Protecting Royal Botanic Garden’s 400 year old trees is a boring job
Established in 1845 and covering 35 hectares, The Royal Botanic Gardens Melbourne play a leading role in the conservation of plants, programs to protect rare and threatened plants and the study of habitats. It also includes Australia’s most comprehensive botanical library.
To ensure the survival of the 20,000+ plant species, the gardens rely on a comprehensive automatic irrigation system which is controlled by a central computer which in turn speaks to satellite controllers throughout the 35 hectares via data cables. Because these cables are approaching 30 years old, they were beginning to show signs of ageing and were becoming unreliable.
The Botanic Gardens trust engaged the services of Century Rain in what was to become known as the Irrigation System Communication Renewal Project in April 2021.
Whilst a very complex project, the brief was simple:
Absolutely all communication and power cables were to be replaced but the installation had to be extremely sympathetic to the root systems of existing plants and trees. There was to be NO mechanical trenching by excavator or chain trencher whatsoever. The roots of the precious plants and trees could NOT be compromised.
Given that Century Rain has arguably the strongest history of working with tree sensitive Parks and Gardens of any irrigation company in Australia, we were the obvious choice for this mammoth, but delicate project.
In fact Century Rain has been the go-to installer for the City of Melbourne, when they need to protect sensitive tree roots whilst carrying out irrigation upgrades and new installations.
Probably the most nerve-racking part of the project came when we needed to bore past a 400 year old tree – there was no alternative route we could take. Arborists from The Royal Botanic Gardens needed to be involved in this tricky part of the project.
Even once we’d bored under the tree and were 12 metres beyond the actual trunk, we still needed to use hydro-excavation to get down to the pipe which had been bored in order to make our connection.
There was a collective sigh of relief when this was over!
Michael Ten Buuren from Ten Buuren Irrigation Designs was engaged to specify this complex project, and was instrumental in assisting with the planning. And there was a lot planning. Our biggest challenge was that we needed to find and prove the location of absolutely all services before boring could commence.
All of the cables were to be installed in continuous length high density polyethyne (PE) conduits coloured orange for power and white for communications. The power conduits (orange) were sized 50mm, whilst the communications conduits (white) were oversized to 63mm. The communications conduit contained the new comms cable, plus a draw rope to enable the possible future installation of a fibre cable network through the gardens.
All of the poly conduits were installed by directional boring and much of the excavation points where junction pits were to be installed were excavated using hydro excavation – again to be sympathetic to invaluable tree roots.
In some instances, where there was a high concentration of services, we used hydro excavation for sections of trench rather than boring.
Hydro excavation is the ultimate form of not destructive digging (NDD), particularly where services cables are present. It involves a high-pressure jet stream of water displacing the soil in the ground. This is followed by a larger bore vacuum hose sucking the mud slurry into a storage container usually mounted on a truck.
Apart from ensuring no damage to large established tree roots, hydro excavation is really the only option when excavating close to or immediately adjacent to utility services. By excavating with a water jet, you simply remove the soil from around a utility pipe without causing any damage to that pipe.
The planning of the various stages was absolutely critical before each section of boring was done. Patience and taking our time was the key to not getting it wrong:
- We had to use 1100 information as a starting point. Then review all of the existing services plans (and existing irrigation plans) from the gardens, plus any local knowledge we could get from staff.
- Following that we would plot a bore path on the ground and mark it with flags and paint for inspection by the super intendant and arborists. Often the bore path needed to be moved prior to commencement.
- Our specialist services locations team would locate and mark the services on the ground, then through hand and hydro excavation would locate and prove each service along the bore path. Occasionally the bore path needed to be changed again – which was all part of the process. We needed to get this right – regardless of how long it took!
- Once everything we could locate and find was proved we would start the boring and avoid all critical infrastructure, including all tree roots. On occasions we might hit an irrigation lateral or old water pipe which was unlocatable. These were simply repaired as we went.
- After each bore the path was surveyed for accurate referencing on As Built CAD drawings for the future.
With high quality continuous runs of HDPE conduits and cables with no joins (joins are always the weak link) the new cable system should ensure the irrigation system is the backbone of the gardens for another 30 years plus.
Whilst this stage of the project only covered approximately half of the Royal Botanic Gardens, it still involved the installation of approximately 2,950m of poly conduits to 9 of the irrigation systems satellites.
Due to the delicate nature of the work, the project took almost five months to complete but what a great feeling it was to successfully complete the project and preserve the important botanic specimens for generations to come.
The Century Rain Team