I am pleased that I received a comment from a reader of the Septic System Answer Man column from February warning contractors to avoid walking in open drainfield trenches during onsite system construction. I will condense some of the interesting comments and respond to them. The comments from the reader may very well be what others are thinking.
Comment: “Yes, foot traffic is not the best thing to do in the bottom of the trench, but in reality, I think in moderation it is probably so small of a concern it barely deserves mention.” The reader states that the weight of the soil will play a part in the compaction at deeper soil depths. The reader calculated the square inches of soil contact of his shoes and divided this number into his weight to get a value of 2.55 psi (pounds per square inch). The reader indicates that this is not very much pressure.
Response: Using the bottom area of my shoe and my weight, I get a value of 5.2 psi on the sole of one shoe, if the entire sole is in contact with the soil. However, this is true only if I walk totally flat-footed so the entire shoe surface comes down at the same time. I don’t walk that way, and most people don’t walk that way.
Walk a few normal steps and you will see that the heel of your shoe touches first and takes most of your weight. Then as you walk, you will roll your weight to the ball of your foot, or shoe. If the heel area of the sole is only 25 percent of the total sole area, the pressure will be four times as great. In my case, this would increase the pressure to 20.8 psi when the heel touches. The pressure will also be greater than 5.2 psi when the ball of my foot is in contact with the ground surface. Such pressures will certainly compress a fine textured soil and seal off the capillary pores where the foot steps. This is particularly true if the soil is relatively high in water content.
If you assume walking on the soil in the bottom of the trench is of little consequence, as the reader has, there will likely be repeated trips back and forth in the trench. It is unlikely that the footprints will be in the same places as the previous tracks. Damage will be done to the soil in the bottom of the trench.
Comment: The reader says the depth of the trench and the weight of the soil will play a role in the compaction of the trench bottom. A calculation is made for the soil pressure at the bottom of a three-foot-deep trench, and the reader comes up with a pressure nearly as great as the one assumed on the bottom of his shoe sole.
Response: Soil is a dynamic and active material. Soil is made up of a variety of sizes from clay to sand. Chemical and biological reactions develop soil structure. Soil forms in aggregates, which are units having particular characteristics of structure and strength. There are continuous capillary pores in the soil, which transmit liquid downward. These are very small and fine pores, which can easily be sealed off by the compaction of foot traffic.
The depth of the soil does have very little effect on the density of the soil in the top part or A horizon, as the soil scientists label it. Soil is not like water, where the pressure increases with depth from the surface. For example, if the water were three feet deep, the pressure would be three feet or 1.3 psi. At a six-foot depth of water the pressure would be 2.6 psi. The same is not true of the soil because the soil has a structure. So the assumption that soil depth contributes as much to the soil compaction as foot traffic definitely is not true. It is the manipulation, compaction and destruction of the capillary pores that are detrimental to the performance of a soil as a waste treatment system.
Comment: “The side raker teeth I have thought about before too, but can see them doing just what the teeth do to the bottom. I think a bucket with teeth angled to the side at the cutting edge is about the best. This way they dig a wider trench than the bucket width. Mine measures 36 inches at the teeth and 34 inches at the sides.”
Response: The reader makes an excellent point concerning the type of backhoe bucket being used. A bucket that has a wider distance at the opening of the bucket than farther back in the bucket is one that should be used.
There are different types of backhoe buckets. If digging a deep trench for laying a pipeline, then you want the side of the trench to be firm and not collapse. For this purpose the sides of the backhoe bucket should be at least as wide as the cutting edges. Then the soil will be smeared and stand up much better in a deep trench.
However, for a sewage treatment trench, leave the soil along the sidewalls of the trench in as natural a condition as possible and avoid smearing the soil. I don’t believe the small raker teeth along the side of the bucket will smear the soil as much as the bottom teeth of the bucket.
Another point, and this cannot be emphasized too much: Do not be digging in the soil if it is too wet! Before you take equipment to a site, go there and dig down with a shovel. Take a golf-ball-size sample of the soil and try to roll it out into a string. If you can get a string an eighth-of-an-inch or smaller, that soil is just too wet. Stay out of that soil until it dries out. You will be doing your customer a big favor. The onsite system will last a lot longer if the soil texture and capillary tubes haven’t been destroyed. The customer would be much better off to have a pump truck take away the sewage until the soil is suitable for the installation of the trench system.
A Final Word
To my reader, thank you for taking the time to write. But, I will tell you and my other readers once again, “Keep your feet out of the bottom of the trench!”
