17 October 2008

Retaining wall collapse, Uganda

Posted by Dave Petley

On Tuesday morning a collapse occurred in a retaining wall being constructed for the foundations of a new tower block in the capital of Uganda, Kampala. Two images have been published online of the failure, which buried nine workers, killing eight of them:

Monitor online image of the Pensions Towers collapse site

New Vision image of the Pensions Towers collapse site

Collapses like this are now quite rare in more developed countries, though not unknown (see here, here and here for example). However, a properly designed retaining wall, based on good site investigation and laboratory data, and a properly executed design, should not collapse in this way. So it will be interesting to find out what has gone wrong in this case. There will be a formal investigation, so we will wait and see, but the pictures and reports do point to a couple of issues.

First, the pictures above show a very red soil. This is of course characteristic of deep weathering – the red is iron oxide. Deep, tropical residual soils like this are often problematic materials because they can be weak and also highly variable. Experienced engineers know to treat them with respect.

Second, the pictures show that the excavation is deep (one of the newspaper articles suggests it was 25 m high!). The wall was being supported, but it appears that this was with nothing more than “wood, iron bars and wire mesh” see here). The images suggest that the wall was just being faced, not supported properly. This is surprising. However, the reasons behind this are perhaps explained by this:

“At the heart of the queries will be NSSF’s decision to change building plans which KCC insists it had advised the developer against. In building Pension Towers, NSSF had initially planned an eight-storey tower consisting of two-basement parking levels. The Fund subsequently changed the plans for the complex, into a 26 storey complex and tripled its costs from Shs36b to Shs120b.”

The suggestion being that as a result of the extra storeys on the tower the excavation was deepened from two to four storeys without taking into consideration the need to change the design of the wall. It has also turned out that:

“The developers of the Pension Towers, whose retaining wall collapsed on Tuesday killing eight people, had earlier ignored a warning from Kampala City Council advising against using unapproved building plans.

The state of play at the site might be indicated by this report into the aftermath of the collapse:

“When the New Vision visited the site around 11:00am, the horrified workers of [the] construction company were desperately trying to dig out their buried colleagues, using spades and hoes. Enraged onlookers demanded that the two bulldozers on the site be used to remove the soil. As fate would have it, the wheel chain of one of the bulldozers broke off as soon as the engine was started by a volunteer as its operator had vanished, while the second bulldozer developed a mechanical fault and caught fire. A third bulldozer had to be rushed in from another…construction site.

Finally, the interplay between western and traditional views of disaster causation on sites like this is nicely captured here:

“Foreman Hajji Kigongo said the accident could have been prevented if they had sacrificed three bulls before construction began. He scolded this boss for failing to perform the ritual, a common practice in Buganda. Susan Kataike, the works ministry spokesperson, said prior to the works, they had tested the soil to establish if it could hold a building of that size.”