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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.
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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.
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