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 Friedman 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:

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

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.

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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