Citygreen / ResourcesFrequently Asked Questions

Frequently Asked Questions

Strata Cell Vs Strata Vault – Product Comparison

What applications would you recommend for StrataCells & StrataVaults?

Wherever engineered void space is required beneath paved surfaces, for quality growing media for trees and/or stormwater treatment.

What are the key differences between the two types of units?

StrataCell is the generation 5 system. StrataVault is the generation 6 system. In essence Stratavault has larger apertures and is lower cost to install and supply per cubic. Please refer attached documents clarifying the design differences.

What is the cost comparison between the StrataCell and StrataVault?

StrataVault is approx. 10-15% lower cost per cubic metre than StrataCell.


Compacted collar & excavation extents

Please advise how the width of 200mm for the reinforcing collar was determined?

This width was determined as the minimum width for the compaction collar. Objective is obviously to keep over-excavation to a minimum and this width is dependant on the machinery the installer has access to. Majority of machinery hire firms will have a 150-280mm vibration rammer available, if the contractor doesn’t.

The purpose of the compacted granular collar is to place the geocomposite material under tension for reinforcement of the matrix, and to minimise possibility of differential settlement of pavement.

Also, please confirm the depth of compaction collar required.

Typically engineers have suggested a benched or tapered design. Majority of projects have utilised the tapered granular collar design to date, for convenience.

The video I found on YouTube only showed the top portion of the collar being compacted. If this is true, how is the soil beneath appropriately compacted?

That method was proposed by engineers a number of years ago, and is still valid, due to the fact that lateral forces from adjacent pavement wheel-loads are calculated as very small at those depths beneath finished pavement. Later designs have moved to an angled design for ease of excavation, either designs can be used.

Please confirm what order the soil media and compacted collar are to be placed. Previous comments advising that the collar is to be compacted first, however this is inconsistent with the installation video.

Both methods can be done, however the latter is most practical and followed currently.

Please confirm if the sides of the strata units are to be vertical of tapered. All standard drawings show a tapered slope, however I’m uncertain how the wedge formed at the base of the units is to be suitably stabilised/compacted.

Please refer comments above (point 4).

What options are available to achieve satisfactory compaction/horizontal support in conditions where traditional compaction of backfill material or quarry rubble is not suitable?

This is rare, and not ideal, however we have known of projects where they have used self-compacting or cement stabilised fill.

What horizontal support is required when compacting the soil media and compaction collar?

For a recent project we have included a formwork board in order to achieve suitable compaction for the compaction collar. Placement of filler media within matrix and granular media within reinforcing collar should occur simultaneously, to provide anchorage to the skeletal structure of the matrix.


Trafficable suitability and pavement design

What are the limitations on using StrataCell under a trafficable road?

This depends on whether you’re needing to achieve a 5t, 10t or full W80 pavement design. Please refer above URL to full engineers report and sample working sheet.

Have other clients/councils used StrataCells in the road pavement?

Absolutely, within Australia and overseas.

What is the documented pavement design criteria (CBR)?

This was mentioned in the StrataCell technical specification, however I was unable to find it. The engineering report addresses this point.

Can concrete be poured directly on top of the Strata units?

We seek to install kerbing and spoon drain above the units. Yes, certainly. This is a common scenario, and actually reduces the cost of Stratavault application due to Top Grates not required on the cell matrix.

Please advise if the application is suitable under car parks?

Yes, a common application.

What are the limitations of the StrataCells under a loading zone?

See engineers report provided.

Can you please provide example pavement design for strata cells under the road and the associated considerations required when undertaking pavement design near and above Strata Cells?

Please refer to the working sheet provided.



What is the closest distance that StrataCells can be placed to services?

Services are commonly placed within and adjacent to the matrix. Different service providers have varying clearance requirements, however filled Stratavault/StrataCell systems have equivalent support to native subgrade.

What services can be placed through the StrataCells? Are these approved the relevant service authority? How are services through the StrataCells managed?

Where services cannot be accommodated with the cell structure, a gap is left in the matrix, providing a service corridor, and a bridging slab placed to span the gap, to engineering design.


Strata unit placement

Can StrataCells be placed vertically offset from the layer below?


Alternatively, can StrataCells be cut in half or are half pieces available?

Yes, either/or. Very commonly are cut onsite. Due to the discrepancy between underground services and the required location of the tree on the surface, there may not be sufficient space to place a tree ball for the species proposed, without shifting or cutting the top StrataCell beneath the tree. Citygreen would want to see an example, to illustrate your scenario.


Root barrier and geofabric requirements

When are root barriers/Geosynthetic Clay Liner/geofabric/filtergrid liners required?

HDPE Rootbarrier membrane is used to prevent invasive tree root damage. Engineered geocomposite (FilterGrid) is used to provide separation of dissimilar materials and to reinforce the structure. Additionally, how effective is the filtergrid is preventing or deterring root penetration? Whilst FilterGrid is not designed as a root barrier, a recent excavation revealed no root penetration of a vertical layer of this material.

When is infiltration into the adjacent soil acceptable/not acceptable?

If you’re referring to root penetration; Citygreen does not endorse containerising the tree. If there are nearby assets to protect (building foundations, road or pavement, or services) in these instances it is wise to protect that relevant edge, however not the complete pit. Our objective is to provide the soil volume required for the life needs of the tree.


Underground drainage

What measures can be used above the StrataCell to reduce the total thickness of the pavement above the cells?

Wherever possible utilise rigid pavement. With much of our underground drainage system at shallow depth, we are seeking to minimise to total depth of the units in order to achieve a suitable connection to the local stormwater network.

When are agg drains required and not?

This depends on the purpose; Aeration and water distribution within the soil cell matrix: – Structural soil cell tree pits always require slotted pipe to run throughout the matrix to provide both oxygen and water to the soil environment. This is crucial for the health and longevity of the growing media.

Drainage of surplus water from beneath the soil cell matrix: – Standard (non-water harvesting) treepits require positive drainage via ‘agg drain’ if the surrounding soil was not free draining. WSUD treepits require positive drainage via under drains.

Are you able to confirm if the filter sock around the slotted agg pipe is required, as mentioned in the installation video?

We are currently reviewing the option of excluding the filter sock / geofabric as it has a tendency to block up. This is dependent on the amount of water flowing through the pit (ie if a WSUD tree pit), and the designer’s preference. Mindful that the piping serves a dual purpose, and that volume of water flow through these pipes is dependent on the design purpose of the pit, that is Council’s decision.

What possibility is there to have a flat base to the tree pit instead of the 5% grade proposed in the drainage?

Whilst encouraging drainage is preferred, this may not always be achievable due to the extra height required to create the grade. We recommend a minimum of 2% grade is provided at base of pit, however adequate drainage can also be achieved by setting the drainage line lower than the main base. This is a case-by-case consideration, with the main purpose being to eliminate the risk of the tree pits becoming water-logged, affecting the health of the soil and trees.

What do you propose when wanting to plant trees with shallow stormwater network?

Given that ideally we would seek to provide an aggregate drain and associated connection to the local stormwater network. We would raise the depth of the overall pit, and if possible, bring the matrix closer to the surface. We can also bring the drainage layer within the lower portion of the matrix.

Can the aeration pipes be incorporated into a stormwater inlet pipe or do they need to remain separate?

We would combine these. For sediment catchment prior to reaching the matrix, we recommend pre-filtration points such as gross pollutant traps, rain gardens or porous pavement. Why? Maintenance advantages.



What passive irrigation techniques are recommended/possible?

A number of methods can be utilized, either altogether, or selectively as required; Porous pavement; kerb inlets, strip drains, rain gardens -to name a few. As mentioned above – the capture and periodic removal of TSS needs to be incorporated into the design.


Tree planting & rain gardens

How does the size of the required tree pit vary?

The size/volume is dependent on the needs of the mature tree species, and the water modelling of the site. What is the ideal configuration? Ideally wide and shallow, symmetrical in shape with tree centred in the matrix, to mimic the nature of tree root development. Narrow and deep, VS wide and shallow, etc. What is the maximum recommended height and width of strata unit for use with trees? Again, this is purely dependant on the projected mature species’ needs. Please see attached industry formulas for calculating this volume.

What happens if we are unable to achieve the recommended tree pit size?

The volume and quality of growing media directly influences the longterm size and health of the mature tree canopy. If faced with a ‘worst-case’ scenario of not achieving target due to infrastructure constraints, the pit can be moulded to fit within the space available. A reduction in soil volume per tree will result in reduction in both life expectancy, and ultimate size of the tree.

What soil composition do you propose for the Strata units when placed in conjunction with a rain garden?

Dependant on the flow rates needing to be achieved, we modify the soil specification to suit. The standard soil specification was designed by SESL, renowned soil scientists. Refer attached soil specification for your perusal. The current plans ask for sandy loam with the inclusion of organic matter, however a rain garden requires a mixture of sandy loam, transition media. On a recent project on Waymouth Street (attached) we have proposed an alternative rain garden soil media to meet the requirements for the tree plantings within the StrataCells. Very good, yes that is a similar approach we’ve taken. We’d more than happy to provide comment on your alternative specification for potential use within future Citygreen tree pits.

What is the minimum required clearance to services?

As briefly addressed in the earlier portion of this email, this is a question for the relevant service provider. Our system has been placed around, adjacent and within a short distance so there is no limitation from our perspective.

What alternative options can be used in lieu of a compacted collar? Can we utilise a concrete or similar material to provide suitable support for the StrataCells and pavement above in narrow space?

See above responses to Point 4.

Can the Strata Cells be placed within the trafficable road?

Certainly. What limitations are there on this? See above responses to Point 9 and attached engineering report.

What are the limitations of the StrataCells under a loading zone?

See above responses to Point 9 and attached engineering report.

Can concrete be poured directly on top of the Strata units?

Yes, with FilterGrid (reinforcement) and builders membrane (protection of treepit soil from cement slurry) between.

Can the aeration pipes be incorporated into a stormwater inlet pipe or do they need to remain separate?

Refer above Point 26.

What services can be placed through the StrataCells?

Services with a pipe diameter up to 140mm can be run through the StrataCell. Services with a pipe diameter up to 230mm can be run through the Stratavault. Alternatively a gap can be left in the matrix, or vertical supports can be removed with engineer approval – Please refer Point 17 above. Are these approved the relevant service authority? How are services through the StrataCells managed? Much the same way as traditionally – excavation and dismantling of the matrix is straightforward. We have a water service which we would ideally like to place through the units to obtain more growing space.