Construction guidelines for tree trenches and tree boxes

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The anticipated construction period for this page is through June, 2014

Standard keys to success in bioretention construction apply to trees for bioretention, including, for example, the following.

• Plan for feasible temporary and permanent erosion and sediment control techniques and sequencing
• Plan for temporary and permanent erosion and sediment control techniques, sequencing, and pay items, and prepare a thorough SWPPP plan. Examples techniques include compost logs (MnDOT 2573) and plastic sheeting (MnDOT 2575), and diversion berms (MnDOT 2573).
  • Plan to minimize or avoid soil compaction to the extent feasible. Techniques include for example, using driveable mats, using tracked machinery, and machinery with long arms to avoid having to drive in tree trenches.
  • Designate a stormwater supervisor to make sure someone is responsible for erosion and sediment control.
  • Plan for snow storage during (if applicable) and after construction
• Construction administration and communication with contractor
  • Effective communication during pre-construction meeting.
  • Include “check points” in specifications with timelines – points of inspection which must be approved before proceeding to next step of construction. These will vary depending on specific project,but will include, for example, approval of required submittals, and required testing, such as, for example infiltration tests.
  • Ensure checkpoints are approved prior to proceeding to next steps. Where applicable, require signature by Contractor, Designer, and Chief Inspector prior to proceeding to next step.
  • Require submittals for material to be used, including, sources and certifications where applicable.
  • Specify required tests and tolerances. See the bioretention section for other construction guidelines and specifications.

Construction guidelines and specifications specifically for trees for bioretention

In addition to general bioretention guidelines and specifications, the following guidelines and specifications apply specifically to trees for stormwater design.

• Tree material and installation guidelines
• Tree soil quality and volume guidelines
• Tree opening guidelines
  • Tree openings need to be large enough to allow for trunk flare. Minimum recommended tree opening dimension is 5 feet by 5 feet.
  • The use of tree grates is discouraged in order to protect the tree root flare.
  • Tree openings need to be protected from foot and vehicular traffic.
• Tree spacing guidelines
  • For street trees, a minimum spacing of 30 feet on center is recommended for large trees to allow their canopies to grow to their full size. This also makes it easier to provide adequate soil volumes for each tree.
• Guidelines for providing rootable soil volume for tree root growth and bioretention under pavement: where there is not enough open space for traditional bioretention, several techniques exist to protect soil volume under pavement from traffic compaction so that this soil can be used both for bioretention and tree root growth. Examples of these techniques include:
  • Structural cells
  • Rock based structural soil
  • Sand based structural soil
  • Soil boxes