Septic Tank & Cesspool Design Strengths & Test Specifications

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.

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

A 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. 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

  

  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.

  See details at 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.

  
  

  ...

  Continue reading  at SEPTIC TANK DESIGN SPECIFICATIONS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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  Recommended Articles

  • CESSPOOLS
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  • CESSPOOL SAFETY WARNINGS
  • CISTERNS, WATER STORAGE
  • DRIVING or PARKING OVER SEPTIC
  • DRYWELL DESIGN & USES
  • DRYWELL SAFETY CONCERNS
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  • SEPTIC TANK INSTALLATION, FIBERGLASS / PLASTIC
  • SEPTIC TANK DESIGN SPECIFICATIONS
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  Citations & References

  In addition to any citations in the article above, a full list is available on request.

  • 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:gth
  • 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
  • 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). 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 I recommend for professionals--DF.
  • 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.
  • In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
    
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  • Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
    
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