Question:

I installed chambers in a mound system. The soil has pushed up and filled the chambers. The chambers have not sunk because I uncovered all 400 feet and all elevations were okay. Also, the design engineer did a perc test on the mound after the lines were removed in undisturbed soil in the mound, and the soil is rising there also. Does anyone know where I can get information on this?

Answers:

- I am going to guess it is serious mole action. I now put cloth tightly around my lateral cleanouts because so many of the valve boxes got completely filled with soil. I don’t know how you would stop that in a chamber system.

I’m going to respectfully disagree with moles or frost heaves. We have seen this in Alaska where there are no moles.

My completely unscientific input (that is, I’m not an engineer and haven’t done long-term studies on this like some of the colleges) is that it’s very much like the dehydrated sponges you can buy that ‘grow’ the first time you soak them.

With virtually all leach systems except chambers, there is ground loading upon all of the infiltrative surfaces — the entire receiving soil has the weight of the earth above sitting hard upon it. Therefore, the weight of the ground above keeps this phenomenon from happening.

With chambers, the only ground loading that occurs is along the edges of the chamber, where it ­contacts the ground. The rest of the protected receiving soils area, the void space under the chambers, has nothing to force it to stay down. As the soils become damp, they expand slightly, not unlike a frost heave but not necessarily freeze-induced. Over time, this protected area has plenty of inducement and opportunity to expand, but little or no incentive or opportunity to dry out and settle again.

This is completely seat-of-the-pants, but it certainly makes sense to my blue-collar mind. Now, if we can just get someone to prove it.

- A fellow installer had a similar problem about four years ago with a mound. The problem was the water (effluent) could not escape the mound due to soil compaction during the installation (non-track excavator). This limited the surface absorption area and caused the ground to swell (fill chambers about five inches). He ended up redoing the mound at a loss (of course). This took all of five months to show up.

- I am the technical director at Infiltrator Systems Inc. and would be happy to help understand your site. Some questions I would ask are:

• How long has the system been installed?

• What type, make and model of chamber was used?

• What was the depth of soil below the chambers?

• What is the sand specification for the fill?

• Are there other, similar mound installations with chambers in the neighborhood? Are they having the same results?

ISI has numerous chamber installations in mounds all over the country — I would venture to guess in the tens or even hundreds of thousands in our more than 20 years of history, since mounds are quite common in many states.

Chambers filling with soil is very uncommon, so my initial thought on this site is that some non-typical conditions occurred that caused this abnormality. I am very interested to understand the conditions that made this occur so we can learn from it.