Chimney Stack Demolition: Top Down, High Reach or Fell It

A chimney stack is the one structure on an industrial site that everyone can see, so it is the one everyone has an opinion about. Blow it up, nibble it down, pull it over. In practice the method is decided by three things: what is around it, what is inside it, and what it is made of. Get those right and a stack is a routine job. Get them wrong and you have a 60 metre structure doing something you did not plan.

This is how we choose the method for chimney and stack demolition, what will hurt you, and what owners should have ready before anyone prices it.

What you are actually dealing with

• Brick stacks, usually older, often with an inner brick or refractory liner, iron bands, and lightning protection. Brick stacks lean, crack and shed; the condition survey matters.
• Reinforced concrete stacks, from power stations, cement works and refineries, with a steel or refractory flue liner inside and sometimes an air gap between.
• Steel stacks and flues, free standing or guyed, lined or unlined, with the lining often refractory or insulation board.
• Inside the liner: flue dust, acid condensate residue, and in older stacks asbestos in the insulation, the liner joints or the sealing rope. Lead paint on steel stacks is common.
• What is attached: ducts from the boiler or kiln, platforms, ladders, aviation lights, antennas that someone else owns, and sometimes a live flue from the plant next door.
• What is around: live plant, roads, rail, overhead lines, other people's buildings, and the public.

Step one: pick the method from the site, not the brochure

There are three ways down. Top down, where remote controlled demolition machines or a crew on a climbing platform take the stack apart from the top in small pieces and drop the debris inside the flue; this is the method where there is no room to fall and live plant is close. High reach, where a long reach excavator with a crusher or shear takes the stack down from the ground; this is the fastest method where the stack is within reach and there is a clear fall zone for debris. Controlled felling or explosive demolition, where the stack is brought down whole into a prepared fall zone; this is the quickest and often the safest where there is room, and the most heavily approved. The first site walk decides which of these is even possible.

Step two: condition and hazardous materials

A structural condition survey, because a cracked brick stack will not take a climbing platform and a corroded steel stack will not stand a partial cut. A hazardous materials survey inside and out, with liner samples taken at height. Flue dust sampling, because what the stack carried for 50 years is in that dust. Lightning and aviation obstruction obligations checked, and any third party equipment on the stack identified so its owner can remove it.

Step three: isolate and clean

The stack is disconnected from the plant and the breeching sealed so nothing live can enter it. Asbestos in the liner, joints and insulation is removed under the asbestos licence, which on a tall stack is an engineered job in its own right. Flue dust is removed and classified before any method that would release it. Lead paint is managed through controlled cutting rather than open flame.

Step four: take it down

Top down work drops material inside the flue and recovers it at the base through a controlled opening, which keeps debris off the ground outside. High reach work drops material into a fall zone that is sized by the engineer and kept clear. A fell or blast uses a cut and a prepared bed, with exclusion zones, vibration and dust monitoring, and a single controlled event. In every method the engineer sets the sequence and the stack is monitored for movement throughout.

Step five: the base and the ground

The base, foundation and flue tunnel come out last. Acid condensate from a lifetime of flue gas often leaves the base concrete and the ground beneath it contaminated, so the footprint is sampled before it is backfilled or handed back.

What not to do

• Do not fell a stack into a space the engineer has not measured. Stacks are longer than they look lying down.
• Do not put a crew on a climbing platform on a stack with no condition survey.
• Do not start any method that releases flue dust before it has been sampled and removed.
• Do not cut a lined stack without knowing what the liner is. Asbestos at height is a licensed job.
• Do not forget the aviation lights and the antenna. Someone owns them and someone has to be told.

Approvals and notifications

Licensed demolition notification to the state regulator for any stack over the height threshold. Licensed asbestos removal notification. Explosives permits, blast management plan and exclusion zone approvals for a blast. Aviation authority notification where the stack is a charted obstruction. Heritage approval for listed brick stacks, which are more common than you would think. Council consent in most cases.

Who should be doing this

A contractor that owns its remote controlled demolition machines and its high reach excavator, holds the Class A asbestos and demolition licences in its own name, and has a structural engineer it has worked with on stacks before. Perfect Contracting dismantled the steel chimney stacks at the former Corrimal Coke Works in sections by crane, and carries out chimney, stack and industrial structure demolition across Australia from its Sydney and Brisbane bases. Read more about our industrial demolition work and our high reach and remote controlled plant, or send us the details.