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Applied Research Impact Case 5

Predicting Pavement Damages Associated with Urban Trees

Dr Louis Lee, Faculty of Design and Environment


In Hong Kong, space is precious. Only very little space in the city is reserved for urban greening. In urban areas, trees are planted in square or round pits on the pavement which are known as “tree pits”. They are a common urban green infrastructure found across Hong Kong. The problem with these tree pits is that as a tree grows, its roots and flares (the broadening of the tree trunk just above soil) start to protrude and come into “conflict” with the surrounding pavement. The tree’s struggle for space to grow and expand could cause pavement damage, resulting in trip-and-fall hazards for pedestrians, declined health for trees, and high repair costs for the government.


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A tree planted in a square tree pit on the pavement


Dr Louis Lee from the Department of Environment studied the relationship between tree height and trunk flare diameter of common urban tree species in Hong Kong. A total of 1,100 trees of fourteen different species in the Chai Wan district were studied. Factors affecting the presence and magnitude of protruding tree roots and flares were investigated. Data about the trees’ diameter at breast height, trunk flare diameter, tree height, lean angle of the main trunk, to name but a few, were collected. Habitat-related factors such as the size of the tree pit, pavement width and pavement material were also examined.


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Example of protruding tree roots


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Example of protruding tree flare


Using the data collected, quantitative models were developed to predict the amount of space required for the growth and expansion of tree flares of each species. Knowing the minimal planting space needed for each type of tree could avoid or reduce any conflicts between tree roots and flares, and the pavement. Moreover, habitat condition is a main cause of the problem. The findings showed that a square-metre increase in open soil would prolong the protrusion of roots by 0.154 to 0.172m, and concrete pavements tend to have a higher likelihood of root protrusion.


This study presented a simple yet replicable method of recording and monitoring the conflicts between trees and pavements for future research in this area. The findings could inform landscape architects, urban planners, pavement engineers and other related professions to optimise urban greening, and pavement design and protection.


(Acknowledgement: This project was supported by the THEi Seed Grant Scheme, Project No.: SG1920114.)