A few months ago, I was posed with a seemingly simple question: Are there standards for septic tank burial depths? My initial response was, well, of course there are standards and they are related to the strength of the materials used to construct the tank and their ability to withstand all of the forces vertically and laterally on the tank where it is installed. I also related to the questioner that most state codes deal with the issue and then, of course, the manufacturer should specify that information when their product
is used.

As I thought more about the question, I began to consider all of the different types of tanks and the different manufacturers, and thought I should look a little closer at the requirements. I looked at a couple of state codes to see how they handled the question of tank strength.

In the codes

The first one stated: "Tanks, fittings, risers and apertures (openings) must: Be capable of supporting long-term vertical loads for the conditions in which the tank will be placed. These loads include, but are not limited to, saturated soil load, based on 130 pounds per cubic foot; be capable of withstanding a lateral load for the conditions the tank will be placed; with proper maintenance and venting, not subject to corrosion and degradation from sewage or sewage gases, including risers and maintenance hole covers; and be structurally capable of withstanding exposure and stresses from freezing conditions."

The second code said tanks must be "structurally designed to withstand all anticipated earth or other loads. The tank is designed so that: All septic tank covers are capable of supporting an earth load of 300 pounds per square foot; and if the top of the tank is greater than 2 feet below finish grade, the septic tank and cover are capable of supporting an additional load of 150 pounds per square foot for each additional foot of cover."

Other sections of both codes specify that prefabricated concrete septic tanks shall meet the "Standard Specification for Precast Concrete Septic Tanks C1227-03,'' published by the American Society for Testing and Materials. For prefabricated fiberglass and polyethylene tanks, they shall meet the "Material and Property Standards for Prefabricated Septic Tanks, IAMPO PS-1 2004," published by the International Association of Plumbing and Mechanical Officials.

The state agencies with responsibility for these codes indicated copies of these standards incorporated by reference into the code could be obtained at their state office and through the library system. If you go directly to the publishers, these standards may have additional updates and changes after the date of the incorporation; and copies are available for a fee.

Precast concrete manual

The National Precast Concrete Association (NPCA) has an online Best Practices Manual. It generally covers all aspects of a precast operation, from selecting materials through producing a watertight, structurally sound product; storage; transport to an installation site; and installation procedures. It is a valuable resource for anyone working with precast concrete products.

The manual states a septic tank must be manufactured to withstand a variety of loading conditions during manufacture, installation, testing and while in service. Loads to be considered include vertical soil backfill; concentrated wheel loads in anticipation of traffic over the tank (something we want to avoid); lateral loads; soil-bearing capacity; and buoyant forces. These calculations should be done by a qualified professional engineer or tested in accordance with ASTM C1227.

The manual provides insight on concrete thickness, mix design and reinforcement. Thickness must be enough to meet minimum reinforcement cover requirements and withstand design-loading requirements. So here the codes come into play by specifying the pounds per square foot for the soil backfill over the top of the tank. The concrete mix must have a compressive strength of 4,000 psi at 28 days.

Readers are encouraged to consider methods to reduce permeability, improve durability and increase strength. The manual discusses materials that can be added to improve these elements. It also highlights standard water-cement mix ratios to ensure the concrete strength. Proper reinforcement is important to withstand the loads applied to a septic tank during its installation and service. It is critical that the product can stand empty, then withstand the soil pressures exerted vertically and laterally, and, for my part of the world, be able to withstand temperature changes.

The manual also provides specific installation instructions including bedding and backfilling, which need to be followed to avoid applying additional stresses and loads to the tank. Not following these instructions can lead to cracking the tank during installation or to problems with watertightness down the road.

Other tank materials

For fiberglass and polyethylene tanks, manufacturers' information sheet on their products will provide data explaining the maximum depth of bury to avoid deformation. A common figure for some tanks I have been involved with is a maximum bury of 30 inches. Also, because these tanks are generally lighter than concrete (one of their selling points for hard-to-reach areas), more attention needs to be paid to buoyancy problems during and after installation. Each manufacturer has preferred ways of dealing with this issue; most involve tying the tank down to some type of curb. To avoid lateral stresses, each manufacturer has specific backfill instructions and requirements to fill the tank "as you go" during installation.