hopefully you can meet the requirements and provide a practical, economic alternate for the owner.

RE: Parallel Retaining Walls

(OP)
Thank you all for your input.  The links in the previous reply were especially helpful.  I'm sure my co-worker will suggest doing away with the two wall system unless there is some arguemnt against having a single wall due to site conditions.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources