Excavating to turn a crawl space into a full basement

[mfstructural]

I have a project where my client has a half full basement and half crawlspace in his home. He wants to expand the full basement to have more space. I have existing plans that show the full height of the crawl space foundation walls as 4' and full height of basement walls as 8.5'. I'm planning to have them excavate the soil in 4' segments to cut off the inner portion of the strip footing even with the interior face of the existing fdn wall and dowel into the underside of the existing wall. The dowels will have to be able to withstand the hydrostatic forces on the wall, particularly because of the construction joint in the concrete.

The other issue is the interior columns, pictured below. shoring will be required to support the existing steel beams (and wood joists). Once shoring is installed the existing columns can be removed, soil excavated, new footings poured. Longer steel columns will then be installed.

Any thoughts on other things I should be considering? I've attached a rough sketch of what I'm thinking without dowels.

 

[WARose]

I've worked on a few and one of the biggest issues become quality control. What the contractor will do is get under there and undermine the existing foundations by excavating right up to the edge of it. I've been under a few where the guy left a soil "column" 6' tall. (Approximately the size (in plan view) of a couple of 8" masonry blocks that were supporting some support beams on the interior.) So watch for that and make it clear to him how to excavate. A lot of the contractors who do residential work don't have a clue.

As far as the wall footing plan goes (as per your picture), it's not going to be easy to slip that in. It's going to take a lot of (expensive) shoring work. It might be easier to see if you can talk him into letting the wall footing stay as is. (I.e. not excavating near it or cutting it off.) How far back you could begin excavation would be the call of a geotech. It's up to the homeowner if he really needs that space around the perimeter.

 

[oldestguy]

You are planning what is called underpinning. The ideal job would be a new wall that is as strong or more so than if were one pour. Thus you should consider how to transfer some load into the new lower section and reinforce that part of the total wall to take some outside earth pressure. I'd add some vertical reinforcing on the inner third of the wall width full height of lower, grouted well into the upper part. Ideally a new wall would have sufficient depth of penetration up there and grouted so that at the joint full tension could be taken by the re-bars. You are not likely to get it that well, but try. You also have to load up that lower new part. A common way is to stop the pour of the lower part a few inches from the upper part. Then, dry (damp) pack in a high strength mortar or concrete mix, in attempt to create some loading of the old upper one onto the new. How you form the outside might be a question so as to later have that wall capable of carrying some outside fill load. Forms might just be left in place. A good job of underpinning also ties each block to the next with re-bars.

For a real belt and suspenders job you would have some room between old upper and new lower so that a jack or wedges could be used there to really load up the lower and hold it while the dry pack sets up.

I'd also bet that this has been done so many times that some one has written up methods that work.I just did a google type search with your title and many articles and videos came up.

 

[thaidavid40]

Another option could be to set some shores under the joists, near their ends, and excavate and remove the entire existing footing and stub wall in that area. These would support cribbing girders bearing under the joists. Then you could excavate and remove the existing stub wall, and repour (or lay up 12" CMU block coursing) to fit the occasion, then backfill. That would give you a clean, continuous surface to waterproof on the outside also.
Dave

Thaidavid

 

[Shotzie]

The interior portion of the existing strip footing could be cut flush with the existing foundation wall, and then provide underpinned concrete that goes from inside face of existing wall to outside face of strip footing. You may need to go thicker with the underpinning at the base of the new foundation in order to achieve the original strip footing width for bearing. You should dowel into the underside of the existing strip footing as you've mentioned with staggered epoxied dowels to develop moment across the joint. Waterproofing that joint would need to be addressed, as well as new drainage and waterproofing on the outside face of the existing and new foundation wall. Once the new floor slab is in, you can build a 2x6 stud wall on the inside face of the foundation, and provide horizontal strapping at 1/3 points between the studs and the foundation wall. Ensure the new studs extend up and get nailed to the sides of the existing floor joists/blocking, and use Simpson A35 clips at the base between the sill plate and the studs to laterally support them. This creates a secondary form of lateral support in case the hinge you created in the foundation wall isn't constructed adequately.

Alternatively, just build a new full height reinforced foundation wall on the inside face of the existing foundation. It would extend over the existing strip footing (ie, 10" thick for lower portion and 16" thick in upper portion due to strip footing thickness. That would allow you to excavate from the inside face only of the wall and leave most of the exterior soil intact likely.

An important note, when underpinning, do it in 4' sections, using either an A-B-A-B pattern or A-B-C-A-B-C pattern, where you excavate all A's first, then underpin, then all B's, then underpin, etc. Also, starting at corners or under point loads is probably the best option.

 

[XR250]

http://www.eng-tips.com/viewthread.cfm?qid=429287

Similar topic a few months ago..

 

[mfstructural]

Thanks for all the replies, I am going to talk this over with the homeowner and contractor. I think both constructing a new wall at the interior and underpinning the existing wall will work. I do think think there is a waterproofing issue with at the exterior though in both cases. Even if a new wall is constructed, the outside face of the bottom portion in addition to the junction between the two foundations (the jog in the concrete) will need waterproofing.

Shotzie, I wasn't really following what you were saying about the horz strapping at 1/3 points between the wall and basement studs.

There is no geotech on this project at this point, I wasn't planning on enlisting one particularly if we're going deeper and underpinning in 4' sections.

This is definitely going to be an expensive project.

There is another thing...contractor wants to remove a 4/5' portion of the foundation to get a small bobcat in to excavate. I'm planning on having them only excavate a trail perpendicular to the foundation and not disturb the soil adjacent to the removed portion of the foundation until the majority of the underpinning is completed.

Thanks

 

[jike]

A couple of questions and then a few comments.
1) What type of soil do you have? Clay or Sand or Silt or some combination? Ground water level?
2) Is there any existing drain tile? If so, how do you intend to tie into it?
Comments:
1) I would recommend using a foundation wall that is much thicker...and eliminate the lower footing. What you have detailed would be very difficult to build and more risky than a one step pour. You want to pour your underpinning and get the excavation closed up as quickly as possible. Sluffing and blow-ins can happen suddenly.
2) You might consider making your wall 24" thick. Labor costs are very high and a little more concrete will have little affect on the overall cost. By knocking off the existing footing projection on the interior and having the interior face of your new wall 5 to 6" from the interior face of the existing wall, you will have a space in which the contractor can pour your underpinning wall. This would eliminate the contractor having to build special chutes, grinding and finishing the wall afterward or using some other method.
3) I recommend vertical dowels into the footing/wall above as well as dowels into adjacent underpinning sections. Horizontal dowels would be pounded into the soil and will be lapped onto other dowels when the adjacent section is poured.
4) If ground water is a problem, you might consider using some sort of drainage fabric on the exterior and drain tile on the interior.

I hope this helps.....

 

[Shotzie]

The strapping I was referring to in my previous post would be fastened to the foundation wall horizontally along the full length of the wall. The third points meant that you would have two horizontal lines of strapping spaced equally between the floor slab and top of foundation wall. The new stud wall would then get built tight to the strapping in order to brace the foundation wall.

 

[EDub24]

If you go with extending the existing footing per your sketch, you'll need to consider how the horizontal bars will tie into each for each segment. Easy to draw on paper but not easy to construct.