In my last blog in this series on the 17 items to include in a foundation inspection I covered soil conditions. Part of this process includes considering soil moisture in foundation inspections. Below, I’ll explain how the Thornthwaite Index is used to determine the effect moisture has on the respective soil, and therefore, how it can be used as a resource for determining outcomes in foundation inspections.  

Using the Thornthwaite Index to Determine Potential Effects of Soil Moisture

In order to understand what is happening with the soil, we first need to understand how the historical moisture patterns have affected that soil. Over very long periods of time, the evapotransperative cycle will make long term moisture levels fall within its normal parameters. Water from rain and snow makes its way into the water table and into the soil. Therefore, the moisture levels in the soil within general parameters will be somewhat constant.

However, depending on the particular area, there may be more or less average rainfall. Thus moisture content in the soil will vary. This is why it is important to know the history of the site and the geographical area when considering all aspects of soil moisture in foundation inspections. 

See the map, from the blog for the Community Collaborative Rain, Hail, & Snow Network, below.

The above closely mirrors the Thornthwaite index, but in color, making it easier to understand.  

Understanding Long Term Effects of Soil Moisture in Foundation Inspections

The soil, as shown in the above illustration, has been wet for many thousands of years. A structure is built on top of those wetter soils effectively trapping moisture. Normal fulgurations will affect those soils as follows:

1. Additional more than normal rainfall will not inject additional moisture, as those soils are already near maximum. 
2. Drier fluctuations allow trapped moisture on the edges to dry out. As the clays dry out, they shrink, resulting in settlement of any structures above. This is the paradigm that comes to mind for most people when they think of foundation problems. 

The Effects of Changing Soil Moisture Should Be Considered

Below are four illustrations that show how moisture accumulates under a foundation in a more arid climate. 

The above illustration shows how clay suction pulls moisture under a foundation in drier climates. Heavier than normal rain soaks down into the soil and is pulled by negative pore pressure. Drier than normal climate will not dry the soil enough to pull any significant moisture back out. Considering the history of the site and understanding these effects informs a clearer picture when analyzing soil moisture in foundation inspections. 

The above illustration shows how clay suction pulls moisture under a foundation in drier climates. Heavier than normal rain soaks down into the soil and is pulled by negative pore pressure. Drier than normal climate will not dry the soil enough to pull any significant moisture back out. 

The Stack Effect: The Science Behind it

The above illustrates the stack effect. Hot air draws up into the attic and out the vents. It is replaced by gasses from around the house, including the gasses in the soil below the foundation. Water vapor then travels with these other gasses.  As it reaches the cooler areas it condenses and becomes a liquid and less energetic. This makes it more difficult to flow through the slab cracks and pores, effectively trapping the moisture.  

The above image illustrates how moisture is trapped as it makes its way as a gas from the water table. This can happen with any other sources of moisture as they make their way toward the sky as a part of the evapotransperative cycle.

As this moisture comes in contact with clays, the clays swell, resulting in the dome heave configuration as depicted below. All of this shows us the bigger picture, which is why all aspects and scenarios should be included when analyzing soil moisture in foundation inspections. 

The Rate of Saturation Matters When Considering Soil Moisture in Foundation Inspections: Snow Melt

Yet another factor to consider when analyzing soil moisture in foundation inspections is the rate of saturation. For instance, in wetter climates snow tends to have less of an immediate effect. Over time, slowly melting snow can create higher moisture content as it drains into the soil. Whereas, in rain events, water runs off before it is fully absorbed and retained in the soil. In historically drier climates snow can have an outsized effect by injecting more moisture into historically drier soils.

Another overlooked factor that can affect soil moisture is the presence of trees. Remember, trees have root systems that pull moisture from soils around the roots to feed the tree. The roots will typically follow the moisture to seek out more water for the tree. So the presence of trees should be factored into expectations of soil moisture in foundation inspections as well. 

Of course, some species of trees have more invasive roots than others. But I will leave the plant kingdom to arborists. Though a good rule of thumb is to expect the root system to be one and one half times larger than the tree canopy.

Seasonal Cycles Can Also Affect Moisture in the Soil  

Almost all geographies will have seasonal cycles that will cause the soil moisture to vary during the year. Sometimes these seasonal cycles will result in seasonal shifting. This seasonal shift will be more pronounced in wetter climates. This is because the edge drying during dry times can be more immediate.

In drier climates it can take up to 20 years to accumulate moisture under a foundation. However, wetting can cause a more immediate effect of edge heave. This is especially true with uniform thickness post tension type of foundation systems that typically have very little edge barriers to resist moisture migration.

Considering All Aspects of Soil Moisture in Foundation Inspections: An Example in Flagstaff, AZ

I remember a particular project in Flagstaff AZ that is a case for considering all aspects of soil moisture in foundation inspections. At this site, the floor had a bowl shaped low in the middle. We had three reports from outside engineers identifying the problem, which is common in Flagstaff, of a base course of cinder rock under the slab. In these cases, these weak irregular rocks break down and consolidate over time.  The cinders were about 24” deep. Yet no one, including us, thought there could be a second problem. 

Our solution based on this first problem we saw? We mud jacked the cinders and raised the floor to a more level position. Unfortunately, new damage later occurred in the interior walls. We had missed a key possibility. This, again, is why it is crucial to factor in all elements when analyzing soil moisture in foundation inspections. 

It wasn’t until after a soil investigation that we discovered very expansive clays below the cinders. It turns out that the roots of the  large Pine trees surrounding the house had reached under the foundation. Also it turns out that the gutter systems were plumbed into perforated style pipe by a previous contractor.

What We Overlooked

The winter snows had melted slowly, and this, along with the water injected under the foundation, caused the clay under the slab to swell. Then, during the summer those Pine tree roots sucked the water back out, shrinking the clays. Each year the floor rose and fell in a continuous cycle that continuously re-cracked the repairs. Do you see how all of these factors can work together, and why they should all be considered? 

As with all aspects of foundation inspection, soil moisture needs to be carefully analyzed. Using the tools and data available you can avoid overlooking something that can cause more work and cost more money in the long run.