What measures can you take against buoyancy on a building?

[Olivaw]

There is this proyect on my office: a 6 story building with 2 basements and a high water table (about 20 ft of water pressure). We have enough dead loads to resist uplift from hydrostatic forces, but ASCE 7 guidelines says that we need to check both earthquake and soil pressure loads (E and H load cases are included on the same load combinations).

The problem is that there is just not enough weight on the building to prevent overturning from both earthquake and hydrostatic forces. I made a model on SAFE to check those foreces but it takes forever to run and it always have some sort of instability. I guess having so much uplift can do that to nonlinear models.

We have come up with the following ideas:

- Lower the foundation to add more weight to the building
- Account for friction between soil and basement walls
- Account on passive soil pressure from the walls
- Add some type of anchorage to take all the hydrostatic uplift

What do you usually do when you have a lot of upllift on your foundations?

 

[hokie66]

Either more mass or anchorage will do it. And make sure the lower basement floor structure, and the basement walls, are strong enough to resist the uplift.

Friction, passive pressure, those are not in my list of considerations.

 

[Enhineyero]

I find that this becomes a problem for some buildings if pad footings are used and stability check is done locally. If that is the case, I would probably recommend changing the isolated footing to a thick raft foundation connecting all the columns and walls. The check for overturning then becomes more of a global stability check.

The other instance I see this is when you have a small building footprint for a tall building. I would usually go for adding in more weight in the building and/or utilise piles for their skin friction.

 

[blihpandgeorge]

for bouyancy, there is a few options:
1) increase building selfweight
2) anchor down via either piles or tension anchors. Tension anchors can make sense if you are using pads and good quality material like rock is not too far away.
3) install a cut-off wall to stop water from getting under the slab. this can be done where there is an impermeable geotech layer beneath the basement and the perimeter retention wall / basement wall can be extended into this and sealed against it. some form of drainage and continous pumping below the basement level may be required for seepage.

lowering the foundation will increase the uplift, so this is can be inefficient way. often the opposite occurs where you remove a basement to reduce the loads.

friction on the basement walls could be included pending some specific geotech advice and the type of wall and how they are constructed. Insitu retention like piles may generate some but walls that are constructed from excavation and backfilled against i dont consider reliable. This really need geotech advice though. Passive resistance would rely on some sort of horizontal toe to engage the soil selfweight, this has a big impact on construction as it means over excavation to install compared to piling.

 

[r13]

Question, where is the 20' water pressure from, underground spring? Or over the height of 2 level of basement? The building might need founded on piles.

 

[DaveAtkins]

I agree with Enhineyero, use a mat foundation. If that is still insufficient, extend the mat footing beyond the exterior walls in all directions--the soil overburden (it is saturated soil) will add the additional dead weight you need.

DaveAtkins

 

[KootK]

I also like Enhineyero's concept of engaging the substructure globally for overturning. At 6+2, I wonder if one might take advantage of the natural backstay mechanism to achieve that. In concept, it's very much like the virtual outrigger scheme in ultra highrise construction.

 

[r13]

Note when permanent ground water stays height, increase foundation area and go deeper might not be cost effective, as uplift pressure increases as well. Adding weight can offset some of the lift force, my next choice will be pile foundation to keep rotation under control.

 

[molibden]

What about extending mat foundation horizontally on all sides, thus adding weight of the surrounding soil to the overall building weight (similar to retaining walls).

 

[Olivaw]

A few days ago we solved this problem. We had made a SAFE model of a raft foundation, but the concrete material had the wrong density on the program, so it wasn't adding all the weight from the foundation slab. Once we fixed that, we had enough weight to control overturning.

It wasn't my most proud engineering moment but Im sharing it so other people may learn from my mistake. The lesson here is to run some simple hand calculations and then check the model. I would have saved a lot of time if I had checked the model from the start.

Edit: Thanks to anyone who posted recommendations, I am going to keep those in mind for other proyects.