The demolition of the annealing furnace presented a problem that was largely due to the fact that the structure itself was sheeted in asbestos cement sheets that were each approximately 6’ x 4’ in size.
The structure was approximately 33m in height and had base dimensions of 15m by 12m. The main structural supports were twelve vertical legs.
The structure itself was positioned quite centrally on the Outo Kumpu site however there was a railway track that run within 5m of the site boundary.
There were basically three available means available to demolish the structure as follows:
1.The first was to open up the structure at the base and put pre-weakening cuts in the legs. Wire ropes would then be attached to the top of the structure and it would then be pulled over using heavy machinery. Once at ground level it would then be processed up.
2.The second would involve the demolition of the structure using a mechanical high reach machine capable of reaching to heights of 40m. This process would be completed following the manual de-sheeting of the structure in the first instance which would require two men to be working at a height of upto 30m for approximately four weeks.
3.The third was to use explosive charges that would kick out the supporting legs and cause the structure to collapse in a controlled manner. Once at ground level the structure would then be processed up.
The first option was ruled out for several reasons including the fact that other structures were still in situ making it difficult for the machines to have a long enough exclusion area available and also the fact that they would be required to track on concrete which would make it difficult for the machines to maintain a grip on the surface.
The second option would be possible but this method involved a significant amount of repetitive manual handling whilst working at height. These activities are both covered under separate pieces of health and safety legislation which requires the employer to avoid these where practicable.
The third option was considered the most feasible as not only did it remove the need for significant amounts of work at height and manual handling it also meant that the structure would be at ground level within a matter of seconds which allayed any fears from Network Rail that loose sheets may blow onto the railway track during the other two options.
Whilst the decision had been taken internally and provisionally agreed with the Client there was still the issue of collapsing a structure that was sheeted with asbestos cement sheets. A method was developed that would involve removing as many low level sheets as possible and then wetting down the area where the structure would collapse. In addition to this the area of collapse was the concrete slab area of some former structures where the roof sections had been removed with the gable walls left in situ to act as a protective barrier. As a final reassurance technique, background air monitoring would be undertaken during the demolition to determine whether any asbestos fibres had become air borne.
A meeting was arranged and involved members of the Health and Safety Executive (HSE) and also a representative from Network Rail to discuss the demolition proposals in greater detail.
Although the HSE never give formal approval of working methods they did not raise any significant concerns with regards to the demolition and left site wishing the contractor good luck for the work.
Network Rail had been involved during all the planning stages but confirmation of the work was made and three Network Rail supervisors were requested to accompany the sentries that would be put in place along the railway track.
Several notifications were made in the proceeding days to the demolition as follows:
Sheffield City Council - although a section 80 demolition notice had been made, an additional specific notification was made.
South Yorkshire Police - an incident number was created and a temporary road closure would be put in place along an adjacent road.
Local Residents - a local mail shot was issued to the surrounding residents and local businesses.
Air traffic control - a verbal notification was made to ensure that the works would not have any effect on any flight activities planned for the night.
The tower itself was part of other surrounding structures that were still in place. These other structures were divorced away from the base of the tower to enable it to be stood on its own.
Once the tower was stood alone the inside workings around the base were all removed to fully expose the 12 legs that would be cut and eventually where the explosive charges would be placed.
The front and rear sides of the tower were de-sheeted to a height of the surrounding structures. This totally opened up the base to enable access to be made for the cuts within the legs and also to place the explosive charges.
Once all the preparation works were complete the explosive charges were put in place and final connections were made.
An exclusion zone was established around the site which would prevent unauthorised access into the area where the demolition work would take place.
This zone was established by positioning six sentries around the site, each one having a line of vision to the next.
Communication was provided between all sentries and the main control by use of battery powered mobile radios.
The works involved the following personnel:
·Demolition Site Manager and Supervisor who carried out the final cuts
·Six operatives to act as site sentries (three of the sentries were each accompanied by Network Rail supervision as they were required to stand on the railway track)
·Managing Director to make any final decisions
·HSE Manager to coordinate the exclusion zone clearance
·Explosive engineers to position the explosives, carry out connections and initiate the detonation
·Two police officers to temporarily close the adjacent road
·An asbestos technician to provide background air monitoring equipment
Once the annealing furnace was at ground level the first stage was to carry out a clear up of the concrete area where the structure fell. This involved operatives working with full disposable overalls and FFP3 type masks to wet down the area thoroughly using water. Once the area was wet down the asbestos was collected using a machine fitted with mechanical grab. The asbestos was placed directly into skips ready for transportation to a hazardous waste landfill.
Background air monitoring was established during the clear up of the asbestos to confirm that the possibility of any fibre release was being controlled.
As the asbestos was cleared the furnace structure was mechanically sheared into smaller sections ready for removal from site.
The process of demolition including the two weeks preparation took three weeks to complete with no accidents or near misses occurring.
Thorne Case Studies: Annealing Furnace, Explosive Demolition
Explosive Demolition of an Anneling Furnace: Outo Kumpu