Drywell or Seepage Pit Failure Detection & Causes

How to detect a drywell or seepage pit failure; What causes seepage pit failure? What causes drywell failure; How to avoid seepage pit or drywell early failure? What care is needed for a drywell? What is the failure criteria for a drywell?
GREYWATER SYSTEMS
    DRYWELLS for GRAYWATER
    DRYWELLS as SEEPAGE PITS
    DRYWELL SAFETY CONCERNS
    ARE DRYWELLS DRY?
    PIT SITE REQUIREMENTS
    PIT AREA & SIZE
    PIT CONSTRUCTION DETAILS
    PIT COLLAPSE or FAILURE
Questions & Answers about how to diagnose a failed septic system or drywell or seepage pit.
References

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This document explains the signs and causes of seepage pit or drywell failure and discusses how to avoid early failure or "fill-up" of the seepage pit.

This article series discusses What are Septic Drywells, What Drywell Maintenance is Needed? How do Drywells Fail? We warn readers that while installation of a drywell to receive graywater at a property may be a good idea, the use of drywells for this purpose may be a warning about the capacity and remaining life of the septic system.

Also see DRYWELL DESIGN & USES and Questions & Answers about Drywells. This material is a chapter of our Septic Systems Online Book: This document explains septic system inspection procedures, defects in onsite waste disposal systems, septic tank problems, septic drainfield problems, checklists of system components and things to ask. Septic system maintenance and pumping schedules.

Watch out: Readers trying to diagnose and deal with sudden soil subsidence or yard collapses should see CESSPOOL SAFETY WARNINGS as those hazards can also apply to drywells and septic tanks, and also see SINKHOLES, WARNING SIGNS. Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information in electronic form, in books or pamphlets for sale is reserved to the author. Some technical review by industry experts has been completed-reviewers are listed at References. Further review comments and content suggestions are welcome. Home buyers who want less technical advice should see the Home Buyer's Guide to Septic Systems. Also see The Septic Systems Home Page.

How Seepage Pits or Drywells Work, Fill-up, and Eventually Stop Working

Photograph of dump truck after it drove over and collapsed a seepage pit

Seepage pits, drywells, or cesspools can collapse and thus be dangerous. The photo shows a contractor's truck after it drove over and collapsed a seepage pit being used to receive effluent from a septic tank. This case is discussed later in this section. The general stages of pit life and failure are discussed first.

Stages in the Life of a Drywell

In a perfectly functioning, new seepage pit or drywell, wastewater enters the pit and seeps at first out of the pit bottom into the surrounding soils. The wastewater may be septic effluent if it's coming from a septic tank, or it may be graywater from other building drains if the pit is being used just for graywater disposal.

Regardless of the effluent source, as wastewater enters the seepage pit or drywell, it contains some non-dissolved solid particles. This debris settles to the bottom of the pit. The bottom of a seepage pit, drywell becomes clogged with scum and debris fairly quickly (and in the case of cesspools, very very quickly.

Watch out: falling into a collapsing drywell, cesspool, or seepage it can be a quick death. See SEPTIC & CESSPOOL SAFETY

As the pit bottom becomes sealed with settled debris, effluent entering the pit can no longer drain out immediately through the pit bottom surface. So as the seepage pit is used more and more, the level of liquid in the pit/drywell/cesspool rises. The rising wastewater then seeps out through the pit sides into the surrounding soil.

As the seepage pit ages, the soil around the seepage-out area of the pit bottom and lower sides becomes clogged and stops accepting wastewater. This causes wastewater inside the pit to rise still further, where it can exit the higher sides of the pit which are in contact with soil.

At the end of its life, the soil under the bottom and around the sides of a seepage pit has become clogged all the way up to near the very top of the pit. Then it's time to dig a new one.

Damaged or Collapsing Drywells, Cesspools, or Seepage Pits

When a contractor accidentally damages a seepage pit, such as when the dump truck shown in the photo above collapsed a seepage pit cover, the best repair would be to excavate the pit and repair any damaged components. If only the top were damaged it could be replaced. If the sides of the drywell were damaged on a site-built pit (such as one constructed of large stones or of concrete blocks), it may need to be rebuilt.

If a damaged drywell or seepage pit is also already near the end of its life (static liquid level near the pit top) it would make more sense to fill it in and relocate a new pit.

One correspondent wrote that after the truck (in the photo) collapsed her seepage pit cover, the contractor "repaired" the system by partially filling it in with gravel, saying that this would make no difference or would even improve the system. Now I've seen a few drywells that were filled with large rubble as insurance against having the sides collapse inwards. But I'm not sure I agree with the contractor.

If instead of a rebuild the contractor dumps gravel into the pit s/he may be simply covering up debris which fell into the pit when it was damaged - the broken cover or other components. By filling the pit and reducing its liquid volume capacity, we are forcing effluent to exit the pit higher on the pit's sidewalls and thus moving it more quickly to the end of its life.

So filling in a seepage pit with gravel both reduces its capacity to receive, buffer, and then dispose of effluent (you have less septic system capacity) and probably shortens its remaining life.

See Cesspools for more in-depth information about those systems.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

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.
DRYWELL DESIGN & USES in-depth information about drywells

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

Inspecting Septic Systems: Online Book, Inspection, Test, Diagnosis, Repair, & Maintenance: our Online Septic Book: Septic Testing, Loading & Dye Tests, Septic Tank Pumping, Clearances, details of onsite waste disposal system inspection, testing, repair procedures.

Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389 (2006), Advanced Onsite Wastewater Systems Technologies. Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going for homeowners but is a text we recommend for professionals--DF. I>
Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP; Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP; Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.


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