Equipment pits overlapping column ZOI

[EDB9]

Not sure if I should post this in geotech or structural so I’m going to post in both. I’m one of those geotechnical structural engineers...

I got a call from a construction company that we work with...this one stumped me.

They’re installing a new equipment pit that’s about 20’ deep. Immediately next to said pit, are a line of columns that look to be spaced about 20’ apart (I don’t have measurements, just guessing from pics). The site is a state over or I would look at it myself.

They initially wanted to install strip plates along the slab to tie some of the columns together at the bases and wrap around to help distribute some of the load. I don’t see this working since now I don’t have any columns I can tie back to. I have no clearance whatsoever between the soon to be pit wall and the footing under the column. They are just now starting to do exploratory excavation.

I’m trying to brainstorm ideas on how to reduce the ZOI pushing on the soil nailed walls that they’re going to be putting in. They need something that is going to keep the walls stable. I know they want to avoid putting in piles because my first thought was tie backs. They want to know if there is another solution that’s easier to install.

Of course today is the day EVERYONE is out of the office too. Any ideas are welcome! I attached a picture of the column in question and this is just the first layer of excavation.

 

[JAE]

Are the existing columns on spread footings? Is that what your photo is showing?

I would be VERY reluctant to excavate 20 ft. deep next to a large column/footing like that. What was the pit designer thinking?

It isn't an issue here with the pit stability. It is the issue of undermining a perhaps significant structural column.

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[EDB9]

That’s what I said! Those columns are designed for 200kips each. The actual loads are closer to 180kips, but still. They have a about a five foot deep footing that looks like a strip footing that follows the column line.

This is what I love about not being the design engineer but the fixer engineer to another firm’s design.

I said the same thing initially that this was crazy to have such a deep pit next to the line with zero separation. They are doing in stages of about 4-5’ sections and installing the retaining wall/soil nail walls. I’m concerned about the impact during construction too! This is a facility that’s going to be an aluminum smelting plant.

I really want to tell them to install a boatload (official term) of tie backs to try to stabilize the wall but I still can’t come up with a solution to the fact that my ZOI is still near the mid-section of the soon to be pit wall.

Here’s a few more pics...

 

[WinelandV]

So, first thing is to assess whether your firm actually wants to deal with this.

Assuming you do, I'd ascertain what that actual current load on the column is. It appears that there is an overhead crane here. It would make sense that the crane is what is driving the 180 kip load (possibly coupled with some roof loading, i.e., snow). ASSUMING that the crane will NOT be used during construction, the actual load on the column may be something semi-reasonable to actually get the pit in (though 20' deep is REALLY DEEP this close to a column).

I'd certainly want some Geotech support on this. I don't see how the 20' pit is going to work once said crane is back in action. I'd be leaning toward a support of excavation design more robust than soil nails - possibly sheet piles coupled with internal horizontal "rings". Or go the REALLY expensive route and use a concrete secant pile wall.

Is the final pit size more or less the size of the hole in the slab? Or is it much smaller (more along the lines of an elevator shaft)?

 

[EDB9]

I am also assuming those are the overhead crane loads as well given the nature of the facility. A lot of the walls I believe are going to be cast in place but for some reason they wanted to build right up against the column line. On the design drawing it looks like they have more clearance but in reality, they don’t since their technique required extra excavation.

As for the pit...the best way I can describe it is it’s going to look like a Tetris game. These holes are just exploratory holes that are at some of the locations of the giant pit to be. This was an old building that they’re retrofitting and I get the impression they don’t know what’s all buried under the slab.

This picture is looking at the first hole location (first picture in the OP)... the thing I don’t get is why are they at the column and not between them like shown on the drawing...almost makes me wonder if they’re in the right place.

 

[JAE]

Does not appear that they are in the correct location when comparing your drawing to the excavation in the photo.

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[EDB9]

So I’m not losing it! I don’t know if the drawing is to scale but they should have had at least 3-4’ of clearance easily. The pit should be between the columns not at the column. At least thats how I see it. I think I have a longer list of questions now!

Thank you for all of your inputs JAE!

 

[TehMightyEngineer]

Any chance you can make the pit round? Much easier to transfer the lateral thrust from the column through the pit walls if you don't have those hard 90° corners.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[EDB9]

Sadly, my firm isn’t the firm who designed this. We offer construction support and this was a problem brought to us. I honestly think this was a bad design to start with but I don’t think they want me to redesign their pit. I do think they’re going to have to install a few more piles because of how ridiculously close to the columns they “explored”.

 

[PEinc]

See additional comments in the Foundation Engineering forum. FYI, posting in multiple forums is frowned upon.

http://www.PeirceEngineering.com

 

[EDB9]

I was unaware posting in two places wasn't permitted. I won't do that again...