footing resting on backfill

[silephbur]

can we recommend that column footings may rest on compacted backfills? is it common to practice? do we need to do geotechnical tests to compute for the bearing capacity of backfill?

 

[MotorCity]

Footings rest on compacted backfill all the time. If the native soil is not adequate, it is removed and replaced with better soil (i.e compacted backfill). Work with your geotechnical engineer to determine a reasonable bearing capacity for the specified backfill and require that a geotech engineer be on site to verify that it is properly placed and compacted in order to achieve the required capacity.

 

[silephbur]

i am currently investigating a wall footing. from the natural grade, they elevated the area by 1.5m. the designer rests the wall footing by 0.6m from finish grade, which means that it doesn't rest on the native soil. is that okay? aren't we expect a large immediate settlement for this?

 

[Ron]

Read MotorCity's post one more time. It's done routinely. Further, I would suggest that you learn a bit more about how geotechnical parameters affect your structural designs.

 

[TehMightyEngineer]

In short, if they use proper material and compact it properly (you need to determine what "proper" is, or get a geotechnical engineer to do this) then you can get a certain bearing pressure capacity out of the soil and design for that. If they don't compact it properly or use the wrong backfill then you may indeed have settlement issues (either immediate or long-term).

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[oldestguy]

To me "backfill" is the material being held up by a wall. Therefore, even though the backfill is well compacted, there is a lateral pressure applied to the wall in addition to that from the earth alone. That's where the geotech comes in to come up with what that added force is. Sometimes it means that the added pressure is too great and the footing should be set lower.

 

[silephbur]

thank you all for the response!

 

[JedClampett]

I'd mildly disagree with all of you. Just because the backfill has bearing capacity, doesn't mean it's not going to settle. An old timer told me a while back that all compacted backfill is going to settle. Even well compacted backfill (unlikely near buildings), is going to settle 2%. Sometimes that's not a problem. If you have one meter of compacted backfill, that's about an inch (sorry about the unit conversion). Does an inch of settlement bother you? Note that some of it will happen immediately during construction, but some will happen over time.
If this is a problem, we often go with a CLSM (controlled low strength material) or slurry for backfill adjacent to structures.

 

[TehMightyEngineer]

Jed, so if you're saying I took a meter of soil and compacted it and then let it sit there with no load it would still settle 2%? Is this rule of thumb just accounting for our compaction methods never being perfect?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[XR250]

I'd mildly disagree with all of you. Just because the backfill has bearing capacity, doesn't mean it's not going to settle

I have seen this occur on numerous occasions.

 

[MotorCity]

I don't think anyone is disputing the possibility of settlement. The OP just needs to determine if the expected settlement is acceptable. Even if you use CLSM, it may reduce the settlement compared to compacted backfill, but it does not guarantee that you still won't have settlement. At some point, you have to bear on the native soil which could settle.

 

[oldestguy]

The "old timer's" comment brings up another question. When compacting soil for structural support, why do some (most) specs require some high percentage density in the field against a lab standard when footings also are designed to rest on natural ground of the same type of material,not compacted? Or, why don't specs say just compact to the same density as natural ground? That commonly is only 80 -85% of the lab density. Maybe the answer is,"If 85% is OK, then 95% is better".

 

[JedClampett]

First of all, as far as unloaded soil settling, that's exactly correct. Sooner or later, it's going to get saturated, weigh more and settle. It possibly would settle anyway. If you look, you see it all the time. My mother's house has a back step. What started out as a 6 or 7 inch step is now 14 inches easy. I guess the backfill for the crawl space is finally reaching it's native state. We add additions to existing structures all the time and my biggest concern is the interface. If the new building settles, it's real noticeable.
oldestguy, I've never thought about that. We usually ask the contractor to scarify the first eight inches and re-compact, but that does nothing for the remaining soil.
And here's a question that will blow everyone's mind; We ask for foundations to be carried down to native soil all the time. How long does it take for backfill to become native? Ten years? Fifty years? One hundred years? Geologically, everything was fill once. Maybe a glacier did the compacting, but it still was backfill.

 

[oldestguy]

OK, how about dumped fill? I know of at least two cases where fill was 10 years old or more and test borings came up with good numbers so building was built on at least 10 on one side and 20 ft. on the other. No problems. I've been involved with surcharging fills with footing bearing pressure equal to that extra pressure. Again no problems, but some building features allowed for differential support.