Why are Soil Vault systems superior to compacted rock and soil mixes?

Posted February 24, 2023

The Citygreen soil vault system known as Stratavault has long proven itself as a superior and preferred choice for successful urban tree pit design.


Trees growing in nature thrive from the huge availability of uncompacted soils around them.  In the built-up urban environment of our towns and cities, plentiful soil around our trees is a lot more scarce due to the constraints of roads, footpaths, buildings, services and even waterways for drainage.  In fact, in most cases if soil is provided for the trees, compaction of some degree is required for engineering purposes or otherwise natural compaction is caused by foot traffic of pedestrians.

In the decades prior to soil vault technology being introduced, trees have been planted in urban areas using many different methods to obtain optimum soil volume while trying to protect the infrastructure from root damage etc.  One common method was structural soil where soil was mixed with rock and compacted to obtain a desired base around the tree so that subsidence was minimized and surrounding traffic areas were protected.  This method had its benefits but also showed major shortcomings especially in climates such as Australia and New Zealand where our temperate climates for some parts of the year promoted very high growth rates compared to areas in North America and Northern Europe.  Other challenges included drought resistance and the ability to get water to the main root system during hot and dry periods which are also extremely common in Australasia.

The challenge structural soils of any kind and method have is that while the rock acts as the support base, there is still only going to be up to 30% maximum soil volume in the mix for the tree to thrive off.  Any higher soil volume will prevent specified compaction and create instability for the top and side traffic loading requirements while less than 30% soil volume will prevent the tree from growing to its full potential, if at all.  Recent studies and experiences have proven this including here in Christchurch, New Zealand where there was a high young-tree fatality rate in structural soil pits during a dry summer.  The compaction of the rock and soil was at a point where water could not reach the root system and just pooled on the surface.  


So why Citygreen’s Stratavault?

A soil vault system like Citygreen’s Stratavault provides an answer to all the challenges mentioned earlier including guaranteed in excess of 90% uncompacted soil volume, water and ventilation supply to the root system, and top and side traffic loading ratings for road engineering requirements.  The Citygreen Stratavault system ensures that there is consistency without the uncertainty of whether structural soils are being mixed reliably, over or under compacted, obtaining sufficient soil volume etc.


Tree in soil vault yields 3x larger shade canopy than in conventional structural soil

A Citygreen study running since February 2011, features two different tree pits that were planted at the same time with the same tree species (Chinese Elm) at the Ashfield Civic Centre in Sydney, Australia. The tree in the centre forecourt was planted using the Stratacell™ suspended pavement system while the two trees near the sidewalk were planted in structural soil (rock/soil matrix). Images below show just how much a hot dry climate can affect a tree in compacted rock/soil versus a soil vault system.  In a warm climate like Sydney we have seen a faster reaction of the trees in their respective soil environments than other areas of the globe. 








Over a year later, the canopy density of the tree planted using the Stratacell system (suspended pavement –  right) is significantly thicker than the one on structural soil (rock/soil – shown on left).












Nine years after planting - the results are compounding!



These results from the Ashfield project are well supported by a fascinating study conducted by E. Thomas Smiley, Lisa Calfee, Bruce Fraedrich and Emma Smiley, comparing structural and non-compacted soils for trees surrounded by pavement.

In the 2006 study, Snowgoose cherry and Bosque lacebark elm were planted into five different soil treatment options under pavement, including compacted soil, gravel/soil mixture, Stalite, Stalite/soil mixture, and non-compacted soil.

After 14 months, the study found that there was more trunk diameter growth with the non-compacted treatment than the other treatments. There was also more twig growth, higher relative chlorophyll rating and more root growth compared to all the other systems.

Download the study HERE

The ability of Citygreen's Stratavault system to provide more soil volume for trees is also evident here in New Zealand where we recently noted trees planted in Queenstown.  Trees planted into narrow open pits versus trees planted in a soil vault system in the car park of a supermarket showed remarkable contrasts in the health and vitality of the trees.  In the case pictured below, the tree planted into the soil vault system has considerably more soil volume available to it as well as water and air ventilation.