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A FIRM FOOTING

As buildings become more complex and expensive, the need for reliable, sustainable and cost-effective technologies in foundations is gradually growing stronger. Jerusha Sequeira examines the options


As important as a building's design and structure are, there’s no denting that what lies underneath is just as crucial. Technologies for building foundations have evolved and improved over the years, and alternatives to traditional deep piling methods have emerged. But among contractors and government entities, there remains a tendency to rely on the familiar, perhaps to be safe rather dian sorry. 
Big Project ME decided to 

speak to professionals to find out where the industry' is heading and what alternative solutions can be offered to deep foundations. For the high-rise construction projects increasingly dominating skylines across the country', piling remains a must. The most common method in Dubai remains rotary bored piling, says Peter Titus, managing director of Total Foundation Solutions (TFS), a company supplying specialist equipment for deep foundations and piling across the GCC. 
However, alternative piling methods such as continuous flight auger (CFA) are becoming more widely used, due to their cost-effectiveness, Titus says. “In Western Europe, the United States, parts of South America, CFA is the main method you would use because it is so much cheaper. Everyone is trying to trim the cost of their overall building, not just the foundations. And they're looking to find ways to do this faster and cheaper, so CFA is the main medrod used in France, Spain & the UK."
CFA piles are formed by drilling a continuous flight auger into the ground. The sides of the hole are supported at all times by' the soil-filled auger, eliminating the need for temporary casing or bentonite slurry. Upon reaching the required depth, concrete is pumped down the hollow stem as the auger is steadily withdrawn, and a reinforcement cage is placed into the wet concrete.
In addition to being cheaper, a key advantage of CFA is that it's much faster, Titus notes. "The time you spend making each pile is reduced. You can auger the pile, pump the concrete, lift the cage and drop it in, say, one fourth or one fifth of the time it would take you to excavate using a kelly bar system."However, CFA does pose limitations in terms of the depth and diameter of the piles.
CFA piles can be drilled down to 41 metres deep and 1 5m in diameter using high torque specialist CFA rigs. “If you get very tall buildings like the Burj Khalifa, you have piles much deeper and much larger in diameter than this can provide,'1 Titus ex

plains, adding that in this case, the rotary bored piling machine works best. Micropiling machines are also becoming more popular, he points out. “All these different machines are used to be able to create holes in the ground, 

temporarily or permanently," he says. The problem is that soil is not uniform or homogenous. Additionally, in coastal areas the water table is quite high, and once a contractor excavates below it, water begins to appear. While contractors have traditionally overcome the issue by dewatering using pumps, this is quite an expensive process.
A way around diis is by casting a jet grout bottom plug. Jet grouting is a technique of creating a pile using in-situ soil and grout mixed together under high pressure to create a soil-reinforced column. Jet grouting improves the soil’s mechanical characteristics while decreasing permeability, Titus notes. A jet grout bottom plug can be created well below ground level by casting multiple short jet grout columns in an overlapping arrangement to cut off the influx of water into a future excavation.
Commenting on how the market is shaping up, Tims says that while piling and diaphragm walling machines were mainstays in the past, micropiling machines are more in demand now as construction needs evolve.
“We have a lot of buildings here which are valuable and need to be retained. Sometimes some of them have had some issues with settlement or differential settlement.” Additionally, some clients are also looking to extend existing buildings, which will need foundations to be strengthened.“The trend is heavily moving towards micropiling, so to have something small enough to put in through a building, adjacent to a building, under a building, involves using smaller machines. The same machine can create a jet grout column, when used with a very high pressure pump.”
These techniques are becoming popular as the market realises that drilling large piles isn't always necessary, he adds. “Definitely that’s the new demand, where we have buildings that might require extensions or remedial work if there's some problem. There is more and more understanding that perhaps you don't have to give a large diameter pile under a villa.’’



 
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