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CU-Structural-Soils

Overview

Structural soils are specially prepared "soils" that can be compacted without limiting plant growth. Typically, structural soils (also called skeletal soils) consist of:

  • 20 % clay
  • 80% gravel coated with hydrogel to prevent separation from the clay components

 

Structural soils were developed by Cornell University for city trees in the mid 90's. The idea has since spread to a myriad of places using local or proprietary materials. 

  • Some Californian cities use fist-sized rocks
  • Vancouver uses large granite rocks
    • Use of granite eliminates a common issue with structural soils: very high pH's
  • Orjan Stahl's Swedish Structural Soils
    • Large stones at the bottom of the tree pit
    • Soil is washed into the voids
    • Repeat for the next layers with slightly smaller stones at each layer

 

Some notes for structural soils

  • Structural soils can easily fail if not properly constructed
    • Stones must be large and angular enough to create abundant voids
    • Soils must have sufficient water holding capacity (otherwise the soils will be too dry)
  • Structural soils are not "better" than native mineral soils. If anything, they are worse.
    • Structural soils are only meant to go under pavement! They are not a substitute for native mineral soils.
  • They are useful for constructing "break out" zones around tree pits or for creating paths to a larger breakout zone.