What Is a Septic Percolation Rate and What Are They Used For?

The Importance of Percolation Rates

Anyone who is even remotely familiar with the design, installation or repair of septic systems has probably heard of the term percolation rate, or the shorter version, perc rate mentioned during a discussion. As you will read in the following article, soil percolation and perc tests are very important and significant in the area of septic system design and function.

Even new homeowners who reside in an area that require on-site wastewater systems have come across that term. Just because it's almost 2018 doesn't automatically mean there is a municipal sanitary sewer system connection available right around the corner from your new building lot or sub-division.The speed at which water or liquids flow through a particular soil or soils, in known as the percolation rate.

Perc rates are also directly affected by the particle size of the subject soil. Particle size is very important when it comes to achieving an optimal perc rate for a new gravity or low pressure distributed septic system.

Clay type soils are comprised of smaller particles and therefore have more surface area.
On the other hand, sandy soils have characteristics quite different from hard pan when it comes to soil permeability.

The actual particle size of a sandy type soil is much larger than that of clay, but the surface area of the sand particle is very small. Percolation rates are important in determining the rate at which water passes through your soil sample. This rate is called the percolation rate and is important in farming, gardening, civil engineering, initial septic system design, and leach field construction.

These physical characteristics of the particles although quite different, can ultimately mean the difference between a septic system and leaching fields that will possibly fail in a short period of time, or drain fields that will last for 30 years or more.

Does Soil Particle Size Really Have That Much Of an Influence on Percolation Rates?

By understanding a little about the soil's physical properties and its relationship to soil moisture, you can make better soil-management decisions. Soil texture and structure greatly influence water infiltration, permeability, and water-holding capacity.

Soil texture refers to the composition of the soil in terms of the proportion of small, medium, and large particles (clay, silt, and sand, respectively) in a specific soil mass. For example, a coarse soil is a sand or loamy sand, a medium soil is a loam, silt loam, or silt, and a fine soil is a sandy clay, silty clay, or clay.

Soil structure refers to the arrangement of soil particles (sand, silt, and clay) into stable units called aggregates, which give soil its structure. Aggregates can be loose and friable, or they can form distinct, uniform patterns. For example, granular structure is loose and friable, blocky structure is six-sided and can have angled or rounded sides, and plate-like structure is layered and may indicate compaction problems. Polymer soil stabilization is conducive with the previous statement about particle size and function in testing for overall perc rate.

Soil porosity refers to the space between soil particles, which consists of various amounts of water and air. Porosity depends on both soil texture and structure. For example, a fine soil has smaller but more numerous pores than a coarse soil. A coarse soil like sand, has larger particles than a fine soil, but it has less porosity, or overall pore space. Water can be held tighter in small pores than in large ones, so fine soils can hold more water than coarse soils. Soil and water relationships are an important aspect of septic design as well as identifying soil horizons out in the field.

Soil Structure from Wetting and Drying Cycles

The structure of a soil is a description of the shapes soil assumes in different parts of its profile over time. Structure is formed largely by cycles of wetting/drying and freezing/thawing, the soil’s chemical composition, and the aggregating effect of some soil microbes.

Well-structured soils have large amounts of interconnected pores that accelerate water and air movement. Weakly structured soils have less continuous pore space, which slows water and air movement. Structure is ranked on a scale of 0 to 3 (structureless, weak, moderate, strong). Structure size is also determined, and classified as fine (f), medium (m), and coarse (c). Blocky, prismatic, granular, and single grain structures are generally favorable for septic systems in soils with low to moderate clay contents. Soils with a weak structured particle arrangement are not very structurally friendly in the profession of civil engineering.

Texture of Septic Test Soils

Texture is a group of terms that describe the amount of sand, silt and clay present in soils. These terms include sand (s), loamy sand (ls), sandy loam (sl), sandy clay loam (scl), loam (l), clay loam (cl), silt loam (sil), silty clay loam (sicl), silty clay (sic), and clay (c). In general, as silt and clay content increases in a soil, the permeability decreases.

Silt loam and silty clay loam textures are very common in Illinois, having formed in loess parent material. When clay content in soils exceeds 35% (heavy cl, heavy sicl, sic, or c textures), the soils are generally poorly suited for conventional septic systems because of slow permeability.

Consistence Measurements

Consistence is a measure of how easily soil can be crushed between the thumb and forefinger. Classes of consistence include very friable (vfr), friable (fr), firm (fi), very firm (vfi), and extremely firm (xfi). In general, as soil consistence increases in firmness, permeability decreases due to a diminishing volume of pore space within the soil.

Soil Drainage Class

Drainage class describes the relative wetness of a soil prior to modification by perforated drain pipe or other means. This designation is not precisely defined, but is broken into seven classes: very poorly, poorly, somewhat poorly, moderately well, well, somewhat excessively, and excessively.

The main factors considered when determining a certain drainage class are soil color patterns, texture, and landscape position. Bright soil colors combined with high and/or sloping landscape position generally indicate a drainage class of moderately well or better.

Now to the final question posed in the original title of this blog post!

Why Are Perc Tests Needed for in the Design and Installation of Septic and On-Site Waste Water Systems?

Traditional septic systems only work if the soil in the leach area is sufficiently permeable that it can readily absorb the liquid effluent flowing into it.

Also, there must  be at least a few feet of good soil from the bottom of the leach pipes to the rock or impervious hardpan below, or to the water table. Less commonly, a site can fail because the soil is too permeable, allowing the effluent to reach the groundwater before it is fully treated. Very steep slopes are also unsuitable for a conventional leach field and will often lead to a failed percolation test forcing you to seek other options to deal with the failure.

The specific standards vary from town to town, but any of these characteristics can prohibit the use of a standard gravity-fed septic system.

In some cases, a more expensive alternative septic system may be allowed. To determine is a building site is suitable for a septic system, a percolation test (typically called a “perc test’ or “perk test”) is required.

No Percolation Test Results, Failed Perc Test, No New Home!

In rural sites without municipal sewage systems, a failed perc test means that no dwelling can be built  — which is why you should make any offer to purchase land contingent on the site passing the soil and perc tests.

As prime building sites become increasingly scarce (or prohibitively expensive) in many parts of the country, rural sites that will not pass a percolation or perc test are increasingly common.

In general, soils with high sand and gravel content drain the best and soils with a high clay content or solid rock are the worst. Most soils fall somewhere in the middle with a mix of course sand and gravel particles, small silt particles, and tiny clay particles – the smallest.

To get a rough idea before investing time and money in testing, dig below the top few inches of topsoil (loam) to the lighter soil beneath. If you can take a handful of the damp subsoil and roll it into a thin, flat shape or worm shape that holds together, and it had a sticky firm texture, the soil has a high clay content and will probably fail a standard perc test.

The two main tests used to determine a site’s suitability are a soil evaluation and percolation or perc test. Testing requirements vary greatly from state to state and often from town to town, as most states allow individual towns to establish separate rules within state guidelines.

So make sure you talk to your town health officer about what tests are needed, when they can be done, and who should perform them. Whether or not a licensed professional is required, it’s a good idea to hire an seasoned expert with local experience as many of these tests have a bit of wiggle room. In the state of Connecticut for example, a (B100A) soil test is required for prior to any subsurface sewage system can be installed on a commercial or residential building lot.

Daniel Sexton III,  and everyone at Septic Tank Pumping Pros have done our best trying to explain the importance and relationship between percolation testing and the composition of virgin ground soils located on the land being considered for a residential onsite sewage or engineered septic system.

Please comment below or contact us for more educational septic pumping blog posts and related information on septic system design, repair, installation or septic tank pumping and cleaning.

Thank you for reading this latest article related to on-site wastewater percolation testing, sewage treatment and tank storage.

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