Excavating to turn a crawl space into a full basement
Excavating to turn a crawl space into a full basement
(OP)
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.

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.







RE: Excavating to turn a crawl space into a full basement
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.
RE: Excavating to turn a crawl space into a full basement
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.
RE: Excavating to turn a crawl space into a full basement
Dave
Thaidavid
RE: Excavating to turn a crawl space into a full basement
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.
RE: Excavating to turn a crawl space into a full basement
Similar topic a few months ago..
RE: Excavating to turn a crawl space into a full basement
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
RE: Excavating to turn a crawl space into a full basement
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.....
RE: Excavating to turn a crawl space into a full basement
RE: Excavating to turn a crawl space into a full basement