Foundation on small site

[SteelPE]

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.

 

[jdonville]

SteelPE,

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

 

[SteelPE]

Thank you for your reply.

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.

 

[JedClampett]

I would drill piers and fill them with reinforced concrete. It's really not that expensive (although the groundwater might require the use of casings) and if they're so short, you can easily inspect the bottom. Getting the foundation below the foundation of the existing structure might avoid any issues regarding construction caused settling.

 

[hokie66]

Assuming you are not trying to excavate a basement and the "ledge" is just the bearing strata, I agree with Jed. Drilled piers are simple to construct, and you don't have to excavate the soil. Driven piles would probably cause vibration issues with the existing building.

 

[apsix]

I don't know the full story of course, but my first pick would be bored piers/piles rather than structural fill, regardless if there are adjoining buildings or not.

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.

 

[oldestguy]

In some communities the owner of the adjacent building is responsible for holding his earth in place when an excavation is made at the property line. This sometimes has some rules as to that nearby depth triggering the responsibility transfer.

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.

 

[swearingen]

I'm going to pile onto the drilled pier bandwagon - it appears to be the best solution considering the info you gave us...


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[SteelPE]

The drilled pier idea seems very reasonable especially since the geotechnical engineer recommended 8 tsf for foundation bearing on the ledge. The drilled pier should allow us excavate down to frost elevation (where the foundations are for the existing building) and place our grade beam. We would then be able to drill down to ledge. Looking at the pictures taken at the site during soils testing it appears that there is some pretty bad soils onsite which will require casing of the hole. The casing should add to the stability of the hole.

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.

 

[JedClampett]

Usually, the holes are no smaller than 24 inches, but 36 inches might be better. Even though that seems large, it allows for inspection of the hole bottom with a camera or lowering someone in there.
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.

 

[csd72]

Jed,

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.

 

[born2drill]

I doubt you'd need anything as large as a 24" or 36" hole to support a 4 story building. Plus equipment to drill holes of that size can be very large, which might not fit in your "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.

 

[SteelPE]

Thanks for your help guys.

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.

 

[JedClampett]

I know the geotechnical engineer is insisting on removing the fill and replacing it. I would suggest to the architect/owner that possibly rather than going to the expense of shipping in and compacting dirt, they might want a basement instead. You could essentially add 20% of space to the project with very little additional cost. Just put in a good groundwater mitigation system.

 

[fattdad]

+1 on drilled shafts. Why the geotechnical engineer is "insisting" on replacing the surface soils is bewildering to me. If the goals of the project can be met with drilled shafts, why wouldn't this be considered a viable option? It makes no sense, especially in light that there are neighboring buildings on the same materils.

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!

 

[SteelPE]

Jed

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.

 

[hokie66]

SteelPE,
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.

 

[csd72]

I would suggest you look into using cement stabilised sand as an option. It does not require compaction which is one of the major time/money issues with engineered fill.

You could do this for whatever portion of the site you need it for.

Worth a thought.

 

[rowingengineer]

I can't understand this geotechnical’s need for risk, normally they are a conservative. The hit and miss method will need a dilapidation survey of next door.

When in doubt, just take the next small step.

 

[swearingen]

hokie66,

I think SteelPE's 2-part reply was to putting in a basement, not shafts.


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