ROWHOUSE Basement lowering - Contractor OVEREXCAVATED 3

[Ben29]

I am working with a contractor who never performed basement lowering (basement underpinning). I agreed to provide underpinning sequence drawings. I get to the house to take measurements so that I can create the drawings. I get there and find out that they totally excavated the entire basement, 15 inches below the existing foundation walls!!! I told them to stop digging immediately. But the entire basement is now dug out. I am sending them temporary bracing details to keep the end walls from caving in (rowhouse is an end-unit next to an alley = vehicle surcharge on endwall!).

aside from the bracing detail, what do I do to temporary support the walls from below? good chance we won't start underpinning for a couple of weeks yet. or Do we just start underpinning as this has now become an emergency repair?

 

[Ben29]

 

[phamENG]

Yes. This is an emergency repair. I would suggest reaching out to the town building department and getting the ball rolling to close down that alley and prevent that vehicle surcharge. There are several well documented building failures caused by improper underpinning sequencing.

Best bet is probably to have the contractor replace and compact the soil around the perimeter for at least 3 feet. Installation of any other temporary shoring that causes vibrations could have a detrimental effect on the rest of the structure. You can do it when the sequencing is correct because the rest of the building is still stable. In this case, it isn't.

 

[Ben29]

@pharmENG, are you suggesting that I replace the soil in lieu of installing the wall bracing? or in addition to? here is the wall bracing detail that I started.

 

[phamENG]

I'm saying they need to get something in there to stabilize the wall now. If they can get in and start doing that right now, then so be it...but even if they did that work today it would do anything for a day or two while the concrete sets up.

How's the drainage in the area? The storm that had us under a flash flood watch this morning is rolling into Maryland this afternoon.

 

[Ben29]

I feel like they are going to ask me if there is anything they can use in lieu of placing back soil. I wonder if they could put back CMU block and fill the cores with sand, or use solid block. My thinking is that, at least they could easily disassemble it and carry it back out and reuse it in the future.

I do not know how the drainage is. It is close to Baltimore inner harbor. There was no water in the basement when I was there yesterday. There is no signs of a sump pump ever being in the basement.

 

[bones206]

For the sake of time, maybe you could drive some steel stakes to pin the braces for now.

What was the original embedment depth of the foundation wall? Just wondering how much passive resistance it had on the basement side to begin with.

 

[HTURKAK]

Er. Ben29 (Structural),

I would strongly recommend immediate temporary backfill ( with gravel, sand bags etc.) before starting the underpinning.

The soil under the wall may spread suddenly if not protected with berm etc.

I painted the proposed berm on your picture.

Good luck

 

[Ben29]

Bones: According to the contractor, the bottom of brick wall was at top of existing dirt floor..... so according to him, existing condition provided no passive resistance.

 

[phamENG]

To the wall, perhaps. But to the soil under the wall? Yes, it did. So if anything increases the lateral load on the soil beyond what friction can do for you, that building's in a world of hurt.

 

[Ben29]

pharm - you are right.

and to think.... this was going to be my 'easy' job this week...

 

[XR250]

Have them pour the slab with an integrated short retaining wall ASAP. Probably just as fast as doing anything else.

 

[Enable]

At this stage you should not be debating about what is the best thing to do. Rather, your focus should be on getting this thing secure and working around the clock to do so.

Steps:

1 - As phamENG said call the city and tell them no vehicles in the pathway but that is a curtesy call. You are doing this now. As in, pylons are out and quick fence is barricading the drive lane. Instruct the contractor to do this immediately. Fuck traffic concerns.

2 - If you can get concrete in there today, do it. XR250 is on the money with that. If you cant, get 3+ feet of 3/4" clear stone slung in (or some other sort of soil that doesnt require a great compaction effort). I agree with phamENG that temp lagging is fraught with issues, such as soil not being in full contact with the boards especially after vibration due to install. This could lead to movement of the soil prior to the lagging engaging, and at that point, she's gone.

3 - Decide how best to re-excavate the basement and proceed as per previous plan.

 

[BAretired]

I agree that this is now an emergency job, and because it is, there is some danger to the workmen attempting to take appropriate remedial measures. Wearing a hard hat and steel toed shoes is not sufficient protection if the soil suddenly gives way, causing a collapse or partial collapse of the structure. Make sure that the contractor and his men are aware of the potential danger.

BA

 

[JStructsteel]

Is there cases where the outside retained height is at the first floor, or is it all alley as shown. If so, some small rakers and rebar pins will hold the soil load. What is it, 2.5' soil height?

 

[3DDave]

They seem very incautious for not having done such work before.

 

[PEinc]

Someone has been very lucky so far! If the new basement slab is to be poured over a crushed stone base, put the stone in now, but heap it up in a berm against the 15" vertical excavation. This could help stabilize the vertical excavation. Later, after the underpinning is finished, spread the stone over the proposed slab area and pour the slab.
Nice safety comment, BAretired.

http://www.PeirceEngineering.com

 

[rb1957]

surely (?) you're not going to leave it as per the first pic ? Won't you stabilize the vertical 15" of "dirt" ? I'd've thought that you'd replace this "dirt" with block, replacing it piecemeal around the perimeter. I guess I wouldn't've profiled the dirt with a vertical face, but rather a 45deg face (losing some floor space).

Sure, undercutting the sides has made it Way worse than the pic.

another day in paradise, or is paradise one day closer ?

 

[phamENG]

Ben29 - what's the story? What have then done to fix it?

 

[Ben29]