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  4. Allerton Oak

The Allerton Oak in Liverpool’s Calderstones Park is one of the oldest trees in Northwest England and has been around for up to 1,000 years according to local legend. The famous tree won The Woodland Trust’s English Tree of the Year Award in 2019, was a European Tree of the Year runner up in 2020, and has witnessed a wealth of history during its life.

Location

  • Liverpool, North West England, UK

Client

  • Bartlett Tree Experts and Liverpool City Council

Project Status

  • Completed 2023

As the land around it has changed, the Allerton Oak has been part of a royal hunting forest, the privately-owned Allerton Estate, and a public park. It’s thought the ancient tree, a Sessile Oak, was the setting for a medieval court, and was damaged by the shockwaves from a gunpowder ship explosion in the 19th century, before becoming a popular visitor attraction from the Victorian era onwards. During the Second World War, local soldiers were even sent its acorns and leaves as tokens of good luck.

Among the facts and folklore, what’s less well known is the story of how our structural engineers helped save the tree, with a prop design our clients believe could become an industry leading approach known as the “Liverpool Solution”.
  

Even though the scope of this project was a single tree, I never imagined I’d be part of something so big in terms of its significance. Especially now, when we are so focused on sustainability and reducing our carbon emissions, this tree is a testament of nature’s endurance and an extraordinary example of structural engineering in its own right.

Inspired by nature: The Allerton Oak’s bespoke props

A force of nature

The project began in 2019, led by Bartlett Tree Experts, in response to a competition by Liverpool City Council to design and build a bespoke engineering solution for the Allerton Oak that would last 50+ years. Having collaborated with WSP before, Bartlett reached out to our structural engineers for help.

The tree had needed propping soon after it was damaged in 1864 and split into three independent living units, possibly by a gunpowder ship exploding in the nearby River Mersey, or a lightning strike. Either way, its existing solid steel props were failing as a structural support, putting the Allerton Oak at risk of collapse.

Historically, the tree had only been propped vertically to stop it falling over. However, trees also grow horizontally, to the left and right, and twist, rotate and cantilever like a diving board. This movement had taken the Allerton Oak’s old props with it, causing them to bend and break.
 

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This 1847 illustration shows how the Allerton Oak was a prominent feature of the parkland.  
Image Courtesy of The Reader Organization.

Timeline:

1440

  • The Allerton Oak has been dated to this year by Bartlett Tree Experts. 

1864

  • Tree damaged by gunpowder or lightning.

1907

  • First props installed.

2019

  • The Allerton Oak is crowned English Tree of the Year and Liverpool City Council launch a competition for a new prop design.

2022

  • New props installed to support the tree – enabling an ongoing healthcare plan to be put in place.

A solution designed to grow

Our structural engineers worked with Bartlett Tree Experts and their partners to develop a simple, elegant design for the new props that can be adjusted as the tree grows. The solution combines vertical and diagonal props with cradles made of durable galvanized steel that won’t rust. Unlike the old prop cradles, the new cradles are also padded to prevent direct damage to the tree, using a bespoke technique unique to the “Liverpool Solution”.

As part of the process, the team used advanced analysis software to model the Allerton Oak’s main branches and explore how the design would respond to the potential movement and growth of the tree, including all the stresses and forces the props will need to withstand.

The analysis also included exploring future scenarios, with climate change forecast to increase the frequency and strength of forces such as wind and storms.

The girth of the tree (m)
5.5
Lifetime of the new prop solution (years)
100+
All the way through the design stages I thought the structure of the props was too “flimsy”. It pleased me having to eat my words on the day they were delivered. As soon as they were installed, I knew we had it right. My ambition was to develop a structural solution to support the Allerton Oak for years to come, but I also hope the “Liverpool Solution” can be built on to produce a national industry standard for similar projects.

Standing the test of time

The props were fabricated offsite by James Engineering Constructions Ltd, which meant careful measurements had to be taken to ensure they were exactly the right height to fit the tree and remain stable. They sit on the ground between the Allerton Oak and its shallow horizontal roots on perforated base plates, rather than in foundations like a building, because the soil is vital to the health of the tree and can’t be disturbed.

The project was interrupted by the Covid-19 pandemic, but finally the props were installed over three days in June 2022. The team used temporary props to protect the tree while the old ones were removed and the new props fitted, and geotextile membrane to protect the soil.  

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A lifeline for the future

Whilst the new props will support the structure of the ancient tree, Bartlett Tree Experts and Liverpool City Council are still working together on a tree and soil health-care plan to improve the growing environment and soil conditions for the tree. As the tree has had a relationship with the soil for hundreds of years it can’t be altered too quickly, and improvements are completed slowly, through a monitored step-by-step process.

An acorn from the Allerton Oak has also been planted in Calderstones Park, known as “Allerton Oak the Younger”. It’s fascinating to think what history the new tree may witness as it grows.

Bartlett Tree Experts have recently presented the unique prop design at the annual Arboricultural Association Amenity Tree Conference, as an industry leading solution to supporting ancient trees like the Allerton Oak.

Using the point loading software blew the walls out of anything we had done in the tree industry before and identified the need to use outrigger props as well as the main structural supports. Exploring the challenge together from both an engineering and arboricultural perspective is what made this project so interesting, and we’ll definitely look at future projects from this angle.

What makes the “Liverpool Solution” stand out

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Future proof

Simple and elegant design that can be adjusted as the tree grows and moves. 

  • Galvanized steel props include cradles with adjustable “jaws” so they can be easily adjusted and moved, extending their life indefinitely.

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Inspired by nature

Design solution supports the tree’s structural integrity enabling an ongoing tree healthcare program to improve its health and vitality. 

  • Outrigger props and main structural supports withstand the stresses and forces impacting the tree.

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A unique collaboration

Prop solution combines structural engineering and arboricultural expertise to work with tree’s unique form and behavior, rather than against it.  

  • Analysis software used to model the tree, design the solution and test scenarios, including the impact of climate change.

Old propping system

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New “Liverpool Solution”

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Other challenges that we have overcome