This project renovated the Grass Lawn Park parking lot adjacent to 148th Avenue NE by addressing pavement failures, root eruptions, and ADA deficiencies resulting in a safe, level surface for vehicles to park and visitors to walk and roll on. New ADA parking stalls follow current ADA guidelines and requirements. In addition to renovating the parking lot, failing storm drainage pipes were also replaced.
The project constructed new curb ramps, sidewalks, stormwater pipe and catch basins, parking stalls; repaved the entire parking lot; replaced 22 significant trees, and installed new landscaping and irrigation.
The 22 trees within the parking lot and along the frontage of 148th Ave NE were removed and replaced with trees that are better suited for the site conditions (“right tree, right place”). Leaving the existing trees and merely cutting off roots would cause trees to become unstable, and they would have the potential to fall on the road or sidewalk in a wind event.
The trees that were removed were American Sycamore/London Plane (Platanus Occidentalis) which were not ideal for the site and soil conditions. The strong tree roots had severely impacted the parking lot pavement, causing uplifting and eruption of the asphalt, uplifted curb and gutter sections, and they had also infiltrated the storm pipes. Over the years Public Works maintenance staff had performed repeated root cuttings within this outdated pipe to remove blockages.
Storm pipe damage by tree root intrusion.
The London Plane trees were replaced with better suited tree types that include the use of structural soil. This structural soil is composed of gap-graded crushed stone (3/4” to 1-1/2”) highly angular with no fines, clay loam, and a small amount of hydrogel to prevent separation of the stone and soil. This medium provides a high strength base for pavements, yet still allows for deep root penetration spreading over a large area which helps prevent sidewalk and pavement heaving and encourages vigorous tree growth. The replacement trees include Redpoint Maple, Emerald Avenue Hornbeam, and Magnolia ‘Elizabeth’. The replacement trees along 148th Ave NE match existing trees in the corridor (Red Maples).