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Gap Graded Structural Soil

Gap Graded Structural Soil is formulated for use in areas where trees are planted within or near sidewalks, streets and driveways. This mix consists of a sized rock, clay loam, and a soil aggregating polymer Soil Drain/P.A.M.  This combination allows architects and engineers to achieve the proper compaction for placing concrete or asphalt over the planted surface, while still providing proper pore space for root development.  Using this Gap Graded Structural Soil will allow potentially invasive roots to grow within the over sized planting pit without causing structural damage to the solid surfaces above.

Since a treated clay loam soil is used in this blend, this increases the water holding capacity of the medium. This mix should not be used for drainage purposes, because the polymer/topsoil combination does not support rapid drainage. It is specifically designed as a planting medium that can be compacted without compromising the root development of the tree. Typically, the soils under sidewalks and pavements are compacted, which can inhibit the migration of the roots. These roots can also damage the sidewalk. The pore spaces between the rocks will allow the roots to grow. The clay soil that has been treated with fertilizers and a linear polymer provides the necessary nutrients.   


  • Dig oversized plant pit 2-3 times the container width and at least 12” deeper.  Install soil mix in 12” lifts. Each lift must be compacted by a non-vibratory roller.  Scarify the top 2 inches of each lift after compaction and prior to placing the subsequent lift.


  • Amended clay loam topsoil, 3/4” sized rock and Soil Drain/P.A.M. (polyacrylamide).

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