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Septic system collapse and truck rescue (C) Daniel FriedmanDo not drive, pave nor park over Septic System Components

  • DRIVING or PARKING OVER SEPTIC - CONTENTS: Is it ok to have septic piping or a septic tank under a driveway or parking area? Can we drive over septic piping or tanks?Can we drive over the drainfield? Can we build, pave, or put astroturf over a septic drainfield?
  • POST a QUESTION or READ FAQs about drainfield damage due to vehicle or equipment traffic
  • REFERENCES
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Is it ever ok to drive-over or pave over a septic system, tank, or absorption bed?

This article explains the problems that occur if septic components are located under a drive or parking area. We explain why you should not drive cars, construction equipment, or other heavy machinery over the septic drainfield and in some cases also not over the septic tank.

Our page to photo shows a big effort underway to pull a dump truck out of a seepage pit. Luckily no one was injured but the cost of extracting the truck and repairing the septic system was significant.



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Do not drive over the septic tank, septic piping, or septic drainfield.

PHOTO of a septic seepage pit collapse when a truck drove over it.Unless special provisions have been made such as protection of sewer piping and septic tanks from damage, vehicle-rated septic tank covers, or similar steps, do not drive vehicles over septic system piping or septic tanks.

Driving over septic tanks, septic piping, or drainfields risks costly damage to the septic system and may also be dangerous.

The bulldozer in our photo (left) was called to help remove a truck which drove over septic system components leading to a surprise collapse.

A property owner may not immediately recognize a septic system problem when piping has been run below a driveway, as crushing and blockage of the line may not happen until a heavy vehicle enters the driveway (such as the loaded septic pumping truck arriving to pump the septic tank). Or a septic line may be broken, permitting soil or roots to enter to complete the clogging process.

Watch out: as we report at SEPTIC TANK ACCIDENT REPORTS, driving over and damaging a septic tank cover or lid can result in a later cave-in, fall-in, or extremely serious hazards. While it is possible to purchase septic tank covers that are rated for withstanding heavy loads, even vehicles, as-installed residential septic tank covers are not normally capable of withstanding such traffic.

Also see SEPTIC & CESSPOOL SAFETY.

How do we run a sewer line under a driveway to get to the septic tank?

Photograph of new septic line house to tank

If a septic line must be run under a driveway, for example to pass from a building to the septic tank, the line must be protected by choice of materials (schedule 40), or placed in a covered and protected trench at adequate depth (such as with concrete covers over the trench) to avoid damage to the piping.

Our photo (left) shows a new sewer line being installed down a hillside, connecting a house to its septic tank.

The original terra-cotta sewer line lasted for decades until the building owner (DF) hired a landscaping company to mow the lawn. The fourth time that a heavy lawn mower drove over the original sewer line it was crushed and broken, leading to a costly sewer line replacement job.

The new sewer line was bedded in sand and protected from damage.

If a sewer line is run below a drive or parking area without proper choice of materials, protective measures, etc. it is an improper installation likely to fail.

Also see CLOGGED DRAIN DIAGNOSIS & REPAIR.

Can a septic tank be located safely below a drive or parking area?

Photograph of  rusted out and dangerous steel septic tankIf a septic tank is to be located below a drive or parking area, it too must be properly designed and protected from collapse.

The tank must be constructed of proper materials and provided with a cover rated to withstand the weight of heavy vehicles.

If the septic tank is steel, site-built, home-made, or even pre-cast concrete but lacking a cover rated to withstand vehicle traffic, driving over the tank risks collapse and even a potential fatality.

Our photo (left) shows a rusted-through collapsing steel septic tank cover that nearly led to a fatality to a home inspector.

See SEPTIC TANK DESIGN STRENGTH SPECS for details about the strength requirements for safe septic tanks, cesspools and drywells.

 

Home made or "site built" septic tanks often have a cover made of wood or other flimsy materials, and depending on the tank construction (dry-laid concrete blocks or stones) the sides may also be likely to collapse if exposed to the weight of even a small vehicle.

The site-built septic tank shown in our photo below was collapsing as well as impacted with solids.

Photograph of a collapsing, impacted, failed home made septic tank.

Driving even a lawn mower over this tank was likely to lead to a catastrophe.

Can a septic drainfield be located below a parking area, pavement, driveways, patios, decks, or other structures?

A septic drainfield should not ever be located below a driveway or parking area.

Driving or parking on a drainfield will prevent proper drainfield operation due to soil compaction and also due to loss of proper evaporation of moisture through the surface, as well as almost certainly leading to crushed broken piping. In sum, driving over the leach field in any vehicle larger than a child's bicycle is a bad idea.

Heavy vehicles may actually crush buried leach field lines, or they may compress the soils around the leach field, either of which leads to failure. Driving on or parking on leach fields will destroy them.

Paving over a drainfield, or installing patio stones or astroturf or any other material that blocks proper soil transpiration interrupts the evaporation of moisture from the drainfield, interfering with its ability to dispose of effluent. Furthermore covering a drainfield may result in inadequate soil oxygen, thus inadequate bacterial action, thus inadequate treatment of septic effluent, thus leading to ground water and possibly local well contamination.

LARGER VIEW of this
costly surprise caused by building a swimming pool over the drainfieldBuilding any structure over a drainfield risks damage to the drainfield from:

Our photo shows what happened when a swimming pool was constructed over the edge of a septic drainfield in Poughkeepsie, NY. The gray water shown leaked from the drainfield onto the pool perimeter when the homeowners did their laundry.

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 reviewers are welcome and are listed at "References."

This is a chapter of SEPTIC SYSTEM INSPECTION & MAINTENANCE COURSE an online book on septic systems.

Generalizing the Reasons to Keep Equipment, Grazing Animals, Paving, Coverings and What Have-You Off of Septic Drainfields

NiceRink ice skating rink (C) Nicerink 2013 used with permissionReader Question: I would like to put a skating rink in my yard over the septic field (mid-Dec. to mid-March, i.e. northern Illinois) because it is the flattest part of my yard. I read everywhere on the internet and your site that one shouldn't put "anything" on top of a drainage field, at least the channels where the pipes ("tiles") are buried.

Photo at left: a NiceRink™ ice skating rink installed in a level area, photo used with permission. [Click to enlarge any image]

My question is exactly how a skating rink can damage field. Has research been done on this specific situation? Is the problem of a skating rink perceived or real? I haven't read of a failure due to a skating rink, though skating rink are relatively rare. I thought about the various issues: weight and permeability. Are there others?

[... content and detail moved to ICE SKATING OVER SEPTIC]

Reply: worries arise if building a skating rink atop a septic drainfield

Regarding your assertion and question

My question is exactly how a skating rink can damage field. Has research been done on this specific situation? Is the problem of a skating rink perceived or real? I haven't read of a failure due to a skating rink, ​​

​​Of course not. The number of possible SNAFUs humans can come up with is near infinite. We are not likely to find a study of skating rinks over septic drainfields because of their rarity. And some reasons for their rarity may be founded in common sense: the various objections to placing anything over a drainfield or soakaway bed can be generalized so that a requirement that experts explicitly address everything imaginable that one could place and should not place in that location is, with respect, well I'll leave out the adjective.

The reasons to stay off of a drainfield, including keeping equipment from driving over the septic fields in just about all regards are outlined at SEPTIC FIELD FAILURE CAUSES

Details of the pros, cons, & warnings about building a temporary ice rink over septic fields are found at ICE SKATING OVER SEPTIC

Cesspool Cover Strength & Safety in the U.K.

I have an extra large cesspit serving 2 properties on my land (19m x 4m) and is a pit only no outlet . Its brick built and was installed early 70’s, it has 2 chambers running back to back, and has a concrete cover on it which spans the whole length and width of the tank having 2 cast iron lids for emptying in the middle.

Question is – I inherited this with house from my Grandfather who used to drive over it with a small vehicle to get to his garage when the pit was still under a lawn area.

Could you please advise if you would consider that this was the type of cess that was constructed for driving over, and would still be ok??? - L.H. 3/25/2014

Reply: warnings about driving over the cesspool

Watch out: driving over a cesspool risks sudden collapse and even a possible death or injury.

My advice is to stay off of the cesspool entirely until it has been excavated and its construction and strength have been evaluated.

We moved the full text of this question and its answer to SEPTIC TANK DESIGN STRENGTH SPECS

Proposal for Paving over Septic Drainfield using Chamber Systems & Pavers or "permeable paving"

Concrete grass block sold at Home Depot Stores - at InspectApedia.com 2017A reader who needed to expand the footprint of his house and had too-little space wrote in a bit of a snit, stating that [paraphrasing] surely there must be a septic design that permits putting the leach field or absorption bed under a driveway.

He proposed installing a paved-over septic absorption bed using a design combining "permeable" materials such as "permeable pavers" or "permeable asphalt" or "permeable concrete" combined with a chamber septic design system itself buried and covered "with permeable surface material".

Indeed there are interlocking, "permeable" concrete pavers that permit stormwaters to soak through or grass to grow up through their openings. The vendors assert that permeable paving mimics the way natural land absorbs water - so I agree that this sounds like a possibility. - (Belgard 2017)

Shown here: an 11.5 x 17.25 x 3.5 inch concrete grass block paver such as sold at Home Depot stores, Lowes, and at your local building supplier. These blocks are intended to assist in absorption of stormwater but are not described as a product intended for use over septic absorption fields or soakaways.

[Click to enlarge any image]

Really? We reviewed several permeable paver products to look for a claim that the paver was approved for use over septic systems without finding nary a one.

While it is certainly possible to build a chamber system whose soil absorption area below the chamber won't be compacted by vehicle traffic, and perhaps where the chambers themselves are engineered to carry the weight of the homeowner's 2000-4000 lb car, I'm doubtful that a poly septic chamber can carry the weight of a 30,000 lb. septic pumper truck loaded and ready to haul away.

Furthermore, an examination of the list of reasons it's smart to avoid driving over or paving over the septic shows that compression or compaction or collapse are not the only worries. If you solved the construction and engineering problem of weight and compaction your paved-over septic absorption field also needs to permit transpiration (disposal of a portion of its effluent by evaporation up through the soil) at a rate comparable to a conventional soil system or at a rate specified by a design engineer and approved by local septic authorities.

Another worry with a paved-over septic is the same as the worry with a septic system buried quite deep into the soil: the level of oxygen in the soil below the drainage trenches or chambers is so low at depth that it is insufficient to support the aerobic bacterial action needed to break down pathogens in sewage.

In sum we may successfully dispose of the sewage effluent but we're probably not treating it to an adequate level. The result is ground-water and environmental contamination by sewage pathogens.

We have not found a septic authority nor a storm drainage or grass block paver manufacturer who describes their product as intended for use over drainfields. Use the page top or bottom CONTACT link to let us know if you find such a design.

Watch out: We hope that most readers would not willingly construct an out-of-site-out-of-mind septic system that disposes of effluent but doesn't treat it: thus contaminating their own environmental as well as their neighbors. But for those who don't care about that sentiment, we warn in addition that in many jurisdictions paving over the septic fields will also be tagged as a building code violation.

Permeable Pavers for Stormwater Absorption (not for septic absorption field designs)

Note: In contrast, it is often possible to construct other portions of a septic system that can be driven-over or parked-over, such as a septic tank and piping between the building and septic tank or between the septic tank and the absorption field.

Using appropriately-rated septic tanks, covers, and schedule 80 or stronger piping can work for that situation, as we describe next.

How to Modify a Septic Tank for Vehicle Traffic or Driveway Passage

Septic system collapse and truck rescue (C) Daniel FriedmanQuestion: can I use boards to drive a 14,000 lb RV over the septic tank

2017/06/06 Rhonda said:

Can you lay down boards to distribute the weight of an RV (14,000 lbs) to safely cross over septic tank?

Reply: probably not: here are requirements to make a vehicle traffic-rated septic tank

Rhonda,

You ask an important question for which I don't have an absolutely authoritative answer, in part because I have no idea what boards are to be laid down and thus how much they will flex or will distribute or not distribute the RV weight. I also don't know what sort of septic tank you want to cross over.

A septic tank sporting a cover that is made of concrete reinforced with steel re-bar is a far different case from an old steel septic tank with a sheet-metal lid. In the latter case I'd say that no board-crossover-plan is going to be safe. In the former case ... it's still doubtful from the vehicle weight you cited.

Watch out: driving a heavy vehicle over a septic tank, even one rated for "vehicle traffic" may exceed the tank's rated strength, resulting in a septic tank cave-in and collapse. Falling into a septic tank is most-often quickly fatal. Our photos in this article demonstrate a truck falling into and being pulled out of a septic tank.

If you know that your septic tank is a "traffic-rated" tank and know who constructed it, you would still want to give the manufacturer a call to be sure that their "Traffic rating" included a 14,000 lb. vehicle and you'll need to know the dimensions of the tank and the spacing of the vehicle's axles. Why?

Consider that unless you have installed a septic tank with a "vehicle traffic rated" or Highway Traffic Rated strength cover, a typical concrete residential septic tank, following the University of Minnesota design guide (as a typical standard) is built to carry the weight of the soil covering the septic tank and a vehicle wheel load of 2,500 lbs. That is a far cry from your 14,000 lb RV.

Let's pretend that your RV is so long that only two wheels of it are on the septic tank at a time while you drive across - you're putting a 7,000 load on the lid. Your boards would have to be both long enough to span well more than the septic tank AND to be so thick and rigid as to not deflect to place a significant load on the tank itself.

Septic tanks designed to withstand vehicle traffic ("HS20 vehicle traffic rated septic tanks") have not just a thicker lid but heavier tank sides and bottom too. HS20 loading is defined by the American Association of Highway and Transportation Officials (AASHTO) as follows:

A vehicle with an 8,000-pound (3,630 kg) front axle and one or more rear axles weighing 32,000 pounds (14,500 kg) each and spaced at least 14 feet (4.3 m) apart. This is a key measurement since it means that the vehicle's axle spacing is greater than the tank width, so only one wheel set is on the tank at a time. HS20 and H20 are used interchangeably.

OK so what now: I consulted with an expert source on concrete septic tank design, the National Pre-Cast Concrete Association who, as is no surprise, have looked at the question of how to make it possible to driver over an existing septic tank. Here is an excerpt from the associations article on this that I'll cite below:

Engineering analysis demonstrates the tank design shown in Figure 1, [in Barger's article] initially designed for non-traffic use, can be made into a HS20 traffic-rated tank with careful engineering and many preconditions. Delta Engineers, Architects & Surveyors of Endwell, N.Y., performed the engineering analysis. The eight conditions are as follows:

  1. 1 to 2 feet (0.3 to 0.6 m) of soil shall be over the top of the tank.
  2. No more than one axle or two wheels shall be over the tank at any one time.
  3. The minimum concrete strength will be 5,000 psi (35 MPa) compressive strength.
  4. The top slab thickness shall be increased to 6.5 inches (165 mm). However, the bottom slab and wall thicknesses cannot be modified.
  5. The tank contains a monolithic compartment wall.
  6. The water table shall be at least 3 feet (0.9 m) below grade when the tank is empty.
  7. The walls are supported at the roof by an interlocking recess that is cast into the top slab.
  8. Reinforcing bar size, spacing and location shall be installed per Professional Engineer recommendation.

Please also see the articles on septic tank covers and septic tank design strengths listed below at "Continue Reading"

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Continue reading at ICE SKATING OVER SEPTIC or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see DRIVING OVER SEPTIC, FAQs posted originally at this article

Or see SEPTIC TANK DESIGN STRENGTH SPECS

Or see SEPTIC TANK COVERS

Or see PLANTS & TREES OVER SEPTIC SYSTEMS

Suggested citation for this web page

DRIVING or PARKING OVER SEPTIC at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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