Conical slide at Zimplats runs like Austrian clockwork

The 160-metre high Conical Smoke Emission Stack, that forms part of the Stefanutti Stocks scope of work for? Zimplats SMC SO2 Abatement and Smelter Project in Zimbabwe, is currently being slipformed. This construction method was chosen as it was deemed the most efficient and safest route, and is being executed with the assistance of Gleitbau Salzburg , a specialized sliding sub-contractor from Salzburg in Austria.?

Stefanutti Stocks and GLEITBAU collaborated to produce a detailed formwork design to execute the project successfully. The slip forming equipment was shipped from Austria to Durban in South Africa, from where it was transported via road to the site in Zimbabwe.?

The 160-metre high conical chimney tapers up from an inside diameter (ID) of 13.33m to an ID of 7.9m ID. The last 45.65m are linear.?

Stefanutti Stocks’ Liebherr Group 200 EC-H 10 Tower Crane was erected for the assembly; operations during the slide; and the dismantling of the sliding equipment. “The tower crane has been erected to its maximum freestanding height of 69.24m for the first?section of the stack,” explains Stefanutti Stocks project manager Hendrik Moller, “and will have to be ‘jacked’ five times, to a final hook height of 170m high.” With every stop crane tiebacks will be installed to anchor the tower crane mast to the stack.

A single cage Alimak , with a hoisting capacity of 2000kg or 20 persons, is being used for access onto the sliding platform.

The octagon shaped base foundation of the stack measures 23m by 23m, and is three-metres thick. The conical chimney being slipformed onto the foundation has the following dimensions:

• Basis outside at +1267.00??? 14,050m
• Basis inside at +1267.00??? 13,250m
• Wall thickness basis at -2,000m 0,400m
• Top Walls outside at +1427.00 ? 8,300m
• Top Walls inside at +1427.00??? 7,800m
• Wall thickness on top??0,250m
• Basis height of chimney??+1267.00
• Top height of chimney??+1427.00

“We are sliding two shifts per 24 hours, seven days a week, with an average sliding speed of approximately three metres per 24 hours,” says Moller, “so the slide should take approximately three months to complete, including the rest periods required to give the concrete appropriate time to set.”

A 10-ton tower crane is available for the lifting of reinforcement, concrete, and other materials required during slip forming operations. The concrete is lifted to the top deck in a bucket and distributed by wheelbarrows or concrete buggies directly into the concrete hoppers. These are connected to a 20cm diameter flexible hose that hang down into the slipform shutter. During the concrete filling of the slide the layers are kept between 20-30cm, with a tolerance of 5cm.

“The setting time of the concrete must be controlled to ensure that the first layer is set when the form is filled 100cm,” explains Moller. “Every metre should be filled in four layers over a period that corresponds to the designed setting rate of the concrete.”

Most of the major structures of this mining infrastructure construction project should be completed towards the end of the year, and a small Stefanutti Stocks team will return in 2024 to complete minor works, where access is dependent on the SMPP contractor.