when to just replace the whole foundation?

[SLTA]

ok folks,

I visited a house yesterday where the soil has been dug away from the retaining basement CMU wall, to waterproof the outside. The house was built in 1958. There was no waterproofing at all originally and the blocks are unfilled, no rebar - typical 50s construction around here. The soils are the typical red clay we get here in Western NC, but then there was a ton of "pot clay" directly up against the house - pure grey, literally the stuff you make pots from. The guys digging out the foundation said that the stuff was suctioned to the walls. There is also a natural spring just below the foundation level that dumps water against the house, and into their repair ditch.

The CMU was in decent condition as blocks, but there are TONS of cracked joints, sheared blocks, and movement everywhere. The original owners installed some wood posts and CMU buttresses inside the basement in the early 60s to stop the movement, which was happening even then. There is up to 4" out-of-plumb in some locations. I've attached some pics of the worst of it.

At what point does the foundation need to just be replaced? I'm thinking we're there...

thanks!

Please remember: we're not all guys!

 

[SimpleJohn]

Looks like a fun project- and the answer to your question will probably be a financial one. If you think that a repair can be made using existing portions of the foundation, then go for it, but if you have concerns, then rip it out. It may be less expensive in the end to build all new and not try to tie in with the old foundation. The bids would tell that tale. It looks like portions of the wall could be salvaged, with reinforcing and grout added through strategic openings in the backs of certain blocks. Might consider full replacement in some areas, with repair for others.

 

[dhengr]

SLTA:
The black stuff they have on the wall now is damp proofing, not really water proofing. I’m surprised that they got the outside of the wall as clean as they did, so the damp proofing might stick. It damp proofs individual block faces, as long as they don’t crack. The better water proofing systems involve plastic or rubber sheets (or rolled sock) adhered to the block, which will span/cover a crack when it occurs. There are multi-layered systems with dimples in the middle which provide a real drainage plane. There should be a drain tile system at the bot. of ftg. elevation, given surface water and the spring condition. Then a couple feet of the backfill should be good draining granular material protected from the clay with a vert. geotextile filter material up to within a foot of final grade. Much of the wall length that you picture should probably be replaced. Any effort at reworking the existing in that condition is going to involve shoring the loads from above, straightening and plumbing the wall, rebar and grouting, which may be more work than replacing the bad portions. A good conc. block mason can tie the new into the salvageable portions.

 

[SLTA]

dhengr, thanks. This is merely the first line of defense that they're planning - they're putting in a full waterproofing system.

Please remember: we're not all guys!

 

[Canuck67]

I have looked at a few of these in the past, with varying outcomes. The first thing I do is to work with a local contractor to determine what the cost of demolition, new footings and new foundation walls would be. Quite often people want to save this entire cost outright, however, once you have finished your investigation and worked with a contractor to determine the repair costs, the difference sometimes is quite small. Long term cost-benefit analysis kicks in. Existing wall and new wall will both require damproofing, drainage tile, excavation, free draining backfill, etc... On a couple of mine, it was shown to be about a $5,000 upgrade for a brand new foundation, over repairs, and in both cases the clients jumped all over it. Your future liability also is reduced with a new foundation.

 

[oldestguy]

The old wall shows that it cannot stand the pressure from the clay backfill. A new plan to fully waterproof the wall should be done with the understanding then the pressure against a sealed wall will be higher, possibly much higher than in the past, assuming the same clay backfill is used (see below). Ordinarily these days any foundation like this with a known source of ground water would also have a perimeter drain system possibly leading to a sump and pump inside.

Regardless of the wall replace or fix, I'd place a perimeter drain system. You don't just throw in a pipe and backfill with gravel or it will likely fail soon. I'd use a 4" diam. slotted pipe ("wrinkled" plastic), with or without a filter sock. However the backfill at the pipe must be fine enough to filter out any clay that will tend to go in and plug it. The best all-'round filter is ASTM C33 fine aggregate for concrete, what is called "concrete sand". Use that at least under and over the pipe, with at least 12 inches of it on top of the pipe. Do not allow any gravel on the job or some well meaning workmen will use that instead. With the large void spaces, gravel will plug early on. I'd also then backfill all the excavation with a bank run sand, no clay. However, to help grass grow and to help divert infiltration the top foot can be clay, with topsoil on top.

For the fix or the replacement consider sand backfill as a fluid at rest, which usually is around 55 pounds per cubic foot equivalent fluid. With clay that number likely will be well over 100 maybe as high as 200 if it is expansive.