Why Does a Foundation Move?
Foundations can move, and this can be a difficult matter for the homeowner to understand.
The study of the movement of foundations involves mechanics, defined by www.thefreedictionary.com as "The branch of physics that is concerned with the analysis of the action of forces on matter or material systems." Therefore, we will be talking about causes and effects, and physical phenomenon.
When we consider how foundations act, we can classify most foundations into three types, shallow pier and beam (typical crawlspace foundations), slab-on-ground (typical new foundations), and deep pier supported (typical repaired foundations). We classify this way because all foundations within a class tend to act the same, and foundations of other classes will tend to act differently.
Read the sections from front to back, do not skip if you want to understand the later sections.
This document is based upon my experiences in most of Texas. It should also be applicable to most of the South and coastal and alluvial plains.
Pier-and-Beam Foundations:
I will consider shallow pier and beam (P&B) foundations first. P&B foundations spread the weight of the house over two to three dozen piers. Each pier has a few feet of area at the bottom to bear on the support soils. Therefore the support soils underneath the piers are highly loaded and the soils are highly stressed. If the bottom of the piers is shallow, and especially if the bottom is at ground level, there is a tendency for the support soils to work their way out of the support position, and this will slowly settle the pier. If the support soils become wet and weak, they will move out of the support position, and they will settle quickly. If the soils are not allowed to move from their support positions, then the piers will not settle (other than from compression of the soils underneath the support soils, which is unavoidable).
Let us look at the ways support soils can move from their support position at the bottom face of the pier. The principle of this movement can be explained by studying a vessel of pressurized air; the highly-stressed compressed air will seek any opportunity to leak to the lower stressed atmosphere. As long as the compressed air is contained within a barrier of material resistant to the stress (steel vessel), it will not leak to the outside. In the case of soils, there is no pressure vessel, just resistance to movement of the support soils.
So how do we resist the movement of the support soils?
1. We can keep the soils in position by making the pier deep, so the outside stress is higher, so there is less stress difference to drive the movement.
2. We can keep the soils in position by spreading the load more, so the stress on the loaded soils is less. If we place more piers under the house, the stress will be lowered. If we place wider faces at the bottom of the piers, we will also be lowering the stress, and in addition, the distance the stressed soils have to move is increased, raising the resistance.
3. Also, the type of soil can be more resistant to movement. Wet clay soils will move much more easily than a moist sandy clay.
4. One thing we want to avoid in the loaded soils underneath the piers is any movement of the soils not associated with the load of the house.
Movement of soils not associated with the load of the house can be a difficult concept to understand. But because it is the reason older slab-on-ground foundations are settled at the outside walls, it is important to understand the concept. The principle of this type of settlement can be explained by studying a block of concrete sitting on sand.
If we use a rod to disturb the sand underneath the block, we will be creating a movement of the sands not associated with the load of the block. What we will be doing is called undermining, used for centuries to bring down castle walls. The more we work the sand, the more the block will settle into the sand below. What is happening is that as we move the sand, it is moving itself in different directions to a location outside of the block to a lower stress condition.
So undermining was a good example to show the principle, but what movement of soil, not associated with the load of the hose, can possibly occur under a pier to allow the support soils to move? The usual answer is: shrinkage and swelling of clays. Clays are called active soils, they shrink and swell when their moisture levels change.
Some clays are more active than others, some soils contain more clay than others, some soils containing clay contain other constituents that may reduce the active effect of the clays. So all soils characterized as clay soils do not act the same.
So when the moisture of a load bearing soil containing clay is allowed to change, this produces a movement not associated with the load of the house, and while the soil is moving due to this effect, it is also moving to reduce the stresses, outside of the load bearing zone.
An interesting effect of trees on the foundations of P&B houses is that the trees normally allow the piers within their influence to resist settlement. This is because surface soils are normally dry, and within the effects of a tree, are kept dry even during rain. Thus there is no moisture change in spite of the weather.
Thus, we have explained the normal mechanisms of foundation movement in P&B foundation houses:
1. Chronic and slow settlement of piers due to the glacial movement of load bearing soils at the bottom face of the piers.
2. Sudden settlement of piers due to flooding conditions making soils, especially clays, more fluid and able to move from their loaded position.
The piers of P&B homes never seem to rise, they only maintain level or settle.
Slab-On-Ground Foundations
Now we will consider slab-on-ground (SOG) foundations. There are two different situations of loading of a SOG foundation. The outer edge of the foundation is a grade beam. This is normally about 12 inches wide and from 12 inches to 30+ inches thick. This beam carries the weight of the exterior walls and some of the weight of the ceiling and roof. The grade beam can be explored by digging at the edge of the foundation. The interior of the foundation is a concrete floor. This is normally designed to be 4 inches thick. This floor carries the weight of the interior walls, some of the ceiling and roof loads, and your furniture, appliances, and you.
The load of the interior of the foundation is distributed across the entire floor, supported by the support soils below. The stress on the support soils is low. Also, the support soils cannot move to a position of lower stress. Normally the interior of the foundation is not a problem.
The load at the exterior beam is high and distributed across a smaller area, so the support soils are more stressed. And the support soils are next to non-stressed soils. Normally this is where problems occur.
In the P&B section we covered four resistances to the movement of support soils. Not all of these are very applicable to SOG foundations. Here we comment on each of these resistance tactics:
1. The grade beam depth is 8 inches or more underground.
2. The stress at the grade beam is much less than the stress at piers of P&B foundations.
3. The soils underneath the grade beam must be undisturbed native soils.
4. Moisture changes resulting in soil movements of the load bearing soils underneath the grade beam is the main and most controllable aspect of SOG foundation performance.
Moisture on the surface at the perimeter of a SOG foundation is controlled by the weather, irrigation or watering, and when it rains, drainage. The effects of the surface moisture on the load bearing soils below is controlled by the clay activity of the soils and the relationship of the surface moisture to the load bearing soil moisture.
We can control moisture at the surface by making sure drainage is positive away from the foundation, and watering the perimeter of the foundation as necessary to keep the moisture content within a reasonable range. We cannot normally control the clay activity of the soils.
Regarding the moisture relationship mentioned above, the deeper the grade beam below surface, the less surface moisture will affect load bearing soil moisture. Also, flatwork and moisture barriers can isolate surface conditions from soils below.
The effect of surface moisture on foundation perimeter load bearing soils is the main condition for SOG foundation performance. Therefore, we would expect foundations whose perimeter is never watered, with negative drainage, going through drought and flood, in high activity soils, to perform much worse than foundations whose perimeter is watered to resist moisture swings, drained, in low activity soils. This expectation is realized in almost all cases.
There is an effect which occurs which is of considerable consequence to SOG foundations. This effect is that of a moisture drive from underground to the surface. This effect can be explained by studying two plants, one in a pot, one planted in the ground. After two weeks of no rain and watering, the plant in the pot will be dead, the plant in the ground will be alive. So where is the plant in the ground getting its water? This can also be seen if a sheet of plywood is left laying on the ground for a week. When the sheet is pulled up you can find slugs and moist soil. Again, where is the moisture coming from? The explanation is that the rain which was absorbed into the ground is now returning to the surface in the form of vapor, which is enough to keep plants alive for a few weeks until the next rain.
This moisture drive is capped at the foundation, just as it was in the plywood example, and so the soils underneath the foundation are constantly moist. This is the reason that foundations are so attractive to trees, they are a consistent and rich source of moisture.
Why is this moisture trapping important to SOG foundation performance? The load bearing soils underneath the grade beam are being fed moisture by the trapping effect at the interior of the beam, and at the outside of the beam is the influence of the surface soils. The point is that there is a beneficial source of consistent moisture to the load-bearing soils under the grade beam, so that control of the surface soil moisture need not be so critical.
This moisture trapping also can have a serious effect on a new foundation. Since soils exposed to the atmosphere are relatively dry, when we place a foundation over these soils they begin to moisturize and swell. This moisturization can cause minor local damages to finishes, called breaking-in, or can cause more substantial damages, which can take years to complete their run. The worst cases are when trees are bulldozed to place a foundation, the very dry soils are then moisturized, the lift can be several inches and last for ten years or more.
How critical need the control of the surface soil moisture be? In my experience, the range of moisture required for healthy grass or plants is enough. It is not required to have a soaker hose or irrigation system timed for daily or twice daily sprays. It is only required to water sufficiently to keep the grass or plants healthy. FYI: If your foundation is being affected by a tree, no amount of surface watering will help.
How critical is drainage to the performance of the foundation? In my experience, negative drainage can be very detrimental to a foundation; it can cause the loadbearing soils to become wet and fluid, and allow the grade beam to sink permanently. But, almost any other drainage condition which does not result in ponding of water at the perimeter of a foundation is not detrimental to foundation performance. FYI: Some engineers and experts find less than perfect drainage is detrimental to a foundation.
If the drainage and moisture around the perimeter of a foundation is maintained in areas of less-than-super-active soils, the foundation can be expected to perform well throughout its life, and may never required foundation repair.
Though maintenance will reduce the rate of deterioration-by-aging of a foundation, aging will occur, and will occur in the following sequence. The outside corners will begin to settle, then the outside walls will begin to settle. After this, there will be little change.
I have said little about trees to this point. Tree roots under a foundation will upset the moisture equilibrium and cause the foundation to sink within the area of influence. The roots are near the surface, so they affect the shallow supporting soils of SOG foundations. Also, many trees use much more water in the summer than winter, so the effect is seasonal.
Settlement can be addressed by repair with piers or pilings. These devices can lift and stabilize a foundation. Since their base is deep, over ten feet deep, they are not normally affected by tree roots.
Thus, we have explained the normal mechanisms of foundation movement in SOG foundation houses:
1. Initial movement of new foundations in the upwards direction due to moistening of the support soils.
2. Sinking and instability of foundations within the influence of tree roots due to the drying effect of tree roots.
3. Instability of foundations due to inconsistent foundation perimeter moisture due to shrinking and swelling of support soils.
4. Settlement of exterior walls at areas of negative drainage due to yielding of support soils.
5. Deterioration-by-aging due to many cycles of moisture changes affecting the load bearing soils underneath the grade beams.
Deep-Pier-Supported Foundation
Which brings us to the third type of foundation, the deep-pier supported (DPS) foundation. Sometimes new houses will have piers integral to the foundation, these are called builder piers. Sometimes older houses will have repair piers or pilings placed under the foundation. In both cases, the foundation is not being supported by the shallow soils, but is being supported by soils much deeper. These soils have more load bearing capacity, and be more resistant to moisture changes. The effect of trees can be eliminated in many cases. The deterioration-by-aging of a foundation can be eliminated.
Epilogue
This concludes this overview of foundation performance. This document should make you a very informed homeowner. However, there is much more to foundation evaluation than is stated above. Almost every typical situation explained above has an exception. There are damages that appear to be from foundation movement but are the result of another cause. There are foundations that differ from the class descriptions. There are houses with multiple problems occurring at the same time. Different trees affect foundations differently.
A typical homeowner is at a technical disadvantage when meeting with a foundation repair contractor. Sometimes foundation repair is not the appropriate action to take. Foundation repair is expensive relative to an expert evaluation.
For all of the above reasons, a foundation performance engineer should be consulted before a foundation is repaired.
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Email: gerard@texashomeengineer.com