John Martin Construction may be best known for its limpet dams (NCE 12 March 1998), but tricky jobs like Castle Mall have always been at the core of its business. So says planning engineer Bob Wright, who adds: 'At around £1M this is our largest demolition-type project to date.'
The capacity of the tower crane used to remove demolition rubble set an 8t limit on the chunks of concrete that could be handled. As-built drawings were still available, so it was relatively simple to divide up most of the existing structure into suitable sections. With no percussive equipment allowed within the mall, John Martin went for diamond sawing as the main concrete-cutting technique, even though most of the beams to be removed were post-tensioned.
As many as six saws would be needed at one time. Setting up a system for disposing of the vast quantities of waste cooling water generated by the saws was a top priority, Wright reports.
'We had to ensure none of it leaked through to the shopping area below. The answer was to adapt part of the existing rainwater system to divert the water to a settlement tank tucked away at the rear of the building.'
Temporary support to the car park slab during cutting operations was provided by traditional scaffolding, chosen for its adaptability, supported off a steel beam grillage, sometimes with trimmer beams back to columns. Wright confirms that concrete in the slabs - up to 700mm thick - has turned out to be very well compacted. Tendons in the post-tensioned beams were well grouted as well, which made cutting up the beams a simple, undramatic operation.
With all but a few awkward sections now removed, John Martin's decision to develop a pair of special lifting frames to remove the concrete sections has been completely validated. 'The obvious solution would have been to drill four holes through each corner of each section and attach lifting chains that way,' Wright explains.
'But that would have been expensive and time consuming. With the frames we simply swing them into position, wrap safety chains around the section and lift it out. We've been able to remove up to 18 chunks a day this way.'
One of the frames is adjustable to deal with awkward shapes. The safety chains are just a precaution, Wright says. 'We did a test in our yard to see if it was possible to tip a section out if the chains were forgotten, but it wasn't.'
The only real problem came from the 'springiness' of the tower crane at its maximum radius. This meant it was unsafe to try and load the 'yo- yoing' sections straight onto lorries. Instead, they had to be placed on the ground and lifted onto the transport by a conventional tracked crane.