Modelling the influence of root position and growth on pavement tensile crack failure when occurring under three thicknesses of asphaltic concrete

摘要

Root-pavement conflicts are a common challenge in urban forest management. Given the expense of pavement installation and the time needed to develop large trees, there is a preference for modelling design solutions to integrate trees and pavement with shared soil volumes. While Finite Element (FE) models are used in pavement analysis and design, and have been used to describe root growth in soils, there are few examples exploring of integration of the two applications with respect to designing urban root zones under pavements. Four series of FE models were developed to test the influence of an asphaltic concrete wearing surface layer thickness and base layer thickness (the top two layers in a pavement design) in surface crack formation from a growing tree root simulation in positions within the top zone of the base layer and below the base layer. The expected pavement damage was defined at places where horizontal tensile stresses exceeded 862 kPa. The testing series were developed from a common group of 18 FE pavement layer configurations using 3 asphalt concrete (AC) thicknesses and 3 granular base thicknesses with a growing root element at 2 root elevations. In the first series, an AC layer of a thickness of 7.6 cm did not exceed the horizontal stresses needed to develop a crack when a simulated root element increased from 3.6 to 5.1 cm when the root element was below the base layer at least 10.16 cm below the bottom of the AC layer. A second series of 77 model runs with a refined root simulation verified the impact of a changed root simulation (shift of element shape with a material modeling change that accounted for soil hardening during deformation). For the second series, a 2.54 cm diameter circular root was expanded to a final size ranging from 3.8 to 10.16 cm. Results of the second series confirmed and added detail to the first, but required a significant increase in computational time to accommodate the added model and data output details. The third series developed root simulations from an initial 0.508 cm diameter (the smallest diameter available in the PLAXIS FEM modeling platform) to a maximum 5.08 cm diameter in a series of 55 tests. The third set allowed a method to observe and compare the colonization step to a series of radial growth steps, which caused minimal influence to the stress state of the AC as a consequence of soil displacements from the root growth. Finally, in the fourth testing series of 58 simulations, we tested a cluster of three root elements at a 10.16 cm spacing 0.635 cm below the AC or the base layer, growing from an initial 2.54 cm to 6.35 cm diameter. Modeling three root elements at the 10.16 cm spacing generated tensile stress-induced cracks in all AC-base-root depth configurations. Displacements in those models exceeded the imposed upward displacement limit of 1.27 cm after doubling in size from an initial 2.54 cm diameter.