Auckland City Council have successfully undertaken testing of the structural integrity of RootCell cellular load-bearing modules that ensure room for growing tree roots in urban landscapes.

RootCells, from Arborgreen Landscape Products , are filled with quality, uncompacted soil for healthy root growth. Rootcells permit the tree root systems to utilise the area under the adjacent pavement without lifting or breaking up the surface.

Trees in the Auckland CBD were previously planted in open tree pits, although specially-constructed blockwork pits for root containment have been specified recently.

The Council commissioned exhaustive testing of RootCells by independent consulting engineers, Jawa Structures, to ensure that the root modules could stand up to the weight of motor and foot traffic above.

Off-site trial pits containing the RootCell system were constructed, with a typical CBD footpath with a concrete sub-base and basalt paving on one side and a CBD carriageway with kerb and channelling on the other. Both surfaces were then loaded with more than 200 passes of an 8.2 tonne Benkelman Beam truck on the footpath side and 400 passes on the carriageway side.

The Benkelman Beam measures the deflection of a flexible pavement under moving wheel loads, using an accurate and easy-to-use direct-reading dial indicator.

The results showed no visible subsidence under these test conditions and from a cost, constructability and arborcultural perspective the RootCell system was applicable to Council’s requirements in Auckland’s CBD.

RootCells have now been trialled on-site in the recently completed upgrade of Auckland’s Swanson Street, which features one tree, and in an upgrade of St Patrick’s Square, where five trees have been planted.

The RootCell system enables the construction of large growing areas to facilitate long term development of trees so that they can attain a large, mature size.

The RootCell system that underwent testing consisted of 300mm2x 120mm high root protection modules made from 100% recycled polypropylene (PP). Each module contained nine vertical interlinked cylinders, each approximately 50mm in diameter.

The modules were clicked together in 600mm matrices so that the top of the matrix was 600mm below the top of the paving to suit the 500mm height of the 1.0m2 HDPE root director and 100mm depth of grate. Modules were arranged to form a 2.7m long x 1.8m wide rectangle with a central depression to receive the tree rootball.

The cells were initially filled to the base of the rootball with planter mix that was vibrated with a small plate compactor, and then completed to the top of the cells and within the root director with planter mix once the tree was in place.

The modules were designed to provide the necessary vertical and horizontal load-carrying strength around the rootball and within the planter mix to carry pavement loading from vehicles.

The RootCell system was designed to accommodate a nominal 1000L rootball from a mature Nikau Palm measuring 1.15m diameter x 0.7m deep. Some trimming of the rootball would be needed to fit it into the root director. A precast concrete panel was placed along half of the pit length to verify whether a shield was necessary to protect the rootball or modules from horizontal loading.

The 500mm deep gap between the top of the cells and the underside of the paving slab was filled with compacted granular material that in an actual pit would be material not conducive to root growth. To better simulate a larger rootball diameter, a flexible strip rootguard was placed around the outside of the square root director with a 50mm gap loosely filled with planter mix. The tree centreline was placed with a 1.1m offset from the kerb face.

A commonly-adopted 40mm thick basalt paver system, with trade mortar containing 30% megalastic onto a 125mm slab centrally reinforced with 663 mesh, was constructed across half of the area of each of the two pits. The second half was constructed with the recently-adopted exposed aggregate 150mm un-reinforced concrete slab.

The road carriageway adjacent to the precast kerb and channel was constructed in 35mm Mix 10 asphalt on 150 TNZ AP40 on 300 GAP65 sub-base, which is a typical pavement design on CBD roads.

Testing procedure - A Benkelman Beam truck was commissioned to conduct the following tests over two days:

  • 200 passes along the carriageway, with the centre of the kerb side wheel 500mm offset from the kerb face.
  • 200 passes along the carriageway, with the centre of the kerb side wheel 250mm offset from the kerb face.
  • 200 passes along the footpath, with the tree side wheel 530mm from the RootCell tree pit edge.
  • 40 passes along the footpath, with the tree side wheel 220mm from the edge of the RootCell tree pit.

Test results:
The Benkelman Beam measurements were taken at 1.0m intervals after each 40 passes along the road carriageway and at two locations adjacent to each tree pit. In all tests, no structural deterioration of either the road carriageway or footpath pavement occurred.

No differences were noted in the results that would have indicated subsidence or softening of the pavement beneath. No rutting of the asphalt or cracking of the concrete slabs or basalt pavers were noted. In addition, the concrete shields constructed along half the length of each of the pits did not provide any noticeable advantage.

Arborgreen RootCells have already been installed in key landscape projects. RootCells are combined with root deflectors that train the tree roots to grow downwards and into the root cell structure. When combined with aeration devices, such as a perforated pipe throughout the root cells, the roots can receive both water and aeration.

RootCells are modular and can be stacked in the excavation to provide a generous layer of available root zone, relevant to the type and size of the tree. The cells are made from recycled material with high structural integrity.