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
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
RE: Parallel Retaining Walls
RE: Parallel Retaining Walls
RE: Parallel Retaining Walls
RE: Parallel Retaining Walls
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
RE: Parallel Retaining Walls
RE: Parallel Retaining Walls
All of that being said, I'm not sure I would use it.
RE: Parallel Retaining Walls
The "outer" wall will bear the brunt of the retained soil loads. The "inner" wall will see a reduced lateral pressure.
See:
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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
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
RE: Parallel Retaining Walls
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
RE: Parallel Retaining Walls
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
We had floods near my work because of frozen storm drains.
csd
RE: Parallel Retaining Walls
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 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
RE: Parallel Retaining Walls
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
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