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12 foot high retaining wall info for backfill

etifad (Structural)
12 Oct 12 18:25
We have a 12 foot high retaining wall (12" blocks). We installed drains around the concrete footers. To fill behind the wall will be approximately 4 feet wide and forty feet long and of course 12 block high. It will sit against a moist hill. Should the back fill be all gravel and if so does it matter what size? The block are filled with mortar for added strength. How long should the wall sit before we back fill?

Thanks
JAE (Structural)
12 Oct 12 19:07
Are there vertical reinforcing bars in the wall? If not you have a problem.

JedClampett (Structural)
12 Oct 12 19:07
This sounds like a failure in the future. Post when this falls down or hire an engineer to design it.
concretemasonry (Structural)
12 Oct 12 20:52
etifad -

Filling the block with mortar (grout should be used for grouting) and just filled cores with no rebar might increase the shear strength very slightly, but provide literally no help with flexure and that will be the major problem with a cantilevered wall. - By the way, 12 courses of normal concrete block is only 8' high.

A 12" SRW retaining wall block wall with geogrid will easily handle a 12' height, but unfortunately, they cannot be installed on next already installed concrete footing for structural purposes.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

fattdad (Geotechnical)
12 Oct 12 22:48
the wall will remain intact after it tips over from the horizontal earth pressure. It really does sound like a failure about to happen.

f-d

¡papá gordo ain’t no madre flaca!

bridgebuster (Civil)
13 Oct 12 19:32
Maybe the wall can be converted to a gravity wall so it's not a total loss.

BTW etifad - where are you located? If you're in the US you could look at your state DOT standards for backfill requirements.
etifad (Structural)
13 Oct 12 21:33
Yes there are vertical rods every third row and horizontal in the blocks also. There will be not water from rain as this wall will be under roof. Would you think the wall would at least hold the weight of gravel until a structure is built on it?
ukengineer58 (Civil/Environmental)
14 Oct 12 4:22
That comment sounds like your trusting to luck rather than analysing it. check it out if it dies it dies. ust a note of caution last week in uk a builder was procecuted for a wall collapsing on and killing a yhree year old as it was not strong enough for the backfill
JAE (Structural)
14 Oct 12 15:07
etifad - are you an engineer? It doesn't sound like it since you apparently have no idea on how to check a basic reinforced block wall in flexure.

I'd strongly suggest you go hire a structural engineer to check your design.
boo1 (Mechanical)
15 Oct 12 15:32
You dont indicate what zone you are in (earthquake zone). but
Attached is analysis from Digital Canal summary.
TXStructural (Structural)
16 Oct 12 16:36
To answer the original question, do not backfill with gravel. It is cohesionless and will not provide proper resistance to overturning. Since there is drainage on both edges of the footing, the resistance to sliding is provided only by friction with the underlying soil. You mention a roof, but normally a roof would be on the excavated side of a wall. In this situation, rainfall on the backfilled side would run through your gravel, to the footing. If the drain is not sufficient, you could lose sliding resistance due to the new moisture and it could all slide to the excavated side. Drainage behind the wall is very important, but you don't want encourage runoff to pool at the drain.

If the wall was designed and built as a proper cantilever (see the CRSI Design Handbook for examples), then your backfill should be lean clay, clayey sand, silty sand, or a similar material Avoid organics and expansive soils.

It may well be intended as a basement foundation wall, which relies for the floor diaphragm at the top for resistance (carrying the load from one side of the structure to another, rather than resisting the entire soil load as a cantilever (think flag pole.) Backfill partially unless the wall is designed to be a true cantilever. Construct the floor or other diaphragm, then finish backfilling.

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