Strutting your stuff

Greg Whitaker , June 10th, 2009

A site in Dubai brings in heavy equipment to build an unusual foundation method

Annoying your neighbours is never good, but it is particularly easy to do when working in the middle of the city.

However, besides the noise, mess and a takeover of the pavement, one thing that is certain to get your neighbour’s goat is if your foundation works cause the ground to be unstable, or if your anchor pins protrude into next door’s basement. Such problems are inevitable in cities though, particularly as existing structures get replaced with taller buildings with a heavier footprint.

This brings us to the site of a new Emirates Bank branch. The plot is near the Dubai-Sharjah border where it is surrounded by buildings on three sides with a road at the front. This building has a four storey basement, which is about as deep as it is possible to go in this part of Dubai, before serious problems with water occur, while the building itself is a medium rise with a ground loading of around 20kp2.

The most common way of securing such a structure would be with large ground anchors, but these would intrude into the boundary, and possibly into the next building’s basement. In any case, an NOC was unlikely to be issued for such works, so an alternative solution needed to found.

The answer came in the form of a foundation method known as ‘strutting’. Unlike other methods, the foundation walls are triangulated by enormous beams, so that while being built up, the foundation pit becomes its own support. 



For the groundworks on this project, the bank chose Lebanese geotechnical firm Edrafor Emirates. Projects director George Saliby explained: “In order to support this retaining wall there are two methodologies. Either you use what are called ‘tie back anchors’ which means you are installing something beyond your property limits. In this particular case, if we were to install such anchors they would have been intruding on the property limits. If the guys next door had a basement then you would not have been able to install it, because then we would be breaking into the
basement wall.”

He added: “So the main reason we have to use struts on this building is that [the client] could not get an NOC from neighbours.

“The ground loading is around 20kp2, but it is a combination between this and what footing the building sits on. It will impose load on the retaining wall. And if it is an old building, you would have to go back and find out what kind of wall it is, what it was really designed for and all that – it is a very tedious process and sometimes it is not even possible. You would have to go back to the original developer and get hold of the plans, which could take up to a year and developers just don’t have time. So it is in our interests to use this method – it is much quicker. You don’t need anything from the neighbours, you just install the struts and work can proceed.”

Struts

The beams are very heavy and a minimum eighty tonne crane is required. Edrafor brought in a lattice-boom crawler crane of 100 tonnes and closed the road so the struts could be installed. Saliby stressed that it was most important to get the positioning exactly right. “If it is out, even by a few centimetres, it could delay the project by a week,” he said.

“Once the ‘Captain Pin’ is in place it is a relatively easy process, perhaps less than a few days, to actually put one in and weld it properly. The plate that it sits on is cast with the ‘wailer pin’ so the only thing is to put the strut in the location and properly weld it. With a good welder, that will only take a day on each side.”

To the author’s non-engineering ears, this seems to be a very complex process. After all, if the pins are triangulated in the wrong position, there is the risk that the entire excavation might collapse. However, Saliby assures us that it is, “quite straightforward other than the struts and the timing.”

After the struts have been installed, they don’t need to stay put. Once the internal walls of the basement have been built, they can be cut away.

Piled Wall

While this project is very interesting, there are other methods of building that should be taken into consideration. One of these is piled walls. These methods of retaining wall construction employ bored piling techniques – normally with the continuous flight auger method (CFA) or rotary. They provide special advantages where available working space dictates that basement excavation faces be vertical.

 Both methods offer technically effective and cost efficient temporary or permanent means of retaining the sides of bulk excavations even in water bearing strata. When used in permanent works, these walls can be designed to accommodate vertical loads in addition to movements and horizontal forces.

Construction of both methods is the same as for foundation bearing piles. Contiguous walls are constructed with small gaps between adjacent piles. The size of this space is determined by the nature of the soils.

Secant piled walls are constructed such that space is left between alternate ‘female’ piles for the subsequent construction of ‘male’ piles. Construction of ‘male’ piles involves boring through the concrete in the ‘female’ piles in order to key ‘male’ piles between them. The male pile is the one where steel reinforcement cages are installed, though in some cases the female piles are also reinforced.

All types of wall can be constructed as free standing cantilevers, or may be propped if space and sub-structure design permit. Where party wall agreements allow, ground anchors can be used as tie backs.

Rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site. In particular, whether boring is to be undertaken in ‘dry’ ground conditions or through wet but stable ones.

Upon reaching the design depth, a reinforcing cage is introduced, concrete is poured in the bore and brought up to the required level. 



Back at the bank site, like most projects, this one was on a very tight schedule, and the crews had to work all shifts to ensure it was delivered on time. However, it was, in spite of the amount of technical challenges and paperwork common to all early stages of building.

Support

Of course, there have been revisions, as Saliby illustrated: “On this particular project, after we designed the system, there was a revision to the elevation to the concrete slab.

"After the first floor slab, the level coincided with the level of the strut at the bottom. 

"This was a problem, so we had to install these inclined struts (at the bottom) so we had to provide these inclines twice in order to remove them. This was not part of the original design, but sometimes there are changes.”

With the foundations now complete, and Edrafor moving on to other projects, we look forward to seeing the bank rise out of the ground soon.

Every geotechnical company needs pile boring machines, and Edrafor has a fleet of 16.

Over the years, the firm has bought equipment from the likes of Bauer, Casagrande and IHC. However, service is equally as important. George Saliby says: “When you are spending hundreds of thousands of dollars on such equipment, the support the company provides is vital. You need people with the equipment and the parts on hand.”