Active clay soils are often misunderstood. They expand greatly with the addition of water (swell). When free water available to the clay soils, especially when they are in a dry state, they will expand. When the water is removed or dries up, it does lose some volume due to where the water particles left but the clay never shrinks or goes back to its original size. It is called “rebound” as it expands and only contracts a small amount. In other words, it is like a helium balloon, when it expands due to air, it gets bigger. When you let the air out, it gets smaller, but it remains bigger than the original shape.

The most important part to know, is that the soils under the house, goes through what is called “moisture equilibrium” within the first year to year and a half What that means is the state of the soil from the time the slab is poured will slowly begin to gain moisture content as the surface below is cooler than the exterior and will absorb moisture from the exterior to the interior But after a period of time, it stabilizes and does not continue to increase the moisture content. If there is no free water under the foundation such as a plumbing leak, then once the soils heaves and reaches moisture equilibrium, it does not move any more nor will it go back down or lose the moisture content.

Regarding foundations on active clay soils, slab-on-ground construction is the most common foundation system used within Texas and many other regions. It is very common and has been proven over time to be effective and technically sound. But since so many slab-on-ground foundations are built every year, there is also a high rate of foundation issues. In fact, of all foundation types, slab-on-ground is the highest rate of foundation issues as compared to suspended slabs (pier and beam, void box slabs and Tella-Firma/Slabtek foundations).

Many think that foundations are designed to stop soil movement when in fact, foundations are designed to accept the soil movement and to try to control the amount of distress within the structure to cosmetic distress only. It is similar to a raft having move as the waves of the water run under it. This may sound odd, but the design standards defined by the Post-Tension Institute and the current building codes realizes that even with a good design, the structures will experience cosmetic distress only.

What makes slab-on-ground even more problematic is the truly unknown characteristics of the global soil conditions under the structure. Yes, a geotechnical report provides a sample, usually a 4” diameter core, that gives a good perspective of what type of soil is there, but it cannot and will not ever be fully consistent with what is across the lot. In addition, all geotechnical engineers will tell you how the soil is “predicted” to react, but it is not an exact science.

To combat soil movements, there are four (4) methods to reduce the potential vertical rise (PVR) within the soils of which each have varying success and costs associated. The best, in CES opinion, is Chemical Injection which is a Potassium/Lime chemical process that is injected/mixed with the soils. It bonds to the clay particles and does not allow the clay to absorb water, therefore no expansion. Great product and very expensive and rarely if ever used in residential. It is more commonly used on Airport Runways, Commercial Projects and some major highways.

The next is Moisture Conditioning of the soil which is a process of cutting out several feet of clay soils, usually 5 to 10 feet, and then moisture conditioned and added back/compacted in 6-to-8-inch lifts. Then the ground is covered in poly with a 12-inch cover layer of soil. The intent is to pre-swell the soil and the poly is to trap the moisture. This has been performed very successfully and has proven to be a good process. It is expensive and you have to have the room to pile the soils for the proper treatment. This method will not hold the moisture content forever and construction should take place within a 2 year period of time, otherwise, the soils could lose their moisture content.

The next is Water Injection of the soils. This is the most common method but it is also the most problematic. Water injection is quite debatable as no one truly knows how the water is dispersed below the ground. Even though a boring can be taken to verify proper moisture content, it will not give an accurate depiction of the moisture content across the entire lot. It is pouring money in the ground and hoping it helps. Again, the soils have to be covered with poly and 12 inches of soil. Usually this process will not hold moisture content for more than 1 ½ to 2 years so all construction needs to be completed within a reasonable time frame.

Finally, there is a water/chemical injection process which is most commonly known as Condor which is a theoretical approach to using both chemicals and water. It is better than water injection and not as good as chemical injection. It will reduce the PVR but does not eliminate it. It also has large variation of success as there is no way to truly verify if the water/chemical solution was dispersed properly just like water injection. This process is more expensive than water injection but less than chemical injection.

All of those are costly and take time and still, only one (1) of them comes close to stopping any soil movement and that is Chemical Injection which usually obtains a one (1) inch PVR at most. If the desire is to have a slab with no movement, it must be suspended.

Suspended slabs have been around a long time and have historically been installed using void-boxes. Void-boxes work quite well but due to the methodology of installation, it leaves itself open to construction defects which does not properly provide the void required to resist soil movements. The most problematic is all the voids between the boxes which can allow concrete to go down and fill in voids thus eliminating the void space. They are also very expensive.

The Tella Firma/Slabtek system is a suspended slab, built as a slab-on-ground and then lifted like a void-box slab without any void boxes. The void created is a true void across the entire foundation. The lift is usually 1.5 times higher than the predicted PVR to allow additional safety factor above and beyond what is expected. This is the same type of structural systems seen in many commercial projects with columns holding up a two-way slab.

In regards to plumbing, historically the plumbing lines would be buried in the ground and extend up and through the foundation. This was true for slab-on-ground and suspended foundations. When the concrete was poured, the concrete would shrink and bond to the PVC plumbing penetrations. This “bond” would not allow the plumbing lines to move which is a good thing. The problem with this though is the concrete, over time, could cause problems with the contact with the PVC and also the ground does move and since the ground moves, ,the plumbing would get what is called “bellies” in the lines. This would not be easily found as the plumbing generally moves close to the same as the house does. So when the house moves, the plumbing moves but when the plumbing moves, it can often move upward where the slope is no longer positive away from the house. This is nothing new and is a common issue within active clay soils.

Then, the code made a significant change whereas the plumbing stacks can no longer be in contact with the concrete. This is to avoid the possible deterioration of the PVC plumbing stacks. Therefore a separation was required. When this came into existence, a new series of problems began to occur, mostly in suspended slabs. Since there was no positive connection between the foundation and the plumbing, the plumbing easily moved when the soil moved. Secondly, based upon various elements that were used to separate the PVC from the concrete, caps were left open that allowed stagnant air to infiltrate into the house.

Therefore, new methods of plumbing had to be researched to address the above concerns. Currently, there are known methods both published and proven through use to avoid plumbing problems with both slabs on ground and suspended slabs. These are as follows:

• The use of sand or pea-gravel under the drainage lines. This is required per code and also provides a cushion against the plumbing from soil movement. See the attached documents on more detailed information regarding trenching.
  • The top of the plumbing should use local spoils to cap the top of the plumbing lines to the surface of the pad. This is required for several reasons. First, it acts like a clay dam to stop moisture from penetrating the plumbing trench. It also provides a cap to hold down the plumbing from heaving.
• Securing toilet flanges to the concrete. Although the code also requires this, it is rarely done within the residential industry all due to a lack of QC.  But, securing the toilet flange to the foundation is critical. Again, refer to the attached documents for more information on this item.
• Putting a clay end-dam at the entrance of the plumbing lines to the foundation.
• Installing a void-box under the elbows of the vertical elements.
• Finally there are approved plumbing sleeves that allow for movement that can be used and we will send over various products that should be considered.

Sometimes, due to construction of homes being performed by human hands, construction deviations occur which may in turn cause plumbing stacks to act differently than intended. If the source of the problem is known, then it is easy to fix. Most issues are found at toilets where the flanges are not tied to the foundation as required. Others occur as there is either a water source or a channel in which water travels under the foundation and gets in the plumbing tranches. In most cases, if the plumbing penetration is cut back down, sealed to the slab with the proper materials, the problem does not reoccur. This is especially true if the home is past a year old and the soils have reached equilibrium.

All of the above are in regards to the foundation. Yet, the soils around the house which are exposed to air and water constantly will act differently. The use of flat-work is required for all residences and the code provides the direction on how these are to be installed. They are not “engineered”. The are a code specified product. Since these are resting on the ground, they are susceptible to movement and is hard to control if not impossible to control.

The standard approach is to use 4’ concrete with either light rebar or wire mesh. If the soils move at all, which they will, the flat-work will move. It will usually move up and not down. If there is a lot of fill on the lot, the soils can consolidate and the flatwork will go down, but that is not as common. As a rule of thumb, compacted soils will further consolidate by 1% of the compacted fill within the first year or so. So a four (4) foot fill will still consolidate by roughly ½”.

There are methods to control a “step” between the flatwork and the foundation which requires the use of a pivot connection between the flat-work and foundation. Also, the use of 5” flat-work has proven to perform much better than 4” although more costly. It is possible to inject the soils as noted above, but that is almost never done. CES has specified the use of Condor for pools and flatwork around pools and that has performed quite well.

The bottom line of driveways is that it is code prescribed and not engineered and unless you suspend it as well, there is no way to control movement within flatwork. Even if you did, the soil would heave around it causing problems. This is all part of the battle of living with active clay soils.

See the CES Foundation Maintenance Procedures as well for how to maintain your foundation.