Deepest Underpinning

ch0mpie

I'm going to be inspecting the subgrade in some underpinning pits the next couple of days and was wondering what was the deepest you guys have ever encounted. The one I was in today was 4'x3'x24', not for the clostrophobic. For what ever reason I've always been the guy who gets to "go down the hole", but this was a personal best for sure. I swear I could hear people speaking chinesse if I listened hard enough

rino1494

We stay away from that type of work.

I hope that the pit was properly shored.

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ch0mpie

I believe it was, atleast everything was to the specs that I could tell, but I'll admit I know alot more about soil than about shoring. Besides the obvious, what would you look for as a warning sign? I have to inspect about 15 more pits on this site.

dayexco

if you know soil, you know shoring.

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someday, i'll be as patient as Nick.

ch0mpie

Let me put it to you this way. I know how to design shoring/support of excavation, and like I said, everything looked ok to me. The thing that scares me about underpinning pits is that only 2 people go in there. Me and one of the contractors grunts (who may or may not be properly supervised). With this in mind, if he was going to take some shortcuts, that probably would not be noticed by someone who didn't build shoring everysingle day, what would they be?

dirtdiggencat

soil is "all class C" or worse, and treated as such on our diggs. use a box,,,, n stay in it!
that's all i know bout soil n shoring.

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tgeb

My guess would be someone would cheat on cross bracing and put in too few for the conditions.

You are going 27' deep in a 4'X3' hole?
How do they dig that?
How do they do the shoring?
I would want steel plates around me, welded together and braced by my welder.

I want to refer you to a recent post of mine.
http://www.contractortalk.com/f62/tragedy-11258/

Don't take unnecessary risk, it is not worth it. Make sure that hole is bullet proof!

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Tom

www.gebcon.com

ch0mpie

I wish you were the contractor tgeb. I'd feel alot safer in your steel box.

yes the hole is 23' deep and is 4' by 3' the whole way down. Basically an area adjacent to the existing building to be underpinned is excavated down to the existing foundation level. Then a four foot wide area is excavated about four feet below the foundation level. From there it is all hand digging, the 4' by 3' opening is directly beneath the wall footings of the existing building. As the hole goes deeper bucket and pully are used and boxes made of 2x12's with 2x4 keys are added on. When the pit is deep enough it is filled with stiff concrete. When dry the building is jacked, and the space is filled with steel shims and dry pack. Then another four foot section is excavated and a pit is dug and the process repeats its self until the wall footing is extended to the new foundation level. Next time I go to this site I will post pics.

ch0mpie

I read this post last week and believe me if I had any incling that something was done wrong I would not go in there. The problem is I've never seen a pit this deep and have no feel for the shoring is sized correctly. I'm just going by the design drawings and the specs and everything seems right. I guess thats why I posted about it, to see if anyone else had experience with pits this deep.

dayexco

you are aware, that before you or anybody you're responsible for goes into an excavation 20' deep or more.....that you have to have an engineer sign off as to the design of the shoring mechanism you intend to use. trust me, it's not my intent to be a smart ass. i just don't want to see people hurt.

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someday, i'll be as patient as Nick.

ch0mpie

The support system, dewatering, underpinning was all designed by a registered PE, and was reviewed by my boss (also a PE). But it still doesn't make it any less scary when you have to go down there.

PipeGuy

I'm surprised 2X material is used at that depth. We typically use 3Xrandom width material to sheet excavations deeper than about 14 feet. Are timbers used to form a frame for the sheeting to brace against or are the 2X12's the only lumber used?

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ch0mpie

I was also expecting 3x material. 2x12s are the only lumber used, no sheeting.

tgeb

http://www.osha.gov/pls/oshaweb/owad...RDS&p_id=10933

The above link is to OSHA standards for timber trench shoring.
From what I read you should not have 2Xs in that trench, more like 3X6s vertical and 8X10s and 8X8s if you are using Douglas fir or equivalent.

You also need to be taking precautions relating to confined spaces.
Are air samples being taken?

http://www.osha.gov/Publications/osha3138.html

A little info on confined spaces you might want to review.

I would want to see the design, make sure it is stamped, and have the engineer review all the shoring on every pit.
Make them sign off on it!
They can say "yup looks good should be fine", but if something happens they will deny ever seeing it.

Don't take unnecessary risks.

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Tom

www.gebcon.com

ch0mpie

Thanks for the info tgeb. I checked over the design (which is sealed by the PE), and basically since it is a pit and not a trench, it is continuously cross braced and 2x lumber is acceptable for the material(loose sand). Air monitoring and all other confined space procedures are being followed, and I feel a little safer having looked into all of this. However, being in a pit like that is still pretty wild. I'll try to post some pics I took tomarrow. I was talking to the contractor's forman today and he told me that his crew has done underpinning as deep as 60'!

tgeb

Looking forward to some photos.

Stay safe Ch0mpie.

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Tom

www.gebcon.com

ch0mpie

enjoy

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ch0mpie

here is a couple more interesting ones. The last one is of the underpinning being jacked.

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tgeb

That is quite a project!

Does not look safe to me.

Is your means of egress the 2Xs nailed across the corner?

Could be bad news, be careful ch0mpie.

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Tom

www.gebcon.com

ch0mpie

[QUOTE=tgeb]Is your means of egress the 2Xs nailed across the corner?[QUOTE]

yes