The Process of Soil Stabilization

The question here is why do the clay soils in the Central North Texas area shrink and swell so much. In order to understand why these clay soils shrink and swell, you need to understand the difference between the water and clay molecules at the basic atomic level of electrons, neutrons and protons. Each element has a specific number of electrons, neutrons and protons. The electrons carry a negative charge and the protons carry a positive charge. The neutrons are, of course, neutral. The neutrons and protons are held in the nucleus of the atom while the electrons are circling around the nucleus like moons. Each water molecule, which we often refer to as H2O, is made of two hydrogen atoms and one oxygen atom. The water molecule is actually neutral, meaning that it has just as many electrons in the molecule as it has protons. In the water molecule, both of the hydrogen atoms shares its electron with the oxygen atom. When the water molecule is formed, it develops the shape of a "V" where the oxygen atom is located at the point of the "V" and the hydrogen atoms are at the ends of the legs of the "V". The angle between the legs of the "V" is a constant 105 degrees.

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 the two electrons that are shared with the hydrogen atoms, develops a negative dipole or negative charge. Positive and negative charges on atoms, although electrostatic, can be compared to the poles on a magnet in that opposites attract. Water, of course is a liquid and has a depth and a surface. On the surface of water the water, the negative charges on each water molecule are attracted to the positive charges on the other water molecules. This creates a week surface tension called hydrogen bonding, meaning that the water molecules stick together. The hydrogen bonding, which creates the surface tension, only acts on the surface of the water (whatever the shape of the surface may be from flat as in a pond to the shape of a round droplet) and does not exist below the surface of the water. Most of the various clays in this area have negatively charged molecules. The negatively charged molecules attract the positively charged ends of water molecules, that is, the hydrogen atoms. As a result, water begins to stick to clay molecules and the clay begins to swell. The water molecules do not actually bond to clay molecules creating some kind of new molecule, they just stick to the clay molecule through a week electrostatic bond called adhesion. But remember, water molecules also tend to stick to each other, a process called cohesion, the first layer of water molecules held to the clay molecule by adhesion, attracts additional layers of water molecules via cohesion. The end result is that the negatively charged clay particle can end up being covered in several layers of water. This can cause the clay soil to swell extensively and create a tremendous uplifting force on concrete foundation slabs.

How a particular type of clay soil particle or molecule interacts with water is strongly affected by its shape and size and the electrical charges of the clay molecule. Clay molecules tend to be tiny flat plate like objects. Each clay molecule has a lot of surface area relative to its volume has a high negative charge. Being flat and having a high negative charge means that such clay molecules can attract a lot of water relative to their own volume. Common examples of clay soils that are flat and are very negatively charged include bentonite and motmorillonite clays.

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 and, the clay particles begin to push apart. As the clay particles are pushed apart, the water molecules are held in place by cohesive forces (water being attracted to water) and 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 can become soft and malleable. In extreme cases, hydrating clay soils can cause over a foot of vertical movement and too much water can hydrate the soil to the extent that it loses its bearing strength. At that point, it then begins to exhibit the properties of a liquid and it can flow like a liquid as in a mud slide.

Because water is only electrostatically sticking to clay, as opposed to chemically bonding to clay, the water can be easily removed and the clay can be dried through evapotranspiration ( meaning the water simply evaporating into the atmosphere along with the process of plants using the water to live on and releasing or transpirating the water into the atmosphere as water vapor ). As clays dry, they shrink and crack and tend to lose some compaction. If a clay doubled in volume while it was absorbing water, it could only lose less than half its expanded 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 on 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 several inches wide and extend to depths in excess of eight feet and more.

Therefore, water can be very destructive to concrete slab foundation because of its constant swelling and shrinking between the wet seasons and the dry seasons of the year. The question then becomes, how can this swelling and shrinking of the clay soil minimized to the extent that a foundation can withstand the soil movement. The simple answer is to provide a substitute for the water that also has a positive charge but, is not drawn away by the roots of plants or will evaporate away because of the heat of the sun. This substitute for the water is ground up lime commonly called hydrated lime. Hydrated lime can be injected into the soil by a process called liquid lime injection where the hydrated lime ( a powder ) is mixed with water until it looks like milk and then pressure injected into the soil around and under the foundation of a house.

All clay particles in soil have a negative net electrical charge(-). With this negative charge, clay particles will react with components of lime that are injected into soil as positively(+) charged particles, or cations. The hydrated lime particles dissolve and release positively charged calcium (Ca++), (or magnesium(Mg++) in the case of dolomitic limestone), cations into the clay soil. These positively charged particles, or cations, are attracted to the negatively charged particles present in soils. This process occurs when hydrated lime is applied to soil, and it reacts with soil moisture to dissolve. The rate at which the hydrated lime dissolves, releasing these particles, is directly related to how fine the limestone is ground, and the chemical form of the limestone. After the hydrated lime dissolves in soil, the calcium or magnesium particles in the hydrated lime are attracted to and held captive by clay particles, thus neutralizing soil acidity and raising the soil pH which in tum lowers the plastic index of the clay soil. Due to this captivity, lime does not move downward in soils very rapidly, especially in our clay soils. For this reason, lime can be applied anytime without any losses from soil leaching. The basic difference between the water and the lime is that the sun will not evaporate the lime away and plants will not eat it to live on like they do with the water. Therefore, once the negative charge on the clay molecule is neutralized, it will no longer react to water and the water flows on by without being captured by the clay soil molecules.

Please understand that liquid lime injection is not an overnight cure. This stabilization process may take several years. Although it takes an undetermined period of time for the lime to become effective, an acceptable measure of soil stabilization can normally be observed within sixty to ninety days. The injection process may require pulling up the flooring in places so that holes can be drilled through the floor for the insertion of the tool used to inject the soil stabilizing agent into the soil. However, an acceptable measure of soil stabilization can often be achieved by only doing a perimeter treatment. After the soil pad has been stabilized, any desired cosmetic repairs can then be made to the sheet rock and brick veneer siding of the home.

Barney Ballard

Registered Professional Engineer

2023 ©  Article by The Foundation Doctor / © 2023 The Barney Ballard Corp