Frost Damage 1

[CANPRO]

A friend of mine is building a house and he recently noticed his foundation walls have started to push inward. I looked at the walls, which had cracked badly and he showed me the line he had strung to measure the deflection of the wall. Three of the four corners of the house have cracks starting in the bottom intersection of the foundation walls and extend diagonally upward the top of the wall. Pictures are attached (left and right referenced from standing in the driveway and looking at the front). The right side wall has moved over 1.5". I don't think the file names are coming through with the link...most of the pictures are along the back right corner and the right side wall.

I told him that the wall can be fixed/braced, whatever needs to be done. But first he needs to determine the cause. I recommended he talk to a Geotechnical Engineer to determine the cause and then I could help with the structural fix.

I have attached a few pictures, a grading plan and the basement plan. Some other relevant information:

•Local Frost Line is 4 feet.
•He described the native soil as a heavy clay.
•The foundation was poured in the fall, it set for 1 week and then was backfilled. He said he took precautions to prevent frost from getting into the ground when he poured the footings and the foundation.
•Shortly after backfilling the ponywalls and first floor were built along with the rest of the house. No significant variations in the foundation noted.
•We have had a couple stints of unseasonably warm and cold temperatures this winter. The mean temperature in the weeks leading up to the noticeable damage was about -6C to -7C.
•The walls are 8" thick poured concrete. He says they put a few bars into the walls but nothing that you could call reinforced.
•Basement has been unheated until he noticed the movement in the walls (a few days ago). The excavator operator said it was likely frost and he should keep the basement heated. He has had a propane heater in the basement since.
•When I was on site, it was around 0C and the previous day was above 0C for most of the day. The ground was very wet.

I wasn't sure if it would be a waste of money for him to talk to the geotech, it seems likely the damage is from the frost. I don't know much about clay and thought that might complicate the problem/fix. I figured at the very least the problem would be properly documented.

I am looking for opinions on what you think could be the likely cause. I would also like to know if anyone knows how the lateral force produced from frost compares to the vertical force.

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Back Right Corner - Ext.jpg

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Back Right Corner - Int.jpg

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Basement.png

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Right Side.jpg

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Site Plan.png

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Typical Crack.jpg

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Back Left.jpg

https://dl.dropboxusercontent.com/u/29304000/ENGTIPS/Back Right.jpg

 

[Ron]

Why do you have a 6-8 ft high wall that retains soil with no reinforcement? Makes no sense. Of course the wall will crack and particularly so with potentially active clay soils under high moisture being retained by the wall. Someone didn't think this one through.

 

[CANPRO]

Its times like this I wonder why they don't require minimum steel for residential foundations. I really don't think it adds that much expense. Overall it seems like a great investment.

Unfortunately, no one thought this detail through. This is the first I saw of his plans and even without seeing the tall walls, I would have recommended min reinforcing. At this point, I'm just trying to help him with the next step.

 

[wannabeSE]

The top of wall elevations are all different. This, plus your mention of pony walls and the excessive deflection at the top of wall leads me to believe the floor is not bracing the top of wall. Is the footing big enough for cantilevered walls?

 

[TXStructural]

Drainage is the number one thing I would look at, and is the chief cause of cantilever retaining wall failure. Obviously, the more water in the soil when it freezes, the greater the risk of frost damage (actually, the pressure exerted by the freezing of water in the soil.)

Unless it is a fairly thick section, unreinforced may be common, but is not a great practice.

 

[SkiisAndBikes]

Sounds to me like the walls may have been backfilled before the wood framed floor system was in place. While contractors do this all the time, it is very risky and not recommended.

Is there a wood framed pony wall on top of the concrete foundation wall? If so, this creates a hinge and is a bad detail for basement walls this height ... no lateral support at top of wall. The soil pressure would have pushed the wall in at the top.

Frost could also be an issue. Once backfilled, the interior of the footings either need to be insulated or heated to prevent frost heave during winter construction. The interior footings typically only have a few inches of soil cover on the interior.

Based upon the Canadian/Ontario Building Codes, one should be able to construct an 8'2" high, 8" thick foundation wall, retaining about 7'-7" of drained backfill without any problem, it's done all the time, provided it is laterally supported top and bottom. Beyond the parameters in the code, this becomes an engineered design.

 

[ztengguy]

Since its new construction, have him contact the original engineer that stamped it for his advice...oh wait...it was more than likely not stamped.

 

[XR250]

Also consider if the floor joists run parallel to the top of wall, there is still effectively nothing bracing it.

 

[Splitrings]

I can't see the pictures(gov. computer doesn't like dropbox) but would guess frost action. I would guess you likely have a frost susceptible soil, more than likely a silt. Heavy clays tend to be too impermeable for frost action. Keep in mind there is a difference between freezing water and frost. Freezing water alone doesn't expand enought to cause the displacements that frost(formation of ice lenses in soil) does. It takes 3 things for frost action; extreme cold for many days, frost susceptible soils and a water table below. The water that causes these ices lenses to develop is actually drawn up from below by capillary action. To prevent frost, you have to remove one of the three.

Unreinforced basement walls are the norm. Regardless, you can't reinforce (not economically anyways) against frost. Remove the soil and or install a capillary break.

 

[CELinOttawa]

SkillsAndBikes:

I'm no Part 9 memory wizard (and I don't really feel like looking it up), but isn't the 8" thick wall at 8' height reinforced? It is a 10" wall that the OBC/NBCC permits to be unreinforced.

 

[cvg]

so has anything been done correctly?
backfill with heavy clay against unbraced, partially cured, un-reinforced concrete walls in an unheated and un-insulated structure. pictures also tend to indicate that grading for drainage away from the building was not done, it appears the ground slopes towards the building.

The cause is not frost heave or expansive clay, it is design error and construction fubar. It is likely expansive clay or possibly heavy equipment and lateral loading during the backfilling operation that cracked the walls. The cracks might not have shown up right away.

 

[Canuck67]

CEL,

OBC Part 9 - No reinforcement in an 8" wall, to 8'-2" wall height. 15MPa/20MPa concrete is permitted. The only reinforcement specified in 2006 edition is for masonry foundation walls and icf walls.

 

[CELinOttawa]

Wow... Learn something shocking every day. OBC Div B Table 9.15.4.2.A. (2012) Propped top and bottom, 15MPa Concrete, Height of 2.5m and less, 8"thick unreinforced wall may retain not more than 2.15m (7 feet) of free draining backfill.

I had to look it up to believe it... *sigh*

Better complete the propping of the top before the wall is ever backfilled. Crazy...

 

[dcarr82775]

If it is clay soil it could be it had enough cohesion that it was more or less self supporting. Then something changed and the soil got weaker so it loaded the unreinforced/unbraced wall and broke the wall. I have seen failure cracks like that before, but that wall had reinforcing.

I don't think it is frost or expansive soils. Bad design/construction

 

[CANPRO]

Thanks for the replies. I agree with the bad design/construction opinion, if the problem didn't show up now from the frost it would have showed itself over time. I think this was a case of multiple trades and designers doing things that aren't 100% proper but will get away with 99% of the time.

I was also surprised with how much retained soil was allowed by unreinforced concrete in the NBCC. Its interesting that the Ontario code specifies free draining backfill because I did not see that in the National code...seems like a fairly obvious requirement that they should have in there. I will have to look into my provincial building code.

I know nothing of the person who prepared the plans, there weren't likely reviewed by an engineer and did not have a stamp (prepared by a drafting company). I'm interested to hear opinions on the attached wall section. This is fairly typical around here and is routinely approved by the City. As stated already, part 9 of the code allows more than 7 feet of soil to be retained by an unreinforced wall if it is braced top and bottom. In order for the wall to be considered braced at the top, it needs to have a floor system fastened directly to the foundation (it even says parallel framing is acceptable). If the foundation wall is not braced at the top, it can only retain 1.2m of soil. The attached wall section clearly shows no acceptable means of lateral support at the top of the wall and they show a MINIMUM of 1.22m of retained soil. Of course their first note is that all work must conform to the NBCC 2010. It seems to me that they should be on the hook for the cost of repairs.

http://files.engineering.com/getfil...-4aa9-b08a-147e4a0f0b45&file=Wall_Section.png

 

[CELinOttawa]

Appears to be both a code violation (failure to require effective lateral bracing to the top of the wall) as well as a negligent design (failure to exercise due care and skill expected of a reasonable and competent person operating in your field of endeavour).

I'd use the term "profession" rather than the obviously linguistically painful "field of endeavour", but as per ztengguy's barb there is no professional involved here.

Oh, and since you're new to our profession I'll spell one more thing out to you: The drawings must be constructable, and must include any and all details typically provided by your peers. That means that slapping "All work shall comply with (INSERT)" is never, ever, going to provide you anything more than a minor negotiation point. It won't save your license, and I certainly won't save you from liability.

Time for your friend to lawyer up, mind you they may want to start with a licensed and appropriately trained and qualified P.Eng's forensic report. I'm pretty confident you've got the right answer here, but no one can give you a professional opinion online.

 

[CELinOttawa]

I just love how horrible that detail is... So many things wrong! *sigh*

If I had an intern bring me that, I'd make them draw it correctly from scratch every day before they'd be allowed to go on lunch break, for at least a week.

I've not had a reputation for being a patient or forgiving mentor.

 

[CANPRO]

CEL, I appreciate your input. I think you may be misunderstanding my position in all of this. I saw the plans for the first time when I was on site looking at the cracks in his foundation. I am just trying to help him clean up the mess. Structurally, I know very well why the walls failed...that's pretty obvious. I originally posted because I wasn't sure what loaded the wall to failure. FYI - I removed a lot of the detail from that wall section so that it would be more generic.

 

[SkiisAndBikes]

I agree the detail is poor. No engineering required when working within the confines of the prescriptive requirements of the NBCC (part 9).

As per 'code' (NBCC), this detail is permitted in certain situations. For an 8" wall, you are allowed something like 3'-11" of backfill height against a fdn wall that is laterally unsupported at the top. Not my idea of a good detail, but it does meet 'code'. Note that the backfill height is measured starting from the top of the slab-on-grade on the interior. The height of the concrete wall can be higher, its only the backfill height that is limited.

Of course, the detail is devoid of any limiting dimensions.

 

[CELinOttawa]

Hi CANEIT,

I think you're trying to help your friend, which I respect, and obviously this provides a learning opportunity... Just be careful not to do anything that would constitute the practice of engineering without a licensed P.Eng. supervising. Many an Engineer has regretted doing a favour for a friend.

As to the detail, this is typical of a disease rampant in the Construction industry. It seems like we are seeing more and more "detail" in drawings and less and less meaning... There is something about the human mind that makes us think we're doing a more complete job when we put more information in, whether that information is pertinent or not. I half expect to pick up a drawing in a few years and have it contain something akin to Galileo's cantilever. Just sad.

I have been, however, potentially unfair here. If the wall had been properly limited in height, and if the soil had been suitable, construction sequence monitored and correct, etc, etc, etc, this would have likely been another "fine" residential construction.

But I still don't think the designer has the faintest clue why this failed or what they could have done to prevent the failure. I'd like to ask them where they would put reinforcing bars to try to make this detail better. $50 says they'd want them on the inside face of the wall, because that's what they've always seen in foundation walls.

Just sad how little our skills are valued this day and age.