Structural soil cells and tree pit systems
Soil cell installation in Adelaide: room for street trees to grow under pavement
Load-bearing soil systems that let a street tree reach full canopy in the middle of a paved plaza.
SC-01
Pavement and roots want opposite things from the ground
A footpath needs its base compacted hard so it won't sink. Roots can't grow through soil that dense. So a street tree in a standard pit is stuck with the cubic metre or two of loose soil it was planted in. It stalls, or it sends roots under the pavers looking for more and lifts them.
SC-02
What a soil cell is
A soil cell is a modular frame of rigid plastic, stacked like a skeleton under the pavement. The frame carries the pavement load down to compacted ground. Inside it is loose, uncompacted planting soil. The pavers sit on the frame and the roots grow in the soil, so both get what they need from the same square metre.
One tree pit, top to bottom
- Pavement on a thin aggregate base
- Geotextile and geogrid to spread the load
- Deck and frame carrying the pavement
- Planting soil, uncompacted, where the roots live
- Compacted sub-base under the frames
- Root barrier along kerbs and services
- Inlets for water and air, plus stormwater if designed
SC-03
What you get for the money
The canopy on the drawings. Trees reach the size the design and the planning conditions assumed.
Level pavement. Roots stay in the soil, so you're not relaying lifted pavers.
Stormwater used on site. The pit can take runoff from the pavement and hold it for the tree. This is water-sensitive urban design (WSUD) at street level.
Services left alone. Root barrier keeps roots out of pipes and conduits under the footpath.
SC-049 items
Our scope
- Tree pit detail and soil volume review, before tender if you like
- Excavation and sub-base compaction
- Soil cell frames installed to the manufacturer's guide
- Planting soil placed in lifts, left uncompacted
- Root barrier, plus irrigation and aeration inlets
- Stormwater inlets to the civil engineer's levels
- Deck ready for the paver, or paving by us
- Tree planting with grates or permeable surrounds
- Establishment maintenance after handover
SC-05
Systems and standards
We install to the manufacturer's guide and the engineer's pavement design. Hold points are agreed with the head contractor before the first frame goes in: the sub-base before cells are placed, and the frames before the deck closes ASYS hold points and ITP.
Load capacity comes from the system and the pavement over it. Citygreen Stratavault is rated to the HS20 highway loading standard. Citygreen Stratacell comes in a 300 kPa series and a 600 kPa series. The pavement engineer matches the system to the traffic.
Trees are supplied to AS 2303, the Australian standard for landscape tree stock ASYS specifies AS 2303 stock.
SC-06
Who we build these for
- Developers
- with street tree conditions on a paved precinct.
- Head contractors
- who need tree pits signed off before paving.
- Councils
- upgrading streetscapes without losing the trees.
- Landscape architects
- who want the detail checked before tender.
SC-07Placeholder projects
Soil cells on site
SC-085 questions
Common questions
How much soil does a street tree need?
It depends on the species and the canopy size in the design. Design guides commonly allow 15 cubic metres or more of uncompacted soil for a medium street tree to reach maturity. A standard tree pit gives it one to three. Soil cells let you provide the full volume under pavement.
Can vehicles drive over soil cells?
Yes, when the system and the pavement over it are designed for that traffic. Some cells suit pedestrian areas and light vehicles. Others, such as Citygreen Stratavault, are rated to the HS20 highway loading standard. The pavement engineer confirms the design.
What's the difference between structural soil and soil cells?
Structural soil is a gravel and soil mix compacted to carry pavement, with roots growing in the gaps between stones. Most of its volume is stone. Soil cells carry the load on a frame, so almost all the volume inside is usable soil. Where the design asks for a big canopy in a small footprint, cells usually give the better result.
Can a soil cell tree pit take stormwater?
Yes. An inlet can direct runoff from the surrounding pavement into the soil, where the tree uses it. It's a common water-sensitive urban design detail. The civil engineer sets the inlet and overflow levels.
When should we bring you in?
At design development if you can. Soil volume and pavement levels are much easier to resolve on paper than in an open trench. We're happy to review a tree pit detail before it goes to tender.
Send us the tree pit detail.
We'll check the soil volume and load rating against your drawings.