Foundation design for a challenging site: old foundation wall, potential differential settlement, and unknown foundation depth

[highlander50]

Dear Engineers,

I want to present a problem I’ve encountered in one of our projects.

We have a client planning to build a house on a plot of land. The plot is quite large, and he only intends to use half of it.

The area where the house is to be built is partially covered by an old swimming pool of unknown depth, filled with demolition and rubble materials, which prevents us from positioning any machine on it to work effectively. We also don’t know the exact shape of the pool in terms of depth-wise, whether the bottom is completely flat or if it slopes to one side.

On the other hand, this half of the plot is at the same level as Street 2 (see attached diagram) but sits at a higher elevation than the other half of the plot.

The other half of the plot was initially used to construct a 6 floor building, but due to financial issues, the builder left the project halfway through, with a concrete slab installed at a depth of 12 meters (as we’ve been informed) and some of the initial diaphragm walls in place for later enclosure. However, when the construction company went bankrupt, the site was left open, and it was later partially filled with anthropic backfill without any compaction or quality control.

The fill doesn’t occupy the entire “hole” left from the unfinished construction, resulting in a level difference of approximately 8 meters between the two halves of the plot.

The walls that retain the soil mass have had their reinforcement exposed to the air for at least 20 years, and in the area furthest from the planned construction site, they seem to be deteriorating based on their current condition.

As an additional note, I haven’t noticed any drainage elements in these walls, which were initially intended to house the parking structure of a building that was never completed.

At this point, I have several questions to correctly approach the problem, and I’d like to get your opinion on it.

1. The house will have its foundations at different levels. In areas without the pool, continuous footings are planned at a depth of about 1.00 meter. However, the foundation depth in the pool area is unknown until the debris is cleared. How would you approach this part?
2. Given the foundation at different levels, with the deepest level currently unknown, I understand that differential settlement shouldn’t be an issue if we design based on the materials closer to the surface, as the materials below the pool should theoretically be more competent and likely overconsolidated (due to the weight of the water it once held). Do you think this assumption is valid?
3. Regarding the pool, I’m uncertain how to face it. Without knowing the foundation depth, it’s challenging to suggest that the pool should be supported by a semi-deep foundation, but I’d like to hear your thoughts.
4. We have a diaphragm wall, apparently without drainage, with exposed reinforcement for 20 years, an 8-meter elevation difference, and now a house will be added on top, increasing the soil pressure against it. My inclination is to investigate this wall's dimensions to ensure it can withstand the load over time, but I’d like to know your opinion as well if you consider that can be an issue.

Attached are diagrams and photos.

Thank you very much in advance for your comments.

 

[Lomarandil]

Oof, you're not kidding.

We can think about some rational, conservative assumptions to make, but I'd be pressing hard for more investigation if that's at all possible. You'll end up saving the client money in the long run.

 

[highlander50]

Oof, you're not kidding.

We can think about some rational, conservative assumptions to make, but I'd be pressing hard for more investigation if that's at all possible. You'll end up saving the client money in the long run.

Yep.. seem it'll be hard to discuss with the CLient as no one wants to have an increase in the initial budget but definetly this can save a lot of money in the future.. it's a complex sitation.

Thanks for answering!

 

[LittleInch]

Well based off your photos the most alarming / urgent issue is the collapse of the retaining wall on what I think is street 1 with signs or some slabs already broken and undermined. Urgent shoring or compacted fill is required to prevent collapse of the street and injury to the public or worse if there are things like gas mains in that location.

I think based off your description and sketches you really need to excavate the pool back to its known elements (you have abig hole next door to dump the contents into, then either properly demolish the pool and ensure you can't get water trapped in there and then fill the pool with materials you actually know about and are not guessing and able to compact properly. you will probably find a few surprises in there so allow for some transport of some material to a special waste site.

Doing anything else is just madness IMHO as you are just guessing and that isn't good for the foundations of a building which could then crack and move and you get sued for saying the ground was ok. Really diggers cost nothing to just dig out the crap they put in there.

But fix that wall NOW!

 

[geotechguy1]

Technically it sounds like an interesting project in terms of challenges, there's alot going on. The problem is to do it properly you need to either do something extraordinarily conservative or apply a high level of rigour in investigation and modeling. Likely your client wont want to pay for or understand the need for either.

I'd refer the job to a competitor to be honest

 

[highlander50]

Hi engineers,

Thank you so much for advises. It all goes a little bit with the main intention I had from the begginig.

Regarding the wall in Street 1, this should be addressed immediately, or at least this is going to be my indication to both the Client and also the authorities.

The project itself, as properly said, is quite challenging, which makes it very interesting, but at the same time, it is going to be expensive. This week I have a meeting with the Client, and my recommendation will be for him to consider building a basement in the house. This will reduce the amount of weight applied at the top of the wall, will also reduce the uncertainties in terms of settlement as all the foundations will be supported on natural ground, and parallel to all that, I will recommend increasing the scope of the study for the wall in terms of current status and lateral loads. For sure, all must be done with isolated foundations instead of continuous footings.

I think the increase in terms of cost for the building itself due to incorporating the basement won’t be huge, as all the materials in the pool must be removed and the space will already be prepared. If he doesn’t want to do it, he will have a nice house with deep foundations.

Regarding the chance of refilling the old pool with adequate material and compacting it to support the foundations, I don’t really like the idea. It’s quite difficult to maintain proper control of the compaction to avoid generating differential settlement (in my opinion), and it’s very complicated to properly compact the material in the current configuration of the site.

Regarding the pool, my recommendation will be to use a prefabricated one, as this (I think) will reduce the possibility of having water leaks and will make it much easier to detect and prevent them. The new pool must avoid the old one, or if not, the foundation will need to be semi-deep, which I think makes no sense.

We’ll see his reaction because, in the end, it seems that everyone knows what they’re doing except the professionals involved. Everything is very expensive, and they don’t want to pay for knowledge.