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• Septic seepage pit design manual

More Information on Building Diagnostic Inspections and Repairs

(2) Site Requirements for septic seepage pits

(i) If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used.

(ii) A minimum three foot vertical separation must exist between the bottom of any pit and the high groundwater level, bedrock, or other impervious layer.

(3) Design Criteria for seepage pits used to dispose of septic effluent

(i) The required "effective seepage pit area" is obtained from Tables 6 and 7 which are shown below.

(ii) No allowance for infiltration area is made for the bottom area of a pit or the surface area of impervious soil layers (percolation rate slower than 60 minutes/inch).

(iii) The effective diameter of a pit includes the diameter of the lining plus the added diameter provided by the annular ring of aggregate. Any area surrounding the liner with rock smaller than 2 1/2 inches in size shall not be included as part of the effective diameter.

(iv) Effective depth is measured from the invert of the seepage pit inlet to the floor of the pit, with the thickness of impervious layers deducted.

Site Distance Requirements for Seepage Pits

This sketch (from New York's Wastewater Regulations) shows the recommended site clearances between a seepage pit (or cesspool or drywell) and other building and site features. A more detailed list of site clerances to septic system components is listed at "More Reading" just below. Notice that seepage pits not only need to be at a sufficient distance from the building, from wells, from property lines, but also if multiple seepage pits are installed, they should be adequately separated from one another.

More Reading

• Clearances for Septic Components Required - Well distance to Septic, Septic clearance from well, trees, lakes, streams, property boundary, etc.
• 75-A.0 New York State Regulation of Private Septic Systems

Absorption Area Requirements for Seepage Pits

The table below shows the required soil absorption area for seepage pits as a function of soil percolation rate and anticipated daily wastewater flow in gallons.

TABLE 6 - SEEPAGE PITS - REQUIRED ABSORPTIVE AREA (IN SQUARE FEET) FOR HOUSEHOLD SYSTEMS

Seepage Pit Size Requirements to Obtain Necessary Absorption Area

The table below shows the size of seepage pit necessary in order to provide the required soil absorption area for seepage pits. First use the preceding table to determine the necessary seepage pit absorption area, then use this table (below) to determine the necessary seepage pit size to provide that absorption area.

Seepage Pit Construction Details: Linings, Separation, Strength, Connections

(v) Seepage pit linings may be precast concrete, cast-in-place concrete, or built in place with unmortared hollow cinder or concrete blocks. Concrete shall have a minimum compressive strength of 2,500 psi and 3,000 psi is recommended. Material with comparable structural strength, determined in accordance with commonly accepted sewage construction standards, principles or practices, may be allowed on an individual basis to prevent unreasonable hardship, provided public health is not prejudiced.

(vi) The separation between the outside edges of seepage pits shall be three times the effective diameter of the largest pit. This separation is measured as the undisturbed soil between pit excavations.

(vii) Pits shall be designed with sufficient structural stability to withstand lateral soil forces as well as vertical loads.

(4) Construction of seepage pits for septic effluent disposal

(i) Laterals leading to each seepage pit must be at least four inches in diameter with a minimum slope of 1/8 inch per foot.

(ii) Seepage pits shall not be connected in series. A distribution box shall be required where more than one seepage pit is installed.

(iii) The pit excavation is to be raked to minimize sidewall smearing that may occur and reduce infiltration capacity. If groundwater is encountered, the pit shall be backfilled with the original soil to a level at least three feet higher than maximum groundwater and adjustments made in the pit dimensions.

(iv) The linings are placed upon a concrete block, poured concrete, or precast footing and surrounded by a six inch minimum annular ring of large aggregate (2 1/12 - 4 inches in size).

(v) The rock is covered to prevent soil from filling the void spaces. Building paper, a four inch thick layer of hay or straw may be used.

(vi) The seepage pit cover shall be structurally sound and capable of supporting 300 pounds per square foot at the weakest point. Covers may be precast concrete or cast-in-place and shall be reinforced. A manhole with an opening of at least 20 inches in the shortest dimension shall be provided.

Seepage Pit Collapse

A seepage pit, drywell, or cesspool can be collapsed as shown in this photo, where a seepage pit receiving septic effluent was driven over by a heavy dump truck. Such collapses are dangerous and can be fatal if someone falls into the collapsing opening. In general, while there are heavy duty septic system components and covers for pits and tanks, you should not permit traffic to drive over septic equipment nor on septic fields.

More Reading:

Drywells (our main page on this topic) for more in-depth information about seepage pit and drywell systems.
SEPTIC SYSTEM DESIGN BASICS
How Big Should the Leach Field Be? includes a practical example using sample calculations and a table of soil percolation rate vs. field size
Design Basics for Septic Systems: Choosing Septic Tank Size, Leach Field Size - basic septic system volume and absorption system design guides.
Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
Sketches of the Septic System Components Private Sewage Disposal Systems - Septic Drawing Library
Cesspools for more in-depth information about those systems.
Drywells for more in-depth information about those systems.]
SEPTIC SYSTEM DESIGN ALTERNATIVES

The Septic Systems Online Book - Where Are We?

More expert information on this topic

More Information on Building Diagnostic Inspections and Repairs

• DRYWELLS for onsite wastewater disposal our more complete discussion of seepage pits, drywells, leaching pits
• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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