Stormwater Tree Trenches: Specifications

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Specifications for Stormwater Tree Trenches
Material Specification
Growing Medium
  • Should be Canadian Soil Classification System sandy loam with combined silt- and clay-sized content between 18-35%; and sand- to fine gravel-sized content (0.074 to 5 mm dia.) between 65-82%.
  • pH value (6.0 - 8.0).
  • Salt level < 2 mmhos/cm.
  • Percent organic matter shall be 3-5%, by dry weight.
  • Growing medium compacted to 80-90% below the tree root ball to prevent settling.
  • Bioretention filter media may be suitable for use as growing medium, depending on climate and tree species (see Bioretention: Filter media).
Modular Soil Support System
  • Structures are designed to be filled with growing medium for tree rooting and support a vehicle loaded pavement up to and including AASHTO H-20 and Ontario Building Code standards for sidewalks.
  • Critical to modular soil support system design is that each structure or layer of structures be independent of all adjacent ones, such that one or multiple layers can be removed to facilitate future utility installation or repair.
Structural Soil Medium
  • Structural soils are installed in the trench adjacent to tree planting pits under permeable or impermeable pavements.
  • Structural soils consist of 3 components, mixed in the following proportions by weight: crushed stone (79.07%), clay loam soil (20%), and hydrogel tackifier (0.03%).
  • Total moisture at mixing should be 10% as per AASHTO T-99 optimum moisture.
  • Crushed stone (granite or limestone) should be narrowly graded from 20 to 40 mm diameter, highly angular with no fines.
  • The clay loam soil should conform to the Canadian soil classification system (gravel <5%, sand 25-30%, silt 20-40%, clay 25-40%). Organic matter should range between 2 to 5% by dry weight.
  • The hydrogel, a potassium propenoate-propenamide copolymer, is added in a small amount to act as a tackifier, preventing separation of the stone and soil during mixing and installation.
  • Mixing can be done on a paved surface using front end loaders. Typically the stone is spread in a layer, the dry hydrogel is spread evenly on top and the screened moist clay loam soil is the top layer. Them entire pile is turned and mixed until a uniform blend is produced. The structural soil is then installed and compacted in 150 mm lifts.
Structural Concrete Panel
  • Structural concrete panel is 250 mm thick, contains rebar reinforcements and sits on equal-sized concrete footing supports on rows of modular soil support structures or structural soil medium, which are supported by a minimum 150 mm base of compacted granular material.
  • Decompact native subgrade soil under tree openings and between granular bases of modular soil support structure rows during installation for better infiltration drainage performance.
Aggregate Base
  • Aggregates are used in modular soil support systems below the structures as the trench base layer, and sometimes, on top of the structures, as the pavement base layer.
  • Specifications for aggregate base materials determined by the designing Engineer based on varying levels of structural loading and hydraulic requirements.
Geotextile & Geogrid
  • Geotextile, geogrid or combinations are typically used on top of modular soil support structures and along the sides of the trench to separate growing or structural soil mediums from native soil or aggregate backfill. Geotextile and geogrid should not be installed on sides adjacent to pervious landscaped areas to provide opportunities for tree roots to grow outside the trench in these locations.
  • Geotextile material specifications should conform to Ontario Provincial Standard Specification (OPSS) 1860 for Class II geotextile fabrics.
  • Geotextile, geogrid or combination products in contact with modular soil support system structures should be according to manufacturer’s specifications.
  • Geotextile installed on tree trench sides and around perforated distribution and underdrain pipes should be woven monofilament or non-woven needle punched fabrics. Woven slit film and non-woven heat bonded fabrics should not be used as they are prone to clogging.
  • Where a root barrier is needed to prevent the migration of roots out of the tree trench, use impermeable ribbed barrier material with a thickness of 1-2 mm.
  • Should be minimum 150 mm dia. perforated HDPE or equivalent material, smooth interior wall and continuously perforated with geotextile sock.
  • A solid standpipe connected to the underdrain pipe and extending to the growing medium or pavement surface can be used for inspection and maintenance access. The top of the standpipe should be covered with a sealable cap or plug and secured with a vandal-proof fastener.
Stormwater Distribution Pipe
  • Perforated pipe should be minimum 150 mm dia. rigid, smooth interior wall HDPE or PVC with perforations on sides, wrapped with geotextile sock, with capacity and perforation specifications confirmed by the designing Engineer based on hydraulic requirements.
  • Solid pipe from inlet structures should transition to perforated pipe once 300 mm inside the trench.