One of the characteristics of clay soil is that it shrinks significantly when it dries. You might have noticed such shrinkage in the gap that has opened between your foundation and the soil. Keeping the soil evenly moist around the foundation helps prevent or even prevent shrinkage. However, when watering the soil, don't just water a narrow band of soil. I normally recommend watering a band of about 6 feet. Now, 6 feet is not a magical width; I just mean to water a wide enough band. If you water too narrow a band or soil, the surrounding ground will suck up the water rapidly, essentially countering your good intentions. You should also not spray water directly on the foundation, which is never a good practice.
How much to water depends on the soil composition, type and conditions. Checking the soil using the old finger probe similar to the way you would check the soil in a planted pot is possible. You just push your finger into the soil to a depth of about an inch and, if the soil feels damp to your finger, it is probably wet enough. Then again, not being able to push your finger into the soil probably also tells you that the soil is too dry. You can also use the simple plant moisture meters available in many stores for use on potted house plants. If you want to get real sophisticated, soil moisture sensors that actuate an irrigation system are also available. You can find these by Googling "irrigation soil moisture sensors". Creative homeowners who don't want or can't have an irrigation system installed around their foundations should be able to make up a system using soaker hose, the moisture sensor and control system and an irrigation valve.
So, what is the big deal about the soil being too dry around the foundation. (Truth is, maybe I should not even be telling you this preventive measure because in the past, we have had a lot of jobs from homeowners because of this issue. But, that is not our way.) First and foremost, the foundation can settle and develop cracks and some of these cracks can be severe. The most common reason for cracks is that the foundation settles unevenly, which is a particular problem with the type of foundation that everyone wants nowadays--the walk-out basement. Heck, I like 'em too, even though I don't have one. With these types of foundations, the soil may be dry to the same depth all around the foundation; but the soil around one part of the foundation may be dry to below the depth of the footing while the soil around another part of the foundation is not. Dry soil shrinks; so some of the soil around the foundation shrinks while it doesn't around the other parts. This condition results in part of the foundation moving while part of it doesn't. And that difference in movement can result in cracks.
Besides walk-out basement foundations, stair-step and concrete block foundations (both of which may also be a walk-out basement foundations) are especially prone to developing cracks under uneven soil drying conditions. Another susceptible foundation construction, which is VERY common, is the crawlspace, stem wall or porch foundation that is poured continuous with, but which is much shallower than, a basement foundation. The same logic can be used to explain why cracks could develop. The shallower foundation settles while the basement foundation doesn't. Basement foundations that are not walk-out types, stem wall and crawlspace foundations aren't totally immune from developing cracks either. The conditions dictate the development.
Slab on grade foundations are also susceptible to developing cracks in the slab, depending on the slab construction. In the case of these slabs, the soil around the outer edges of the slab could dry while the soil farther under the slab dries slower. In this case, the soil around the perimeter of the slab shrinks while the soil farther under the slab doesn't. The perimeter of the slab settles, while the parts farther under the slab do not. In this case, the slab might develop cracks as well as heave.
The second reason is that what does down might need to come back up. Just as clayey soils shrink when they dry, they re-expand when they moisten. If the foundation settles due to dry, shrinking soil, the foundation might not move back the same way when the soil moistens and re-expands. In moving back with the expanding soil, the differences in the amount of movement around the foundation could occur, creating stresses that cause cracks. The soil also might have shifted and exerts pressure on the foundation differently than before. Soil that shrinks may not re-expand as much as it shrank because the spaces between the soil particles are squeezed reduced as it shrinks. When the soil re-expands, the spaces between the soil particles might not be the same as before, causing stresses that create cracks.
A third reason for keeping the soil around the foundation evenly moist is one I hinted at earlier in this post. The gap between the soil and the foundation becomes a water channel when it finally rains. This issue is a problem mainly for basement foundations. Don't let anyone kid you, nearly all foundations are porous to a varying degrees, even those with some exterior water barrier coatings. Water running down the foundation wall is absorbed by the foundation materials and transferred to the inside of the foundation. If the foundation happens to have a crack in the right place, the water will have an open path through the foundation wall.
Water that isn't absorbed by the foundation materials or doesn't flow through cracks can cause issues around the foundation by eroding soil around the foundation. In most areas, building codes now require that homes have a foundation drainage system. Those systems work to capture a great deal of water around the foundation IF they are installed properly. On the other hand, water has a mind of its own and seeks the path of least resistance. Therefore, the water might choose to flow under the foundation and not into the foundation drainage system. If enough of it travels through one area, soil erosion can occur, causing the whole uneven foundation support thing, which can case cracks, and/or worse shifting, of the foundation.
A final, not so directly an issue, is loss of vegetation around the foundation. In particular for homes built on hillsides, loss of vegetation could result in loss of soil erosion control. With soil erosion could come soil sliding. Cracks in dry soil could also open channels to deeper soil layers that are not as well bound together as the upper layers, resulting in slides.
A couple of other things should also be kept in mind. The foundation may not the only thing that has moved when it settled. Everything sitting on the foundation might have also moved. Foundation movement might be imperceptible but still be enough to cause cracks in other interior and exterior building materials. Further, movement is greater higher up in the building, which means that cracks might be found in upper floor materials without be found in lower floor materials.
Another thing to keep in mind is that movement creates stresses in the foundation and other building materials. Those stresses might not cause cracks right away. Those stresses could still be present only to cause cracks later on, such as when another condition creates additional stresses—the old straw-camelback syndrome.
I have thrown a lot at you in this post—and without pictures. (Over time, I promise more pictures.) The issues can be complex. Still, they do not mean you need a structural engineer to look at every crack. If you have doubts, then hire an engineer to check things out and hopefully provide assurance. On the other hand, you might try keeping the soil around the foundation evenly moist and avoid having the need for an engineer.
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