Urban tree management in the development and maintenance of urban landscapes.
Project Number 17311 Starting 10/01/2002 Ending 09/30/2007
The objectives are to develop a model for pressure distribution and set the foundations for discussing a design modulus for root presence under pavement. An empirical model of root pressure distribution through Styrofoams, geotextiles and soil-type materials will be developed to objectively evaluate protection strategies and encourage understanding of the root-pavement interface. Loads similar to roots pushing on concrete and changes in pressure distribution resulting from increasing distances between the pressure source and the concrete section will be measured. The project also looks at one protection strategy, foam layers, to evaluate its effectiveness from a mechanical, rather than observational approach. A method of interpretation is thus provided to begin discussions to deal with tree-infrastructure conflicts. The study can also identify the critical levels of pressure needed at given distances to cause damage, thus providing a method of evaluating species root intrusiveness or protection under specific situations. Root growth pressure will be directly measured through the development of root pressure collars for lab and field use. The collars will be fabricated and tested as a method to monitor radial root growth pressures over time. Once calibrated to known pressures, the tools will be used on existing tree roots. As the tools are developed and used, measured pressures will be added to the distribution model development. Additional testing will work to define soil environmental impacts on root expansion pressure and onset of eccentric cambial growth. Information from the studies will be used in continued research on pavement protection issues and the mechanical role roots play in pavement damage. APPROACH: A grid of load cells can develop stress distribution maps with measured loading sequences. Working pressures will be initially established from the existing literature on young roots. Loads will be imposed from an expanding cylinder and a one inch wide uniform load equal to the total root growth pressure, as eccentric root cambial growth may focus root expansion upward where there is less resistance. A time component will be factored in as the expansion of the root presents a long-term continually increasing stress on an aging pavement wearing surface. Measurement of load distribution at the pavement interface will be collected as the distance of the root analogue through given materials is manipulated. Rigid foams which have been observationally tested as a protection layer, will be used in three layers to look at the change in loading distribution with increasing layer thickness in a uniform medium. An additional geo-textile layer will be imposed to evaluate their probable use in spreading the load over large sections of the concrete. Phase two testing will expand the testing into an aggregate system and may use preliminary root growth pressure data generated from other sections of the greater research effort. Data will be coupled with existing methods for evaluating the flexural strength or stiffness of concrete materials and methods to evaluate these materials as a function of service life age. The root pressure couples will use temperature compensated load cell signals placed under a contact gasket within the removal collar. Once set, data logging of signals will provide growth pressure data to logic check the pressure distribution model. Soil environmental parameters, mostly water and internal shear strength will be used to manipulate the pressure exerted in radial root growth to displace soil solids, prior to deforming from eccentric root growth to move around rigid objects.
While tree roots are associated with displaced sidewalks, it is clear that root presence does not necessarily result in pavement damage, and repairs to pavement often damage the tree. Several methods have been advanced as solutions, yet little is known regarding root growth pressure, or its dissipation through materials between the root and the pavement surface. This project begins to define the mechanics of root expansion effects on pavement to evaluate current and emerging tree-pavement solutions.