Maximum Concrete Underpinning Pit Depth 3

[cap4000]

Typically I have seen the maximum depth of concrete pits to be 4 foot deep by 4 feet wide in a 1,2,3,4 alternating sequence. Deeper than 4 feet typical specs call for bracing the structure above. Contractors I have talked too, say 8 feet deep can be done in one lift, no problem. Can someone clarify this issue for me. I would appreciate any feedback.

 

[jheidt2543]

It depends on basically two things:

1. Will the soil stand up long enough to accomplish the underpinning without falling into the pit from under foundation or building.

2. OSHA regulations require worker protection when the pit is over 4' deep. There are a number of ways to do this; with and without bracing.

 

[SlideRuleEra]

Having been a heavy construction Contractor at one time, it is a fine line between what is "no problem" and what is "legal". There is great temptation to selectively disregard safety requirements. Strict compliance with these regulations is costly (in both time and money). Many times they truly seem "ridiculous" under certain field condition.

For the type situation that you describe, safety is a statistical matter, like playing a win-loose game such as "Russian Roulette". If procedures are ignored, most of the time nothing "bad" happens...
but every now and then all hell breaks loose and there is the devil to pay.

The Contractors that you are talking to have most likely been "winners"... so far...

http://www.SlideRuleEra.net

 

[PEinc]

It seems to me that cap4000 is talking about the dimension of the pit which is perpendicular to the face of the wall to be underpinned - not to the actual excavated vertical depth.

The depth back from the face of the footing being underpinned can be almost any dimension, even greater than 4', as long as you don't undermine so much of the footing that you cause settlement. I've done pits that were about 6' along the wall face by 8' back under an existing, large, continuous, wall and footing which supported an urban passenger train terminal. Large pits need to be done carefully. This meams proper shoring of the pit sides with extra thickness lagging boards. Usually, normal pits are shored with 2x10 dressed lumber. Larger pits may need rough cut 3" or 4" nominal thickness lumber. Extra consideration must also be given to the corner connections for extra long shoring boards. A few nails may not be sufficient. As long as you don't undermine too much of the footing, shore the sides every four vertical feet (or more frequently if bad ground conditions), and use proper shoring, you should be able to successfully underpin the structure.

 

[cap4000]

PEinc,

I am talking about vertical depth in this case.

 

[PSlem]

No need to brace the structure if you limit the number of pits open at any one time. I have done pits over 20' deep under 10 story buildings with only 2x8 lagging, never exposing more than 1/4 the footing.

 

[DRC1]

The purpose of underpinning is to support the structure, so unless the footing is made from loose or poorly mortared rubble, or highly distressed concrete, the foundation does not need support. The underpinng can go as deep as nesseary, using wood lagging to construcrt a box all the way down. We have done pits 15- 20 ft deep. I have read of pits 45 feet deep. Once pits get over 6-8 feet, bracing of the pit itself may be needed. Pits are never dug on next to another, but are dug in a hopscoth patteren to allow a pit to cure and take load prior to digging an adjacent pit. Underpinning pits should not be used below a water table. Underpinning is an art and there are few people who do it well. Many concrete contractors will excvate a hole, esp. if it is shallow, say 4 feet or less and pour concrete, but that can be risky and is not true underpinning.
Hope somwhere in there is an answer to your question

 

[cap4000]

PSlem,

What you are describing sounds alot like the underpinning method that Spencer, White & Prentis used for the renonvation of the White House in 1950. Essentially they added a 20 foot deep basement to the building. However, this renovation job was so large that lateral X-braces for wind are shown throughout the structure with steel beam struts bracing the new concrete pit walls. Also, the cresote sheeting that formed these pits was left in place. Looks like groundwater was never encountered in the process. Appreciate your expertise and tip. Thanks.

 

[cap4000]

DRC1

I think you bring up an excellent point about what is really true underpinning and how few contractors actually know how to do it. I can't even imagine stepping into a 45 foot deep pit let alone actually work in it. Stay Safe. Nice Job.

 

[PSlem]

I'm an old SWP employee. They did write the book.

 

[cap4000]

PSlem

How long did you work for them and what year? What a coincidence as I have their book. Can you explain to me how a 20 foot deep pit actually proceeds downward and how long would one 4ft. x 4ft. x 20ft deep pit take to complete assuming dry sandy conditions.

 

[PSlem]

I was there '76-'80. Back then these pits were $1000/CY so slow. Dirt was shoveled into buckets and taken up with well wheel. Only connection in corner was a vertical 2x4 in 4' lengths. Ladder rungs were nailed diagonally into one corner. Memory is a little fuzzy but I think 6'/day with 3 man crew.

 

[PSlem]

Actually the pits were 3'x4' and the rings were nailed together up top and lowered in and leveled.

 

[cap4000]

PSlem

Thanks for the great info. I think often when people hear of the word underpinning they just think about the concrete and have no idea of how critical the carpentry work really is. Your bucket scenario is how the SPW book describes it. They even describe an engine system pulling up the 2 buckets. 3 men getting 6 foot a day I think is really a hard days work. You must of slept good those nights. I think todays unit price is over $1500.00/cyd.

 

[PEinc]

PSlem,

Actually, the production should average about 3 vertical feet per shift or less if the pits get significantly deeper than about 30 feet or if the material being excavated gets rocky. The deeper the pit, the longer it takes to pull the bucket up out of the pit. If you can set up a mechanical system to pull the buckets, you can speed up the operation a little, but machanical systems are rarely used.

I realize that some underpinning crews nail the rings together and then lower them donw in one piece but that's not really how it should be done. If the pre-assembled ring can be lowered, then the area where that ring is to be installed has been overexcavated. The pit should be excavated neatly to the rear faces of the ring boards and the boards should be placed tight against the dirt face when assembled in the pit.

 

[cap4000]

PEinc,

I think your production rate of 3 feet is what I would expect. You bring up a very interesting issue regarding the boards being installed tightly to the soil. The SPW books talks alot about the soils "Aching" capabilties. They rationalize it and compare to a post drilled down into the ground and then withdrawn. As the soil now has a perfect hole in it, they credit the arching action of the soil that allows that to occur. As with H pile design this soil arching action can be very beneficial and in regards to underpinning a structure deep in the ground the end result could be catasrophic without it. Thanks for addressing this not so well understood phenomena.