Clay Soil Cracking Foundation ProblemsTo understand why soils shrink and swell, you need to understand water molecules and clay molecules. Each water molecule is made of one oxygen atom and two hydrogen atoms. Each of the hydrogen atoms shares its electron with the oxygen atom. Thus, water molecules are shaped like a “V” with the oxygen atom at the point of the “V” and the hydrogen atoms at the ends of the legs of the “V.” By sharing a negatively charged electron with its oxygen neighbor, each hydrogen atom develops a partial positive charge. At the same time, the oxygen atom, by adding two electrons that are shared with the hydrogen atoms, develops a negative charge. Positive and negative charges on atoms act like poles on magnets. Opposites attract. In part, water is a liquid because the negative charges on each molecule are attracted to the positive charges on other water molecules, so the molecules stick together. This is an example of hydrogen bonding.

Certain types of clay molecules have negatively charged locations on their surfaces. The negatively charged points attract the positively charged parts of water molecules, that is, the hydrogen atoms. As a result, water sticks to clay molecules. The water does not chemically bond to clay molecules, it just sticks to it, like tape on a window. The layer of water molecules that sticks to a particle is said to be held by adhesion. Because water molecules tend to stick to each other, a process called cohesion, the first layer of water molecules held by adhesion, attracts an additional layer via cohesion. The end result is that a clay particle can end up with the negatively charged locations being covered in several layers of water.

For a given soil particle, how it interacts with water is strongly affected by its shape and size and the electrical charges on the surface. Clay molecules tend to be tiny flat plate like objects. Each molecule has a lot of surface area relative to its volume. Some clay minerals are both flat and have lots of negatively charged locations on their surfaces. Being flat and properly charged means that such molecules can attract a lot of water relative to their own volume. Common examples of clay minerals that are flat and negatively charged include bentonite and motmorolinite.  

Because clay molecules are often tightly packed together, there is very little space between particles. That is to say, there is very little pore space. At the same time, because of the lack of channels between particles, it is very difficult for water to pass through clays. As a result, clays are said to have low porosity and low permeability. As water is added to a clay soil, it first attaches or adheres to the negatively charged points on each molecule. As more water is added, the layer of water held by adhesion grows to a thickness of several water molecules. Of course, adding water to all of the particles in a lump of clay means that, as water is attached to each particle, the clay particles begin to push apart. As the clay particles are pushed apart, porosity increases, but permeability does not. Permeability stays low, in part because the water molecules are sticking to the clay particles. As more water is added, water molecules held in place by cohesive forces (water being attracted to water) continue to thicken the water layer around each clay particle. The increasing volume of water continues to push clay molecules further apart.As more and more water is added, the water layers become thick enough to act as a lubricant between adjacent clay particles, and a mass of clay becomes soft and malleable. In extreme cases, hydrating clay soils can cause over a foot of vertical movement in north Texas.

Because water is sticking to clay, as opposed to chemically bonding to clay, water can be removed fairly easily. Primarily, clays dry through evaporation. Plants can accelerate the process. As clays dry, they shrink and crack. If a clay doubled in volume while it was absorbing water, it would lose less than half its volume when it dries out. As clays expand, they form relatively solid masses with few or no voids. In contrast, as clays dry, they shrink unevenly, forming cracks and fissures. The result is that a dry consolidated clay that hydrated might expand six inches but only contract three inches as it dries. This is one reason why homes that suffer upward movement caused by plumbing leaks often never return to their original level positions.

The processes of clay soils absorbing water and releasing it proceed slowly. It takes time for water to pass into and through the tiny spaces between clay molecules. It also takes time for water to push clay molecules apart. Watching clay soil hydrate is like watching grass grow. As a practical matter, what we can see is soils shrinking during dry periods. Previously smooth areas become cracked. Cracks in clay soils caused by dehydration can be many inches wide and extend to depths of many feet. The author has measured cracks in Irving, Texas, that were up to six inches wide and into which a tape measure could be inserted 8 feet.

Halliburton’s Baroid product line includes Benseal, a clay material that takes advantage of bentonite clays’ hydrophilic (that is, water-attracting) properties. For those who are interested, Youtube has several short videos on the structure of water.

Evaluating Soils and Determining if You Need Foundation Repair

Sometimes, it is pretty obvious that your house needs some work done on the foundation. But, most of the time, it is not. In order to ensure your home stays safe for you and your loved ones, it’s important to leave this work to the professionals. Contact Advanced Foundation Repair today for a free, precise inspection. With years of experience, a transferable lifetime warranty, and the willingness to work with you on a payment plan to make sure your home gets the repairs it needs, you won’t be disappointed.