Septic system collapse and truck rescue (C) Daniel Friedman Septic Tank & Cesspool Design Strengths & Test Specifications
     


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Septic tank covers & strength specifications: what are the strength & design requirements for septic tanks, cesspools, drywells and similar structures? What are the design loads in psf associated with vehicle traffic, soil loading, and other live and dead loads on septic tank or cesspool or drywell covers?

This article describes loading and strength requirements for concrete and other septic tanks or cesspools and includes citations of ASTM standards as well as industry experts.

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Septic Tank, Cesspool, Drywell Strength & Cover Requirements, specifications, codes

Caravan site debris and site hazards (C) Daniel FriedmanA septic tank, cesspool or drywall, including the tank or container structure and the cover must be strong enough to withstand all loading conditions including:

  • The weight of the surface cover or "surface surcharge"
  • Concentrated wheel loads - vertical loading from above if the system is to be installed where it must be driven-over.
    Watch out: just about every expert source simply says do not drive over nor park over the septic tank nor drainfield. Also see DRIVING or PARKING OVER SEPTIC
  • Lateral loads from soil
  • Soil bearing

Typical top loading strength requirements for concrete septic tanks (that in our opinion should also apply to septic tanks, drywells, cesspools of any construction, range between 500 and 1100 psf or a top load of 2,500 lbs wheel load + soil & other loads, with the warning that for systems subject to heavier vehicle loading or other heavy loading the strength of the entire assembly (top, walls, bottom) must be adusted accordingly.

In addition to PSF specifications, reinforcement, such as steel re-bar for concrete cesspool, drywell, or septic tank covers is required to avoid tensile strength failures and collapses.

Watch out: site-built septic tanks, cesspools, drywells are most likely out of the range of engineering calculations and applicable standards because of their unknown condition. For example dry-laid concrete block or brick cesspool, septic tank, or drywell construction may be particularly vulnerable to sudden even fatal collapse hazards independent of the strength of a reinforced cover that may have been placed over that structure.

References for Drive-on Rated Safe Septic Tank Covers

  • ASTM C1227 - 13 Standard Specification for Precast Concrete Septic Tanks, Active Standard ASTM C1227 | Developed by Subcommittee: C27.30, http://www.astm.org/Standards/C1227.htm - Abstract:

    This specification covers monolithicor sectional precast concrete septic tanks. The following materials shall be used for manufacturing concrete septic tank: cement, aggregates, water, admixtures, steel reinforcement, concrete mixtures, forms, concrete placement, fibers, and sealants. The precast concrete sections shall be cured. Structural design of the septic tanks shall be by calculation or by performance.

    Concrete strength, reinforcing steel placement, and openings shall also be considered in the design. The physical design requirements include: capacity, shape, compartments, influent and effluent pipes, baffles and outlet devices, and openings in top slab. The following tests shall also be done: proof testing, leakage testing, vacuum testing, and water-pressure testing.

    This abstract is a brief summary of the referenced standard. It is informational only and not an official part of the standard; the full text of the standard itself must be referred to for its use and application. ASTM does not give any warranty express or implied or make any representation that the contents of this abstract are accurate, complete or up to date.
    • Septic Tank top load (cover loading and load on walls) calculation: assuming 300 psf live loads added to 240 psf earth loading for a 2 ft soil cover over the septic tank = 540 psf x 150% = 810 psf required strength
    • Septic tank floor load calculation: 715 psf x 150% = 1,073 psf required strength
    • Septic tank wall load calculation: 180 psf (soil pressure) x 1505 =n 270 psf required strength
    • Wagner (below) npotes that since floor load is the highest force that figure can be used for vacuum testing, 1,073 psf = 14.9" Hg.
  • ASTM C 990 - septic tank sealing against water intrusion.
  • ASTM A615 or A706 reinforcement requirements for concrete septic tanks
  • Dan Wagner, "Fabricating a Watertight Precast Concrete Septic Tank", Milan Vault Inc. 10475 N. Ann Arbor Rd. Milan, MI 48160 Phone: 734.439.1538 Toll Free: 1.800.821.0265, Website: http://www.milanvault.com/, Document source: http://mowra.org/Watertight%20Septic%20Tanks%20Why%20&%20How%20-%20Wagner%20-%2001-13-11.pdf retrieved 3/25/15
    • Vacuum testing specification for water tightness testing: 4" of mercury (Hg) for 5 minutes, must hold 100% to be certified. ASTM C 1227-10a
    • Top loading simulating a concentrated single wheel load of 2,250 lbs over a septic tank assumed to have tywo feet of soil cover (of un-specified compaction and soil weight) is calculated ast a live load of 88 lb/sqft. Top loading = 325 psf (earth plus live load weight) x 150% = 488 psf required
    • Floor loading (500 psf) x 150% (safety factor) = 750 psf required. Author notes that "Since floor load is the highest load force it can be used for vacuum testing at 750 psf = 10.4" Hg
    • Wall Loading (220 psf x 150% = 330 psf required
    • Vacuum testing for wall loads: 330 psf or 4.6" Hg.
    • Top weight loading: 420 lbs/sqft (this simulates soil and live loading, NOT vehicle loading)
    • Assumed septic tank weight: 7,000 lbs.
    • Author also cites 300 psf life load from the ASTM C1227 standard described above
  • Dan Glasgow, Editor, "Design and Specification Guidelines for Low Pressure Sewer Systems", Technical Advisory Committed for the State of Florida Department of Environmental Regulation, June, 1981, retrieve 3/25/14, original source: http://www.dep.state.fl.us/water/wastewater/dom/docs/guide_lowpres.pdf
  • "Minimum Standards for Design and Construction of Onsite Wastewater Systems", State of Kansas Department of Health and Environment Bulletin 4-2, March 1997, Bureau of Water—Nonpoint Source Section Forbes Field, Bldg. 283 Topeka KS 66620 (785) 296-4195 In Cooperation with K-State Research and Extension, retrieved 3/25/14, original source http://www.kdheks.gov/nps/resources/mf2214.pdf - Excerpt:

    D. The top of all tanks shall be designed and constructed to support a uniform load of 400 pounds per square foot plus 2,500 axle load. When buried more than 2 feet deep, the tank, especially the top, shall support an additional 100 pounds per square foot for each foot of soiil or portion thereof in excess of two feet.

    E. If the tank is placed in an area subject to any vehicular traffic it shall be certified to meety H-20 highway loading by a Kansas licensed structural engineer.
  • " Design Criteria for Septic Tanks and Individual Disposal Systems", WTS-22, Rev 2002, Nevada, retrieved 3/25/14, original source: http://ndep.nv.gov/bwpc/wts-22.pdf
  • University of Minnesota Onsite Sewage Treatment Program, "Section 7: Septic Tanks, 7-21 Structural Soundness", http://www.septic.umn.edu/prod/groups/cfans/@pub/@cfans/@ostp/documents/asset/cfans_asset_131292.pdf
    Excerpts:

    From MN Rules 7080.1980:
    A. All precast reinforced concrete sewage tanks must be constructed to meet the requirements of this chapter. Information on best practices for tank construc- tion is found in the National Precast Concrete Association’s best practices manual, Precast Concrete On-site Wastewater Tanks (2005) B. All fiberglass-reinforced polyester and polyethylene tanks must be construct- ed to meet the requirements of this chapter. Information on best practices for these tanks is found in the International Association of Plumbing and Mechanical Officials (IAPMO), Material and Property Standard for Prefabri- cated Septic Tanks, Standard PS 1-2006 (2006).

    All septic tanks must be structurally sound in order to prevent collapse. They must be able to withstand handling and transport after manufacturing and not be susceptible to damage during installation.

    Additionally, tanks must be capable of supporting anticipated soil loads as well as a 2500-pound wheel load, and be able to withstand both internal and external hydraulic pressure. Regardless of the materials used in the production of septic tanks, structural integrity depends on good design, use of quality materials, proper manu- facturing methods, and careful construction techniques. MN Rules Chapter 7080.1910 spells out the requirements for all sewage tanks
  • "Residential Onsite Wastewater Treatment Systems Design Handbook", New York State Department of Health, Bureau of Water Supply Protection (2012), original source: http://www.health.ny.gov/environmental/water/drinking/ wastewater_treatment_systems/docs/design_handbook.pdf
    Health Education Services Health Research Incorporated P.O. Box 7126 Albany, NY 12224; (518) 439-7286 http://www.healthresearch.org or http://www.healthresearch.org/store/water-treatment-sewage-manuals For an electronic copy of this Handbook go to: http://www.health.ny.gov/environmental/water/drinkin g/wastewater_treatment_systems/design_handbook.htm or contact your local health department or: Bureau of Water Supply Protection New York State Department of Health Corning Tower, Empire State Plaza Albany NY 12237 Tel: 518-402-7650
  • "COLORADO ONLY - plastic triple-wall Infiltrator brand heavy-duty septic tank kits. ", https://www.thenaturalhome.com/septictank.htm

Reader Question on Cesspool Cover Strength & Safety in the U.K.

Septic tank cross section sketchHI, I've been interested in your Cesspool info on your site and wonder if you have a UK version that offers consultation as well? - L.H. Middlesex, U.K., 3/5/2014

Reply:

I have been working for some time to add variations of wastewater treatment (and other InspectApedia topic) to our website and I'd welcome the opportunity to improve the UK-pertinence of our information.

Usually I find that the actual construction and operating principles are the same in most countries but that we use different terms that are unfamiliar across borders. (A water heater in the U.S. is a geyser elsewhere for example.) Other differences are in the details of building standards and codes. And of course when we conduct that research inevitably that allows us to add names of product and suppliers in the country of interest.

We do not offer for-pay consulting but we do provide pro-bono research and assistance for people of limited means. In addition, even if you were of - what should I say - more capable? - means I'd be glad to research specific questions that you may have. That work allows us to identify information that I'd want to add to InspectApedia anyway.

Are you looking for help specifically with a cesspool problem or is the topic more broad. Let me know what's up and I'll see how we can help.

Reader Reply:

In answer to your question, yes I do have a query that I wonder if you can answer, although may be a little unique?

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.

As you can tell its quite robust and visually inside looks to be in good condition and is also maintained/emptied yearly with the emptying company stating it is in good order.

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 lawned area

Years later, with having to reduce size of garden meant the only way to get to the garage was to construct a crushed concrete/shingle driveway over the length of this cesspit.

I know my Grandfather used to drive over it periodically years ago, and before he had an extra bit of land to the side which he then used for the driveway, now this land has been removed and hence the above having to happen , I am driving over it but a little worried knowing the size of it could swallow 2 small cars! However it does have a 6” thick concrete lid spanning the length of it and then additionally covered with crushed concrete and shingle making this cover about 12” thick now ….

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

LARGER IMAGE: unsafe septic tank cover discovered by simple exploration - we roped this area off and placed heavy plywood over the opening - it was by a children's play area.

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

  • Your system has too many unknowns for anyone with any sense to promise you that it is safe. Here are some of the variables that are apparent just from your message:
  • We have a home-made constructed cesspool and cover of unknown components, strength, design
  • The fact that the system was driven-over in the past in a small vehicle is no assurance that the cesspool and its cover are safe. Driving over it in the past may even have weakened or damaged the structure.
  • Providing a new marked passage over the old cesspool increases the risk of a sudden collapse by inviting more traffic and perhaps traffic by heavier vehicles.
  • Quoting from the article above

Typical top loading strength requirements for concrete septic tanks (that in our opinion should also apply to septic tanks, drywells, cesspools of any construction, range between 500 and 1100 psf with the warning that for systems subject to heavier vehicle loading or other heavy loading the strength of the entire assembly (top, walls, bottom) must be adjusted accordingly.

In addition to PSF specifications, reinforcement, such as steel re-bar for concrete cesspool, drywell, or septic tank covers is required to avoid tensile strength failures and collapses.

Watch out: site-built septic tanks, cesspools, drywells are most likely out of the range of engineering calculations and applicable standards because of their unknown condition. For example dry-laid concrete block or brick cesspool, septic tank, or drywell construction may be particularly vulnerable to sudden even fatal collapse hazards independent of the strength of a reinforced cover that may have been placed over that structure.

My advice is to stay off of the cesspool entirely until it has been excavated and its construction and strength have been evaluated. It might be possible to install a vehicle-safe cover over the system if the rest of the structure is capable of bearing all of the loads involved.

Meanwhile, I would not even walk over the system. Keep people off of it until its condition is known or appropriate repairs or improvements have been made. I know I sound like a scared old maid, but then, I've consulted in a few septic and cesspool collapses that involved fatalities (SEPTIC TANK ACCIDENT REPORTS) . Even if you think the risk of a collapse of your system is low, the penalty can be too severe to take a chance.

 

 

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Or see DRIVING or PARKING OVER SEPTIC

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