Foundation on small site
Foundation on small site
(OP)
I have a site that has undesirable soil with a proposed 4 story building. These soils are fairly shallow and were recommended to be removed by the geotechnical engineer and replaced with structural fill. Depth to ledge is around 10'-0" +/-. The site is very small requiring the building to be place close to the property line. An existing building (on the other property) is already located tight to the property line. Foundations for the existing building are located 4'-0" below grade. This makes removal of the unsuitable soil very difficult as this method would require shoring the existing building during removal of the unsuitable soil (I don't know how this could be done with ledge 10 feet below grade).
Hopefully I explained this situation properly.
Are there any other types of methods I can use to construct this foundation? Or are we going to be forced to underpin the existing foundation?
We would like to avoid any situation that involves remediation to the existing structure as the two owners do not get along.
Hopefully I explained this situation properly.
Are there any other types of methods I can use to construct this foundation? Or are we going to be forced to underpin the existing foundation?
We would like to avoid any situation that involves remediation to the existing structure as the two owners do not get along.





RE: Foundation on small site
Depending on the soils underlying the undesireable layer, you could do deep foundations (drilled shafts, helical piers, augercast piles, etc.). If you really need to undermine the adjacent existing strcture, you will need to underpin. Rather than traditional underpinning (very expensive per ft2), you might consider micropiles or a tieback wall.
Discuss options with the geotech of record. They should have given you alternatives to excavate and replace in the report.
Jeff
RE: Foundation on small site
In this instance piles just seem ridiculous to me. The depth to ledge is 10-0" +/-. Seems like a waste to use piles for such a short distance.
The initial thought was to use sheet piles on the property line (not really sure this could have been done with out damage to the existing building). We quickly realized that there was no soil below the proposed excavation to hold the sheet pile in place (don't really think they can drive them into ledge).
The geotechnical engineer did recommend an alternative foundation design which was a structural SOG supported on grade beams which were supported by spread footings on the suitable soil or ledge.
To me this system doesn't make sense because of the proximity to the property line/existing foundations. Putting footings on competent soil requires excavation below existing foundation elevation. We run into the same problem of how to prevent undermining of the existing foundation.
Oh, and there are ground water problems with this site as well.
RE: Foundation on small site
RE: Foundation on small site
RE: Foundation on small site
The compaction required for the structural fill could also cause problems to adjacent buildings supported on shallow footings in 'undesirable soil' with high ground water.
RE: Foundation on small site
That 10 ft. depth excavation for those "caissons" might even be done by a utility auger without casing, depending on the soil type. I've seen some pretty low cost work done that way and it worked.
On one job the contractor even used a drill with with big auger diameter.
RE: Foundation on small site
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RE: Foundation on small site
I only have one more question. How large to they typically make the holes for these drilled piers? Are their standard sizes or can I do anything I would like?
Thank you guys for you help.
RE: Foundation on small site
Drilled shafts without casings are very cheap. Even though everyone makes a big deal about the casing cost, I'm not convinced even that adds that much.
RE: Foundation on small site
I assume you would need a casing if you were going to lower someone down there? It would not be allowed in this country - health and safety dictates that you always use the safest method, which in this case is CCTV.
RE: Foundation on small site
I'd think micropiles would make more sense. Typical sizes range from 5.5" - 13.375" and they can be installed with very compact drill rigs if need be. Depending on your local building code, you may need to video inspect the rock sockets or possibly run a compressive load test.
RE: Foundation on small site
After all this talk about drilled shafts the geotechnical engineer is insisting on removing the fill. He has already come up with a procedure he believes will allow for excavation next to the existing building. He plans on removing the existing soil in sections. This will be slow but it is the way the owner wants to proceed.
Thanks again.
RE: Foundation on small site
RE: Foundation on small site
Bear in mind that your column/walls loads may be greater then the neighbor's loads, both of the structures likely include a ground-supported slab, which would be the only building element affected by the surface soils.
The incremental removal of soils (and the corresponding replacement and compaction efforts) doesn't sound that protective of the neighboring structure without the use of excavation bracing.
Just some of my thoughts to go along with the other appropriate replies.
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation on small site
When we received the geotechnical report this option was discussed but dismissed for 2 reasons:
1) Depth to ledge at the shallowest point is 6'-0". Raising the building is not an option as they have an OH door at the front of the building.
2) Depth to ground water. Even though this can be resolved with a proper drainage system I am still very cautious of systems the ground water issue is resolved by a perimeter/under slab drainage system.
Fattdad
As I understand it, the geotechnical believes removing the soil will be less expensive than providing grade beams with a structural SOG. The footprint of the building is not that large and he believes they can safely remove this material without any effects on the existing structure.
RE: Foundation on small site
At the risk of talking through my hat about a site I know little about, I would say your geotech is talking through his hat.
What do you mean by "ledge"? Is this a rock stratum where you intend to bear? If so, why does the rock being shallow make piers less attractive? At that depth, you could possibly build piers with backhoe excavation.
The proposed method of removing the existing soil in sections means the new material will have to be placed and compacted in sections. I wouldn't want to found a four storey building on that type fill.
You asked for advice, no one here has recommended removing the material. It's not contaminated, is it?
Remember, if there are settlement problems with the building, it won't be just the geotech who is blamed.
RE: Foundation on small site
You could do this for whatever portion of the site you need it for.
Worth a thought.
RE: Foundation on small site
When in doubt, just take the next small step.
RE: Foundation on small site
I think SteelPE's 2-part reply was to putting in a basement, not shafts.
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
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RE: Foundation on small site
It seems alot of money is potentially going to be thrown around and I'm not sure it's needed.
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation on small site
RE: Foundation on small site
You can design the structure, advising that you give no guarantee of foundation performance.
RE: Foundation on small site
That approach will not keep him out of the firing line if damaging settlement does occur. He may not be found liable, but by that time he will have spent a lot of money defending.
RE: Foundation on small site
RE: Foundation on small site