Undermine Foundation?

Drbobp

Anyone have any experience digging parallel to an existing foundation at an elevation below the footer? How would you stabilize the existing foundation while digging the new basement footers and pouring the walls? Trying to get educated before talking to an PE.

Looking at a job to put an extension on a slab w/4' frost walls built 50's rambler or ranch. House is currently an "L" configuration. Plan is to demolish the short leg and build a two level and full basement addition. Problem is the footers on the addition will need to be 5' below the existing footers (assuming there are footers or a grade beam where the two segments meet.

For commercial work I would drive sheet pile or pinning rods to stabilize the foundation before digging. That seems like overkill for a small residential foundation. House would probably shake apart before the piles reached depth.

concretemasonry

The usual rule of thumb is that the excavation sould not be closer/below than a 45 dergree line outward from the bottom toe of the footings. Different soil types and moisture conditions will change this.

This is assuming the weight of the footings spreads out at 45 degrees (1:1) from the bottom toe, so you are not removing any soil that is carrying any load from the foundation wall.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

Blas

Soil solidification under the existing footings if it is sandy. another way around it is underpinning the footing with grout or lean concrete. All these options are kinda pricey but necessary if you don't want the building in the hole with you. (Soil solidification is basically a glue injected into the sand or sandy clay that hardens it like a rock. It can be applied by drilling and injecting or simply injecting during excavation.

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"It is what it is"

rbsremodeling

We do this in DC all the time. We lower basement floor 2-3 feet to make the basement useable. We dig out the dirt level to the existing footing, shoring up as necessary. Repour and secure all the corners. Then dig out 4 foot sections and underpin the existing footing all the way around the perimitter walls and then pour a new basement slab. It is very costly because we usually have to remove the dirt by hand. Six guys and a whole lot of home depot buckets and dumpsters of dirt.

Corners run about 2-3k and underpinning the foundation 200-300 LF plus the dirt removal to get down to footer depth

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Blas

Yea underpinning is not fun at all. I always exclude it if I can

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"It is what it is"

tgeb

All the above advice is dead on.

I have done a few underpinning jobs. With engineers involved.

Excavate to within not more than a 45 degree angle, cut out sections not more than 4' wide. Place concrete footing and wall, or footing and masonry wall. Leave a 1-1/2" to 2" gap to be filled with dry packed non shrinking grout.

Do the next 4' section.

In large areas we would have to break the job into no less than 3 phases sometimes 4. That is do 4', skip 8' or 12' to the next section.

It's a real pain, but it's fun!

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Tom

www.gebcon.com

Drbobp

Thanks for the input guys. couple of follow up questions.

So you are essentially pouring a footer under the existing footer 4' at a time using th 45 Degree slope to maintain the existing footings?

Is this a two step pour with grouting to make sure the existing footer is continuously bearing on the new footer?

Are you shoring the building to unload the footer or are you bracing the newly poured footers to prevent movement?

What do you mean by secure the corners? Are these poured columns that are proud of the existing walls?

rbsremodeling

after re reading the post. It seems this is not a job you should perform yourself. Why not hire someone to come out and give you a bid to do the underpinning and sub it out?? Watch them while they do the work so you can have a visual understanding of what needs to be done. I subbed out underpinning for years before I was comfortable doing it myself. A mistake could be costly if it is done wrong

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ultimatetouch

Rory,

what does this cost you? I have a 38x20 home I have just purchased. We will dig it out one foot. I am amazed by the numbers that are coming in.

Do you find typically that when you go down 3' that you will be far below the footing. You guys are really digging those suckers out. I imagine you guys are below the footing and you have to repour a wall underneath the footing.

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Ultimatetouch
Illinois Remodeling Company, Kitchen Remodeling Chicago, Room Additions Illinois

rbsremodeling

Mark We usually start around 20k I have charged as much as 50k. We go 2' below the footing on average. underpin the existing footing in 4' sections and repouring the slab we drill 1/2 rebar into the old footing that was not underpin and tie it into the slab so when we pour cement is shot under the old footing. We typically have a brick and cement type of footing so it really is useless after we have dug down so far underneath. It kinda takes some skill. Often we use adjustable steel jacks and 4x4 under the existing floor joist to keep the as much weight of the perimeter walls until we are done.

I have really made it sound easier than it is. It took years before I was comfortable doing it myself.

With out seeing it a basement that size I would say atleast 20-30k min without seeing it. Some of them we can use equipment, some of them its all bucket and back. Usually takes us 30-60 days with inspections

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Drbobp

I don't intend to do this myself. I contract every trade, so I don't have the manpower to do it myself even if I wanted to. I guess its the engineer in me (electrical not structural) that wants to know all the details before I will sign off on something as possible.

I have been reading up on the subject. All the books give pretty simple explanations of how its done. That was my first indication that this will be 90% art.

ch0mpie

Heres two treads were I posted pics of underpinning just to give you an idea of what the procedure and final product look like
http://www.contractortalk.com/showth...t+underpinning

http://www.contractortalk.com/showth...t=underpinning

Hope this helps

ultimatetouch

I have one bid in for 22k. I thought that it was kinda high.

I think the oppisite is true of what you said above. I think its a lot easier then it sounds. Its a lot of digging and man power. Then again I understand where the stress points are on a house. I just want a firm understanding of how it works. You have a lot more experiance with this sorta thing then I do. How does this sound Rory? I am not so concerned with taking weight off the building because its a single story with attic.

1. dig out 5' of each side of corner and underpin each corner. dig out sequential 5' sections to underpin on all sides.

2. dig for new steel columns while leaving the old ones in tact.

3. pour footing and install steel I beam and columns. finish under pin.

4. Remove old columns and beam. Dig out entire basement to desired level.

5. Install drain tile and pit.

6. install 4" gravel and 4" concrete for new floor.

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Ultimatetouch
Illinois Remodeling Company, Kitchen Remodeling Chicago, Room Additions Illinois

tgeb

I would not go 5' intervals unless approved and stamped by a structural engineer.

And I would never do both sides of the corners at the same time like that. But that's just me.

Make sure you don't skip the dry packing of grout by filling the entire segment with concrete. I have seen it done that way and it is a recipe for disaster. Concrete shinks as it cures, not much but enough to make a difference, and that is the purpose of the grouting procedure.

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Tom

www.gebcon.com

rbsremodeling

Mark

I agree that you have a better understanding than the average homeowner so I overlooked that.

It was a reflex to having to explain the cost to and process to homeowner.

It is very labor intensive I have 6 guys that I use that go all day if I can't use equipment and usually I can't and its all buck and back.

Average guy wont last a day doing the hand/manual digging.

Your theory is sound to me and I think you have the concept down pack.

I agree with Tgeb also. In DC you can only dig so many holes before you have to get it inspected by the city and pour sections and then continue digging pits.

We have to pack with non shrink grout and we have to place 1/2" rebar pads in the pits before we inspect and pour. Maxium pit width here is 4' wide per engineer plans. The engineer usually shows how the pads are dug and poured.

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ultimatetouch

I interviewed four guys and none of them said anything about drypack grout. So what is it and where do you get it? Should my architect be specking this out or should I talk to an engineer personally?

Rory do you guys dig the corners at the same time?

How much floor space do you guys typically lose from the curb that goes around the perimeter?

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Ultimatetouch
Illinois Remodeling Company, Kitchen Remodeling Chicago, Room Additions Illinois

rbsremodeling

The grout was never specified on any drawing by anyone, I learned about it and its puropose from the guys that orginally did my underpinning. We dig the corner last after we underpin the walls. then we do the corners after everything is setup. the corners don't always need to be done we do it if they crack and shift and we determine if it's needed it is usually never sped'ed out to do the corners, we do it based on how the corners look to us. Ususally we lose about 6" because we frame in about 2" off the original footing which is still in place from alternating the pits which leave some of the original footing intact. Hope that makes sense. if not I will clarify

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ultimatetouch

Rory,

I am getting a lot of conflicting answers from the concrete guys I had out today. I will use them for there man power but I want a firm understanding so I could manage and set the plan up. They will carry it out. There my muscle.

One guy says that he will pour along side the existing footing leaving the dirt underneath the actual footing. He says I will lose six inches on both sides. This sounds rediculous.

From my understanding the other guys will dig beneath the actual footing in sequential stages every 4'. This means that it would be hollow those 4 feet directly beneath the footing and dirt. Is this correct?

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Ultimatetouch
Illinois Remodeling Company, Kitchen Remodeling Chicago, Room Additions Illinois

rbsremodeling

Second guy sounds like he is on the right track. The hollow is usually filled when the slabed is poured we stick some 1/2" rebar and tie it in the slab. there really isn't a hallow per sey but that's the easiest answer. First guy sounds like he doesn't have a clue

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ultimatetouch

Rory,

I met with an architect regarding a renovation I am going to be doing this evening. It seems there are many different ways he has seen it done. I really prefer to do it the way it will take up the least amount of space in the basement.

I am also going to meet with a guy from permaseal (waterproofing guys) just to pick there brains. There is a plastic nippley thing that keeps the water that get through the foundation away from the framed wall and diverts it down to the drain tile. Im interested in that product. I believe there is one right way to do this. Would it be possible to show me an elevation of what you are refering to. You could fax it to 708-366-9539. If its possible. Thanks for all your help!

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Ultimatetouch
Illinois Remodeling Company, Kitchen Remodeling Chicago, Room Additions Illinois