The “art” of concrete cuttingPublished 10/7 at 14:27
Within the precincts of Africa’s most valuable real estate, V&A Waterfront at Cape Town Harbour, was a clutch of grain silos, part of the long-unused Grain Silo Complex built in 1922 for servicing exports and imports through Table Bay. The complex was a protected historic landmark, its future periodically debated as the luxury retail, restaurant and hotel tourism magnet rose around it, until an ambitious solution was proposed that challenged local concrete cutting expertise for 30 months. PDi’s Africa editor Kevin Mayhew reports.
The Grain Silo Complex, within sight of Robben Island, the incarceration facility for African icon Nelson Mandela, captured the imagination of London-based architects Heatherwick Studio, local preservation societies, city fathers, V&A management and a wealthy Kenya-based German entrepreneur and collector of continental and diaspora African art, Jochen Zeitz.
The US$38M (€33.7M) solution retained the industrial façade of the complex while sculpting the interior to accommodate the grandest collection of contemporary African art in the world. Its design includes an atrium higher than the Louvre in Paris, and will be known as the Zeitz Museum of Contemporary African Art, due to open in 2017.
The intricate cutting work was undertaken by contractor WBHO, alongside Cape Town-based specialists Concrete Worm. Ross Demolition was responsible for the post-cutting dismantling and extraction of debris.
Concrete Worm managing director, Peter Fink, said that the project was the most ambitious of the company’s 20-year history, a sharp learning curve that took up to 10 teams on site at any time in certain phases. “There were three distinct elements that each posed challenges and had to be undertaken with many restrictions,” said Fink. “Not least of these was to preserve the integrity of silos built nearly a century before, and minimising dust and noise inconvenience at South Africa’s prime local and overseas tourist attraction over a protracted period.”
The first focus were the 42 circular silo tubes packed in six rows of seven silos, each about 40m tall. The initial task was to saw cut a slot under the outer wall to allow for a raft slab to support the new structure. These slots were phased through the entire silo bin area until all the bins had a new support slab. Only then could the contractors begin cutting the internal walls into blocks to create a void inside the bin area.
All the walls were curved, which made it difficult to mount wall saws, and scaffolding work meant special training for staff to work safely at height. Much of the scaffolding was suspended from the top of the silos to allow machine dismantling and construction to continue below. This meant all equipment, including wall saws using up to 1.6m diamond blades, had to be hauled to the roof then carried down the scaffold to the area of work.
The round bins where the decorative scallop cut had to be made were the most challenging. Here a surveyor, templates and 3D drawings were used to indicate where to cut. “The challenge was cutting a constantly changing curve through multiple round chambers. Most of these cuts were done by hand, as no rail mounted machine could follow the cut line,” said Fink.
From a dismantling point of view the initial removal was that of a tower atop the silo building, which required a 5t excavator to cave the walls in with steel cutters that followed to remove the steel, according to Ross Demolition site agent Roland Mountjoy. The company used 175 staff, six foremen, three operators and six cutters during the long and complex project.
A steel and timber platform was erected at 6m intervals for the crane to lower the 5t excavator to the various platforms to demolish the internal walls of the silo bins down to ground level.
Second focus was the tall elevator building that contained rectangular silo bins for the bottom half and five upper level slabs. The building’s very small windows had to be enlarged. There were 120 openings 4m x 3m that had to be created for windows. Wall saws cut the top and sides of each window opening. This was done to free the walls from the existing columns before dismantling. Due to the age of the structures it was considered safer to cut the walls first, which ranged from 200mm to 480mm thick, before breaking. Each floor was done separately and could only start once certain structural elements were installed by WBHO for construction.
The rectangular bin walls had to be removed to make way for slabs and rooms. This was also a challenge as entry was from the top via suspended scaffolds. Some of the bins were only 2m x 1m shafts. These tight, dark areas required a high level of skill to complete. The walls were cut free from the columns before Ross Demolition could dismantle by hand and remove a staircase and lift shaft with cutting torches and jackhammers.
Final task was drilling and saw cutting through the tunnels and basement of the structure where the walls and slabs were the thickest. These 700mm thick concrete slabs were reinforced with railway lines. Openings through the slab were required for new foundations. New door openings and decorative cuts had to be made through walls ranging from 600mm to over 3m thick. Some of the cuts had to form a curve and was achieved by constantly adjusting the wire saw pulley system, a first for Concrete Worm.