Keep Your Trench Rock Clean

Dirty rock can contribute to closing the voids that keep effluent moving through your conventional trenches

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A colleague of ours at the University of Minnesota often said it was hard to get people to fund research on the most basic onsite systems. And yet, it was easier to make a case and get research grants funded for new systems or ideas that were often much more complicated and expensive than our basic technologies.

He said everyone thought they knew how those elaborate systems worked, while there were still a lot of unanswered questions about how the gravity-fed rock-and-trench system functioned. This was not a knock on the research done on new technology; he just highlighted that there was — and still is — a lot we don’t know, and it would good if there could be another comprehensive look at simpler systems to improve our knowledge about how to better site, install, and manage them.

This was brought to my mind by a short paper that was provided at the WWETT Show and done by a company in Canada that manufactures early-alarm systems for sewage treatment systems, among other things. They instrumented a conventional gravity rock trench system. What they found was extremely interesting, and I think it highlights why we as an industry need to continue to research and look at our most basic treatment solutions.


The company installed its monitoring devices in three locations within the treatment trenches. They found areas within the trench that were ponded to depths that were close to failure (surfacing), while other areas within the trench showed no evidence of ponding at all. Since the trench bottom was level, this did not make sense. If the liquid could flow unimpeded through the rock, it should spread gradually across the bottom of the trench, forming a biomat and ponding above the biomat to a somewhat uniform level.

When they looked at the rock, they reported that it was excessively dirty. They speculated that with the rock being dirty, biomat bridged across the void spaces, effectively blocking off part of the trench from liquid. This is a plausible explanation when we consider that with gravity distribution in 4-inch pipe, effluent will run out just a couple of the half inch holes, so effluent essentially dumps out in one place without using most of the distribution piping.

In our view, this is evidence that rock used in sewage treatment trenches or beds must be clean. Another way dirty rock can impact system performance is as the effluent moves around the rock, washing off the finer particles and depositing them in the bottom of the trench. This action physically seals off the soil infiltrative surface, reducing the ability to accept effluent. For pressure distribution systems where rock is used in trenches, mounds, or at-grades, avoiding this action is important to maintain system longevity.

Criteria used to define clean rock is specific: No more than 5 percent by weight should pass through a 3/4-inch sieve, and no more than 1 percent by weight passing a No. 200 sieve. These criteria ensure that the rock used is smaller than 3/4-inch in diameter, which prevents the use of pea gravel as a rock media. Pea gravel is subject to plugging due to biomat formation across the voids between the rock. It also ensures the rock doesn’t have a lot of fine soil particles, which can be subject to washing over time and plugging the soil infiltrative surface.

In terms of rock size, on the high side it should have no more than 5 percent by weight larger than 2 1/2 inches in diameter. Specifying an upper limit means the rock is workable during installation and has a large volume of void spaces. Since the two primary purposes of drainfield media are to hold the sides of the system and provide space for effluent to be stored, using larger rock makes it more difficult to work and doesn’t provide as much storage space.


One final comment on rock criteria: It needs to be durable, so it should be crushed igneous rock or similar insoluble material. If rock is not durable, it will break down over time, sealing off the infiltrative surface. An example of unsuitable rock is crushed calcite limestone.

From an installer point of view, contractors should make sure they get clean rock delivered to the job site. This means ensuring the supplier is not providing pit-run gravel, which will generally have a large percentage of fine material.

Once the rock is delivered, it is up to the installer to keep it clean. This means being careful about where and how it is stockpiled so soil is not picked up and deposited with the rock in the trench bed. If you are going to pay a premium to have clean rock, keeping it clean only makes sense.


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