Testing of Structural Urban Tree Soil Materials for Use under Pavement to Increase Street Tree Rooting Volumes

"Quercus robur L. seedlings were grown in compacted stone-soil mixes known to meet engineering standards for pavement base compaction and strength. Root penetration into these materials was greatly increased in comparison to an equally compacted clay loam which was also one component of the test material. Oak root penetration in a clay loam decreased 50 % from 6 grams to 3 grams dry root weight, as the bulk density increased from 1.24 to 1.55 Mg/m3. Severe root impedance was observed when clay loam bulk densities exceeded 1.5 Mg/m3 (90 % standard AASHTO peak density), a situation produced after 20 % of the standard compaction effort was imposed on the soil profile. At the standard AASHTO peak density for the clay loam (1.67 Mg/m3), which would be the norm in a sidewalk installation, root growth was entirely stopped. In contrast, structured stone-soil mixes compacted to 100 % of their respective standard AASHTO peak densities (between 1.85 and 2.07 Mg/m3) did not restrict root penetration with mean root dry weights between 4 and 6 grams and demonstrated satisfactory bearing strength (California bearing ratios between 40 and 80)."