Figure 2: Silva Cell Illustration

The oldest installations of suspended pavement of which we are aware were installed in 1985 in Bethesda, Maryland , and in Charlotte, North Carolina. In both cities, the trees have performed significantly better than the average urban tree that only lives to be 13 years old.

• 
  Average tree height was 12.2 to 13.4 m (40 to 44 feet)
  
• 
  Average diameter at breast height (DBH) was 356 to 508 mm (14 to 20 inches)
  
• 
  Average soil volume was 17 m³ (600 cubic feet) (not counting soil sharing)
  

• 
  Average soil volume was 17 m³ (700 cubic feet) (not counting soil sharing)
  
• 
  167 out of 170 trees planted (98%) are still alive 26 years after planting.
  
• 
  Average height is 13.4 m (44 feet)
  
• 
  Average DBH is 0.4 m (16 inches)
  
  

A study by Bartlett Tree Research Laboratories has been comparing tree growth in natural soil under suspended pavement compared to growth of trees grown using other ways to prevent rooting volume compaction under pavements: stalite soil, and gravel soil (ie structural soil), as well as to trees grown in compacted soil. Each tree was provided 5.7 m³ (200 cubic feet) of rooting space. As of 2010, the 6^th year of the study, Elm growth was best in the suspended pavement with natural soil (see Figures 4 and 5).