The impacts of soil consistence on septic system installation

September 16, 2021

By Sara Heger, Ph.D.

Consistence describes how easy it is to deform or break up the soil. Consistence is used to determine if the soil contains clay that swells greatly when wet or shrinks greatly when dry (also known as shrink-swell clay). If enough shrink-swell clay is present in the soil, the soil pores swell shut when the soil gets wet. If the pores are closed, the amount of wastewater the soil can accept is near zero. If the water cannot be accepted, it also cannot be properly treated. Installers can also use this information to estimate how susceptible the soil is to smearing or puddling. 

Consistence is determined by two conditions: wet and moist. Moist consistence is measured in terms of how easy it is to break a ped when the soil is moist. If it is difficult to break a ped when squeezed between thumb and forefinger, the soil is considered to be very firm. This corresponds to the soil having enough shrink-swell clay to cause problems with the acceptance of wastewater. 

Wet soil consistence is described in two ways: stickiness and plasticity. When the soil is so sticky that it stretches between thumb and forefinger as one tries to pull them apart (it feels as if the fingers are glued together), then the soil likely contains a high amount of shrink-swell clay. This is referred to as very sticky. When the soil can be rolled in a wire about the size of a regular pencil lead (2 millimeters) and not break when picked up, the soil likely contains a high amount of shrink-swell clay. In many areas, a soil horizon that is very firm, very sticky, or very plastic is considered unsuitable for wastewater treatment and dispersal. 

From an excavation standpoint, if a soil has a moist consistence that is loose or friable, it is unlikely that the trench will be smeared even when soil moisture is high. This means that water movement will not be impeded after installation is complete. However, friable soils may be a safety hazard for personnel because they may collapse under the weight of excavation equipment. Soil collapse can also occur in soil that may be smeared or compacted during installation or that may become fluid and create dangerous working conditions. Certainly, the installer should be aware of how the soil feels in the bucket and how it falls out of the bucket. Very sticky or very plastic soils feel harder or tougher and do not break up when they fall out of the bucket. On the other hand, a loose or friable soil breaks up easily when dumped out of the bucket. In the course of excavation, the degree of these characteristics may change across the landscape.

When the term plasticity is used relative to soil consistence, it refers only to the size of the wire that can be rolled and is related to the clay mineralogy. Soil consistence is a property of the soil that will not change from day to day due to changes in the soil moisture. The plastic limit and penetration resistance are terms that the installer must know relative to excavation safety. These terms are different from consistence terms. Plastic limit and penetration resistance figure prominently in compliance with OSHA standards for maintaining safe conditions. 

Determinations of plasticity and penetration resistance are two of the manual tests for determining OSHA soil class relative to excavation safety. These tests must be performed in the field at the time of installation as they may change from day to day based on soil moisture content. Therefore, the installer must follow OSHA safety procedures on the day of excavation. The results of these tests in the field determine the nature of trench excavations and relative need for sidewall sloping and other protective measures.