Conventional Septic Drainfield Design
Specifications / construction details

a. Site Requirements for conventional septic drainfields

(i) The minimum distances that absorption fields shall be separated from other facilities are shown in the sketch below.

[Click to enlarge any image]

See details and tables of septic component separation distances from various site features found

at CLEARANCE DISTANCES, SEPTIC SYSTEM

(ii) A minimum of four feet of usable soil shall exist above bedrock and groundwater with a minimum separation of two feet to the lowest part of any trench.

(iii) Absorption fields shall not be built under driveways, parts of buildings or under above-ground swimming pools or other areas subject to heavy loading. Surface waters shall be diverted from the vicinity of the system.

b. Design Criteria for conventional septic drainfields

(i) The required length of absorption trench is determined from Table 4A based upon the percolation test results and confirmed by the soil evaluation. The maximum trench width for design purposes shall be 24 inches. Only 24 inches shall be allowed for absorption area calculations. W

here trenches exceed 24 inches in width, calculations of absorptive area shall be based on a width of 24 inches.

(ii) Adjacent trenches shall be separated by at least four feet of undisturbed soil. Individual trenches shall be constructed parallel to the ground contours with trench bottoms as near level as possible. They need not be perfectly straight but abrupt changes in direction shall be avoided.

Table 4A of Septic Drainfield Trench Lengths Determined by Soil Percolation Rate and Daily Wastewater Input Flow

Notes to the Septic Drainfield Trench Length Table Above

[1] Original source: New York State NYS75-A.8 Table 4A [image file]

[2] Conditions that require more than 1000 feet of septic drainfield trench must have an alternative dosing system design.

An Alternate Table for Determining Septic Drainfield Size

The following is adapted from our engineer's article summarizing "How Big Should the Septic Leach Field Be" found at

HOW BIG SHOULD THE LEACH FIELD BE?

Determining the required size of a leach field is a bit more complicated. The first thing to consider is the nature of the soil in which the leach field is to be constructed. Because water has to be absorbed in the soil, we need to know how fast it can be absorbed.

This is called the percolation rate and is expressed as the time it takes for water in a test hole to decrease in level by one inch (minutes/inch).

We must also know the type of soil and whether seasonal changes in the natural level of groundwater will interfere with the satisfactory operation of the system. Seasonal groundwater must be more than four feet from the bottom of the leach field trenches.

Judgments regarding the soil conditions and percolation rates are best left to a professional. If the soil percolates very quickly, (less than one minute per inch) or very slowly (greater than 60 minutes per inch) it will not be possible to install a standard leach field in the existing soil.

We must now determine the amount of water that has to be absorbed each day. As with the septic tank sizing, there are also "rules of thumb" that can be used to find out how much water must be absorbed each day for each bedroom in the house (expressed as gallons per day per bedroom).

• For older houses (built before 1979)
  
  we must allow 150 gallons per day (gpd) per bedroom.
• Homes with 3.5 gpf toilets:
  
  For houses where the toilets are limited to no more than 3.5 gallons per flush and the faucets and showerheads are limited to 3 gallons per minute or less, we must allow 130 gpd per bedroom.
• Homes with 1 gpf toilets:
  
  For houses with water-saving toilets that use only one gallon per flush we allow 90 gpd per bedroom.

The required flow rate is found by multiplying the appropriate flow by the number of bedrooms (in this case, we do not have to count a garbage disposal as a bedroom).

Knowing the rate at which water can be absorbed by the soil (the percolation rate) and the flow rate (in gallons per day), we can use the following table to calculate how many square feet of absorption field is needed.

Readers will notice that this table is similar to but less detailed than our typical state or board of health table above at TABLE 4A.

This table specifies the allowable wastewater application rate into the soil of a conventional septic system drainfield as a function of the soil percolation rate for percolation rates between 1 minute per inch to 60 minutes per inch.

Table 4B - Septic Drainfield Soil Application Rates for Non-Standard Wastewater Flow Quantities

This table specifies the allowable wastewater application rate into the soil of a conventional septic system drainfield as a function of the soil percolation rate for percolation rates between 1 minute per inch to 60 minutes per inch.

In other words, the table shows the allowable volume of wastewater in gallons that can be applied per day per square foot of septic field size.

Soils with a percolation rate of less than 1 minute per inch should not be used for a conventional septic drainfield.

Readers will note that this table considers only the dimensions of the bottom of the drainfield trench in considering the effective soil absorption area.

Typically a conventional drainfield trench is 2 ft. wide, so the effective absorption area is simply 2 ft. x field-length in ft.

Beware that in some jurisdictions, when calculating required drainfield trench lengths the authorities may consider the trench width to be a nominal one-foot wide regardless of its actual width.

Notes to the table just above

Original source: New York State Septic Design Code TABLE 4B - APPLICATION RATES [Image File]

• Soil with a percolation rate of less than 1 minute /inch or more than 60 minutes/inch is unsuitable for a conventional septic system design: the wastewater will be absorbed into surrounding soil too rapidly to permit adequate treatment.
• Required Septic Absorption Field Area = Flow Rate (GPD) / Application Rate (GPD / Sq. Ft.)
• Required Septic Absorption Field Length = Required Area (Sq. Ft.) / 2 ft (assumed trench width)
• Note that this table considers only the dimensions of the bottom of the drainfield trench in considering the effective soil absorption area. Typically a conventional drainfield trench is 2 ft. wide, so the effective absorption area is simply 2 ft. x field-length in ft.
• Note that some jurisdictions may set a maximum length of 100 ft. (or another figure) for absorption field trench length.
• Beware that in some jurisdictions, when calculating required drainfield trench lengths the authorities may consider the trench width to be a nominal one-foot wide regardless of its actual width.

c. Materials Used for conventional septic drainfields

(i) Perforated distributor pipe shall be used in the trenches.

(ii) Solid (non-perforated) pipe shall be used between the distribution box and the trenches.

(iii) Perforated pipe shall be made of rigid or corrugated plastic and be labeled as fully meeting ASTM standards for use in septic systems.

(iv) Corrugated plastic pipe delivered in coils is not to be used unless provision is made to prevent the recoiling or movement of the pipe after installation.

(v) Aggregate shall mean washed gravel or crushed stone 3/4 - 1 1/2 inches in diameter. Larger diameter material or finer substances and run-of-bank gravel are unacceptable.

(vi) The aggregate shall be covered with a material that prevents soil from entering the aggregate after backfilling, yet must permit air and moisture to pass through.

(vii) The preferred material for covering the aggregate is a permeable geotextile. Untreated building paper or a four inch layer of hay or straw is acceptable. Polyethylene and treated building paper are relatively impervious and shall not be used.

d. Construction of conventional septic drainfields

(i) Septic drain field trench locations and depths

should be marked by stakes before the trenches are excavated. The natural surface shall not be significantly disturbed.

If the site is re-graded or similarly disturbed, the soil shall be allowed to stabilize and new percolation tests conducted.

(ii) Septic drainfield trench depth shall be as shallow as possible,

but not less than 18 inches. At least six inches of aggregate is placed below the distribution line and two inches above the line. The earth cover over the aggregate should not exceed 12 inches in order to enhance natural aeration and nitrogen uptake by plant life.

(iii) Septic drainfield or soakaway trenches shall be excavated

to design depth with bottoms practically level.

(iv) Heavy equipment

shall be kept away from the field because the weight may permanently alter soil characteristics due to compaction, cause trench cave-ins, and/or mis-align and break pipe.

(v) Septic drainfield or soakbed trench bottoms

are to be raked and immediately covered with at least six inches of aggregate.

(vi) Septic drainfield / leachfield trench walls:

Any smeared surfaces on the trench walls are to be raked.

(vii) Septic drainfield effluent distribution lines

are carefully placed on the aggregate and covered with aggregate to a depth of at least two inches over the top of the pipe. Additional aggregate may be required to bring the top of the aggregate to within six to 12 inches of the surface.

(viii) In gravity distribution septic systems, the pipe shall be carefully sloped

at between 1/16 inch and 1/32 inch per foot. Grades shall be determined by an engineer's level, transit or carpenter's level.

(ix) Septic drainfield / soakbed trench cover:

After the upper aggregate is placed, the geotextile, untreated building paper, hay or straw is to be immediately installed and the trench backfilled with native soil.

If the drainfield trenches cannot be immediately backfilled, they should be temporarily covered with an impervious material such as treated building paper to prevent sidewall collapse and siltation into the aggregate.

(x) Septic drainfield / soakaway trench backfill:

The earth backfill is to be mounded slightly above the original ground level to allow for settling and after settlement the entire area should be graded without the use of heavy equipment and seeded with grass.

[See our warnings at CONSTRUCTION OF SHALLOW SEPTIC SYSTEM ABSORPTION TRENCHES]