Looking for the most efficient design for Foundation on fill 3

[brinni]

A two story duplex townhome was demolished 20 years ago on a project built in early eighties, because of foundation settlement, as much as 5 inches differential. Ten years ago I had substantial geotech investigation done, and as it turns out, there is poorly compacted fill under the building pad, ranging from 9 feet to 28 feet in depth over a foundation area that is probably 50 feet by 50 feet. The foundation design which was done based on the report was prohibitively expensive with deep poured in place 12 diameter piles on three to five foot spacing on exterior and interior beams
Now that time has passed and maybe some consolidation may have occurred, I was wondering whether removing a uniform layer say ten feet deep and replacing with compacted graded fill whether I can create a stable pad to allow for a conventional slab on grade to be built. Any other reasonable suggestions based on your experiences will be considered. If it helps I can look for the geotech report and the foundation design and attach.

 

[hokie66]

Please don't duplicate postings.

 

[fattdad]

Tell us the soil description, the N-value (or better yet jam a dilatometer into the ground) and the function of the building (i.e., is this retail construction that doesn't really have much distributed floor load or is it a manufacturing or storage facility that does) and we'll have a better way to approach your inquiry.

I'm not replying to the thread in earthwork and grading as this is a foundation question.

f-d



¡papá gordo ain’t no madre flaca!

 

[oldestguy]

I'd look at surcharging the site combined with an undercut under footings. If the surcharge is 10 feet high and one can assume 1,000 psf pressure, then the bottom of the undercut should not have an imposed pressure exceeding that (coming from the footings). Chances are you could use compacted granular fill in the undercut and assume 4,000 psf or so for the foundations.

Chances are the surcharge would show no movement after a short period, assuming not highly plastic fill.

I'd do the undercut first with that filling by compacted fill.

I've found this method works well even for uncompacted (usually old) fill in most cases. The data from settlement platforms under the surcharge tells a lot. If you see slow settlement, then you can adjust surcharge to a higher height if necessary. Don't forget that your surcharge top must be at least as wide as the building, preferably wider. In any case monitoring what the surcharge does is necessary, starting from before surcharge is placed.

 

[brinni]

To Oldestguy.. What is the most economical way to surcharge the site, and over what period of time does it need to be monitored? and also typically what depth to undercut the footings?

To others:
I have posted the soils report from 2000 ( eleven years old) online at
Your file's link is:

http://files.engineering.com/getfil...-fa33b4478a94&file=soils_report_2000_sec1.pdf

 

[fattdad]

Not much to go on in the geotechnical report. there is no pocket penetrometer data or SPT N-values for the fill. It is shown as fat clay and/or elastic silt (by lab testing) and we know it's compressible.

Surcharge is an option. Whether or not you need 10 ft is a design question. Heck you may only need 5 ft. Not sure how long it would need to stay in place, but a settlement monitoring program would be needed no matter. I'd think if the ground water table is below the fill (seem like it's abou 12 ft), then compression would go rather quick.

By all means, you'd need an engineered subbase below the footings. Whether that means 4 ft of dense-graded aggregate would result from knowing foundation loads (i.e., how wide the foundation would have to be if sized for 3 or 4,000 psf).

I'd think if you proofrolled the subgrade with a fully-loaded dump truck and got passing results, the slab on grade subgrade would be o.k.

Then again, I'm not registered in California. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[brinni]

I would guess that a possible solution is to surcharge using the same material that we would use for the subgrade... Then all the excess spoil from the excavation can be moved off site or elsewhere on site?

 

[molerat2210]

Here are my options for you:

1) Surcharge (as suggested by others) Think about esimtating expected settlment. May want tube samples for consolidation tests or CPTs to use with a Schmertmann analysis. It will be difficult to estimate expected settlement with the data you have. Need the estimate time-rate: are you looking at 6 months of 6 years? Client may not want to wait. How will you monitor the settlement?
2) Grouting. Cost effective and fast. Results can be spurious with clay but level of confidence probably adequate for residential work with some QC.
3) Undercut and replace (my choice). May only need to undercut a portion of the fill to have some comfort that settlement is manageable. Use a geotextile and layer of crushed stone at bottom of backfill to help distribute load. There is some benefit of previous surchage from old building. This work can be done fast and does not require a subcontractor or any specialized QC (only eyeballs and a ruler).

 

[oldestguy]

OK, as to what, details I would use, I first see that the CH soil is likely to be significantly affected by shrinkage and swelling as moisture changes. That can happen even if everything is compacted per some spec. Thus, the foundation system has to be such that it will not be affected drastically by these ground movements in the future. For such conditions, and for a light building, I have recommended that any part of the site be considered as not having any support for an area 10 ft. by 10 ft. That gives the structural engineer something to work with along with some figure for subgrade modulus. For a well compacted granular undercut fill that might be 300 to 500 ppci. I commonly go 4 feet below footing grade with the undercut. Not sure what your site clay would have for subgrade modulus if no undercut. The structural result is probably a well reinforced mat foundation under the whole site. It can be done also with a grid of "grade beams". It means all underground pluming goes in first or is set within chambers in the foundation. This generally is the "least expensive" and may not need the undercut and compacted fill, knowing that you still have lots of CH clay below it, subject to those moisture changes. The undercut will "help some" but not eliminate shrink or swell.

Then, assuming that the fill was poorly compacted, the supercharge may help, but is not likely to have any effect on those future moisture changes. However, with moisture sitting near the plastic limit, meaning getting it to lose water by surcharge, means knowing something of drainage paths and the likelihood of any such change taking a long time. That's unknown. This is not a sloppy soft clay by any means either, so what you can get is consolidation is not much anyhow.

Thus, I'd still surcharge for a short time and get what I could out of it by pushing out air by "compaction" , to more full saturation, not expecting any "consolidation". A rolling surcharge across the site with about a week per location would be about all I'd go for if some use is found with the test fill. This is merely a compaction job, not getting rid of possible settlements due to consolidation (look up the definition of that word).

Start by a test fill maybe 10'x 10' at the top before deciding on undercut also and if no reasonably rapid movement takes place in a week or two, I'd not surcharge and think about undercut also, but still would go for a foundation that can tolerate differential movement. Without undercut, no pressure to the ground over 1,000 psf or so.

Finally,as to landscaping, use plenty of paving around the building will help keep moisture changes minimal and place no fast growing trees or shrubs anywhere near the building.

Building details should have plenty of control joints in all walls, inside and out. As you can see, that CH clay is the main item to control what you do, not necessarily previous compaction work.

 

[fattdad]

I wouldn't react quite as much on the presence of fat clay or its "expansion" potential. The liquid limit is right at 50 and the plasticity index is less than 20. Sure, it classifies as a "fat clay" but no where near where I've seen serious shrink-swell potential. Rather I'd attribute the earlier building damage to the lack of compaction.

I'm not saying surcharge and undercuts below footings are not warranted, however. What you really need is a better definition on the soil modulus. I think the load and settlement response will likely be "elastic" and that's best described by modulus. A dilatometer will give you pretty good modulus values that you can then consider when looking at loads and deformation.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

One thing I forgot (old memory). When the moisture contents are near the plastic limit, with low Liquidity Index, the soil is quite precompressed. That means that surcharge would not do much consolidating, even if you waited a long time. So past problems were due more likely to not good compaction and probably shrink-swell action also. The test fill will tell much about the prior compaction factor.

 

[oldestguy]

More thoughts. Is the soil one that has potential of some degree for shrink-swell? Perhaps check on area problems might help answer this also. Trees many times are the culprits.

There are some references out there as to what are the clues to indicate possible shrink-swell potentials. I see some variation in warning flags that are in the references I have.

However, in my experience with clays weathered from loess, even with low PI (near 20) and low LL (near 35), occasional severe problems occur anyhow. Lake clays and those weathered from granite are another matter.
So for this job, I'd think the test fill done first would be quite useful to direct then where to go. If that shows good effect of "compacting" the old fill, and I do not find an area shrink-swell situation to be common,I'd do the surcharge and undercut under footing grade and a conventional spread footing foundation, slab on grade for lower floor, as in my first post.

If I don't see substantial "compacting" effect by the test surcharge, and also by a following site surcharge, I'd opt for the (mat) design which also expects differential support due to future shrink-swell activity. Using the undercut takes away some of the stuff that can shrink-swell.

The thought of doing undercut first to give some soil for the surcharge of course may dictate the order of work to some extent. Hey, it ain't so easy is it. You see I sometimes find lab testing ahead of time as misleading, particularly for predicting times for various actions.

 

[brinni]

thanks for all the valuable feedback. I am going to try the undercut and replace with some surcharging solution, after the original geotech engineer takes another look at it and weighs in on this, since he will be responsible for the foundation design as well...

 

[InDepth]

Why not go with convential belled caissons...and the slab on grade designed as a two way suspended slab system? Interface sidewalks may settle.

 

[grabens]

I have recommended pre-construction helical piers and auger-cast piles with a structural slab, and geopiers for sites on uncontrolled filled. The fact that one building has previously had problems leads me to believe a shallow foundation system is not the best option. The fill is too deep to over-excavate and replace.

 

[EBSEngineering]

grabens has it. Use helical piers and a grade beam structural slab system provided there is good bearing soil at a reasonable depth. We use helical piers in these situations all the time.

 

[oldestguy]

One point about any piers deals with the utilities in and out. Sewers need to be supported inside the building and sloped or otherwise built to allow for ground to settle (or raise) outside of the building. Water service, buried electrical, etc need protection in case the outside area moves and they are damaged where they meet the non settling sections.

Also some form of exterior covering or other to keep vermin from going under the building in settled voids.

Sidewalks approaching the building need some ability to span from settled ground to a non settling building.

Then, what will these conditions, if they occur, do for the value of the building later?

Finally, I'd check the cost estimates.