deep foundations next to building on poor soil

[inaz]

An existing buildng will be razed for a new 4-storey structure. There is an adjacent building with a zero setback distance. The upper 15 to 20 feet of soils are loose to medium dense old fill soils, underlain by a very dense clayey sand stratum. The bottom level will be a parking garage (slab-on-grade).

The existing structure is seated on the upper less quality soils with shallow foundations. The site is downtown along the main business district, so staging area is very limited. I plan on recommmending drilled cast-in-place piers. To limit haul off, I've looked at displacement flight auger piles. Any helpfull suggestions?

 

[DRC1]

I would worry about the flight auger piles caving, esp. in the upper soils. I would consider 8 or 10 inch mini piles. You would get better quality contol and probably higher allowable loads from the mini piles. The $/ft will be higher, but the increased capacity may offset that. Is the new building going to be below the existing building? If so you may need to look closely at earth retention.

 

[Focht3]

You have a difficult problem. I would look at underpinning the adjacent structure before I built any foundations for your new building. You need something that can be installed with minimal disturbance and doesn't involve use of any soil removal. Small steel piles, jacked into place, would provide ideal underpins for the problem you have described.

Next you will need to construct a low loss-of-ground basement wall and temporary retention system. A drilled shaft wall would be a good candidate; if caving is an issue I have seen some pier rigs that have an auger and casing that are advanced simultaneously and counter-rotate. (Yes the casing is screwed in.) A hole less than 20 feet deep can be supported by a cantilevered drilled shaft soldier pile wall - but forget lagging. You will lose too much ground. The shafts will have to be close to limit the risk of soil loss between the shafts.

Mini piles could work to support vertical loads, but are unsuitable for the retaining wall.





Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[inaz]

Great comments Focht. I should point out that there is not a basement level. Finish floor elevation of the lower parking level is at existing grade.

 

[cdh61]

inaz,

You should be transfering the loads to the deeper deposit of dense clayey sand, via end bearing piles. Both cast-in-place or auger piles can work, along with other pile options, but there are advantages/disadvantages of each pile option. For example if using cast-in-place piles, you will have wet concrete in an open hole next to the existing building, and you may need to use a fast set concrete to limit potential problems. Also as DRC1 mentioned soil disturbance from auger piles near surface. What did the geo engineer recommend to limit disturbance of the adjacent building? What is the adjacent building?

In a case like this you will have to watch the stresses from the new structure does not cause further consolidation of the fill soils near the existing building, even though the fill has been in place for many years.

regards

 

[Focht3]

Sorry - posted that pretty late last night. No basement? Then I'd stay with a low displacement solution. I'd avoid displacement auger piles - too much risk of damaging the existing structure. They're a great option otherwise - provided you aren't worried about negative skin friction.

For anyone interested - Dr. James (Jim) Long at U. of Illinois performed lateral load tests on mini piles. He found that the threaded joints are a serious weakness - failure load for one pile was only 16% of the expected value. I expect the mini pile makers and installers to address this issue in the near future.

And for what it's worth, the use of mini piles in the U.S. seems to be pretty regional - mostly along the east coast. I spoke with one of the engineers with a main installer; he indicated the issue was one of cost competition. Interesting -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[1dirtDr]

inaz;
I agree with the fellows here. However my questions are
1-Are you removing the old structure and rebuilding, or
2-Adding to the existing structure?
3 is the next door building foundation higher or lower than yours

if it is same elevation or lower and you are not exposing the footings or undermining them, and
If you doing (1) then you will need under pinning for the existing foundation to bypass penetrate the fill and transfer the load on to the hard/dense clay and sand….this is a must, due to downdrag loads from the new addition, and not to forget the earthquake loads There are several methods for that and all are pretty close in $$. The under pinning rigs are pretty easy to maneuver and they can get in pretty tight places.
If you doing (2) then go with the mini shafts AKA augur cast piles. They are fast kind of messy but cost effective.

Now if the other building is higher and its footings may be exposed, then brace the sucker up just like what Focht3 has recommended.
Then use the above.

YOU WILL have to pay for that little project, I speak from a personal experience… downtown Seattle in the middle of china town 10+ floor condos, with some old buildings on three sides and major street on the fourth side, to top it off, none of the other owners liked the idea of having these condos next door. Subsurface soils…garbage. Literally old dump site, some of the drillers found bottles and old cans from way back then . We used almost every deep foundation system in there, and that was before we started on the foundation for the building itself.
So have fun…, explore every possible option including the removal of the existing structure and then building your new one… if it not too much $$$

I is a dirt squeezing engineer, so I apologies for any misspelled words, bad grammar, sentence structure or language rules (or lack of)
Cheers every1