Construction damage can create many tree hazards, and there are a diverse range of variables that can impact a tree on an active work site.
Visible scarring or branch breakage is easy to spot and is of relatively low risk. Excavations, soil compaction, and the removal of roots can create a significant hazard. There are no definitive applied principals for tree assessors to deliver confident answers on the stability of trees after an excavation. To determine the critical root zone, we can measure the trunk diameter in inches, and multiply that value by 1 to 2 feet. The factor of multiplication can depend upon the age of tree, and who’s method you would like to use. Then it is up to the tree assessor to evaluate the types of roots that may have been impacted by the construction within this zone. In some cases a tree is structurally compromised by construction, and fails years later when everyone has conveniently forgotten that any construction occurred. The time delay to failure in those circumstances results in a lack of accountability and limits any learning opportunities for contractors.
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Grade Changes
The two trees pictured in the slide show above, display some of the counterintuitive problems that can arrive during a visual evaluation of an active construction site.
The tree in the backyard appears to have a huge change in soil profile, bringing with it some serious concerns about tree stability. This tree however grew in that soil profile and the situation appears much worse than it actually is. An existing retaining wall had been removed making it appear that a large excavation of roots has occurred. In the two images of the backyard tree, not a single root was impacted and the soil profile remained the same. This meant there was little to no change to the stability of the root system.
The three images in the front yard, show a tree that looks less impacted than the backyard tree appeared. This is not the case, as the front yard tree had its lower branches removed which changes its ability to dampen wind loading forces. There was an excavation through the root zone to install services for the house, which had been backfilled at the time of the image. The soil of the root zone has been heavily compacted from construction equipment and material storage. Compacted soils can lead to root losses and allow decay to set in. The utility trenching severed and cracked roots, allowing another viable entry point for decay organisms to take hold.
Sometimes construction sites can be deceiving, and a site history is an essential part of identifying hazards. Things aren’t always as they appear, such as the two trees discussed above. The tree in the backyard that appeared to be unstable was minimally impacted, meanwhile the other tree in the front yard had more significant damage done to it.
Click on the link below for more information on tree protection:
- Protecting Trees from Construction: A Homeowners Guide
- Tree Protection Policy and Specifications for Construction Near Trees
Tree Protection Ordinance
Tree protection ordinance to prevent defect creation during construction exists in many municipalities. Their strength and effectiveness at delivering the intended tree protection, is limited to the laws and enforcement that governs them. Some local authorities are limited by county, state, or provincial laws that limit their powers. In the images shown above, the trees being protected during construction are only required to have the small barrier fencing seen in the image. The municipality has no authority to protect any root zone that extends into the private lands. The tree protection bylaw then in effect only offers trunk protection, and does little to protect public trees that border on private development.
On construction sites this same municipality has some jurisdiction to demand that trees be retained on private lands during construction. The municipality can enforce rules to have developers install a new replacement tree, if any that were listed on the development permit are removed. To avoid this rule, many developers now clear cut the lot prior to submitting development permits so they do not have to deal with the restriction. The intended goal of improving tree retention is in fact having the opposite effect of encouraging deforestation. The more dangerous aspect of the regulation for development on private lands, is that the tree stems simply must be retained in order to pass the criteria. Entire root systems could be removed, and dangerous stems are incentivized to remain standing in order to be compliant with the bylaw. The financial incentive in this case is for the developers to either clear cut the trees prior to the development permit submission, or leave the dangerous trees standing in place at the end of construction. In application the tree retention by-law serves to encourage behaviour that creates hazards. Therefore the tree by-law itself is one of the influencing factors in hazard creation.
Click on the link below for the tree protection by-law:
Root Damage
What we can’t see can kill us. Less scrupulous contractors can easily excavate large root systems, and cover it over with soil. Trees can then fail either directly from the root loss, or the subsequent decay that sets in from the damage.
In the images above the developer did everything correctly to preserve the 100 year old tree. Unfortunately an overzealous excavation operator cut further over than was planned to make it easier to access the forms. Thinking he was doing the construction team a favour, the operator was shocked to discover that his actions now lead to this tree having to be removed. Damage occurred to the structural roots that were as large as 8″ in diameter that went right up to the base of the trunk. A large majority of the root system was removed, likely upwards of 50%. The risk of stem decay also increased dramatically because of the close proximity to the trunk that the roots were ripped off by the excavators bucket.
It is interesting how many people would never remove the supports for building or human made structure, but have no issues removing the supports of a tree. In many situations the impact of root removal during construction is not as clear as it was in this case. There are many theories around strength loss inside the critical root zone. As an industry though, there isn’t even agreement on what the critical root zone is, or if it is even an acceptable term. Much more needs to be understood to help tree assessors and construction firms understand what is an acceptable amount of root loss before a tree becomes unsafe.
For more information on root loss during construction click below:
Western Canadian Tree Defects 