Large structures: Big is beautiful

By Chris Sleight09 November 2010

Potain has supplied eight tower cranes to the US$ 3.2 billion Son La hydropower project in Vietnam.

Potain has supplied eight tower cranes to the US$ 3.2 billion Son La hydropower project in Vietnam.

iC takes a look at some of the major projects underway around the world, along with the construction techniques and equipment being used to execute them.

Large-scale construction projects require a very special set of skills. As well as overcoming the logistical challenges of dealing with huge volumes of material, and a large workforce, often in remote places, building such major projects often requires contractors to push the envelope of what's possible.

The pressure is always on to execute projects as quickly and cost-effectively as possible, and this can often mean working in partnership with equipment suppliers to develop new solutions.

For example, Peri has supplied an innovative slab table system to the Canadian South Health Campus medical centre near Calgary to help contractor Ellis Don ensure fast shuttering times and reduced material requirements.

The aim was to construct the 200000 m2 of floors on the project as cost effectively as possible.

To do this, Peri developed 90 m2 tables with only four corner supports. Additional loads are taken by the building's reinforced concrete columns, without the need for intermediate supports and Peri's Deflection Compensating System (DCS) automatically corrects for any bending. What's more, the tables can be moved by hand.

DCS is a self-levelling system, comprised of sensors and hydraulic cylinders, which keeps the slab table level when loaded. Five sensors on each table record the respective deflections during concreting and the hydraulic cylinders automatically react to raise the table back to the horizontal.

On this project the system has a tolerance of about 2 mm and up to ten tables can be controlled at the same time through a common hydraulic unit.

Altogether 57 of these high-tech tables are being used which means 2800 m² of slab area is completed each week. Specially-designed trolleys allow the tables to be moved by hand without any additional means.

The main building of the medical facility is currently under construction and features two basement and eight upper levels each with around 20000 m² of slab area. The 250 mm to 300 mm thick slabs are supported on 762 square reinforced concrete columns on a 9.60 m x 9.60 m grid. Standard storey height is 4.50 m.

The hospital complex, situated to the south of Calgary, is believed to be the largest construction project in Canada. The ground-breaking ceremony took place in 2007 and the structural work for the main building is part of Phase One which is scheduled to be finished at the end of 2011. Work on the entire campus will continue until 2018.

Rock excavation

Over the border in Michigan, US, tunnelling contractor Ric-Man Construction Inc. has ventured into caisson work with construction of a combined 13.2 billion litre sewer overflow facility in Dearborn, Michigan.

The project began in January 2008 with a substantial rock grouting and stabilisation programme. Excavation and construction of the self-sinking caisson began in January 2009, with the structure poured in eight separate vertical lifts.

Ric-Man used six Atlas Copco BBC 34W pusher leg mounted rock drills bore more than 400 rock bolt holes in the rock face, before using them to secure wire mesh. This was then given a 150 mm covering of shotcrete.

Lift 8 of the caisson was on bedrock at 42 m below ground level, and pouring the structure's bowl-shaped slab meant excavating some 11000 m3 of rock across the 32 m diameter. The choice was whether to drill & blast or use breakers.

"Our real challenge with drilling and blasting at this site was the fact that it's a residential area," said Ric-Man Construction COO Duane Mullica.

"The cost and logistics of trying to blast, reassuring the residents and making them comfortable that there will be no damage to their homes and the infrastructure - the complications grew as we talked about it. It's also a safety issue. You need to have so much training and more insurance."

Having settled on mechanical excavation, the company lowered two Atlas Copco ECM 590 drill rigs into the caisson to predrill 600 14 m deep relief holes into the bedrock. These made it easier for the hammers to break the rock.

Once these were drilled, the company brought in a 77 tonne Koehring 1066 excavator. Ric-Man started with a 7 tonne class HB 7000 Atlas Copco breaker, but then opted to bring in the HB 10000 to increase productivity. Production has been 170 to 260 m3 a day.

"The quality, experience and method we chose to complete this project puts us in the running for others of this type," Mr Mullica said.

Dam challenges

Taking a different approach to rock excavation, Turkish contractor AGE Construction is using two new Sandvik DX 700 top-hammer crawler drill rigs in conjunction with the company's RT300 bits for rock excavation on the Kavsak Bendi hydropower plant at Adana, in southern Turkey.

In addition, the contractor has existing Sandvik rigs and a new DD320 jumbo for tunnelling work on the scheme.

AGE Construction as main contractor for the construction and hydro mechanical works of the 180 MW scheme, which is being built on the Upper Seyhan River. Foundation work began in October 2008, and the project is targeted for completion by 2012.

The main elements are a roller compacted concrete (RCC) dam, the main 2,26 km long, 9 m diameter water tunnel, along with two 7.9 m diameter diversion tunnels measuring 531 m and 330 m in length.

The surface excavation is making use of Sandvik's new RT300 type uniface design bench drilling bits, which the company says have a longer life, higher penetration rates and produce cleaner holes than their predecessors.

This means cost savings according to Erdinc Cetin, surface drilling and tunnelling manager of Sandvik Mining and Construction Turkey.

"We can say that the rigs are using about half the fuel that would be consumed if we were using the down-the-hole drilling method, despite the hardness of the ground," he said.

Meanwhile in Vietnam, eight Potain tower cranes, including six special application cranes, are playing an integral role in the construction of the Son La hydropower project in Vietnam.

This US$ 3.2 billion project is the largest of over a dozen major dam-building projects in Vietnam and the largest and most complex energy resource venture ever undertaken in South East Asia.

Located in north-western Vietnam on the Da River, the Son La dam will be one of the world's largest measuring 138 m high, 90 m wide and close to 1 km long. It will have six turbines and require over 3.1 million m­­3 of roller compacted concrete and 1.2 million m3 of conventional concrete in its construction. Due for completion in 2012, the dam will provide 9,500 GWh in hydroelectric power each year - equivalent to 10% of Vietnam's current energy demand.

Commissioned by Vietnam Electricity Corp., construction of the dam is under the control of main contractor Song Da Corp., a state-owned construction company and the largest contractor in Vietnam.

Minh Chi, Potain's Vietnamese dealer, supplied Song Da with the cranes for the project which include two MD 2200s, four MD 900 Bs and two MC 310 K12.

Do Quang Loi, deputy general director of main contractor Song Da 5, said, "Because of its national importance, we had to have the right crane for the job."

Six of the cranes, the MD 2200s and MD 900 Bs, have been on site since August 2008. Since their deployment, they have been working constantly, running three shifts, seven days a week. The cranes are mostly pouring concrete and building the dam's penstock system and water intake.

Bridge expertise

In a different aspect of concrete construction, Gleitbau has applied its slipform and heavy lifting technology to the construction of the two 272m high pylons of the Yi Sun-Sin Bridge in South Korea.

When complete it will be the fourth largest bridge in the world, with a main span of 1545 m, and will form part of the access infrastructure to a new steel production and petrochemical industrial region as well as playing an important part in the 2012 World Expo, which will be hosted by the city of Yeosu.

The two H-shaped pylons consisting are inclined at up to 5.5° and change their cross section with height. Each leg is trapezoidal with a wall thickness of up to 1.50 m and high reinforcement content. They are connected with post-tensioned concrete crossbeams at heights of 255 m and 164 m

In addition to the slipform works, Gleitbau Salzburg was also chosen to lift 1300 tonne and 1400 tonne precast crossbeams into position.

As far as the slipforming technique for the legs is concerned, concrete was pumped up to the topdeck of the form and spread out by small distribution booms. The form also included two small luffing jib cranes to carry out the lifting work, which added to the complexity, because they had to be lifted as part of the system.

In addition storage was needed on the forms, in case bad weather meant the cranes could not lift.

The slipform assembly was incorporated an overlapping steel form connected to yoke frames raised by standard jacks that was able to accommodate geometry changes. A form set was provided for each pylon leg connected by a 66 m long lattice truss bridge, which provided stability in bad weather.

In contrast, the crossbeams were cast at ground level and lifted to their final elevations utilising Gleitbau heavy lifting technology. Immediately after finishing the in-0situ work on the pylons, the lattice trusses of the slipform were dismantled and the support structure for the lifting jacks was assembled on top of the legs.

A large capacity tower crane was assembled and connected to the top of each of the lower crossbeams, and the entire assembly was jacked into place as one - a total weight of some 1300 tonnes.

Better and better

These projects all go to show that even with the most challenging projects, there is room for innovation and better solutions. They also illustrate the fact that contractors often find success when they tap into the expertise of their equipment suppliers.

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