The Need for Trees in Urban Areas
Trees are an important part of our natural life support system: they have a vital role to play in the sustainability of our towns and cities and we need to take better care of them, both now and in the future.
There is a growing recognition that trees and woods can provide a whole range of multifunctional contributions to environmental, social and economic sustainability.
Trees help to improve the quality of life for the millions of people who live and work in urban areas, and since ours is among the most urbanised societies in the world, the UK is a very good place to show how the urban forest can contribute to sustainable development.
Most people agree that trees are a good thing, and yet we see them starved of natural food and water, damaged at their roots by earthworks, particularly in developed areas of towns and car parks.
Problems
Because of the built environment roots of trees can at times struggle to grow through the soils. This would be mainly due to the compaction over the top of the natural root growth.
In a trees search for water and nutrients, it has been known for the roots to cause cracking of the pavements and to interfere with underground services, such as water pipes, gas pipes and electrical ducts.
Buildings themselves have also been affected by the advancing roots as they go in search of food and water.
The effect on the area around the tree can thus be problematic and lead to expensive repair work having to be undertaken. At times the tree may even have to be removed.
(Abridged from text supplied by Trees for Cities. For further information please visit www.treesforcities.org)
Solution
The SEL Permavoid® Tree Protection System offers a range of solutions to the many problems associated with planting trees in the urban environment. The system can be designed to suit site specific requirements.
There are two typical arrangements.
Type 1 – SEL Permavoid® ‘Sandwich’
A single layer of 150mm deep SEL Permavoid® units is laid in a raft arrangement over the potential root zone of the tree. These units are filled with a suitable nutrient rich soil/sand mix and then encapsulated with geotextile (generally Seltex 300R but if there is potential for hydrocarbon contamination then this may be substituted by SEL Sudstex Permafilter® geotextile).
The soil infill will settle within the SEL Permavoid® units leaving an air filled void.
The benefits to employing this construction are;
- The SEL Permavoid® raft will act as a protection to the tree roots. This will promote a uniform directional growth of the roots, thus stabilising the tree.
- The high strength SEL Permavoid® Units dissipate vertical and dynamic loadings to protect the soil around the tree roots from compression and compaction due to trafficking.
- The SEL Permavoid® unit is filled with nutrient rich soil, acting as a container and retaining moisture and nutrients which are released to the tree roots over time.
- The soil within the SEL Permavoid® will settle creating an air layer within the structure. This provides aeration and prevents root growth through the surface, thus protecting expensive surfacing from damage caused by root heave.
Employing the SEL Permavoid® Tree Protection System in this manner closely replicates conditions found on the forest floor by providing a layer rich in nutrients above the underlying tree soil.
Type 2 – SEL Permavoid® "root wall"
A vertical wall of 150mm deep SEL Permavoid® units provides the ideal interface between car park or road construction and the essential nutrient enriched root zone required by trees and shrubs.
The high strength and performance of the SEL Permavoid® units provide the vertical structural support for the constructed surface layers whilst the provision of SEL Anti- shear connectors between the SEL Permavoid® layers provide sufficient lateral restraint to effectively segregate and support compacted sub-base thereby eliminating the tell tale settlement often seen on traditional parking areas near to trees.
The benefits to employing this construction are:
- The SEL Permavoid® “root wall” is modular and fully interlocking. It can therefore be made into planting areas of any size, configuration and depth to suit site conditions and the particular growing requirements of all types of tree species and shrubs. The “root wall” layout can be a unique site specific design with each zone sized individually to accommodate landscaping requirements.
- The SEL Permavoid® “root wall” is a structure with 95% void ratio, providing superior drainage capabilities. The SEL Permavoid® wall base and sides have a high perforated surface area promoting efficient water ingress (infiltration) and egress (exfiltration) through a range of approved SEL geotextile encapsulation options. These properties combine to provide ideal drainage conditions to avoid saturation of the root zone whilst simultaneously providing efficient sub-base drainage to alleviate subgrade softening. In the simplest form, water collected from the SEL Permavoid® “root wall” can soak away directly into the ground where soil conditions are suitable as part of an overall project specific SUDS solution.
- One of the essential requirements for healthy trees is a constant supply of oxygen to the roots. The SEL Permavoid® “root wall” has a 95% volumetric void ratio which acts like a fresh air duct to provide a constant source of oxygen to the perimeter of the root zone area. The ventilation requirements for larger landscaping schemes can be enhanced with the introduction of SEL ventilation units, ranging from ground level vent boxes to stainless steel vent bollards.
- In order to offer irrigation to the root zone, the SEL Permavoid® “root wall” may be constructed with some or all of the SEL Permavoid® units substituted with SEL Permafoam® units. When charged with rain water, the SEL Permafoam® units will retain water up to 80% of their volume. This water is released on demand as required by the tree using natural capillary attraction through the root zone. The SEL Permafoam® units may be charged with surface runoff water directly from the SEL source control system or as part of the overall project specific SUDS solution.
- The SEL Permavoid® “root wall” system is best used in conjunction with the SEL Permavoid® “Sandwich”. The SEL Permavoid® “root wall” will provide the vertical support to the SEL Permavoid® “Sandwich” layer at the interface with traditional construction methods.
- The SEL Permavoid® “root wall” system may be used to define the extent of root growth to effectively control the required landscaping layout. When installed using approved SEL root barrier geotextiles the SEL Permavoid® “root wall” can effectively determine the required demarcation between root zone and traditional hardstanding construction.
