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dealing with subsidance - need an engineer?

Posted by nativeplanter (My Page) on
Tue, Jan 26, 10 at 10:37

I recently found this forum and have learned a lot by reading old posts. We have a 1922 foursquare in SE Virginia, that we bought just over a year ago.

We had some flooding in November during a Nor'easter. The water didn't get into the living space of the house, but did go into the crawl space and was perhaps knee deep in the very front of the house, a little less deep towards the back.

Since then, I think the house may have settled or subsided. There are two new cracks in plaster that are larger than the older ones, and some of the double-hung windows don't want to open (all of the windows were painted shut when we bought the house, these are of the ones we had managed to free). At first I thought they might be swollen from the increased humidity, but they still don't want to open. In addition, one of the doors is sticking now. (Interestingly, two doors that used to stick are now free.)

The soils on our lot are mapped as fine sandy loam, underlain by sandy clay loam (Augusta and Tomotley series). Parts of the yard are pretty poorly drained, I think in part because the land is so flat. The house is on what I think are called piers, with the crawlspace bricked in on the outside. The piers themselves are brick, at least on their faces. There is no basement.

We are assuming that we need someone to come and look at the situation. Our neighbor gave us a contact for a civil engineer. What types of things should we be looking for in a professional to assess our house and determine if and what needs to be done? Apparently, flood insurance does not cover subsidance issues, so we didn't get any hints there at how to proceed. This is our first house and we don't have much experience with foundation-type issues.

Thanks for any advice!


Follow-Up Postings:

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RE: dealing with subsidance - need an engineer?

Certainly a civil engineer would be qualified to study your problem and come up with a solution. There are very likely other homeowners in your neighborhood with similar issues, and they might know of a local business with a good reputation for foundation work.


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RE: dealing with subsidance - need an engineer?

Native planter,

Here in Houston where we have expansive clay soils, the most stable homes are built on top of deep bell-bottom piers. They rely on friction to support the foundation beams and avoid sinking or upheavals due to changes in the water content of the ground or fill. There is no functional bedrock in Houston.

I'm guessing that bedrock is not present in SE Virginia, but I don't know. Older homes like yours did not have drilled piers, and were built "block on beam" in Houston, where the foundation beam was poured on the ground (hopefully compressed fill soil, but who knows) and then blocks of concrete were placed beneath the floor joists to raise the house above grade.

In our 1920 bungalow, we dealt with a sagging chimney and front porch by having bell-bottom piers poured beneath the existing foundation beam. Since the size of the beam and the state of the rebar (non-existent if I remember correctly) was smaller than is currently used, the piers were poured very close together. Then the chimney and other blocks around the perimeter of the house were shimmed up to level.

Because the piers I poured were 18' deep, that part of the old house will be stable for many years to come. It is still important (in Houston) to watch for water drainage and tree placement, since upheavals are often just as damaging as subsidence.

In your area of the country, there are certainly structural engineers (civil or otherwise) who are familiar with the soil conditions and can advise you on how to proceed. You might want to discuss the flood and subsidence with your insurance company - they may pay for an engineering study of your foundation and plot. My guess is that you will be pouring bell-bottomed piers, or if there's bedrock, piers to the bedrock to support your house. You may have to revise the existing foundation beam (if there is one).


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RE: dealing with subsidance - need an engineer?

A knowledgeable civil engineer may be able to help, but a geotechnical engineer will be able to help.


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RE: dealing with subsidance - need an engineer?

Thanks everyone for the advice. We have the civil engineer our neighbor recommended coming next week to take a look and dig some holes, etc.. It's only $250 for the consult, and at least we'd have something to go on. He said it's $500-$700 if he needs to write up a report and recommendations/etc.

It's funny - I work with civil engineers (and a tiny bit with geotechs, but not in my office), but my company does infrastructure-type projects and I don't know how residential stuff might differ (I'm a wetland/enviornmental scientist with a landscape architecture background, so my understanding of this stuff is pretty basic). How does a geotechnical engineer differ from civil for residential housing? How does one go about finding such a person? The yellow pages has soil engineers; it also looks like some foundation companies are listed under structural engineers.

Karen - you are right; we don't have bedrock. The soil origin is marine terraces. Thankfully, we don't have the expansive clay problems that you have in Texas. To be honest, I don't know what it means to be a "drilled" pier (although I can guess), or what "block on beam" means. Do you know of a good book or website where I can learn about these things? My landscape architecture skills only go as far as decks, porches, and gazebos for this sort of topic.

Lastly, if the house has shifted a bit, does that automatically mean that something needs to be done? Obviously, the concern would be of it getting worse or the house being unstable. Other than that, though, it's not like the floors are slanting. (Clearly, this is why someone should come look at the house, but I'm just trying to get an idea of how these sorts of issues work. First house; no experience in such matters).

Thanks everyone!


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RE: dealing with subsidance - need an engineer?

Geotechnical engineers specialize is soil issues.

The civil knows how big to make the footiong based on the load bearing capactity of the soil.

The geotechical engineers specialize in the soil issues, and determine the load bearing capacity.

There is overlap, and many civil engineers know how to perform (at least) basic load bearing tests.


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RE: dealing with subsidance - need an engineer?

Native planter,

Here is a block & beam foundation I found with Google's help. Note the concrete beams are poured on grade or just below. Your house doesn't have the modern construction materials shown in this drawing, but hopefully the general concept is clear.
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Here is a pier & beam foundation. Note the piers below the surface, supporting the concrete beam or the blocks that support the subfloor.

The point at which someone decides whether to address foundation issues is probably subjective. I would say that if windows and doors are not functional, it is time to do something. Cosmetic issues like plaster cracks can always be repaired, but I think you'd probably like to use the doors and windows.

Sometimes in older homes there can be plumbing problems associated with large movement in the foundation, but since your house is raised above the ground, I will bet that no drain pipes will be crushed by this subsidence. But it might be good to check how much your house has moved relative to the pipes.

What would concern me more than anything is the fact that the house is continuing to move. There will always be small movements in any house, but large changes from season to season mean that things are not stable. I think you will enjoy your home more if it is level and unchanging. The cost of bell-bottom piers here in Houston is about $500/pier, and for a 1920 house you might need quite a few, because the perimeter beam is likely undersized for piering. But perhaps there is a targeted approach that would work.

You might also want to talk to your engineer about drainage. A french drain or something similar might be really helpful in keeping your plot from having standing water and further subsidence.


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RE: dealing with subsidance - need an engineer?

Helical piers drilled in are far easier to install on an existing structure than trying to add piers under the footers.

If the movement is from overloading of the soil, the helical piers can spread the load out over a larger area tan the existing foundation.

The entire job depends on what the actual site conditions are.

If the foundation has already cracked more work may be needed to bridge the crack.
Conditions determine if bridging is done before or after lifting.

If the foundation is a bond beam that has shifted, it may be possible to lift the beam back into place and shore it up.

There is no way to tell what is really happening without an actual site inspection, and possibly soil testing.

The repair is driven by the problem.

Do not start trying to decide on how to repair before identifying the cause of the problem.


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RE: dealing with subsidance - need an engineer?

Thanks Karen, that helps a bit. The diagram you found looks like it has a concrete pad on the bottom? I don't think I have that - there is dirt on the bottom of the crawl space. But I think I get the idea.

Brickeyee, don't worry - We're not about to make any decisions at the moment. I'm just trying to get a feel for the issue so I can have a more intelligent conversation with whoever we have come and look at the house. That, and I'm trying to prepare my brain for what we might find out. Sort of a "what are the range of possible scenarios" questions that a newbie might have.

When you mention whether the foundation might already be cracked - if we have dirt underneath, does that mean there is no "foundation"?

Sorry for the silly questions... I just remembered that I have the "Rennovating Old Houses" and "Caring for Your Historic House" books... perhaps I ought to open them tonight and review the foundation sections. Duh!

Anyway, I do appreciate everyones advice and whatnot, and am learning a lot here.


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RE: dealing with subsidance - need an engineer?

Native planter, your house doesn't have any concrete pad, you're right, the interior blocks are resting on the ground. Around the perimeter of your home, though, there's probably a concrete beam. It may be under the ground, or poking through just a bit, or completely above but covered by your siding. You can look around in the crawlspace and dig around with a small spade to see if there's a beam there and how deep it goes.

I did foundation analysis in the 90s and learned a lot, but still had more to learn when we had to work on our own bungalow. You'll be an expert by the time you're done!


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RE: dealing with subsidance - need an engineer?

If you find the civil engineer that's local to be less than helpful, try to find one that specializes in older structures. My Chemical Engineer husband says that you should have good luck since you have found one who's local or has done some work in your area and should understand the structural issues a home could face in it's environment, but sometimes it's even more helpful to find a structural engineer (a type of civil engineer) who specializes in old homes if you need to find creative ways to fix the issue.

Most likely your local guy will do great :) But if not, get a second opinion before you do anything drastic!

So you're telling me all I need is a good flood to get the doors to unstick??? I can sacrifice a couple windows for not having to yank the front door open with the jaws of life when someone knocks (it's so offputting to guests...)


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RE: dealing with subsidance - need an engineer?

EXCELLENT NEWS!

The engineer came today and looked around, both inside and out. He said that everything seems to be OK. He explained that the types of cracks that would be particularly troubling would be coming from the corners of doors and windows, and we only have one window doing that. He looked at the brick outside of the crawl space for cracks and didn't find any. He did say that to be safe in the future (barring, of course, another real flood), we should add a lot of dirt around the foundation to help the drainage (our lot is pretty flat, and water accumulates everywhere), and maybe add a french drain. But he didn't see any major problems with the house that needed fixing.

Whew! Maybe I overworry, but I feel much better for having finally gotten a professional opinion on the matter. So now we can get back to more trivial issues, like figuring out how to get seven layers of paint off the window and door frames!

Thanks so much everyone for all the advice!


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RE: dealing with subsidance - need an engineer ?

Oh, Igloochick - I suppose a good flood might do the trick for your front door, but as they say... "Your milage may vary!" :)


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RE: dealing with subsidance - need an engineer?

Nativeplanter, I'm glad the engineer gave you some good feedback on your situation. It must feel good to put some of your worries away.

I would suggest, though, that you try to establish the contours of your floor. A foundation company can come and take measurements, you can buy or borrow a laser level or a water level and do it yourself, you can use a 6' level and do it, or you can hire an engineer to come and do it. In any case, I think it would be helpful to know how much your house is sloping in any direction, at least so you can track changes.

When I was doing consulting engineering on foundations, I went out to one house where the owner had been meticulously patching and painting plaster cracks for 40 years. You couldn't see anything wrong with his walls inside, you could see some pointing in the brick veneer, but when we measured his slab we found that one side of the house had sunk 6" below the other.


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RE: dealing with subsidance - need an engineer?

Karen,
Thanks for the excellent suggestion. I think we will do just that. Thankfully I do know how to use a laser level and I think that would probably be the fastest and most accurate way for me to do it.

An interesting observation - one of the new cracks that had gone into the wood of a window frame (cracked between the lower horizontal and right vertical wood pieces) has gotten smaller. It's still pretty soupy on the ground, though, due to all the recent rain and snow.


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RE: dealing with subsidance - need an engineer?

We live on Mt. Tamalpais just north of San Francisco in Marin county. One corner of a newer section of my house, which has a seperate foundation than the rest of my home, has subsided. This is visible just by looking. The window and the plaster have cracked.

Has anyone tried just lifting a house, sinking long steel bars and just fixing the foundation?


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RE: dealing with subsidance - need an engineer?

Some things cannot be done by guessing or from the internet information. Serious foundation issues require professional advice.


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RE: dealing with subsidance - need an engineer?

In San Antonio we didn't fix a sloping floor until it was more than four inches out of level. Many older houses had their foundations fixed.


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