Ask Customers Lots of Questions and Listen Carefully to Their Answers

Details from customer interactions can reveal potential issues that could lead to spectacular system failures down the road

Recently we had a question from homeowners who just bought a house and had the septic system inspected before closing. The reported system had passed the inspection with flying colors, and there were no issues. They then asked if it was common practice to have a piece of sheet metal over the top of the concrete tank buried beneath about a foot of soil.

Right away this raises at least a couple of red flags, and you may be able to think of a few more. How many times have we read that someone was walking in the backyard and fell through a faulty cover or lid of a septic tank or cesspool that was hidden by soil cover? Way too often, the person falling in has been seriously injured or died as a result. Our first advice to the homeowners was to avoid walking over the area until a professional can inspect the tank lid and makes sure it is structurally sound.

QUESTIONABLE INSPECTION

Our concern was heightened when the owner reported the tank was approximately 40 years old. It would take a good deal of research to find out how and where this tank was constructed and to what standards. Our best guess is it would probably not meet current standards for best tank construction put forward by the National Precast Concrete Association. Tanks are reinforced with rebar and must be able to withstand soil loads both vertically and laterally.

The homeowner then indicated he thought the sheet metal was placed to deflect or prevent water from moving through the soil to the top of the tank. If this was the purpose, the tank is probably not watertight and this was an attempt by the previous owner to stop water from infiltrating the tank during rainy periods and flooding the drainfield.

These comments also highlight that the tank was not opened and inspected during the real estate inspection. As we have discussed previously in this column, proper inspection for structural soundness and operation requires inspecting both the tank and its contents. Checking the contents before pumping can provide valuable information about system performance. After pumping, the tank can be carefully inspected for deterioration or cracks.

Our recommendation to the homeowner was to have the tank pumped and inspected by a professional to evaluate structural soundness, watertightness and operation. It is likely that the tank will either need to be replaced or significantly repaired to bring it up to today’s standards.

Back to the question about a proper cover over the tank. During installation, proper backfilling and cover around the tank are important to long-term operation and stability of the tank. The tank needs to be installed level in the excavation. The bottom of the excavation needs to be level and free of any large rocks or debris. This will avoid cracking when the tank is set in the excavation and is ultimately filled with sewage. Natural soil can sometimes be suitable for bedding the tank, but often it will require a layer of granular material such as rock or coarse sand.

FINISHING THE SITE

Backfill around the tank should be done using a series of compacted lifts. The backfill should be free of large soil clods, rocks, chunks of frozen soil (in our part of the world) or any construction debris. This is done to avoid any voids in the backfill that over time will result in differential settling, which may put stress on the walls of the tank and/or provide pathways for water to infiltrate around the tank, potentially causing flotation or infiltration into the tank. If the native soil is heavy clay or other unsuitable backfill material, crushed rock or pea gravel 1/2 inch in diameter can be used. When the lifts are compacted, care needs to be taken to avoid damage to the tank.

The top of the tank should not be covered by more than 4 feet of soil unless the tank has been rated for additional depth and is specifically part of the design requirements. Minimum soil cover over the tank should be 6 inches. In most cases, native soil can be used as long as it is not in large clods or contains large rocks. The final cover should be mounded to allow for settling and graded away from the tank so water cannot collect over the tank. Any ponding on or around the tank provides the potential for water to infiltrate the tank. Any surface water should be diverted away from the tank through final grading or installation of berms.

To help ensure the tank is watertight after installation, any risers and lids should be installed using proper materials (sealants) at the joints, as well as having the right type of joint connection between the riser and the tank, usually some type of tongue and groove connection with the application of additional sealants.

For this particular case, it would be useful to review the local inspection requirements at the time of sale and work toward having an inspection requirement that does a complete operation inspection of the system, including opening the tank, viewing the contents, and evaluating structural integrity.



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