Swelling clay against basement wall

[dgillette]

I have an interesting problem which I'm not quite sure how to address. My basement wall is bowing in farther and farther, and I suspect swelling clay. The material is, I believe, colluvium derived from the Denver formation. (My front porch, which is not on piers like the rest of the house, moves up and down several cm with changes in soil moisture. I've underpinned it by crude methods, but the foundation soil has dropped almost 10 cm away from the slab. That's a whole 'nother story.) The house is about 35 years old. I moved in 15 years ago. The previous owner was retired and had lots of time to water the lawn, whereas I've almost let it die from lack of water, which may have something to do with all the settlements since I bought it. Over the last couple of years, the wall movement has accelerated and damaged some of the sheetrock in the basement. There is a distinct depression next to the house on the side with the problem, which may be attributable to poor compaction of the backfill. I wonder if it could instead be a result of the seasonal swell and shrink, as the clay swells pushing in on the wall, then drops as it shrinks, and the process repeats, progressively wedging the wall inward. The same thing may have occurred at a low block retaining wall I built a few years ago, as the top of the wall is slanted more than it was at the end of construction.

Any ideas for a solution? I haven't found it in the text books, and it's outside of my immediate area of practice. I've considered excavating a narrow slot along the wall (by hand, with help from teenage boy) and putting in crushable foam or sand. I should also try to improve the surface drainage away from the house, without putting more fill against the bowed wall.

Also, the floating floor slab has dropped as much as 3 cm since I finished my basement "office" about 10 years ago. As I recall, that happened mostly in a very dry year, and there may have been some increase in well pumping. I could take half a year off work and do nothing but work on the house, if I could afford to.

Thanks!
dgillette

 

[oldrunner]

I suggest that you immediately obtain the services of a geotechnial engineer and maybe more importantly, a structural engineer, as it sounds like you have a failure going on. The structural engineer should be able to provide you with advice on temporary measures and then the geotechnial consultant can advise you on an "economical" solution.

Don't put yourself or the boy in the narrow slot! Cave-in's are usually recovery, not rescue.

 

[dgillette]

Well, maybe slot isn't the right word. I'm thinking of a trench 60 cm deep, 40 cm wide, then a slot the width of a mattock as deep below that as the mattock will reach, just to unload the upper part of the wall where the worst part of the bending moment is generated. That should get me about half the height of the fill on the wall. I am familiar with OSHA regs and trench caving, and I only have so much time and energy for excavating by hand, so that's the limit for this season.

Water does not pond in the depression or anywhere within 5 m of the wall, which doesn't prove anything. The basement sump for the perimeter drains stays 100% dry 100% of the time. However, the wall in question is on the dark side of the house where snow collects (and melts). I don't think the depression is the top of a slope failure because its volume appears to be larger than the displaced volume of the wall movements. Also, the recent movement has been no more than about 1 cm/year and it's now a total of about 8 cm or less. It is not an emergency or a wall failure, more of a question of preventing further movements before I repair the sheetrock. With the exception of one that has 2 mm of aperture, the cracks in the unfinished portion of the wall (where they can be seen) are all hairline and mostly vertical

I've canvassed a number of geotechs on this. One with some direct experience suggests keeping the clay in its present wet state, rather than letting it cycle wet-dry-wet-dry etc. That can be accomplished with a membrane on the ground surface.

Otherwise, thanks for the suggestions. BTW, I am a geotechnical engineer - I just haven't ever dealt with swelling clay against a wall outside my own yard in my 20+ years of practice.

 

[oldrunner]

For some reason I assumed that the bending of the wall was greater than you describe. I'm surprised that you don't run into this problem more, but it's probably the local area. In the SF bay area, these things are pretty common, including creep of whole hills.

I'm a structural who works with a number of geotechs on distressed structures and I'm still learning stuff after fifty years.

 

[dgillette]

I've never run into the wall problem in part because I don't practice where I practice, i.e., while my home and office are in an area of swelling clays, my projects are scattered all over the west. I seldom run into swelling clays at all, let alone against a wall. Where we have run into swelling clays, the preferred solution has been to remove it.

Anyway, where you have seen it, does the wet-dry-wet cycle cause a progressive wedging effect on a wall season after season (like frozen ground against a wall), or is it like vertical movements that roughly track the previous years' movements? I can envision a possible mechanism where the strain path during volume decrease from drying does not simply reverse the strain path from volume increase from wetting. This is what the slot with crushable foam would address, at least for a few seasons.

 

[adtsch]

dgillette:

I have looked at a couple houses with similar distress in the Denver area...Did you ever get to the bottom of it?

 

[dgillette]

I never really got to the bottom of things. I did find that the recent movement was smaller than I thought (as evidenced by lack of distress where I had used Liquid Nails several years back, to stick the little fillet of mortar along the inside of the sole plate back against it). Apparently my memory of when the damage to drywall in the bathroom occurred was not correct. (There is no discernible damage to the drywall that I did in about '95 or '97.) The low area along the wall I concluded was not settlement of an active block, but simply a result of crappy compaction of the backfill. I found I could stick a steel rod quite a way into it with little resistance. It was soft and wet, and I decided just to leave it wet. There was actually more settlement on the sunny side of the house where things really got dried out, judging by the space under the front porch (soon to be replaced if all goes well). I do plan to improve drainage away from the house when I get around to building the rest of the retaining walls for the back yard since I'll have to do a little grading anyway.

The general bowing in of the wall is probably just deflection of a slab that is too thin and not heavily enough reinforced to be 50 feet long with no counterforts or internal bracing. There is some incipient spalling on the inside near the midpoint of the wall, but little or nothing has actually fallen off since I moved in in '91.

There were plenty of other shortcuts taken in the construction, so a little crappy compaction and use of the bare minimum rebar in the wall shouldn't surprise me. As soon as I win the lottery, there is going to be some serious work done on that house.

FYI - This is on the south slope of South Table Mountain, around to the west from NREL.

Thanks for your interest.

DRG

 

[oldestguy]

Hi to home owner with a problem.

I once knew a retired engineer from the US Bureau of Reclamation who lived there. He apparently had much worse cycling movement than you describe. He just lived with it.

However, he was one of the instrumental engineers figuring out how to deal with it in that bureau. In my case we used him as a consultant on a swelling clay problem and his advice was very well founded.

My suggestion would be get a hold of that nearest office and see if they have any advice. In that case they ought to be able to give advice free.

 

[jdmm]

There probably is no easy solution since providing additional support to the wall is likely not an option. If the problem is swelling clay against the wall then the solution is relatively straight forward; remove the clay and replace with sand and gravel. It would be a good time to replace your perimeter drains at the same time. Although a slot might help, theoretically, the clay should be removed in a wedge at an angle of 45+phi/2 measured from the horizontal. For a clay that has a friction angle of 30 degrees then the slope of the excavation should be 45+30/2=60 degrees. This is about 2 vertical to 1 horizontal. For footings at a depth of about 7 feet then the excavation should extend about 3.5 feet away from the wall at the top of the excavation.

In view of the amount of work what I would suggest is that you and your teenager dig a number of test pits down to the footing level and have a geotechnical engineer have a look. It should be possible to assess the problem and come up with a solution with a single field visit.

 

[dgillette]

jdmm: As wet as the backfill is, it would have to go considerably flatter than the active angle for adequate stability of a hole someone would enter. (Think in terms of Su, not phi'.) The clay is tight enough that the perimeter drains are pretty much irrelevant to anything. Decided I would keep the fill wet to prevent the wet-dry cycling, so when I do the grading, I'll put a membrane over the 6-8 feet nearest the house. It's on the north side of the house so %w probably doesn't fluctuate too much as it is. I'll stick with the devil I know, rather than the more-expensive devil I don't know. The wall isn't going to collapse, and I can repair sheetrock if it cracks.

 

[gotwood]

I'm not a PE but im a structural eng. working in denver metro area. I think your foundation was not design properly. One of the straight foundation wall is too long. They shoud have installed a counter fort or buttress to break the wall span in to 2 spans. I think thats why it is bowing. If you think about it, foundation on piers is almost like 1 way slab since there is no fixed end moment at the bottom. I think counterfort using drilled pier was necessary on the wall you are having problem wih.

 

[dgillette]

Thanks. That's what I'd concluded too, as I wrote on 8/28 - the wall is just too long horizontally for its strength and stiffness.

The ideal solution would be to create either a buttress on the inside, or a counterfort or tieback on the outside by Unless it starts getting worse again, I'll just live with it, since neither task is very appealing, and I'll remind myself that it's just a house and not a national monument or a critical investment. I'll live there and make whatever patches are needed until I die, sometime in the next 50 years, and then it will be my boys' problem and not mine. When I have time 5 or 10 years from now, I'll replace some of the sheetrock, or at least patch and repaint in the bathroom where it's been crushed a little by the wall movement. In the adjacent office, which I finished about 10 years ago, the sheetrock doesn't show any distress, although the base boards are as much as 3 cm off the floor since the floating slab dropped. (Not sure exactly why that happened, but it seemed to coincide with several dry summers in a row. It may not be coincidence that it occurred within a few years after I did a little regrading so water did not pond against the house, and the sump for the drains never again saw a drop of water.)

There were a number of corners cut in the construction, in addition to the uncompacted backfill. I even found empty pop cans in the heating ducts. If you own an older house, you don't need to waste money on hobbies, because the house can be your hobby. ;-)

Best regards,
DRG

 

[gotwood]

I just recently bought a house and found 1979 penthouse magazine underneath the dead space under the stairs haha. It sounds like buttress would be a lot easier to do. This is what I would do although I dont have remediation experience ( I just design it correctly at first place ). Take the soil right next to the problem area. Hopefully the wall will be straight again. Build a counterfort and use a hellical pier instead of drilled pier (i think it would be cheaper and quicker). Then maybe i put drainage along that wall since you already removed the soil wrapped with squeegee gravels and material and back fill it. Sounds like you need to put cosmetic repairs and sell it !!!

 

[Baldie]

This post might be too late to help with your fix, but I thought I would share a few thoughts.

I have practiced in the Denver area for over 16 years - mostly in transportation, dams, and commercial buildings. I have looked at a few distressed residences with similar problems.

Your situation appears to be somewhat unique with settlement on one side of the house (due to drying of expansive soils) and swelling soils problems on the other side (due to wetting). Could the wall movement be due to the increase in unit weight and loss of strength of clay backfill that has become nearly saturated (deduced from a previous post by you) rather than wetting of expansive soil backfill? With the age of the house at 35 years, I would have expected expansive soils problems to have settled down years ago. We usually see swelling soils/bedrock problems in homes less than 5 or 6 years old. Changes in drainage or irrigation could trigger new problems (such as settlement caused by your reduced irrigation compared to the previous homeowner).

I would not recommend digging and leaving the wall exposed for too long as this could allow the wetting front to advance deeper (e.g. it will be a potential pathway for more water to get to the soils below the foundation). Then you could have serious problems with foundation movement, not just a wall moving. The same goes for backfilling with sand which would be pervious and allow moisture to get to the foundation.

After you excavate the soil behind the wall, you could straighten the bowed wall by using jacks pushing from inside the basement. These could be anchored to the floor or extended to the other wall across the basement as a reaction. Adding a counterfort (as you are planning or perhaps already did), a "sister" wall, or dead men could be used to strengthen the existing wall.

If it were my wall and depending on how severe the problem is, I would probably just dig out the soils behind the wall and recompact them well above optimum moisture at a fairly low density (ie. 90% of standard proctor).

Good luck. I'm sure this is a frustrating problem to deal with.

 

[dgillette]

There are all kinds of goofy things going on, including (I now realize) enough settlement along the north wall relative to the center line of piers that one bedroom door and the basement door directly below it bind. (This is pretty surprising, since I assume the wall is on piers like like I think everything else in the neighborhood is.) It's not just the floating basement floor slab that has moved, although the wall has settled less than 1 cm, vs about 3 for the slab. There is a bend in the mainfloor floor at the center beam, noticeable in bare feet. If I knew the settlement was done, I could solve the door problems with a little carpentry.

It's very curious that all the settlement is taking place so long after the house was constructed. There may have been increases in well pumping, and some effect may have come from the fact that the previous owner was retired and had lots of time to water the lawn, and I have not done that. Also, probably coincidentally, I did some regrading 10? years ago so water could not pond at the west side of the house. The sump pump no longer runs when there is a storm, and I've never seen water in the sump at all since then. It's theoretically possible that the water that used to get into the foundation that way helped prevent the settlements by keeping the clay from shrinking. Maybe I should try to reverse the process by putting water INTO the sump! ;-) Don't laugh - before I underpinned the front porch, I used a root waterer to inject water so it would rise enough to stop the door binding.

The bowing of the wall does not seem to be getting any worse, so for now I'm leaving the soft wet backfill wet, so it won't shrink and re-expand and get wet again. At this point, it's more nuisance than anything else, certainly not enough to take major structural action. The increase in unit weight in the backfill from wetting should be pretty minor, and I don't think that could explain it.

Ask me again in a few years when I have more observations. Meanwhile, I'm trying not to be too concerned about it.