Tree Roots and Infrastructure Damage

March, 1998

Introduction

Establishing Blame

We are surrounded by operator error! Tree roots are a common invitee to many of our resource concentration and use structures. Biological illiteracy can be masked for short periods by shear mechanical prowess in building and maintaining structures. We have been conditioned to blame tree roots for a host of engineered and carefully designed failures. Among the scientific community, as other questions are being examined under replicated and controlled conditions, we accept a biased view of cause and effect, and help blame tree roots with only circumstantial evidence. It is concepts engineered to fail and their defenders that burden us.

Role of Roots

Tree roots control resource volume or space. In most developed area soils, providing more space for root systems is equivalent to providing more resources for the tree. Roots are designed to carefully sense current soil conditions, and in concert with the rest of the tree, exploit resource space. The growth regulation system of a tree, centered between shoots and roots, assures relatively quick reaction to internal and external environmental changes. The roles of roots are to colonize and hold resource containing space. These roles require elongation, radial expansion, lateral development when needed,
continual maintenance of an absorbing system, material transport, food storage, element processing, and survival through poor growth or poor resource availability periods.

Infrastructure Conflicts

Tree roots contribute to modifying their own environment and developing stress and strain on various types of structures. Sometimes the costs of tree-literate design and engineering is greatly outweighed by the cost of correction and cure. The economics, considering all the indirect and direct costs coupled with benefits over tree life, should be examined closely whenever tree or structural treatments are prescribed. Here, I will not review the various methods for evaluating managerial actions economically, but will examine root control options without cost estimates. I will quickly review the economic perceptions in the literature surrounding tree roots and damage to infrastructure.

Economic Impacts of Damage

In one city 30% (of which 4% were severe damage, 8% were moderate damage, and 18% were minor damage) of all surveyed trees were cited for sidewalk damage and 13% (of which 2% were severe damage, 3% were moderate damage, and 8% were minor damage) were cited for curb damage (57). Of these same trees causing pavement problems, 37% of them initiated infrastructure damage at a distance greater than three feet (57). One community spent $277.78 per mile of sewer line on root control and associated repairs (52). Average costs of tree related sewer repair was cited as $1.66 per tree (35). One community used 40% of its annual tree budget for sidewalk repairs stemming from tree root damage (35).

General costs for sidewalk repair because of tree root damage was estimated to be $500.00 per repair, with a life span of the repair estimated at 5 years if the tree is not removed (53). The most widely surveyed solutions to tree root problems were listed as tree species selection changes, root barriers (only 25% of surveyed believed they worked), and root pruning (poorly accepted and suspected of increasing structural failures) (35). Tree root growth is considered an expensive nuisance and liability risk in many communities.

Physical Aspects of Damage

Tree roots are directly involved with damage to sidewalks, curbs, gutters, and to a lesser degree, sewers (35). One community's replacement cycle for sidewalks continually damaged by trees was every 5-10 years (35). Tree roots are cited as opportunists, utilizing structural faults in infrastructure to capture essential resources (35). Even small diameter roots are able to facilitate pavement damage (28). Tree diameter and species are major controlling factors (80% variation accounted) cited for infrastructure damage (34,35). Managers stated that damage was more site-specific rather than species-specific though (35).

Tree Diameters & Damage

Where trees were continuing to damage sidewalks, cement replacement was required on a 5-10 year cycle whether root cutting at the time of cement replacement was part of the treatment or not (35). Soil water levels (wet vs. dry sites) made no difference in the damage frequency to sidewalks (28). Tree diameter measures and distance of the stem to the infrastructure were part of  uantifying damage risks. Unfortunately, seldom are non-tree-associated failing infrastructures examined to help visualize the scope of root damage (21).

Infrastructure Damage

The force required to damage cement from below can be calculated. Let the cement sidewalk slab for this example be four inches thick, five feet wide, and reinforced with a coarse hardware cloth. The slab laying on the ground can withstand approximately 3000 psi when loaded from the top if the slab lays flat on a smooth under-grade. This slab can withstand 330 psi if pushed up on from the bottom. If a small root elevates the slab only a small amount (one inch) with people continuing to walk on the slab's top surface, only 60 pounds of pressure per inch is required to crack the slab. Over time this a relatively small amount of pressure.

In the case of waste-water carrying pipes, older clay and cement pipes with gasket connections along their length present many avenues for tree roots to colonize over time (35,45). The pipe bed or underlayment can lead to a proliferation of settling faults. Roots growing along the pipes, and developing mass over time, can exert significant new pressures on pipes. Roots pushing into and around gasket connection points radially expand and break seals. Pipe materials that easily transmit temperature changes to their surfaces can provide areas of fracture pore space and available water condensation around the pipe. New plastic pipe materials, solvent welding systems, and proper installation can help eliminate future tree root problems in pipes (45).

Soil Movements

Trees can transpire large amounts of water under good conditions. As water is removed from expansive clay soils, soils shrink. Soil shrinkage from this process occurs wherever roots are concentrated. Soil volume changes, primarily shrinkage, can cause damage to infrastructures not designed or sited properly on expansive clays (20,30,44,49). Most solutions other than changing design, materials, and their use, are effective only over the short-run and usually involve significant damage to any tree involved. Tree removal and abusive, periodic crown pruning practices are inappropriate solutions sometimes used to minimize damage (39).

Growth Environments

Materials used for underlayment of pavements are usually coarse, well-aerated products, like sand. The rest of the soil left in position around infrastructure is usually compacted to some degree, with sub-soil / sub-grade extensively compacted to bear infrastructure weight in use. Many dense building materials
have enough thermal mass to keep them away from immediate temperature equilibrium with their soil environment. Materials being out of thermal equilibrium with neighboring materials lead to water vapor pressure changes and water condensation at the interface (along the surface pore space) (3,5,19,20,45).
Along and under these dense materials, with limited evaporation, the soils can be at or near field capacity for long periods (28,29).

Pipes made of dense materials have additional thermal interactions with the soil because of liquid temperatures moving inside. The greater the differential of temperatures, the greater chance for pore space development at the soil - pipe interface, for increased maintenance over time, and for water accumulation seasonally or daily. Thermal changes also stress joints, gaskets, and connectors providing opportunities for roots to utilize additional resource space.

Conclusions

Literature Cited