From earth to sky
Soil testers Geert Janssen and Karim Khalaf explain the challenges of building Dubai's tall towers.
Dubai is the land of tall towers. And with the Burj Dubai reigning the skyline and a taller one in the pipeline, soil testers Geert Janssen and Karim Khalaf of Fugro Middle East, explain the challenges of being the first to go where no tower has gone before.
What is your role in building tall towers?
Khalaf: The developer, contractor, consultants and designers need specific answers, as they know what they have to build. This is where we come in.
We have to come up with the exact quantification of the ground ability to be able to take the structure including the design of the building, the bridges on top, arches and all the elements that would affect the ground, and this can sometimes be a challenge because a very high structure or a very large arch-shaped building could input a massive load on the ground. And this is where we are expected to give answers - how to prepare the ground to be able to carry this sort of building.
And what if the ground can't take it?
K: It's not that straightforward.You cannot tell a developer:
ÃƒÂ¢Ã¢â€šÂ¬Ã‹Å“Sorry this ground cannot carry your building'. You have to assess what it can take and then make sure what needs to be done to enable the ground to take the load. And this is possible because there are a lot of foundation processes, foundation systems and a lot of techniques. We can add foundation into a ground to enable it to carry a structure, and so we have to present this idea. We don't have the option to say: ÃƒÂ¢Ã¢â€šÂ¬Ã‹Å“This can't be done'. This is our job - to make sure it can be done.
Janssen: But, of course, during a feasibility study we can suggest a site other than what's been selected if the costs can be brought down there. We suggest the most viable option. But if the plans cannot be changed, then we make sure that it is possible on the chosen site.
So one can construct anywhere? J: Well it's not that simple.
K: There are also geotechnical reasons that are limits as well.
J: You can build anywhere but it is a combination of factors then and one would have to do a lot more to make it possible. It would also then cost a lot more.
Did you do the soil testing for the Burj Dubai as well?
J: Yes, but not the geotechnical testing. We did the geophysical testing.
What is the most important factor in soil testing?
J: One of the things that one has to realise in general is that buildings are going higher and higher and another building that is planned will probably be the tallest building in the world, so for that there is no clear answer.
There is no experience, we're pushing limits; we're talking about things that haven't been done in the past. So it's very difficult to know all that needs to be known about soil - nobody knows and that is where the geotechnical engineer comes in.
K: We have to come up with answers on how deep to investigate. It's a very simple question but how deep do we have to go when such a high structure has never been built in the history of mankind. Who would know? So we start somewhere and then see if we're all happy with it, including the client and consultants, and when we're all satisfied, then we go ahead.
Once the depth has been decided then there are a certain number of tests that have to be done to assess the ground. What are they? Are they enough? If we're building something that has never been built before why should the tests that we've done be enough? Isn't there an additional test that has to be done? So again it's the same approach; we start by proposing something. It is reviewed and discussed and we start doing it.
In geotechnical engineering there is no single answer. We have multitudes of different tests and you can never get an answer by one test. You always have to do two or three, combine them, compare them and come up with the best solution. And if they're all pointing in the same direction, we consider it good.
Consistency is the most important thing in geotechnical tests. When you have two tests out of three that are going in the same direction, then you're quite happy because one is explaining the other.
With most projects under secrecy, is it difficult to work without all the information?
K: Sometimes, yes. It's not only for tall towers. When a developer tells us he has 10km2 to develop; 20 buildings of three to seven floors and 1,000 villas that may or may not have basements, it is as difficult as someone who wants to build an extraordinarly tall tower.
But generally a project has a definition - so it's either a tower that is bent or a tower that is going out of the verticality, irrelevant of how high it is or how heavy it will be. Sometimes the footprint is more important than the height. If you have a 10,000m-high tower but it has a 300m footprint it is going to be very stable but if you have a 100m-tall tower with a footprint of 15m, it's totally different.
Do you have a lot of pressure from the client to hurry things up?
K: We stick to the book if we aren't given enough time. We don't consider finances then, we only look at safety. If we're given more time then we work on solutions that would go easy on the budget as well. The design also matters.
If the design were complicated would you need more time for soil investigation?
K: Structurally, the buildings are getting very uncommon designs and shapes and architects are having fun. For us the margin of creativity is not about finding an extraordinary foundation system because there are not so many, but in finding the pertinent parameters to optimise the quantity of foundation. Having time or being allowed to do as many tests as we wish to make it optimal can solve this.