|CONVENTIONAL TREE PITS ARE DESIGNED FOR FAILURE |
Looking at a typical street tree pit detail, it is evident that the tree pit disrupts the layered pavement system. In a sidewalk profile, one can count on a subgrade of existing material. A properly compacted subgrade often can be counted on to be largely impermeable to root growth, water infiltration, and to significantly reduce drainage if large percentages of sand are not present. Above the subgrade there is usually a base material. To maintain a stable pavement surface, a base material is compacted and possesses high bearing strength. This is why a gravel or sand material containing little silt or clay is usually specified and compacted to 95% Proctor density (AASHTO T-99). The base layer is granular material with no appreciable plant available moisture or nutrient holding capacity. Water collection just below a pavement causes pavement failure. Subsequently, the pavement surrounding the tree pit is designed to repel or move water away, not hold it.
Acknowledging that the above generalizations do not explain all of the challanges below the pavement for trees, it it no mystery why trees are often doomed to failure before they are even planted. Excavation of large areas of soil is expensive. For that reason, the compacted subgrade often occurs within inches from the bottom of the pavement surface.
Considering that this zone is often impermeable and usually compacted to levels associated with root impedance, it is no suprise to observe tree roots relegated to the base materials just under the pavement or along faults in compaction. Given the poor drainage below the base, the tree often experiences a largely saturated planting area. Designed tree pit drainage can improve soil saturation, but does nothing to relieve a physical impedance of the material below the pavement.