Thick back fill

[struengineer]

I need to back fill a 50’x50’x12’ (deep) pit and build a pre-engineering building. The max load from each column (Total of 8) is 50 kip. I specify the select structure fill as backfill material. Backfill are required to be compacted (95%)every 8”. However, I am still afraid that this thick backfill will cause exessive settlement. I do not have a lot of this kind of experience. Is any engineer here can give me any suggestion on how to prevent the settlement in this situation? Thank you very much.

 

[Ron]

It shouldn't be an issue, assuming competent material below the fill and proper compaction. If you want to give yourself a bit more confidence, specify 100 percent compaction...that way the void ratio is not likely to change much, thus induce non-elastic settlement.

 

[TDAA]

Assuming your select structural fill is non-expansive granular material, spec a modified Proctor in conjunction with a high percentage of compaction.

 

[dgillette]

If you do spec 100%, just be aware that moisture control will be critical. If the fill is more than 1-2% wet of optimum, it could be quite difficult to get 100%. (On the dry side, you can just beat on it harder to get more density.) If your borrow source is on the wet side, the contractor may have to dry it back, which is a pain in the neck (or closer to the ground), and can really slow things down.

 

[oldestguy]

You worry about nothing. For your loads and at a reasonable bearing pressure of say 4,000 psf you will have probably better support than the nearby natural ground.

95 percent of Modified Proctor is a common very conservative requirement. Now let's say your column load is 500 kips. Then you might measure a slight settlement, but still well within usual tolerances.

Another thing, those metal buildings can take all kinds of distortion. Does the manufacturer give any limits to differential settlement?

The main trouble I have seen with them is excessive snow loads popping the bolts, but no catastrophe with some major footing settlements (like 1.5 ft. in 200 ft.).

 

[BigH]

As Oldestguy said - Your loads are so small, even with a modicum of compaction you will be fine. But . . . that said, ensure that the lifts are placed properly and compacted as they should be . . .

 

[struengineer]

Thank you guys. I feel much better now. I just have never used this thick back fill under fundation. Therefore do not feel confident.

By the way, if I assume the subbase under this thick back fill is strong enough. What kind of bearing capacity I can expect for this 95% compacted select structure fill (NYSDOT spec. item 203.02)? I assume 4000 psf to design footing. Will it be too conservative? I cannot find any info about that.

 

[oldestguy]

As to bearing capacity, what does your local building code assign to dense select compacted material? Some codes require that a P.E. give a written recommendation, or the alternative is no building on fill.

Maybe a local geotech might be contacted in the case of a written recommendation requirement.

What comes from here obviously are opinions based upon limited data, but usually from experienced engineers.

 

[csd72]

How much could 12 feet thick of soil possibly settle any way?

 

[fattdad]

This is not a bearing capacity question (you do need to check it however). As already referenced if the fill is fine-grained and its placed wet of optimum, there can be some excess pore pressure that will attenuate with time. This can lead to settlement under the weight of the fill alone. Do I think it'll be a lot? No. Could it be relavent? Well, maybe. To minimize this concern you can specify coarse-grained fill materials. However, if the soil that you have is fine grained, you can also install a settlement monument (2 ft square metal plate at the depth of 2 ft with a rise pipe that acts as a survey monument) and take elevation measurements two or three times a week for a few weeks. This may not necessarily affect the construction schedule as you can likely still install the footings, providing you can tweek the columns if there is truely a few nano-inches of settlement.

Can you forecast this in advance? Not likely. DM-7.2 (p. 39) gives some data on the anticipated properties of compacted earth fill and you'll see some typical values of compression that can devleop in various compacted soil materials.

So, I'm not sure it's a non-issue, but it may be. I'm pretty sure it's not a foundation issue, however.

f-d

¡papá gordo ain’t no madre flaca!