Digging deeper

As the number of high-rises being constructed in the UAE climbs, the piling and foundation process goes deeper to keep buildings standing for years to come, reports Shikha Mishra.

Edrafor Emirates, ANALYSIS, Projects

As the number of high-rises being constructed in the UAE climbs, the piling and foundation process goes deeper to keep buildings standing for years to come, reports Shikha Mishra.

When it comes to buildings, pilings and foundations have a lot to answer for.

The quality of foundations and pilings has given us one of the most beautiful among the Seven Medieval Wonders of the World - The Leaning Tower of Pisa.

The tower was intended to stand vertically, but because of its poorly-laid foundation and loose substrate that allowed the foundation to shift direction, it began to lean towards the south-east soon after its construction began in 1173.

But not all foundation errors end up becoming national treasures. A miscalculation in the most basic of the construction processes - foundation - can lead to horrific mishaps.

Professor Harry Poulos, senior principal of Coffey Geotechnics in Australia, says that the foundation design process in the Middle East - while not different in principle from that in other parts of the world - has to take into account the relatively weak rock and soil deposits on which tall buildings are founded, the relatively small amount of experience of how foundations behave in these deposits and the extraordinary loads that the new super-tall buildings impose on the foundations.

"As a consequence, designers are being forced to use cutting-edge methods of analysis, design and testing to gain an adequate degree of confidence that the foundations will perform as expected.

Because of unprecedented foundation loadings, tests on foundations cannot be carried out in the conventional manner, by applying loads to the pile head. We cannot get enough load on in this way to test the pile adequately," he says.

"Thus, we are using new techniques such as the Osterberg Cell test in order to be able to apply the required test loads to the foundation piles. We are also taking great care to verify the integrity of the foundation piles, which are invariably constructed of concrete.

Integrity testing is vital because the groundwater conditions in the UAE are very conducive to deterioration in the quality of concrete over short periods of time. Any flaws or defects in the concrete piles may compromise their long-term durability."

The biggest cost variable on a construction project is likely to be what is under the ground. According to a study presented in 2005 by the Department of Building and Real Estate, Hong Kong Polytechnic University, Hong Kong, owing to the complexity and the short contract period, foundation projects bear the highest risk and cost among construction stages.

Prior to the commencement of a foundation project, the estimate of its construction cost is one of the most important activities.

As a foundation contract has a higher risk than other types of projects, many unforeseeable factors, such as ground conditions or inclement weather, can affect its final cost.

A builder should award a project to a contractor offering the highest quality and not the lowest price as poor foundation quality can lead to delays in a project which can further escalate costs, or in rare instances, may cause a serious accident.

Pierre Fayad, general manager of Edrafor, says that the primary foundation concerns in a high-rise that need to be addressed are differential settlement and bearing capacity. Differential settlement is when one part of a foundation settles more than another part.

Differential settlement can cause cracks to the entire structure of a building.

Bearing capacity is the capacity of soil to support the loads applied to the ground. It is necessary that a foundation is not loaded beyond its bearing capacity or the foundation will ‘fail.'

In order to address these problems, geotechnical engineers must design and build piles with very high standards of quality that are technically sound. In order to take on large loads of high-rises (4000-6000 tonnes per pile) the diameter and length of the piles have to be increased.

"We often see such piles in the region that are designed to take on high loads.

In such cases, pile diameter has to be in the range of at least 1.5m in diameter with length up to 50m. Such piles require very large piling machines," says Fayad.

Foundations are generally broken down into two categories: shallow and deep foundations. Fayad says that deep foundations are required for high-rise towers: "High rise towers induce very large design loads that in Dubai get transferred to poor soil layers with high water tables."

The shifting sands in the UAE are the reason behind deep foundations in the area. On certain types of sand, a low-rise building can be erected without piling.

The type of foundation depends on the soil condition, and a thorough geo-technical investigation is done to decide on the type of piling a high-rise building requires.

A spokesperson for PMDC (Project Management and Consultant Development) says that foundations for high-rises are deeper and larger in diameter.

"The two types of piling are compression and tension piles; compression piles are used when the load is higher and tension piling is used when the soil has a high water table. High water tables are a very common phenomenon in this region."

PMDC is currently working on the Dubai Pearl project which, once completed, will reach close to 75 levels. The piling work on the project has begun and the excavating on the site has also been started.

The challenge facing the foundation and piling industry is one which is plaguing the entire construction sector in the UAE - a shortage of trained personnel, labour and escalation of prices for high-quality material.

"There is no simple remedy to these challenges," says Mark Newton, operations manager, Dutch Foundations. "In Dubai it is a fairly recent problem, the escalation of prices in materials is driven by rising oil prices and inflation in the country.

It is a serious problem for contractors as clients are reluctant to write in escalation clauses in contracts, subsequently with the rise in costs, we as contractors suffer.

But, we are now insisting that these clauses get written in. I have noticed material shortages in steel and concrete normally revolve around political issues rather than actual shortages.

In terms of staff, it is always a problem when one is operating in a boom market, everyone is vying for same specialised people and poaching is very common.

Our solution to this problem is to run an in-house training scheme. Though we have a core staff of experienced people, we bring in younger graduates and train them."

Dutch Foundations is currently building Lam Tara Towers on Sheikh Zayed Road with 1.8m diameter bored piles that extend 40m underground, with working loads of approximately 4,500 tonnes per pile. Lam Tara is slated to be the second tallest tower in Dubai after the Burj.

And then there is the highest of them all... according to figures from Emaar, the foundation slab for the Burj Dubai is 7432.2432 m2 in size, while the piling is 50m deep. And given Dubai's penchant for out-building itself, we are sure that figure will be topped pretty soon.

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