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New pit walls adjacent to column line with overlapping ZOI

New pit walls adjacent to column line with overlapping ZOI

New pit walls adjacent to column line with overlapping ZOI

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.

RE: New pit walls adjacent to column line with overlapping ZOI

I'd drive sheeting, perhaps short lengths at a time, welded together, braced on the inside. You might also look at supplemental columns taking the loads off the two, sitting on timbers, back away from the hole. Might have to leave the sheeting in place, depending. Also, I'd plan on compaction grout behind the sheets on all sides, early on.

RE: New pit walls adjacent to column line with overlapping ZOI

They’re planing on pouring walls with soil nails behind is what it looks like on the few drawings they sent me. My concern is these columns are likely for an overhead crane... how do I keep these static designed walls from wanting to fail during dynamic loading conditions. I do not have confirmation if the crane is being kept but I am also assuming since it’s being converted into a recycling plant, it will get utilized.

RE: New pit walls adjacent to column line with overlapping ZOI

If they are looking to use soil nails, I would look into underpinning the columns with micropiles. They will already have the equipment mobilized so the only cost will be time and material.

RE: New pit walls adjacent to column line with overlapping ZOI

That was my first suggestion and they are wanting other options... they’re already installing other micro piles at other locations. I am almost to the point where I want to say I think they excavated to close to the footings and now it’s time to underpin them and add tie backs for additional support since they took the ZOI away when they removed ALL of the soil around the footing.

RE: New pit walls adjacent to column line with overlapping ZOI

I agree with MTNClimber that the columns should be underpinned. I would not drive sheet piling next to the columns. The column footings could be underpinned with micropiles (if you can find a good way to attach the micropiles to the footings or columns) or with conventional concrete pit underpinning (which the owner may not like because this requires more work area and slab removal). At 20' deep, the excavations will need temporary or permanent excavation support with bracing or ground anchors (tiebacks or soil nails). However, will the excavated pits be big enough to fit anchor drilling equipment? Also, the proposed structures look to be relatively close together, or opposite each other, which may prevent use of ground anchors. Rather than using sheet piling, I would recommend using drilled-in soldier beams or pipe piles (using the micropile/tieback rig). I think that the new structures should be designed so that the lateral bracing or ground anchors are temporary. Permanent anchors could also impede future development in the plant.


RE: New pit walls adjacent to column line with overlapping ZOI

They are supporting the all of pit walls with soil nails as they construct. As for the pit, it's going to be at least 30' wide and 50' long. Keep in mind, it's not a rectangular pit, more of a tetris shape I'll call it...

As for the underpinning, they have existing piles (unknown type or depth) under each column. I'm talking to them to try to convince them that installing a second column to tie back to is just going to create an egress problem more than anything because they'd have to put it right in the middle of the only walkway around the pit. Personally, I don't really like that idea too much. I like the idea of underpinning the existing footing and adding battered tie backs... we shall see if they'll go for it.

What a fun Monday!

RE: New pit walls adjacent to column line with overlapping ZOI

If the columns have existing bearing piles, the columns won't need underpinning. I would look at drilled-in soldier beams and lagging. If you have sufficient headroom to install long, heavy, WF or HP soldier beams, you could possibly temporarily brace the soldier beams just above the existing floor level (single level only) which would free up space for the concrete work. If you have insufficient headroom, the soldier beams may need to be spliced or you may need to use drilled-in, thread-coupled, pipe piles with bracing installed at one or more lower elevations because the pipe probably will not have enough bending capacity (compared to a WF or HP soldier beam). Without using permanent ground anchors, the new structures would need to be designed to support the lateral earth and floor surcharge pressures. For my previously stated reasons, I still don't like the idea of using permanent ground anchors (tiebacks or soil nails).


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