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Demolition engineering and a range of temporary works design for the removal of large portal frames, specialist structures and a large chimney from the Shotton Paper Mill site.
Works formed part of a £600m investment to convert the site from a traditional newsprint paper mill into one of the world’s most advanced cardboard and tissue production facilities.
In order to safely and efficiently remove the structures, the methodology utilised a combination of mechanical demolition techniques and controlled collapse methods.
Demolition contractor: John F Hunt
Client: Eren Holdings
Scope Of Demolition
The demolition project’s scope included removing large portal frame structures, RC frames, and specialist structures such as large chimneys.
The majority of the structures were removed using a 90-tonne high reach excavator, with additional excavators to assist with the demolition of lower structures and processing work.
A significant amount of work was also required to remove two paper production machines, which were 100m long and had heavy rollers inside which could weigh up to 75 tonnes.
Removing the structures was a challenging task as the structure was built with heavy gauge materials to limit vibrations that could damage the delicate paper as it traveled through the rollers at high speeds.
Once the innards of the structures were removed, the superstructure was mechanically demolished. This included large portal frames and large columns, which were left freestanding and then felled with a plastic hinge at the base and folding it over, as even with the large shears on the 90tonne machine, the flanges were too thick to be cut through.
Additionally, the project involved removing relics in the ground due to the site’s previous use as a steel mill. This included the removal of foundations, some of which were up to 7.5m deep. Ground works design involving working platform design and slope stability checks for excavation sides were carried out by Andun.
Our main scope of work included providing engineering assistance on the demolition methodology, proving the plant on raised slabs and undertaking weight calculations for crane lifting. The project was recognized by the National Federation of Demolition Contractors (NFDC) in issue 2 of their 2022 magazine.
Steel Chimney Stack Push Down
Towards the center of the site was a 100-tonne, 54 meter tall steel chimney stack that needed to be removed.
The stack was located between a couple of buildings of the main mill. We isolated the chimney by demolishing the structures around it, and then initiated a controlled collapse by preweakening and then pushing from behind and away from the live parts of the site.
The stack demolition included pre- weakening the base at the bottom of the stack to provide the hinge point that would determine the direction of the structure’s collapse. Andun ran stability check calculations on a stage by stage process to ensure that the structure would not come down prematurely during the preweakening process.
The goal of preweakening is to remove enough so the structure can be pushed over with an excavator but not too much that it is blown over by the wind in an uncontrolled manner.
Inside the structure the stack was anchored to a reinforced concrete base with several ground anchors. The preweakening method used in this case was for a birdsmouth cut at the front of the stack and then moved towards the back, burning out the bottom of the anchors in a sequence starting at the birdsmouth and moving towards the back through small letterbox holes.
The ultimate aim of any pull down is to bring the structure to the ground safely and efficiently, so it could be processed using heavy plant equipment on the ground.
By reducing the amount of working at height, we were able to lower the overall risk of the demolition process. This method of demolition is considered high risk due to its dynamic nature, but by implementing a site-wide exclusion zone, we were able to control and manage the risk effectively.
