Common Causes of Foundation Failures that Jeopardizes your Investment

I often hear stories of investors walking away from a deal when foundation issues are identified during the inspection, and for a good reason, since some foundation repairs can set you back tens of thousands of dollars and several weeks in rehab just for a SFH.

But how many of you have ever thought about the causes of foundation failures? Just because you don’t see the symptoms today, it doesn’t mean that you won’t have a problem in the future.

Whenever you buy a house, whether you purchase or rent the land that the property sits on, you are inheriting the soil directly beneath your property. And just like you would inspect your property and vet your inherited tenant, if you do not do your due diligence on your foundation and soil, you will be in for a big expensive surprise in the future.

Now, there are several foundation types and they all look and behave differently. Furthermore, some types are more or less prone to a failure depending on the event in which causes the failure to occur. Since I don't have the time to cover all types of foundation and failures in a single article, today I'm going to focus on the slab-on-grade foundation in conjunction with soil/moisture related failures.

Source: http://www.bettisconstruction.com/single-post/2014...

Slab-on-Grade Foundation

Many residential concrete foundations are placed as a monolithic slab on grade with footings located beneath bearing walls of the slab (i.e. exterior walls). The advantage of the slab foundation is that it is cheap and easy to build. They are designed only to resist bearing and often require minimum reinforcements to prevent cracking and shrinkage.

The structural disadvantage for slab foundation is the proneness to failure due to ground settlement (soil consolidation), shrink-swell soil, frost heave, and scouring. Yikes! Well.. at least the developer got to build the house for cheap :)

Source: http://www.goinspections.com/?page_id=74

Ground Settlement (Soil Consolidation)

In general, settlement is the process where the bulk volume of the soil beneath your foundation is reduced by exclusion of water under long-term static loads and the soil particles become packed together more tightly.

This process is best explained with an idealized system composed of a spring, a container with a hole in its cover, and water. In this system, the spring represents the compressibility or the structure of the soil itself, and the water which fills the container represents the pore water in the soil.

1.The container is completely filled with water, and the hole is closed. (Fully saturated soil)

2.A load is applied onto the cover, while the hole is still unopened. At this stage, only the water resists the applied load. (Development of excess pore water pressure)

3.As soon as the hole is opened, water starts to drain out through the hole and the spring shortens. (Drainage of excess pore water pressure)

4.After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excess pore water pressure. End of primary consolidation)

Source: https://en.wikipedia.org/wiki/Consolidation_(soil)

Since the slab foundation CANNOT be easily repaired, proper soil compaction prior to the pour is extremely important! Oh, by the way, do not confuse initial compression with primary or secondary consolidation. While the initial compression takes places almost immediately, primary and secondary consolidation can take years to complete.

Now, you may be wondering... what's the big deal if the foundation settles a little... and actually... yeah... it's not a big deal if the entire foundation settles uniformly (*slightly). However, we often have a big problem when we have a differential settlement.

Source: http://allaboutfreebooks.com/types-settlement/

How Could This Happen?!!!

Crappy Fill

To create a level grade to build a house, the cheapest way is to cut the nearby mound and fill the low point of the site. Unfortunately, there's a problem that could develop from this approach.

When a proper clearing and grubbing of the existing vegetation is not carried out prior to the cut and fill, the existing vegetation is mixed with the fill. These organic materials do not possess the same bearing capacity of the soil and will begin to decompose over time.

Inadequate Compaction

When the fill is placed to level the site, the soil needs to be properly compacted. Now, you might be thinking that all you need to do is to dump the soil on the ground and run a truck back and forth over the new fill... wrong :(

Proper soil compaction requires a geotechnical engineer to first determine the fill materials and characteristics, required moisture content, lift height (depth of the soil layer per compaction), machine performance, and a number of passes to achieve the required soil density. Unfortunately, too often these vital directions are not followed and fills are improperly placed without adding adequate moisture and proper lift or passes to achieve the specified soil density that your investment will eventually be placed on.

Now, imagine that half of the house was built on top of the cut (over-consolidated soil - the good stuff..) and the other half was built on the crappy fill with poor compaction... Yup.. Overtime, the vegetation mixed with the fill will decompose and poor compaction will begin to settle, and since the foundation was never designed to resist bending or shear stress, you'll have a failure in the foundation which will propagate to the rest of the structure.

Source: http://www.homecrafters.net/calgary-home-inspectio...

There's a lot more to cover (which I will eventually) but I hope this helps you understand a little more about your home, foundation and your inherited soil.