Septic Sytems in Clay Soils, The Expanding Clay Dilema- GroundStone


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Impact of Expanding Clay Soils on Septic System Performance

Septic systems in clay soils, especially expansive clays can bring certain challenges and headaches to any homeowner living with a backyard that consists of clay.

With expanding clays there can be many challenges with not only house foundations but with septic systems, it is important to recognize these types of clays to ensure the right strategy is being used to relieve potential integral problems.

Since the efficiency of a septic system is limited by the ability of the soil to filter wastewater naturally as it passes through the soil particles.

It stands to reason that the overall performance of a septic system will be affected by soil properties such as soil type, texture, structure and mineral composition.

When wastewater effluent from a septic system infiltrates clay soils that have even relatively low levels (5-10%) of expansive clay minerals, the wet conditions will cause the soil to expand.

This reduces the pore space within the soil structure, which will, in turn, diminish the ability of the soil in the drain field to absorb wastewater and hence its drainage capacity.

expanding clay particle

Clay soils with poor drainage will, therefore, have a reduced capacity to absorb effluent from a septic system, and will not be able to process wastewater effluent efficiently.

clay particle

The result is a reduced performance of the septic system, or even system failure, which can cause effluent to pool on the surface.

Furthermore, expanding clay soils with a COLE value greater than 0.06 (i.e. soils that are prone to swelling and shrinking) can cause structural damage to underground septic systems, which may require costly repairs.

expanding clay

Effects of Shrink-Swell soils

Shrinking and swelling characteristics is due to the clay having the ability to attract and absorb water.

Some expanding soils are known to increase 150 times their size.

Expansive clay can be very sticky when wet and it usually cracks under drying conditions.

Therefore during dry seasons cracking of the soil surface can be indicative of shrink-swell soils.

These cracks can be very large and can cause damage to building foundations located on top of theses expanding soils and can cause significant infrastructure problems.

expanding clay damage

The economic damage these types of soils causes are on a worldwide scale.   These expansive soils are responsible for more damages to structures and pavement than any other natural disaster. (Nelson, 1992).

The hydrology of these types of soils can dramatically change the landscape, underwater ponding conditions, water can channel through the cracks.

septic systems in clay soils

The result is the potential to transport water pollutants such as wastewater from septic systems, herbicides and pesticides from agricultural land.

This, of course, has ramifications to groundwater quality, contaminants can flow freely through the cracks and directly into aquifers, streams or ponds.

Managing these types of expansive soils is a challenge, swelling isn’t determined by just one factor.  The amount of clay, type of clay mineral, organic matter, and moisture and (CEC) cation exchange capacity all plays a role in the swelling and shrinking. (Kariuki et al, 2004)

Alternative Options for Septic Systems in Expanding Clay Soils

Some septic system options for expanding clay soils include:

  • An aerobic treatment unit (ATU) will help process sewage before it is discharged into the drain field, to help prevent soil particles from becoming clogged up.

Above ground, sand filtering may(sand mound septic system)  still be required in many regions throughout North America.

  • A sand mound septic system consisting of an above ground mound of porous sand media that serves as the drain field/filtration bed where effluent is processed.

This sand media must be clean and clear of any silts or clay residues.  There are a few acceptable sand media types that are acceptable in standard practice books and codes.

  • A larger drain field provides a bigger assimilation system to make up for the reduced rate of drainage.

 This is a low-tech and relatively cheap option if you have the space available on your site to extend the drain field over a larger area.

In many regions throughout North America, this practice is no longer to code.

  • Cesspools and Dry Wells: This was a common practice for many different types of septic systems in past history.

 Many regions throughout Europe still uses this wastewater dispersal solution.  Much of North America has now diverted from this method of wastewater dispersal because of the poor treatment of effluent.

Many water sources have seen high concentrations of nitrogen due to these types of systems still being in function.

Septic Tanks and Expanding Clay Soils

We’ll get into the dynamics of expanding clays soils further down in this article but I wanted to mention some important elements of installing septic tanks in expansive clay.

Septic Tank Problems in Clay

  • One problem with placing a concrete septic tank directly in the expansive clay is the potential of the tank cracking. 

Just like foundations, expanding clay can wreak havoc on septic tanks.

The constant shrinking and swelling over time will cause cracks in the concrete septic tanks over time. 

septic systems in clay soil

Having a wider and longer excavation around the concrete septic tank will allow for enough bedding sand to support the tank. 

This will allow enough of a buffer to relieve the pressure directly on the concrete tank.

  • Placing a polyethene septic tank directly into expansive clays can cause some serious compromises to the integrity of the tank. 

From enormous side pressure of the expansion in the clay can cause plastic septic tanks to buckle or cave in. 

Many of the plastic risers in the septic tanks can also become ovular in shape and the lids can no longer fit correctly.

plastic tank risers

If poly septic tanks are to be used with expansive clays, bedding the tank with the proper amount of sand will certainly relieve the pressure, provided the plastic tank is not deeper than 3 feet.

Some of the science behind expanding clays

Clay soils consist of layers of mineral sheets stacked one on top of another. The structural composition of these layers determines whether they are likely to expand or not. 

Expanding clay soils typically form deep cracks during dry periods when soils shrink. These open fissures allow water to penetrate deep into the soil in the wet season.

The positively charged water molecules are attracted to the negatively charged clay minerals within the soil, allowing water to move in-between the mineral layers, which causes the soil to swell. As the soil swells, the pores between soil particles decreases, restricting water movement (drainage) through the soil.

expanding clay particle

As the soil dries out, water sandwiched between the clay mineral layers is released, resulting in the soil shrinking and cracks appearing on the surface of the soil.

Areas that have a climate with a distinct wet and dry season are likely to experience a greater rate of shrinking and swelling.

Testing for expanding soil

Calculating the coefficient of linear extensibility (COLE), which is the shrink-swell
potential of the soils natural fabric, intact soil core samples are collected in 3 samples and the
volume change of the core must be measured at moist and dry conditions.

soil analysis

An engineering equivalent of COLE uses a dried ground soil sample and measures the
change in length after drying.  This can be done by forming a cylindrical worm when the clay is moist, then oven baking until the sample is dried.

In Summary

If your lot has clay soils, it is advisable to consult with a professional who can collect a soil sample and have it analyzed to determine its mineral and structural characteristics.

This will give you a better idea of its potential to shrink and swell.

Having a clearer understanding of the soil’s potential to expand will help determine the most appropriate type of septic system to install on the site.

Making the correct choice upfront will help prevent the headache associated with system failure and/or expensive repairs further down the line.


Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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