Putting in the ground work
Out of sight shouldn't be out of mind. New techniques in piling and foundations mean delving a little deeper into a building's construction can turn up piles of innovation.
Before any breathtaking structure can even begin to take shape on the skyline or any ageing building can be given a new lease of life, establishing top-notch foundations below ground must be the first concern.
Far from being a case of digging a hole and filling it in again, piling and foundation works are a highly specialised job that is steadily evolving in the region. With every new and more ambitious structure come equally ambitious innovations below ground. To put it simply, larger buildings require deeper foundations, while offshore construction requires a new way of thinking altogether.
Only a few years ago, the size of structures in the region lent themselves to traditional forms of foundations, and much of the piling in the Emirates remains the more traditional 'rotary-bored cast in situ' method.
This involves a steel casing being inserted into the ground using a vibro-hammer, followed by an auger drilling tool, which is then used to remove the soil within the casing to form a borehole; this is then followed by the installation of a steel rebar cage. Finally, concrete is poured into the hole to form a bored pile, with a vibro-hammer used to extract the casing from the ground.
But with every bigger and bolder creation comes the need for new ground works able to sustain increasing loads and cater for new situations. This, coupled with the increasing number of foreign firms looking for a bit of the action, means innovations are creeping in to the sector.
One company not shy of innovation is Turkey-based contractor Zetas, which laid the foundations for Princess Tower in Dubai Marina, which, at 107 storeys, claims to be the tallest residential building. The basement is 22m deep and the piles 67m below ground - it is believed to be the deepest excavation in the UAE to date.
The following techniques offer refinements on traditional methods, others have been brought in to deal with situations unique to the region.
Continuous Flight Auger (CFA) Piling
During CFA piling the hole is drilled with an auger, which is fitted with an expendable cap. When the hole is drilled to the required depth, concrete is then pumped down the central stem, blowing off the expendable cap under pressure. No casing is required. Maintaining positive concrete pressure the auger is withdrawn - removing the loose material - and the reinforcement is placed into the pile up to the required depth.
"It has particular applications," says Derek King, divisional manager, Dutco Balfour Beatty Piling and Ground Engineering. "Sometimes it's faster, sometimes it suits certain ground conditions. And it's quieter, with no vibration, which is good for an inner city."
King adds that Dutco Balfour Beatty's relationship with UK piling giant, Stent, has enabled the company to provide specialist knowledge and high standards to CFA piling jobs in the UAE.
"CFA piling is where we have advanced skills compared to the local market. All of our rigs are completely instrumented and we have good control on these kind of piles, in line with demand in Europe. And by tapping into Stent, we have been able to offer that to our clients here."
Using Stent's Integrated Rig Instrumentation System (SIRIS) the rig driver is given a detailed picture of various parameters during the construction process; these include auger depth, rotation, concrete pressure, concrete flow and over-supply.
"We did Interchange 4¾ for the [Dubai] Roads and Transport Authority with this method, and they are very particular on quality now," adds King.
"There's no rocket science to this, but if you want to guarantee quality and really instrument the process, then it's good."
Impact Compaction Technology
The development of offshore projects in the Gulf has increased the amount of land reclamation and shoreline ground improvements. For areas that only need to sustain the load of low-rise villas, for example, the need for piling is reduced and other ‘ground engineering' applications can be employed.
Working in joint venture with South African firm, Landpac, Dutco Balfour Beatty has used impact compaction rollers to make improvements to the specific soil types found in the Emirates.
"It's a non-circular roller, and it's very big, so it puts a lot of energy into the ground," says King. "It works by increasing the densification of the ground and improving its bearing capacity. It's very fast and you can get improvements down to 4m."
After treatment by impact compaction, net allowable bearing pressures of up to 175kN/m2 can be used for the design of shallow foundations. King adds that the benefit of the technique is that the treatment/improvement process is rapid - some 4000m2 of site area can be treated in a day.
"The whole of Dubai Maritime City was treated with this method."
And the technique may have applications on future projects in the region, as logistics begins to play an even greater role in the type of piling or ground engineering required.
"The World is going to be particularly difficult to get wet concrete out to. So the options are to use precast and try and avoid any foundation piling," says King.
"You can drive precast piles but it's expensive, and you will have to get your plant out there. But if you can do ground improvement for liquefaction and then roll the top, then you may be able to get away with shallower footings."
Jet grouting is a general term used by contractors to describe various construction techniques used for ground modification or improvement. Grouting contractors use ultra high-pressure fluids or binders that are injected into the soils at high velocities.
These binders break up the soil structure completely and mix the soil particles in situ, to create a homogeneous mass, which in turn solidifies.
There are three types of jet grouting: the monofluid system uses the binder to break up and provide soil mixing of the soils surrounding the drill rods; the three-fluid or ‘Kajima' jet grouting system uses water and air to break up the soil, and it produces ground modification or ground improvement by causing a partial substitution of the finer soil particles; and an intermediate two-fluid jet grouting system has recently been developed to improve the range of influence of the monofluid jet grouting system.
"It is an accepted technology in Europe, has a British Standard and is being widely used in those markets," says Fatih Kulac, general manager, Zetas. "We have seen that we might have a chance to introduce this here to be an alternative to vibro compaction and in some cases piling."
Jet grouting is an intermediate technology that can be applied for low-rise villas and buildings up to 10 floors. Zetas used the technique - along with CFA - to construct secant shoring piles on Sky Gardens in Dubai.
Towards the smaller end of the spectrum is micropiling. It can be defined as small diameter piles, typically less than 150mm, where the pile load is carried structurally predominantly by the pile reinforcement.
Hence micropiling is often relatively heavily reinforced with a substantial full length central reinforcement bar. Micropiles are also normally grouted due to the small volumes. They can be installed in almost any type of ground where piles are required, including rock, and design loads range from as small as three tons to as high as 500-plus tonnes.
Among the advantages to micropiling is that the equipment is small and lightweight. Emitting low noise and vibration, an asset to crowded, urban environment. It is also suitable for restricted access sites, for example those with low headroom. It requires minimal site preparation and can be installed successfully into most ground conditions including rock.
- Dutco Balfour Beatty - Piling and Ground Engineering Division
- Dutch Foundation
- Züblin Ground and Civil Engineering