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• Design manual for gravelless or no rock septic fields
• No-rock or chamber product descriptions, sources
• Septic field product cost comparisons
• Septic field capacity & life comparisons
• Septic Systems - Design, Inspection, Testing, & Maintenance

This chapter discusses determination of a gravelless or "no gravel" or "no rock" septic drainfield systems, offering installation specifications. Drainfields, also called leach fields, absorption beds, soil absorption systems, and leaching beds, perform the functions of septic effluent treatment and disposal in onsite wastewater treatment systems, conventionally called "septic systems".

Gravelless septic systems or "no gravel" septic system trenches use plastic or other prefabricated wastewater distribution systems which are buried in soil without the use of surrounding gravel. Typical gravelless septic systems use a plastic chamber, a geotextile-wrapped pipe, or a polystyrene-wrapped pipe to distribute effluent into the soil. The necessary soil absorption area is provided by the perforated surface of the gravelless septic system components (or by soil at the bottom of a chamber) themselves rather than by the gravel and trench walls of a conventional septic drainfield. These systems can provide an acceptable effluent disposal system for sites with limited space for a drainfield or where gravel is not available or is quite expensive.

Advanced septic treatment methods are discussed in separate chapters. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Gravelless Systems - Gravelless Septic Absorption Systems

There are three typical gravel-less effluent disposal systems in current use and shown in the US EPA sketches below (originally from NSFC) and I describe a fourth variation which is provided by some manufacturers:

Use of Polystyrene-wrapped perforated pipe for Septic Effluent Treatment/Disposal Fields

1. Polystyrene-wrapped perforated pipe buried in an earthen trench. I have been unable to find websites listing this product. Companies offering this product are invited to add your listing here - no fees are involved: manufacturers of alternative effluent disposal systems are welcome to list their product lines and websites here at no fee.

Use of Geotextile-Wrapped Perforated Pipe for Onsite Wastewater Treatment and Disposal

2. Geotextile-wrapped perforated pipe buried in an earthen trench. In this type of system, a large diameter corrugated plastic pipe (eight inches or greater in diameter) surrounded by a hydrophilic geotextile is installed in an excavated trench using only the original soil as backfill. Other systems utilizing products such as galleys, flow diffusers or leaching chambers can be installed without aggregate backfill. One linear foot of these products shall be equivalent to one linear foot of conventional (24 inch wide) absorption trench.(1) For an example of this geotextile-wrapped pipe approach to wastewater disposal using 8" or 10" diameter piping, see Crumpler Plastic Pipe's "No-Rock TM Septic-Leachate drainpipe systems" listed at Product Sources below.

CPP informs us that because the square foot equivalent leaching area when this sytsem is used is not a foot-for-foot ratio based on a 36" trench, the 8" & 10" CPP Filter Wrapped systems are typically longer than the traditional septic drainfield trenches. Although the individual line length may be longer, the CPP system may actually require less total square footage of lot space if the on-center line spacing is controlled by the excavated trench width treatment plume & not an "arbitrary CODE stipulation".

According to CPP, this added length achieves superior distribution to the trench sidewall where the bulk of the septic liquid enters the soil for treatment. [One reason for wider trench on-center spacing advanced by some health departments is to assure that the site affords space for future trench line replacement.--DF].

Most sanitary or health department codes size 8" diameter Geotextile-Wrapped Perforated Pipe to equal a 2 Foot wide conventional septic drainfield trench, and similarly size 10" diameter Geotextile-Wrapped Perforated Pipe to be equal to a 2.5 foot wide conventional drainfield trench.

In determining how much leach field area shall be provided when using Geotextile-Wrapped Perforated Pipe, most state codes (where they have them) will require that the cubic footage that would have been required if a conventional 3-foot wide conventional perforated pipe & gravel leach field system had been installed will be used to determine the size 2 Foot or 2.5 Foot wide gravelless systems as well. Thus if 100 feet of a conventional gravel system in 3 Foot wide trenches is required for one bedroom (or 300 Cubic Ft), then 150 feet of 8" Geotextile-Wrapped Perforated Pipe will be required or 120 feet of 10" diameter Geotextile-Wrapped Perforated Pipe. Sketch of geotextile-wrapped pipe in plan view, courtesy of Crumpler Platic Pipe - CPP - links below.

For OC (On-Center) spacing of parallel lines treating sewage from a 2 bedroom home, a conventional gravel system of 100 foot length would require 1200 square feet of lot surface area if 3 times the actual 'excavated' trench width is used for OC seperation. In comparison, for an 8" Geotextile-Wrapped Perforated Pipe system, which can be placed in a 1.5 Ft(18") wide trench, 1,125 square feet of lot surface area would be required. For a 10" diameter Geotextile-Wrapped Perforated Pipe, which can be placed in a 1.5 Ft (18") wide trench, 900 square feet of lot space is needed.

If a different or as CPP puts it "arbitrary" OC trench width spacing is mandated (ie: 5 ft or 7 Ft OC required regardless of 'excavated' trench width) by a state code, then the lot surface area calculations will of course be different. [Thanks again to Crumpler Plastic Pipe, Inc. for this added detail. Links to this company's website and products are provided below.]

What all this means is that for a given building usage and set of soil conditions, whether or not the Geotextile-Wrapped Perforated Pipe system is going to provide the necessary effluent handling capacity in a smaller total square feet than a conventional drain field would require depends on the on-center spacing requirements set by the local authorities.

Use of Chamber Systems for Wastewater Treatment and Disposal

3. Chamber systems, using pre-formed plastic chambers buried in an earthen trench. Wastewater is delivered to the top of the chamber by a solid plastic pipe. Effluent seeps into soil across the bottom of the chamber. The empty chamber also provides effluent storage volume to handle peak wastewater flow loads. Chambers are available in widths varying from 16" across (at the bottom) to 34" wide and are placed in trenches ranging in width from 18" to 36". Synonyms for this design include "leaching chambers", "galleys", and "flow diffusers". Illustration courtesy of Infiltrator Systems, Inc., used with permission. (Infiltrator Systems Inc. contact information is listed below).

Use of Vertical Thin Wall Chambers for Wastewater Treatment and Disposal

4. Vertical thin wall chambers: A fourth variation on gravelless effluent disposal systems is similar to the chamber system (#3 above), but uses pre-formed but thin plastic "chambers" which are inserted into a narrow slot cut into the soil; these systems diffuse effluent primarily through their sides into the soil.

Site Requirements & Design Criteria for Gravelless Septic Absorption Systems

Site Requirements: These systems shall be used on sites that have been classified as having a design percolation rate of one to 45 minutes per inch, and meet the vertical and horizontal separation distances in Table 2 (of the New York State Standard - for example) shown in 75-A.4 Soil and site appraisal for Septic Systems.(2)

Chamber systems provided by Infiltrator Inc. (links below) have been approved for use in some states, including New York, at perc rates up to 60 minutes/inch, and in some other states, at percs up to 120 minutes/inch. Approval letters are available from that company.

Design criteria: The local health department having jurisdiction shall be contacted prior to construction regarding the acceptability of specific products for use as a gravelless distribution system. (3)

As we described above when discussing Geotextile-Wrapped Perforated Pipe systems for onsite wastewater treatment and disposal, the total lot area in square feet which is required for a given alternative treatment system will vary not only depending on the anticipated daily flow, flow rates, soil and site conditions, but also on what spacing requirements the local or state code officials will require.

Construction of gravelless septic absorption systems

Gravelless distribution systems shall be installed in conformance with the manufacturer's instructions because of the proprietary design of some products.(4) The length, width, and depth of the required excavation will vary by individual product.

The essential difference between a gravelless wastewater treatment system and a conventional perforated-pipe and gravel-trench drainfield is the use of a special product to distribute and release effluent (sketches above) and the absence of gravel in the trench where the distribution system is buried.

The special gravelless products used are designed to avoid soil-clogging right at their surface or they would have a short life. This concern is addressed by use of special materials (such as geotextiles) which resist soil clogging, and by a design that provides an adequate total area in square feet for soil contact.

Each product manufacturer should be expected to provide an installation manual for the product that you are going to use. The list below is generic and incomplete as site specific and product specific details can't be provided here.

• Perform a site survey to determine the size, layout, and materials needed. The manufacturer indicate the disposal area provided by the product (per linear foot, for example) but your engineer and installer need to evaluate your soils and daily wastewater usage to determine the amount of leaching area needed for your site. (Tables of field sizing and soil percolation test procedures are provided at this website).
• Obtain local building code or board of health requirements
• As with any wastewater disposal field, never drive equipment in or over the trenches. Compacting the soil (or after installation, damaging the drainage components) will reduce the absorption system capacity or may destroy it completely. Some systems such as Infiltrator Inc.'s chambers, have been tested to meet an H-10 engineering rating (16,000 lbs / axle), so occassionally if a system built using some chamber products are driven over it won't necessarily be an issue. But Infiltrator Inc.'s technical experts do not recommend constant vehicle traffic over anything but their heavier duty H-20 chambers. These disclaimers notwithstanding, it remains a better practice to keep all traffic off of septic fields. And even where septic equipment has been described as rated for vehicle traffic, heavy equipment traffic can cause some surprising collapses that can be costly to repair as shown in this photo.
• Some sites may need a curtain drain or other special measures to redirect surface or subsurface runoff away from the effluent/wastewater absorption area.

Life Expectancy of Gravelless Septic Systems

To date we have not received study data comparing the life expectancy of a conventional gravel trench drain field with that of a gravelless drainfield in equivalent soils and usage rates. In either type of absorption system ultimately the biomat and soils around the absorption system trench become clogged, the rate of clogging depending on usage, soil characteristics, maintenance of the septic tank (frequent pumping extends drain field life), and what is put into the wastewater (salts, grease, other products reduce drainfield life).

Ultimately the capacity of the absorption system is a function of the area of soil being used to dispose of effluent around the effluent distribution system (trench width and depth, and surface area exposed to effluent).

OPINION: I would expect that for two alternative below-ground effluent disposal system designs such as a gravel trench or a chamber system, if installed in the same soils and at identical original capacity, there will be no difference in the length of the service life of the installation.

There may be differing costs, however, in subsequent system repair or replacement. For example, does the failed system need to be excavated and removed in one case but not another?

Installation Cost for Gravelless Septic Systems

The primary waste line, septic tank, and distribution piping costs are unaffected by the use of these septic effluent absorption system alternatives, except that some of the choices such as the chamber system are asserted (by Infiltrator Systems Inc.) to use fewer linear feet of material in the absorption field (about half) - a factor that must be taken into consideration in pricing the installation cost.

The trenches may need to be wider or in some instances deeper, however - which will affect the excavation cost. Some writers suggest that even if a manufacturer claims that you can use less trench length with a no-gravel system you should instead opt for the larger capacity (and probably longer life) provided by not skimping on total trench lengths. However, for a small site with limited space for an absorption field, the added capacity of a gravelless system may be a solution.

Consumers expecting to save in the installation cost of their drainfield by using a no-gravel or "gravelless" septic effluent disposal system should compare all of the costs involved. The savings in gravel and possibly excavation cost (shorter trenches) needs to be weighed against the added cost for the special products (described above) to distribute the septic effluent to the soil.

OPINION: I expect that in all cases the excavation cost is going to dominate the absorption system total cost. It is possible that option 4 described above will have the smallest excavation cost because it can be installed in a very narrow trench, perhaps cut with a smaller specialized machine such as is used for running buried wires and pipes.

We are polling manufacturers for installation costs for gravel-less drainfield alternatives and will post that data here as it becomes available.

Keep in mind that cost comparisons among onsite wastewater handling systems can be specious. It makes little sense to compare the costs of two different systems when only one of them would actually work at a given site. Therefore I advise that you first determine what your site requires because of its soil, space, and so on. Then you can compare alternatives that would work on that site.

Sources of Gravelless Effluent Disposal Systems

This listing of suppliers of gravelless or "no rock" septic system components is arranged alphabetically. We and our website have no financial or other relationship with any suppliers listed at our website and we do not endorse specific products. The illustration at left shows a typical residential gravelless wastewater disposal system using chambers. Illustration courtesy of Infiltrator Systems, Inc., used with permission. (Infiltrator Systems Inc. contact information is listed below).

• Crumpler Plastic Pipe, Inc. provides fabric-wrapped drainage piping 800-334-5071 Roseboro NC USA - "No-Rock TM Septic-Leachate drainpipe systems" are available in 8" and 10" systems.
• In-Drain from Elgen is a combination of plastic and geotextile used to treat septic effluent. Elgen products have been around for decades using variations on this design. Elsewhere I've recommended their drainage mat systems for foundation waterproofing and other site drainage problems.
• Hancor Gravelless "Blue Stripe" Pipe 888-FOR-PIPE 8" and 10" diameter. Quoting the manufacturer, "When backfilled with native soil, Gravelless pipe provides safe and effective septic tank effluent treatment in sites determined to be suitable by your local health department. The performance of gravelless systems has been verified through independent research performed at the University of Minnesota. This research evaluated the long-term acceptance rate of fabric-covered corrugated pipe for eight different soil types, and the pipe performed well in all soils tested except fine sand. Gravel-filled trenches in fine sand would likely perform in a similar manner."
• Infiltrator Systems, Inc. provides chamber products 800-718-2754 Old Saybrook CT USA. The manufacturer provides state specific design manuals for no-rock chamber systems for specific states in the U.S.
• Add your listing here - no fees involved: manufacturers of alternative effluent disposal systems are welcome to list their product lines and websites here at no fee. Informational listings are welcome, not sales pitches.

Deep Septic Absorption Trench Systems

If you scrolled down in this document to look for information on Deep Septic Absorption Trench System construction, that subchapter is published at Deep Trench Systems. Please continue reading by using this link, or use the web-links at page-left to navigate the online septic systems book or our other online documents.

More Reading:

More Information on Building Diagnostic Inspections and Repairs

• 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 Septic & Well 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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