Question:
We started using chambers about eight years ago. Our soils are mostly loam. We have used fabric over chambers for three years. About four years ago, when we went back to check a chamber system we had installed a year before, we noticed the chambers were about one-third full of silt.
We had walked this field down carefully when we installed it. We used a laser and held to a tight tolerance on the trench. We believe there is no way the silt came in from the bottom — it had to come in through the louvers. After some research and trials with other options, we started using fabric over all our chamber systems. Does anyone know of any independent studies on fabric use on chambers?
Answers:
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I’ve installed a number of chamber systems and have had no problems with overburden infiltration. Your situation sounds like one that an engineering firm technician friend described to me. He told of a chamber system that had been installed and in use for a while. When they opened it up, they found the “bottom had welled up” to fill almost one-third of the space. This was clearly some sort of expansion of the material as there was no subsidence of the overburden and no sinking of the chambers.
The engineer friend has seen a phenomenon that meets your description. He didn’t have an official name for it but “swelling of the soil” would probably suit it. He was at an installation where I was adding a pump chamber to a conventional septic system. It was a 250-gallon tank and lift station installed about six feet from the septic tank.
The ground was damp, silty gravel and at least a little elastic. I dug it to grade, then lowered the tank into position. We shot the top of the tank and found it was about 1 inch high. I pulled it out, cleaned up the bottom of the hole, re-shot the hole bottom, rechecked our figures, and reset the tank. It was about 1 inch high. The result was the same a third time.
Our calculations and digging were right on — the ground was simply rising up each time. We finally set the tank on the bottom (once again it was showing slightly high) and backfilled it recognizing that nothing we did to the bottom of the hole changed the hole. We shot the finished tank and it was on grade.
As for covering chambers with fabric, only time will tell. One of the flags I see there is laying the fabric out, then covering it. If the fabric is not contoured to the chambers, it will bridge low areas and eventually sag, give way and create subsidence.
Information that I have suggests the issue is soils expanding or rising inside the void rather than infiltration through the chamber louvers. Certainly the addition of water into the uncompacted and unpressured surface protected by the chambers would allow for a natural absorption and expansion that is not possible in soils under pressure. I would consider all soil under rock-and-pipe beds and the ground-contacting surfaces of the chamber area to be under pressure and not likely to expand.
There is nothing technical in my comments — just down-home, blue-collar observation based on experience, and some hearsay from a trusted friend. There’s a lot more work to be done on studies to prove or disprove observations installers make.
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I install chambers and we use filter fabric. In chambers without fabric that we have installed, we have seen infiltrated soil in them. I just assumed it got in from the expansion and contraction of the soil.
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Your ground conditions sound similar to what we have in the Fraser Valley of British Columbia. We are on a floodplain, through which the Fraser River runs. When the river is high, we get what we call boils, where an excavation, even a shallow one, can weaken the ground in that spot and allow the groundwater a means of escape. So you dig for a while, and then you see a bit of seepage, then bang, the hole is full of water. Perhaps it’s a similar hydrostatic pressure that is pushing the soil up into the chambers.
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I have replaced two systems on loamy sand where chambers were installed four years ago. Company authorities were there when we dug it up. The sand seemed to come in from the louvers on the sides. You could actually see the layers coming through the louvers. On the brand I use, the company has changed the way they make them. Louvers are slanted more to the outside. I use fabric, but only in sand. Two-thirds of my systems are in clay, and we walk the sides in. Indiana rules use bottom area only.
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When I prepare a set of plans for a septic system, I generally will show a non-woven fabric over the chambers. There are a few very light ones available. I include inspection ports in the chambers and, if the seal is left loose, air can enter and exit through them. I generally pressure dose every drainfield and configure the force main to blow fresh air into the field each time the pump is activated.
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All I can tell you is that what works best to stop soil (heavy geo-textile fabric) also works well to prevent air from getting in. That is intuitive. Prove it to yourself. Lay a piece of fabric out in your flower bed, and cover it with a couple of inches of soil or mulch. Better yet, try several different weights of fabric in different locations.
The heavier material will prevent fines from penetrating, but will stay moist for long periods of time. And, most of the time, the wet fabric loaded up with wet fines will keep the soil beneath moist and anaerobic for long periods of time.
Of course, the lighter-weight fabrics may provide a compromise between the two objectives. Now, remember that the solid plastic portions of chambers also do not allow air or water to pass, shedding precipitation from the surface to the side louvers — which compounds the soil moisture problem. I suspect that this is less of a concern in dry climates than it is in colder and wetter climates.
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I can offer the perspective of a chamber manufacturer. I hope I speak for all manufacturers when I say we have the same goal as contractors and designers: We do not want systems to fail. Our aim is to produce a product that protects the public health.
There are many drainfield products on the market today that are filter-fabric based, and they have a proven field track record as long as they are designed, installed and maintained properly. Our company provides a filter fabric specification for contractors who wish to use it. It is not required, nor will it void the warranty if used.
When we went to provide a filter fabric specification, we researched companies that had experience a track record. Then we invited fabric manufacturers to give input and recommendations to help us select the best product available. The result was the thinnest and lightest material available. A 0.3-ounce, non-woven fabric addresses many concerns, including the need to provide oxygen and to provide the necessary bridging upon backfill with very fine, uniform sands.
Testing has been completed with wastewater (it was limited), and within a test cell (large Plexiglas box).The fabric specification is not something that we took lightly.
In general, the problem we have observed with intrusion through the sidewall has been limited to very fine, uniform sands.
In general, if a system is backfilled with this soil type, then fabric is not necessary. Part of the research included simulating a 100-year rainstorm in a test cell, with chambers installed in fine, uniform sand. No intrusion was observed.
Field research concluded that when sites are left open, possibly awaiting final inspection, and a strong rain event occurs, then very fine uniform sands can be mobilized. This is no ground-breaking research; I am sure that most of you are aware of this, and for that matter, any type of soil exposed to a heavy rain event will erode and carry sediment into a trench, regardless of product type.
So our specification is at the discretion of the contractor because we believe in their knowledge and experience base. Please understand that from our experience with tens of thousands of systems installed each year, we do not have many system failures. Our reported failure rate comes in at a fraction of a percent.
Of the calls we do get, only a small number are intrusion-related. If I had to guess, I would say that I deal with three or four per year. (This response was submitted by Dennis F. Hallahan, P.E., technical director with Infiltrator Systems Inc.)
