Citygreen Strata Cell Specification
The purpose of this specification is to inform landscape architects and installers of the detailed
requirements for using the Citygreen Strata Cell system effectively.
Strata Cell is a highly engineered solution that enables street trees to flourish and prevents heaving
of adjacent pavements and roads.
While the Strata Cell modules have very high strength capacity, the longevity of the pavement
structure is contingent on all components being correctly installed in line with relevant design
criteria.
The following Specification addresses each key step in the planning and installation processes.
Further information may be obtained from www.citygreen.com USA/Canada 1866 282 2743,
Australia +612 6578 8250.
Citygreen Strata Cell Specification
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4/5/2011
Table of Contents
1.0
Definitions ............................................................................................................. 4
2.0
Site Preliminaries ................................................................................................... 5
2.01
Product Storage .................................................................................................................. 5
2.02
Services ............................................................................................................................... 5
2.03
Tree Pit Layout .................................................................................................................... 5
2.04
Tree Pit Depths.................................................................................................................... 5
2.05
Soil Analysis ......................................................................................................................... 5
3.0
Tree Pit Excavation and Drainage ........................................................................... 6
3.01
Excavation ........................................................................................................................... 6
3.02
Reinforcing Collar ................................................................................................................ 6
3.03
Drainage .............................................................................................................................. 6
3.04
Sub-base Preparation.......................................................................................................... 6
3.05
Installation of Aggregate Sub-base below Strata Cell Matrix ............................................. 6
4.0
Matrix Installation and Soil Filling ........................................................................... 7
4.01
Strata Cell Assembly............................................................................................................ 7
4.02
Lateral Pipes ........................................................................................................................ 7
4.03
Vertical Pipes....................................................................................................................... 7
4.04
Root/Moisture Barriers ....................................................................................................... 7
4.05
Loading Matrix with Filler Soil ............................................................................................. 8
4.06
Filtergrid Separation/Reinforcement .................................................................................. 8
5.0
Compaction and Pavement Layers .......................................................................... 9
5.01
Reinforcing Collar ................................................................................................................ 9
5.02
Tree Pit Opening ................................................................................................................. 9
5.03
Granular Base Course.......................................................................................................... 9
6.0
Materials ............................................................................................................. 10
6.01
Strata Cell 60 Series .......................................................................................................... 10
6.02
Strata Cell 45 Series .......................................................................................................... 10
6.03
Strata Cell 30 Series .......................................................................................................... 10
6.04
Filter Grid 3030 ................................................................................................................. 11
6.05
ReRoot Linear Root Barrier ............................................................................................... 11
6.06
RootStop Linear Root/ Moisture Barrier ........................................................................... 12
6.07
Aeration Pipes ................................................................................................................... 13
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6.08
Filler Soils .......................................................................................................................... 13
6.09
Granular Base Course........................................................................................................ 14
7.0
Addendum ........................................................................................................... 15
Citygreen Strata Cell Specification
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1.0
Definitions
A. Strata Cell:
Patented, load-bearing, engineered plastic module with vertical and lateral
interlocks, for creating void space beneath pavements.
B. Coloured Indicator Triangle:
Coloured insert in top face of Strata Cell module, used to
enable correct alignment of modules. Indicator verifies module strength.
C. Strata Cell Matrix:
Assembled and interconnected volume of Strata Cell modules
D. Lateral Pipe System:
Perforated pipe system for circulation of air, and distribution of
water and nutrients, connected to surface grating.
E. Vertical Pipe System: Large diameter plastic pipe system for inspection, circulation of air,
and connection to surface grate.
F. Rootstop Root/Moisture Barriers:
penetration.
Linear membranes to prevent root or moisture
G. ReRoot Barrier: Linear root barriers with vertical integral root training ribs.
H. Filler Soil:
Correctly balanced soil mix to provide optimum growth conditions for tree
root systems within Strata Cell matrix.
I.
Filtergrid :
road base.
J.
Tree Pit Opening:
High strength geogrid bonded to filter fabric for reinforcement of granular
The pavement opening into which the tree is planted.
K. Granular Base Course: Compacted granular material to approved regional standards for
support of pavement to relevant Pavement Load design.
L. Tree Pit:
Excavated space filled with quality soil media for tree planting.
M. Reinforcing Collar:
A trench between Strata Cell matrix and surrounding soil, lined with
Filtergrid, filled with granular base course and compacted.
N. CBR:
California Bearing Ratio
O. ASTM: American Society for Testing and Materials (www.astm.org)
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2.0
Site Preliminaries
Assessment of site conditions and planning for installation.
2.01 Product Storage
Strata Cell modules must be transported and stored on Citygreen pallets with pallet wrap intact until
ready for installation. Pallets should be positioned on firm level base, so as not to impede traffic or
work flow.
2.02 Services
Installer must obtain accurate service locations from all providers and discuss potential conflicts with
tree pit location prior to commencement. Any amendments to tree pit numbers, dimensions or
location to be approved by the principal Landscape Architect in writing.
Installer must observe statutory clearance and cover measurements for service pipes and conduits.
2.03 Tree Pit Layout
Tree pit locations and dimensions, as detailed by the specifying Landscape Architect, must be
accurately surveyed and marked using string lines, survey pegs and marking paint. The Landscape
Architect is required to inspect and approve the tree pit layout, relative to project details, including
granular collar detail.
2.04 Tree Pit Depths
Installer to confirm excavation depths from finished pavement elevations, including provision for
drainage and base course layers.
2.05 Soil Analysis
Site soil analysis to be conducted by a Soil Analyst and submitted to Landscape Architect to ensure
adequate drainage of surplus water from base of tree pit. Where drainage is inadequate, provision
must be made using land drains connected to main storm-water pipe, or grade to soakage pits.
Design of soakage pits to be approved by Landscape Architect, prior to commencement of works.
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3.0
Tree Pit Excavation and Drainage
Excavating the pit, providing adequate drainage and meeting required load
capacity at floor of tree pit.
3.01 Excavation
Installer to excavate the tree pit accurately to the dimensions of the detailed plans, allowing 200mm
(8”) additional clearance in length and width. Side walls of excavated pit to be clean, straight, and
within 15° of vertical. Tree pit length, width and diagonals at base of pit to be measured to ensure
that correct dimensions are being obtained (measurements shown on tree pit detail plus 200mm).
Installer to confirm that correct depth has been provided, measuring from finished pavement level,
including any drainage layers. Base of tree pit should be flat unless there is a drainage layer to be
provided, in which case the subgrade surface must have a grade of 5%.
3.02 Reinforcing Collar
The top perimeter of the tree pit must be further excavated to a depth of 300mm (12”) and to a
width of 200mm (10”), or a width sufficient to permit a narrow foot compacting plate (reinforcing
collar) to be installed. Sides and base of this excavation must be clean and straight.
3.03 Drainage
Positive drainage to main storm-water service from base of tree pit is mandatory if the design
incorporates water harvesting and is strongly recommended if the site soil is not well drained. The
base of the tree pit must be graded at 5% to the perforated collection pipe in trench. This collection
pipe is to be wrapped in filter sock and connected to the storm-water system. A 100mm (4”) layer of
clean aggregate is to be placed in base of tree pit and leveled.
3.04 Sub-base Preparation
Base of tree pit must be free of debris and level. Installer to check CBR of the subgrade below the
proposed granular pavement layers to ensure it meets the applicable pavement design criteria. If the
subgrade is fill confirm that it is compacted to a minimum of 95% of maximum dry density at
optimum moisture content in accordance with ASTM D 698 Standard Proctor Method.
Compact the subgrade in natural ground with a minimum of three passes of a suitable vibrating
compacting machine or apply other compaction forces as needed to achieve the required subgrade
compaction rate. Apply additional compaction forces at optimum water levels.
If an aggregate drainage layer is specified cover top and sides of with a proprietary non-woven filter
fabric to engineer specifications and ensure the sides of the drainage layer are compacted against
the sides of the excavation to the density specified.
3.05 Installation of Aggregate Sub-base below Strata Cell Matrix
Install aggregate sub-base to the depths indicated in the drawings and compact to a minimum of
95% of maximum dry density at optimum moisture content, in accordance with ASTM D 698
Standard Proctor Method. Compact the subgrade with a minimum of three passes of a suitable
vibrating machine or apply other compaction forces as needed to achieve the required subgrade
compaction rate.
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4.0
Matrix Installation and Soil Filling
Assembling the matrix and associated components, filling with soil.
4.01 Strata Cell Assembly
(Refer illustrated guides in Addendum)
Check and confirm all tree pit dimensions and mark location of tree with surveyor pegs before
commencing assembly of Strata Cell matrix. Rectify any discrepancies or errors.
Ensure base of pit is level and free of debris. Place the first Strata Cell in left corner of tree pit with
colored indicator triangle pointing towards the corner.
Place second module next to the first one with colored indicator triangle aligned towards corner and
snap together, checking that the two connecting edges have matching numbers. Continue
connecting modules from left to right until the first row is complete, then start the second row and
complete from left to right, connecting all modules at adjacent edges, ensuring all numbers are
paired correctly and colored indicators are aligned, until first layer is complete.
The second layer is commenced by placing the first Strata Cell in left corner of tree pit with colored
triangle pointing to left side of pit – i.e. counter-rotated 45° from the first layer. Connect this module
to the first module in layer below and then proceed to complete the second layer. Ensure that all
indicator triangles are aligned and all modules connect together firmly, laterally and vertically.
The third layer is assembled with colored indicators aligned as per first layer and a fourth layer is
assembled as per the second layer, until the specified number of layers is in place.
An opening for the tree root ball (as detailed and dimensioned on the tree pit plans) must be
provided within the matrix.
4.02 Lateral Pipes
As per tree pit design details, perforated aeration pipes should be installed within the top layer of
modules. The pipe must be laid in a complete connected circuit within 600mm (24”) of the outer
edge of the matrix. Junctions and risers should be fitted at spacings of no more than 3.6 metres
(12’). Riser pipes to be trimmed to 150mm (6”) above finished pavement level and supported in a
vertical position by temporary staking. Open ends of pipe to be sealed with caps or weather-proof
tape.
4.03 Vertical Pipes
Where vertical aeration or inspection pipes are specified they should be placed within the large
central opening of the Strata Cell modules, in correct locations. Pipes should be trimmed to a length
of 150mm (6”) above finished pavement level. Open ends of pipes to be sealed with caps or
weather-proof tape.
4.04 Root/Moisture Barriers
Where specified in the tree pit plans, a linear Rootstop Barrier should be installed. Typically this will
be placed between the Strata Cell matrix and the side wall of the pit. Ensure the barrier is inserted to
Citygreen Strata Cell Specification
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the full depth of the pit and is not in contact with any sharp debris or stones that may puncture the
barrier. Any joins must have a 150mm (6”) overlap and be taped both sides using Rootstop tape over
clean dry surfaces. The top edge of the Rootstop barrier should be trimmed with a sharp knife to
level with the top of the Strata Cell matrix.
4.05 Loading Matrix with Filler Soil
Ensure that all required filler soil testing and certification is complete to the satisfaction of the
Landscape Architect prior to loading into the tree pit.
When matrix is fully assembled, with all specified piping and barriers in place, the filler soil can be
loaded into the matrix. Soil should be placed in the matrix using an excavator bucket and spread
with rakes or shovels until the void spaces are filled. Ensure the outer trench for the reinforcing
collar is kept clean and free of filler soil. Matrix is to be vibrated using plate vibration or needle
vibration equipment in order to shake soil into all voids.
Continue loading dry soil, raking out and vibrating, until matrix is filled.
Should the filler soil constituents and moisture content not permit the voids to be fully filled,
assemble the Strata Cell modules in layers and progressively fill, layer by layer.
Note: In some instances an air layer is detailed in the top of the matrix. In this case soil is loaded,
spread and vibrated in smaller amounts, to ensure an even distribution of soil beneath the air layer.
4.06 Filtergrid Separation/Reinforcement
The outer trench for provision of the reinforcing collar should be cleaned and all filler soil and debris
removed. Place the Filtergrid layer on the top of matrix and cut to length, ensuring that the material
fully covers the top of the matrix, the upper side walls of the matrix, and the bottom of the adjacent
trench for reinforcing collar. Any material joins must be straight, free of debris and over-lapped
150mm (6”). Pipe penetrations to be provided by means of two intersecting slits cut with a sharp
knife to form a cross.
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5.0
Compaction and Pavement Layers
Enclosing the tree pit and placing pavement layers, to comply with design
loads.
5.01 Reinforcing Collar
Load the granular base course material into the base of the collar trench ensuring the Filtergrid layer
is not displaced from the base of the trench. Compact the granular material in 150mm (6”) lifts until
the collar is level with the top of the matrix.
5.02 Tree Pit Opening
Confirm the exact required position of the tree pit opening from project details and with reference
to survey markers. Cut Filtergrid layer and fold back to expose the tree pit opening. Position formwork to provide for poured concrete system, or other method as specified in project details.
Place ReRoot linear ribbed root barrier within the tree pit opening with vertical ribs facing inwards.
Ensure bottom edge of ReRoot barrier is placed on the Strata Cell matrix and upper edge is at
finished pavement level. Trim to suit with sharp knife. Ensure any joins are overlapped a minimum of
150mm (6”), are clean and dry and taped both sides with Root Stop tape.
5.03 Granular Base Course
Load and spread granular base course material onto the Filtergrid layer in an even depth of 100mm
(4”). Compact this layer with a vibrating plate compactor with a mass of 1200kg – 1400kg/m² of base
plate, to specified compaction levels. Continue building compacted granular layers to required levels
including the reinforcing collar.
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6.0
Materials
6.01 Strata Cell 60 Series







Engineered plastic modules designed to connect together to create a matrix, for use beneath
pavements. The interconnected skeletal matrix provides in excess of 94% void space for
filling with soil media, or storing/ detaining storm water. Due to the high structural integrity
of the modules, these matrices can be used close to trafficked pavement surfaces- subject to
engineers’ certification.
100% recycled Polypropylene (PP) reinforced with fiberglass.
No steel components- corrosion free.
Ultimate Load Strength 624 kpa (90.5 psi), verified by laboratory crush tests.
50.8 cm (20 inch) x 50.8cm (20 inch) x 25cm (10 inches) in height
0.0645 m³ (2.2778 ft³) per module
15.3 modules/ m³ 11.853 modules/ yrd³
6.02 Strata Cell 45 Series







Engineered plastic modules designed to connect together to create a matrix, for use beneath
pavements. The interconnected skeletal matrix provides in excess of 94% void space for
filling with soil media, or storing/ detaining storm water. Due to the high structural integrity
of the modules, these matrices can be used close to trafficked pavement surfaces- subject to
engineers’ certification.
100% recycled Polypropylene (PP) reinforced with fiberglass.
No steel components- corrosion free.
Ultimate Load Strength 450 kpa (65.26 psi), verified by laboratory crush tests.
50.8 cm (20 inch) x 50.8cm (20 inch) x 25cm (10 inches) in height
0.0645 m³ (2.2778 ft³) per module
15.3 modules/ m³ 11.853 modules/ yrd³
6.03 Strata Cell 30 Series







Engineered plastic modules designed to connect together to create a matrix, for use beneath
pavements. The interconnected skeletal matrix provides in excess of 94% void space for
filling with soil media, or storing/ detaining storm water. Due to the high structural integrity
of the modules, these matrices can be used close to trafficked pavement surfaces- subject to
engineers’ certification.
100% recycled Polypropylene (PP)
No steel components- corrosion free.
Ultimate Load Strength 306 kpa (44.38 psi), verified by laboratory crush tests.
50.8 cm (20 inch) x 50.8cm (20 inch) x 25cm (10 inches) in height
0.0645 m³ (2.2778 ft³) per module
15.3 modules/ m³ 11.853 modules/ yrd³
Citygreen Strata Cell Specification
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6.04 Filter Grid 3030
Composite of a laid geogrid made of stretched, monolithic polypropylene (PP) flat bars with welded
junctions and a mechanical bonded filter geotextile welded within the geogrid structure, used for
reinforcement in many fields of civil engineering including road construction, landfill and hydraulic
engineering.
Property
Test Method*
Unit
Geogrid
30/30 Q1
151 GRK 3
30/30 Q1
Raw material
-
-
Mass per unit area
Max. tensile strength, md /
cmd**
Elongation at nominal
strength, md / cmd**
Tensile strength at 2%
elongation, md / cmd**
Tensile strength at 5%
elongation, md / cmd**
Aperture size, md x cmd**
Production specific
elongation
EN ISO 9864
EN ISO 10319
g/m²
kN/m
Polypropylene (PP),
white
200
≥ 30/ ≥ 30
EN ISO 10319
%
≤8 / ≤8
EN ISO 10319
kN/m
12 / 12
EN ISO 10319
kN/m
24 / 24
-
mm x mm
%
Approx. 32 x 32
O
Geotextile
151 GRK 3
Raw material
-
-
Mass per unit area
Max. tensile strength, md /
cmd**
Elongation at max. tensile
strength, md / cmd**
Puncture force
Displacement at static
puncture strength
Detector tested
Roll dimensions, width x
length
EN ISO 9864
EN ISO 10319
g/m²
kN/m
Polypropylene (PP),
white
≥150
7.5 / 11.0
EN ISO 10319
%
40 / 30
EN ISO 12236
EN ISO 12236
N
mm
1,670
30
-
mxm
Yes
4.75 x 100
* based on, ** md = machine direction, cmd = cross machine direction
6.05 ReRoot Linear Root Barrier



Linear root deflecting barriers to be used in proximity to tree root ball to protect all
pavement layers. Vertical, integral ribs guide tree roots down into StrataCell matrix beneath
pavement.
Continuous rolls to minimize joins and possible penetration.
Widths to suit application and pavement depth
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

100um thickness
100% High density Polyethylene. This resin meets FDA regulation 177.1520 for food
packaging.
Nominal physical properties:
PROPERTY*
*
ASTM D
UNIT
VALUE
Density
1505
g/cm³
0.952
Melt Index
ESCR,F₅₀
Condition B
Tensile Yield Strength
Elongation at Break
Brittleness Temperature
Flexural Modulus
Shore Hardness D
1238
1693
g/10 min
h
0.35
50
638 @ 50mm/min
638 @ 50mm/min
746
790
2240
MPa
%
°C
MPa
-
27
>600
<-90
1310
66
Physical properties reported herein were determined on compression molded specimens prepared in
accordance with Procedure C of ASTM D 1928.
6.06 RootStop Linear Root/ Moisture Barrier




For use within or to line the main tree pit, where moisture or roots must be prevented from
entering an external zone. Commonly used to protect critical services, or road based
courses.
Continuous rolls to minimize joins and possible penetration.
Widths to suit application and pavement depth
100% High density Polyethylene. This resin meets FDA regulation 177.1520 for food
packaging.
Citygreen Strata Cell Specification
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Nominal physical properties:
PROPERTY*
*
ASTM D
UNIT
VALUE
Density
1505
g/cm³
0.952
Melt Index
ESCR,F₅₀
Condition B
Tensile Yield Strength
Elongation at Break
Brittleness Temperature
Flexural Modulus
Shore Hardness D
1238
1693
g/10 min
h
0.35
50
638 @ 50mm/min
638 @ 50mm/min
746
790
2240
MPa
%
°C
MPa
-
27
>600
<-90
1310
66
Physical properties reported herein were determined on compression molded specimens prepared in
accordance with Procedure C of ASTM D 1928.
6.07 Aeration Pipes




Flexible, perforated pipe system for optimizing oxygen exchange within the tree pit soil. This
lateral pipe system is connected to the surface by means of T-pieces and riser pipes and also
forms the means of distribution for harvested storm water, and supplementary irrigation or
nutrient dosing.
100% High density Polyethylene pipe
Moulded T-Pieces and junctions
Cast aluminum ventilation grilles with tamper-resistant grating.
6.08 Filler Soils
Refer addendum: Filler Soil Specification for Strata Cell Tree Pit.docx
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6.09 Granular Base Course
Granular material meeting the standard below, or equivalent (approved by specifying Landscape
Architect in consultation with qualified Civil or Geotechnical Engineer):

ASTM D1241-07, Type 1, Gradation B Standard Specification for Materials for Soil-Aggregate
Sub base, Base, and Surface Courses.
a. Type I mixtures shall consist of stone, gravel, or slag with natural or crushed
sand and fine mineral particles passing a No. 200 sieve.
Sieve
37.5mm (1.5”)
25mm (1”)
9.5mm (3/8”)
4.75mm (No 4)
2.0mm (No 10)
425m (No 40)
75m (No 200)
Citygreen Strata Cell Specification
Percent Passing
100
75-95
40-75
30-60
20-45
15-30
5-15
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7.0
Addendum




Strata Cell Connection Procedure Layers 1 & 3.pdf
Strata Cell Connection Procedure Layers 2 & 4.pdf
Citygreen Urban LEED Performance Assessment.pdf
Filler Soil Specification for Strata Cell Tree Pit.docx
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