Shallow Foundation

[Randy L. Mayo, PE]

I am sure many of you old timers have noticed shallow footings on a hillside home with a walkout basement like the photo below. These basements are typically three concrete walls with the rear wall framed with studs. The downhill footing is either exposed or just a few inched under the ground surface. This is a design flaw IMO that goes unnoticed by many contractors and inspectors. This issue has been around for a long time and there are several publications that address this issue for new construction. You can do a search for Frost Protected Shallow Foundations (FPSF) and find methods to insulate and reduce the probability of frost heave damage. However there are thousands of homes that do hot have the required insulation (old and new). If the house is continuously heated then heat from the house travels through the floor and under the footing which reduces the threat of frost forming under the shallow footing. The problem occurs when the house is vacant or is a seasonal home with the heat turned off or turned down very low. Frost heave will most likely occur about midway along the back of the footing which will lift the floor and typically produces a noticeable crack in the floor. The two photos shows the typical shallow footing and the typical crack at the middle of the slab.

[ROBERT V. YOUNG]

Mr.Mayo.
Totally new to me. I have never witnessed that before foundation practice before.
Montreal Quebec Canada.
Any photos showing the home and the hill it is built into.
I take it is is a shallow slope.1% grade.
Links on education into the practice please.

[Randy L. Mayo, PE]

Robert

These walkout basements are usually on moderate to steep lots.

[ROBERT V. YOUNG]

Thank you Mr.Mayo.
Any links to foundation drawings. I would love to see the layout.
Thanks....Robert

[ROBERT V. YOUNG]

The overhang must be cantilevered.
Love to see the plains.

[Mark S. Tyson]

Randy,
I know i am probably naive in that i live in flat as a pancake and no frost SE Florida, but i cannot understand how that condition can happen unless the home was built before Building Codes were in affect.
Forgetting frost zones, set back from slopes etc. what about the basic requirement that all exterior footings shall be placed at least 12" below undisturbed ground surface

[Randy L. Mayo, PE]

Mark

Rural areas of Missouri have no building codes and the majority of contractors have no formal training in advanced construction techniques. The larger cities have building codes and only a handful of counties have codes.

[Mark S. Tyson]

Originally Posted by rmayo

Mark

Rural areas of Missouri have no building codes and the majority of contractors have no formal training in advanced construction techniques. The larger cities have building codes and only a handful of counties have codes.

And i thought we had problems in Florida, where all contractors must have 8000 hours proven structural supervision and pass an 18 hour protored exam to recieve license and the Building depts, inspectors, plans examiners, etc. must be ICC certified.

[Tim Spargo]

Randy, thanks for the post. Was this an issue discovered during the course of a home inspection? The crack in the slab, is there a reason (deflection) that the crack was of concern?

Dowel and pour?

[James E. Braun, CMI]

If we ever get freezes like we use to, we will see a lot of foundation cracking in Missouri. Sad but true.

[Randy L. Mayo, PE]

Tim

This was found during a routine home inspection, but I have seen this issue many times. The cost/benefit of fixing the problem is too expensive for most home owners. Most just fill the crack in the slab and live with the problem.

[ROBERT V. YOUNG]

You hit the nail on the head Randy. Cost.
It is a quantified question.
I try to explain to my clients that the cost of repairing certain cracks outweight the the effectiveness in financial returns. Re sale of the home.
Is it worth putting that amount of money into fixing the defect in your area.
There are many questions to ask the home owner before you take on a repair.
Mostly I say yes to the defects being fixed.
The types of repairs are as many as to foundation cracks.
A well rounded structural engineer or foundation repair contractor that deals specifically with all the issue causing the defect can fix the problem at a modest or moderate price to the home owner.
IE: wall movement from lateral pressure.
To resit the wall with levers and jacks to push or pull the wall as close to the original position. Then replace existing soil and clean or replace arrogate and french drain.
IE: 2 years ago I worked behind a foundation repair specialist. followed the job through completion. Phase inspection.
Resit a 22 foot span 8 feet high, 12,00.00.
Compared to reformed wall at 22 thousand and that was the lowest estimate. No guarantee of brick damage.
With brick repair and upper foundation water proofed and seams fixed $15,000.
The alternative , reforming the foundation, would incur a greater brick repair cost.
22 large and xx for brick damage.
5 working days for the foundation contractor. Clean job and no leaks to this day. 5 year guarantee.
All done in November, the late fall..

[Dan Bowers, CMI]

Like Randy said, in Kansas City I see this condition with the exposed footings about twice a month in certain areas of town (often on the fringes or burbs). The homes are often 30-45 years old on a sloped walk-out lot. Front foundation wall / full height concrete; side walls combination of concrete (stepped down) and wood short walls; rear wall in my area is MOST often full height framed.

50% of the time NO issue / 50% like Randy said - issue.

Most often its not economically feasible to do the repair for the resultant benefit. Lot of things like that in older houses OR those with lax OR missing codes AND you don't have to get out in rural America to find them.

Many bustling metro area's around me WERE semi-county with lax OR no code enforcement in the late 60's to late 80's.

[ROBERT V. YOUNG]

Randy any trees near the home ?. Next time show some shots from 20 feet away from the home so people can get a look at the bigger picture. Even clay can heave concrete up and displace the footing if not excavated and back filled properly. Go to the city hall and see what soil is in that area. I bet clay or other expansive soils. http://web.mst.edu/~rogersda/expansive_soils/

[Randy L. Mayo, PE]

Robert

I added a few more pictures that may help. You bring up a good point, soils that have high shrink-swell characteristics are capable of producing forces great enough to cause this basement floor to crack. In this case I was hired to do the home inspection and to look into a failed septic system. As part of the septic system investigation I always look at the engineering and physical properties of the soil. The soils at this location are very gravelly/sandy loam with a low plastic index and a low shrink-swell potential but are classified a moderate for frost action. Based on published soils data and looking at the foundation frost action was the most likely cause. Shrink-swell and frost action, for the most part, can be considered independent soil properties. Each of these two soil properties can be classified as high, moderate or low so there is nine possible combinations when combining the two. So in many cases without taking soil samples and having lab tests done, in an engineering report I would have to state the cracked floor was the result of frost action and/or shrink-swell soils. The satisfaction of knowing which soil property contributed the most is usually not worth the added expense. I have to remind clients this is only a symptom of the problem and the real problem was the footing was too shallow. It’s too easy for home owners and new inspectors to focus on symptoms and not the problem. One last comment on soils… Soil properties vary with location, depth and moisture content so working and/or designing with soil is more an art than science. For this reason working with soils is a high liability business and I urge inspectors and engineers alike to use caution when associating foundation failures to soil. Knowledge of local soils and how to build on it is gained by experience you can’t read it in a book.