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| Truck arriving on site with foam injection equipment for root clearing. |
By Kevin Duke, President, Duke's Sales & Service, Inc.
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| Truck arriving on site with foam injection equipment for root clearing. |
In a report published in 1977, the United States Environmental Protection Agency states that: "The intrusion of roots into sewers is probably the single most destructive element that faces those maintaining a wastewater collection system".
There is probably no more unpleasant task in Public Works than explaining to homeowners why their basement is full of raw sewage. Tree roots are the number one cause of main-line sewer stoppages in America. Sewer stoppages flood homes or, in the case of storm sewers, intersections and yards. Tree roots can also be a factor contributing to grease problems and problems with sediment accumulation. Roots impede flow, causing the sewer to become sluggish and septic. This enhances grease accumulation and also the generation of hydrogen sulfide.
Tree roots can cause considerable structural damage to a sewer pipe. In the same way that tree roots can lift and fracture cement sidewalks, they can separate and crack sewer joints. Roots enter sewer pipes as tiny, almost microscopic, hair-like structures. These roots appear to be fragile, but their appearance can be deceiving. If you've done any hiking in the mountains, you've seen tree roots penetrate crevices in giant granite rock. The roots split the rock and keep growing despite the tremendous weight of the rock above them.
One obvious similarity between tree roots and sewers is that they are both underground, and they are not visible to the unassisted human eye. This presents a challenge to Public Works officials seeking funding for underground utility maintenance.
It's easy to see the damage that roots inflict on sidewalks. The damage inflicted on sewer pipes is much more insidious and expensive. Considering the replacement costs for underground utilities, most municipalities have more money invested underground than above ground.
Root problems are often unknown until a basement floods. By then, much of the damage has been done. Even a television inspection doesn't tell the whole story. When a sewer is televised, the roots visible on the monitor are only the tip of the iceberg. It's the heavy, woody root structures inside the joints and girdling the outside of the pipe that cause the structural damage.
How roots grow
Most plants have one of two main types of root systems: the fibrous-root system and the tap-root system.
Plants with fibrous-root systems, such as garden plants and weeds, are not normally associated with sewer problems. Fibrous-root systems generally do not penetrate deeply into the soil. Instead, they occupy the upper layers of the soil and extend outward from the base of the plant. The root shoots are uniform in size and fibre like.
In plants with tap-root systems, the primary root of the seedling grows directly downward into the soil. Branches, or secondary roots, grow laterally from the primary root. Tap-root systems are well adapted to deep soils and soils where the water table is relatively low. The secondary root structures can grow to several inches in diameter and can exert enough stress to break a sewer pipe.
The extent of a root system can be truly amazing. Most plants have as much mass below ground as they do above ground. The primary root of a corn plant, for example, may penetrate the soil to a depth of four to eight feet. If the total root system of a four month old corn plant (including lateral roots and feeders) were elongated into a single strand, the total length would be 400 to 600 feet. For a ten-acre field, it could add up to 5,000 miles or more. A root system will leave no stone unturned in its search for nutrition.
The top of the plant is definitely more dependent on the root system for its survival than vice versa. We have all seen this when trying to remove a willow tree without pulling out the stump. The stump will continue to send up new shoots, despite continual cutting. Roots are the most tenacious and longest-lived life forms on earth. The root systems of some of the grasses of the American Great Plains are thought to have remained alive for thousands of years.
Tree roots have three basic functions, each is detrimental to a sewer system: they anchor the tree and keep it upright, they store food for the plant, and they absorb and conduct water and nutrients. The anchoring role is particularly true of trees and other plants with tap root systems. Trees are subject to tremendous wind forces, and can survive violent storms without being uprooted.
When large tree roots invade shallow sewer pipe, stress can be transferred to the pipe. Similarly, the seasonal expansion and contraction of roots as they store food during winter months can cause structural damage to sewer-pipe joints.
Eighty-five to ninety-five percent of a plant is water, and the soil moisture content is an important determinant of root growth. Below a certain moisture level, the root hairs and feeder roots will die, and a certain amount of water is held so tightly by soil particles that the roots cannot absorb it rapidly enough to prevent wilting. Roots also tend to grow faster and deeper in dry soils or during dry periods. Nearly all water loss in the first six to eight inches of soil is due to absorption and transpiration by plants. During dry seasons, roots burrow downward more rapidly. The activity of surface roots slackens, spurring deeper roots to greater activity. This is one reason, in addition to accumulation of solids, why Public Works staff may experience a rash of sewer problems after a drought.
Some trees like willows and swamp maples have extreme water needs. A full grown willow can consume hundreds of gallons of water per day. On a terminal sewer, which is the first upstream stretch of sewer, a stoppage may go undetected because the tree is able to absorb the water. This situation is similar to a septic tank where the solids are digesting on the upstream side of the stoppage.
Just as roots will wilt below a certain soil moisture level, roots cannot survive where the soil is saturated. The roots of most trees do not grow underwater. For this reason, roots do not normally cause a problem where a sewer pipe is under the permanent water table; however, roots can cause severe infiltration problems where the water table fluctuates.
The leading tip of a root shoot is called the meristem, and this is where the action takes place. The meristem grows one cell at a time in the direction of the most favorable environment. It is this ability of the meristem, to detect very slight differences in nutrient levels and moisture, that enables the root to find its way to the sewer pipe.
The temperature variance between the flow in a sanitary sewer and the surrounding soil causes condensation to form on the top of the pipe. This moisture then evaporates up through the soil causing microscopic vapour trails to form. This is one case where water flows up hill. The meristem encounters this vein and follows it upstream to the sewer. Microscopic openings in sewer joints only a few cells wide enable hair-like roots to burrow their way inside.
Why do roots like sewer pipes so much?
Where a sewer pipe has a steady flow, such as in sanitary interceptor pipe, the roots grow in from the top of the pipe until they reach the flow, and then they form a veil across the top of the pipe. They can then dam up the flow and collect solids until they form a blockage.
Where there is a low flow or an intermittent flow, such as in a storm sewer or a small diameter sanitary sewer, the roots will completely fill the pipe. They grow in from the top, and then they form a structure that looks like a horse's tail and travel down the pipe. In some situations, roots dug up from service connections form a perfect tube of roots four to six inches in diameter and several feet long.
This article was abridged from Environmental Science & Engineeringmagazine, which also contains many more articles not posted on our Web Site. See our home page on how to order your subscription. We regret we can only accept orders from Canada and the United States.