Sinking house cracks (C) Daniel FriedmanSinking Buildings & Floors: Causes of Building Settlement

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Sinking or settling buildings:

This page explains causes of building settlement or sinking (distinct from sinkholes), and gives building and site inspection advice useful in identifying areas where there is an increased risk of building settlement.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Causes of Soil Subsidence & Building Movement Not Due to Sinkholes

Stormwater drainage sinkhole in Pennsylvania - PA DCNR

The bare minimum that a property owner needs to know about sinkholes or any other sudden subsidence of soils at a property is that these conditions might be very dangerous.

Someone falling into a sink hole or into a collapsing septic tank could be seriously injured or even die. If a suspicious hole, subsidence, or depression appears at a property the owner should rope off and prevent access to the area to prevent anyone from falling into the opening, and then should seek prompt assistance from a qualified expert, geotechnical engineer, septic contractor, excavator, or the like.

The photograph (above-left) of a sinkhole opening in a residential yard in Pennsylvania is from Kochanov, W.E. and illustrates the child hazard or even adult hazard that storm water drainage sinkholes can form.

Kochanov, W.E. lists and we elaborate and expand here a variety of subsidences that are not caused by sinkholes, followed by examples of sinking houses not caused by sinkholes, and examples of cesspool or septic system collapses.

Floor collapse pulls away wood post from ceiling - Northridge CA earthquake (C) Daniel Friedman Floor collapse pulls away wood post from ceiling - Northridge CA earthquake (C) Daniel Friedman

Understanding the cause of a sinkhole assists in knowing what to expect in the future and in planning for building or sink hole repairs. An important key to identifying a sinkhole area is to consider the location of carbonate bedrock.

Watch out: significant building movement may be caused by a variety of problems other than sinkholes. Some of these are equally dangerous, and expert evaluation is important. Consult with a structural engineer who is specifically familiar with building and foundation movement and failures. Examples of conditions that can raise serious, urgent concerns for safety and risk of sudden, catastrophic building collapse include at least the following:

Readers concerned about soil subsidence or sinkholes should see SINKHOLES - IMMEDIATE SAFETY ACTIONS, and also see FOUNDATION CRACKS & DAMAGE GUIDE and CESSPOOL SAFETY WARNINGS.

Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY

. Readers concerned about diagnosing building movement from other causes should read this article, and also see SITE FACTORS AFFECTING FOUNDATIONS and FOUNDATION DIAGNOSIS & REPAIR.

If your concern is cracking, settlement, or heaving of a concrete floor slab, see CONCRETE SLAB CRACK EVALUATION.

Sinking Houses Not Caused by Sinkholes

Case Report: Sinking House in Fairbanks Alaska

Fairbanks house settlement (C) Daniel FriedmanAn reader has shared photographs of a sinking house in Fairbanks, along with reports of investigation about the probable cause and remedy of this possibly dangerous condition. From examination of the owners' photographs it appeared that the home was constructed on piers set on permafrost.

The occupants of the home observed an increasing frequency and degree of movement in a new home whose structure was unfinished, including

The house was built by my husband. The walls are 2x6. His idea is to lift the house and build a floor under it. It was built on a concrete slab in 2005. We had Soils Alaska come out a few years ago and check out the grounds. They bored two holes and gave us a report. They did not seem too concerned about the settling. I am concerned because I am seeing movement everyday. Can you tell me the signs to be really worried and to get out of the house? Our neighbor has a work shop that started moving and he found a big six foot hole in the center. - K.R. Fairbanks AK

Signs of house movement (C) Daniel Friedman Signs of house movement (C) Daniel Friedman

The owners were concerned about a possible sinkhole under the house - as sinkholes have been reported in Alaska. But another possible explanation was an effect of global warming and loss of stable permafrost under the building. It was not reasonable to attempt to "diagnose" the cause of movement in this structure by remote consulting - onsite investigation by an expert, probably a geotechnical engineer, was needed.

Reading the owners' description of house movement and examining their photographs, it was apparent that there was significant ongoing structural movement and that the building may be unsafe.

Here was our advice on investigating this moving structure:

Building Movement Analysis Divides Into These 3 Main Categories

  1. Amount & rate of building movement: What is the present extent (how much so far) and rate (how fast) of movement and does either of those make the building unsafe – for example, if movement has moved framing such that framing connections are insecure a floor could collapse leading to serious injury, or if there are sinkholes or a sinkhole from melting permafrost or other cause, a sudden foundation collapse could be dangerous as well. If you are really seeing movement every day that sounds like a significant safety concern.
  2. What is the cause of building movement – is it poor framing connections, improper design, improper site preparation below the foundation, or an innate site problem like a sinkhole?
  3. Structural repair need & urgency: Based on the above, what repairs are needed, and at what urgency and with what alternatives. In cases of unstable buildings where there is a threat of sudden collapse, it may be necessary to immediately evacuate the structure and protect its surrounding area, notifying the appropriate local authorities.

When we understand the cause of movement that will determine what repair actions are needed and will also help understand the level of risk.

Call a soils test engineer (a geotechnical engineering firm had previously done some analysis at this property) as well as a foundation inspector or structural engineer familiar with building failures in your area, to answer the questions posed above as well as to tell us if there are other critical questions to be asked. See CONCRETE SLAB CRACK EVALUATION for more detailed guidance on slab crack diagnosis and repair.

Case Report: Basement Slab Heaving, Residential Home in Edmonton Canada

The following reader correspondence discusses the observation, diagnosis, and repair of a cracking, heaving, and settling basement floor slab in Edmonton in 2010.

Cracking floor slab sketch (C) Daniel FriedmanCracking Basement Slab Sketch & Complaint

I've been reading about foundations and cement slabs on your website and would like to know if you might possibly have some business contacts here in Edmonton, Alberta, Canada I may contact regarding a problem we're having with the basement floor in our not quite four-year old home.

Ultimately, I'm hoping to find someone qualified in the city I reside in and since your website is the only one thus far that actually addresses such issues as cement slabs, I'm hoping you may know someone.

We've had the City of Edmonton, the homebuilder, as well as a company specializing in basement repairs all come in to provide their opinion as to what's happening. Regrettably, not only do none of them appear to agree on what the cause is, none of them agree on how best to rectify the situation.

Further, the structural engineer I contacted informed me that he can perform a “visual inspection” of the existing slab and provide his assessment as well as recommendations for a fee. He has however, cautioned me that he can’t see what’s happening below the slab and has no first hand knowledge as to how the subsoil was prepared, the water table, etc. Since I thought that residential structural engineers would have expertise in this regard and he’s indicating somewhat to the contrary, I question if he will be able to help us rectify the situation we find ourselves in.

I am in a bi-level house that is not quite four years old. All homes in our city are required to have sump pumps. When we moved in, the basement floor had a few hairline cracks in it. Over time the cracks became more pronounced and then during this past year we've noticed that a few of the cracks have developed separations of approximately 1/8" in width and are now intersecting. In addition, where some of the cracks intersect, the concrete slab appears to have almost lifted or risen at the intersection point. I say “appears to have lifted” because the slab along the perimeter around these intersecting cracks is lower than the point of intersection. When they first appeared, the perimeter didn’t seem lower to us, nor did the cracks intersect.

I contacted our builder to request the results of the Soil Bearing Certificate for our lot. It indicates that the engineering firm probed the soil up to 36" below the footing level and found some evidence that 8-10" of soil at footing level was disturbed from its original condition. As such, they required that piles be placed under the footings for additional support in the event that the disturbed area settle in the future. I asked if there was any indication of water and was told that while there wasn’t, water tables change all the time.

I'm lost as to if its a soil issue, a water issue, both, or something else altogether different. A neighbour at the opposite end of our cul de sac has had so many problems with his sump pump and water that even the walls in his house have developed cracks, the drywall shifted as well as problems with the trusses. Right now, I'm wondering if I should just go further into debt and have the whole basement floor removed somehow and redone, though I don't know how since a year ago we had it framed and drywalled. - M.S. Edmonton, Alberta Canada

Some Diagnostic Questions for This Slab Cracking Problem - DF

More Cracking Slab Diagnostic Information from the Homeowner

We have provided a very rough sketch of our basement slab. All cracks are 1/8" or more in width unless otherwise indicated. With respect to the questions you've asked, I know for certain that:

Diagnostic Comments on the Cracking Slab Observations Above - DF

So this basement floor slab is not structural, nor are the cracks a structural threat unless someone failed to make separate piers below any columns;

Your sketch shows that cracks in the slab appear related to not only one end of the building, but points in the slab where there are penetrations. Often these patterns suggest that there was uneven compacting of fill below a floor, perhaps more of a water problem in that area, and that in a stressed floor slab cracks often originate at stress points or points of uneven structural unity, such as at an outside corner in the foundation wall footprint or at openings cut through the floor for pits such as a sump pit or a drain.

Generally basement floor slab crack patterns could be due to concrete slab shrinkage or floor slab settlement - but shrinkage occurs early in the slab life; settlement can occur later; Shrinkage cracks in a slab are discontinuous; settlement cracks are not, as the slab is actually breaking; Shrinkage also won't produce a crack with the slab at different heights on either side of the crack;

So it sounds more as if you have soil settlement under the slab, or if the home were left un-heated, frost heaves.

If we've got settlement, sumps can contribute to settlement by undermining a slab (or worse, a footing or foundation wall) by slowly removing fine soil along with water that it pumps from under the slab (water should not be there - a drainage problem), but more likely this slab was also poured on loose, poorly-compacted fill.

Often fill is dumped at one end of a foundation and then leveled, sometimes just by hand, by a low grade worker. [This was my - DJF - first job in construction]. In those cases perhaps no one compacts the soil, it is just raked roughly flat before the slab is poured. Furthermore, if the original grade was sloped and fill is being used to level up the foundation interior before the basement slab is poured, then there will be more total fill at one end of the floor than the other, making settlement and slab cracking more likely at that end.

The fix for this floor cracking can be costly if you have to break up, compact soil, and re pour; most people wait on that; there's also mud jacking - pumping hydraulic concrete mix under the slab, but you'd need to evaluate the soil and conditions under there first, and mud-jacking is not very appropriate if the floor slab has cracked and broken into many pieces. Mud-jacking works best when the slab is mostly or entirely intact.

Re "one company believes all our problems due to sump pit being too shallow and not cutting in often enough " - that is nonsense; a deeper pit makes the sump cycle less often, sure, but the same volume of water gets pumped out;Moving out more water or moving it out faster doesn't fix a settlement problem, it probably speeds it up; unless you were intercepting water and keeping it from under the slab in the first place;

I think you are going to need to make a hole or two, either by drilling or by breaking up the slab - I'd do it in the worst-cracked area - and see what we've got there. Or start breaking out the slab enough to see what is the condition of soil below the area of settlement.

If it's not a structural threat it's not so urgent; don't trip on the cracks; See CONCRETE SLAB CRACK EVALUATION for more detailed guidance on slab crack diagnosis and repair.

Update on the Edmonton Floor Slab Cracking & Settlement: Diagnosis & Repair Steps Taken by the Owner

An Edmonton Canada property owner was kind enough to provide us with this update in May 2010:

Believe it or not, the more we looked into our heaving floor issue, we found that we were told by several individuals that Edmonton has actually been considered as being in drought-like conditions over the past three years?! As of this April our house is four years old.

One of the engineering firms we contacted suggested drilling holes in the slab and observing the water table under the slab but lost our confidence when they indicated that in the end, they’d still only be providing us with their educated guess as to what was happening and after I asked what good it would even be to provide me with a “certified” report (as you know, engineers aren’t the cheap route, either), they indicated that water tables can change at any time, so this simply made us feel we’d be no further ahead than when we started.

Although it wasn’t our first choice, in March we had someone in to break up, compact soil, and re pour the north facing portion of the slab where it had been heaving. Prior to starting this job, the company we enlisted even told us they wouldn’t be surprised if we had some cracked pipes – they indicated they were finding a lot of this in new homes with problems similar to ours. When they jack hammered the floor they found the soil was damper than it should be, but there was no standing water and no cracked pipes. They took out the sump pit, compacted the soil, drilled holes into the sump pit, put crushed rock in around the pit and ran weeping tile all along the north perimeter of the house where the slab had also been heaving and then they repoured.

Quite honestly, we still don’t know if we did the “right” thing but after all the people we had come by to look at our floor and provide their opinions, we somehow got the feeling that none of them really have the answer as to what’s happening. Of course our hope is that this situation is rectified. However, yet another neighbour three doors down from us in the cul-de-sac is also experiencing horrible problems with his floor and foundation. And, just like the others in the area with issues such as his, they all appear to be two-story homes, while ours is a bi-level.

As a home owner I find it quite shocking and disheartening to learn that it’s the year 2010 and the house building doesn’t appear to have progressed very far in terms of technology, etc. yet this is the largest purchase a person will ever make!

Follow-up comments on the Heaving Floor Slab Case - DF

The observation that on breaking up the slab for inspection the contractor found that soil was damper than it should be suggests that either drought was not a direct cause in the floor settlement, or a cycling of dry and wet conditions was at fault, similar to the expansive clay soil problem that occurs below homes in other parts of North America such as areas of Colorado.

Case Report on Sinking Houses in New York State

In the Wappingers Falls, New York home shown in these photographs taken in 1999, significant ongoing cracking and movement was apparent in the building interior, including

Sinking house cracks (C) Daniel Friedman Sinking house cracks (C) Daniel Friedman

Further investigation into the history of the site indicated a condition that may have been the cause of this movement: the home had been constructed on poorly compacted fill over what had been a stream bed.

Sinking house cracks (C) Daniel Friedman Sinking house cracks (C) Daniel Friedman

A second Hyde Park, New York house settlement case in the 1980's (est) was traced to construction of the home on what had been a landfill. By bad luck, a portion of the home's foundation was constructed on fill while the other half sat on more sound soils, leading to significant differential settlement. Small "sinkholes" that occur in landfills as buried trash, tree stumps, or debris rot and settle can nonetheless be very dangerous, especially to children.

Repairs of that sinking home included an extravagant installation of a custom engineered steel frame and supporting piers. Depending on the cause of house or foundation settlement, other more economical repair methods may be suitable such as the use of helical piers or mud-jacking (pumping grout below a settling concrete slab or walk or even a foundation wall).

Cesspool & Septic System Collapse Hazards

Note on cesspool collapse hazard - question about sudden back yard cave in

Sinkhole or drywell collapse in New York (C) Daniel Friedman

Earlier this evening, after a day of rainfall, our backyard caved in. Currently there is a hole in the ground about 12x10ft and 6-7 feet in depth. After the initial collapse, there was some growth in the diameter but that appears to have stabilized. The closest edge is about 6-7 feet away from the actual house.

[Photo at left shows the New York "sinkhole".]

See Sinkholes in New York for details of this case and our advice on what to do about this sudden yard collapse in New York.

Watch Out: Immediately rope off the area of any soil subsidence or suspected old septic tank or cesspool area, and mark it plainly as unsafe so that a wandering neighbor, adult or child, does not go near nor fall into this hole. It could be quite dangerous.

See CESSPOOL SAFETY WARNINGS for examples of potentially fatal cesspool collapse hazards.

Sinkhole & Subsidence Articles


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