ABU DHABI // Investigators began sifting through rubble and debris yesterday as a full-scale probe began into the collapse of part of a landscaped residential courtyard.
The 1,400 square-metre concrete slab between three tower blocks of Al Rayyana residential development on the outskirts of Abu Dhabi city plunged one floor down into an underground car park at 8.20pm on Sunday. Eight vehicles were damaged but no one was injured.
All five of the development's tower blocks were evacuated and residents, mostly teachers and other staff employed by Abu Dhabi Education Council, were accommodated in the Yas Viceroy Hotel.
Lt Col Mohammed Al Ansari, head of the national urban search and rescue team, said the municipality would take all necessary steps during the investigation, with the full cooperation of Sorouh, the developer.
The probe will include a full review of architectural plans and a soil test. Investigators usually study the soil after a structural collapse, both immediately under the building and a few metres away, to compare the load-bearing capacity.
"Investigations are still under way to determine how the incident occurred," Lt Col Al Ansari said.
There was speculation yesterday that the weight of vegetation in the courtyard, exacerbated by heavy watering, may have contributed to the collapse.
"All the extra weight of soil and plants - a simple overloading would help push though the top slab," Geoffrey Hichborn, a forensic civil engineer who has investigated the cause of collapsed buildings across the United States, said after looking at photos from Al Rayyana.
Mr Hichborn said the biggest clue was how the floor fell. The break was clean, he said, and the slab fell between columns.
Called a punching shear, "it's the most aggressive spontaneous and unpredictable.With beams that fail, you get lots of warning, of days, or months, but there's no warning with a punching shear."
The next stage of the investigation will be to look at the structural design and integrity of the courtyard and its surroundings.
"The structural design has to be looked at," said Dr Farouk Yaghmour, a Jordanian architect. "But it's not sufficient to look at that alone, they have to see the implementation of the project, whether people stuck to the design 100 per cent or not."
The courtyard's location above the car park also made the area tricky, he said, as it was not on natural soil.
"They have to check everything, from the soil test when they started the project up to its handover," he said. "The soil has a bearing capacity and all the foundation has to be designed according to that bearing capacity."
He said in the Arabian Gulf, the soil was critical because the water level was relatively high.
"From my experience, the who, how and cause for this kind of collapse is not easy to judge," Dr Yaghmour said. "Before you go in depth, you must study everything related to the project."
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Soil mechanic companies must now check the database of the courtyard and its surroundings, including the soil tests done from drilling before construction, the type of soil and its bearing capacity.
They will then collect samples from the rubble to test it in laboratories to find out whether it was built according to the design.
"The technology is very sophisticated nowadays," said Dr Yaghmour. "They can use a laser or magnetic tests to check."
And the cause could be many different factors, from leakages in the specifications when building to a flawed structural design.
"But usually, it's related to the structure," said Dr Yaghmour. "If you overload it, it will collapse."
But that could take several months to determine.
"It's not that easy," said Dr Yaghmour. "It all depends on the project, how big it is, what documents they have and who implemented the project to see how long it can take.
"I'm surprised this happened in the Gulf because they are usually quite strict with their controls. From my experience, municipalities in the UAE always check the steel that is involved and ask for tests."
On Sunday night, the project manager at Al Rayyana first notified Sorouh, who then contacted Civil Defence. Emergency officials cordoned off the affected area and evacuated residents.
About seven fire trucks arrived along with a number of police cars and the urban search and rescue team as well as a police bus was at the scene. A field hospital was set up at the site in anticipation of any injuries, and a survey of the site was carried out to make sure there were no people missing.
Residents were evacuated to the entrance of the development and told they would spend the night in a hotel.
Facility management cut gas, electricity and water supplies before conducting a roll call of residents who were waiting outside. Management telephoned those who were not there and informed them of the collapse, Sorouh said.
Ahmed Al Shamsi, division manager of general services at Adec, said 194 teachers and their families were transferred by bus to the Yas Viceroy Hotel and filled 200 rooms.
As a result, teachers were unable to attend class yesterday but schools were notified of the situation.
"Adec has already made the necessary arrangements to help accommodate teachers into a new premises," Mr al Shamsi said. Adec has taken 189 apartments in the new residences.
One teacher said: "We were watching television and we heard a loud bang. We went outside to see what it was and someone said the courtyard collapsed."
The development, next to Abu Dhabi Golf Club, consists of 33 mid-rise blocks with one, two and three-bedroom apartments. Last week Sorouh announced it had signed deals with corporate tenants to rent out 72 per cent of the homes in Al Rayyana.