Landslides along the Panama Canal

The Panama Canal is truly one of the engineering wonders of the 20th Century. This 80 km long waterway was completed in 1914, cutting 13,000 km on the sea route between New York and San Francisco. However, the fearsome cost of the canal in human lives is jaw-dropping - it is estimated that 27,500 people lost their lives during construction, primarily as a result of malaria.

However, it is less well-known that probably the largest technical problem arose from landslides that occurred in a section of the canal route known as the Gaillard Cut (sometimes called the Colebra Cut), which is the continental divide, about 13 km from the Pacific Ocean. Unfortunately, at the time of construction the strength of the rock was over-estimated, primarily because the engineers (understandably) did not anticipate that the materials would weaken due to oxidation after excavation.

The upshot was a series of large and very damaging landslides that caused the required amount of excavation to increase dramatically. There are two wonderful presentations available online of the problems that landslides caused along the Gaillard Cut, including contemporary photographs of the various failures. The first, prepared by J. David Rogers of the University of Missouri - Rolla is available here:

http://web.mst.edu/~rogersda/umrcourses/ge342/Panama%20Canal%20Landslides-Revised.pdf

The second, prepared by Maung Myat, is hosted at the same site as the above:

http://web.mst.edu/~rogersda/umrcourses/ge342/pancanal-rev.pdf

To give you a taste of the contents of these presentations, here are two images from the former, showing slides along the Gaillard cut:


More recently, the Panama Canal underwent a widening programme consisting of widening the watercourse to 192 m along the straight stretches and up to 222 m on the bends. This work was completed in 2001 to budget and on time. A 1988 review of the geotechnical challenges posed by this work is available here:

http://www.pancanal.com/esp/plan/estudios/ep-0008.pdf

Today the satelite imagery gives no indication of the magnitude of problems posed by the Gaillard Cut:

Interesting slide on Highway 97 in Canada

Thanks to Andrew Giles for bringing this one to my attention. Highway 97 in British Columbia is currently being upgraded through the Okanagan Valley (Fig. 1). This project is being undertaken by Arthon - their project website describes this as:

"Four lane roadworks and 1,000,000 m3 rock removal over a 7 km section of BC's main north-south highway corridor. The B.C. Ministry of Transportation awarded Arthon Contractors Inc. a $38.6 MM contract to complete roadway construction by summer 2009".

Fig. 1: Google Earth image of the location of the problematic slope on Highway 97.

The rock removal appears to be a widening exercise in which a substantial amount of rock has been removed to create a platform. Unfortunately, last Friday the contractors noticed that a crack had appeared in the slope above the works at the location shown on Fig. 2.

Fig. 2: Google Earth perspective view of the area of the unstable slope before the slope cutting had commenced.

The crack in question is not insubstantial. I must note here that I am very impressed with the openness of those involved - the contractor has placed images on their website of the nature of the problem (Fig 3.), even showing comparison views of the crack opening.

Fig. 3: Arthon Ltd image of the crack on the slope (from the Arthon Ltd web page)

The block of rock that is moving is large - about 300,000 cubic metres (Fig. 4), with the crack at the rear extending downwards for at least 10 m.

Fig. 4: Arthon Ltd image of the mobile block (from the Arthon Ltd web page)

Unsurprisingly, the authorities have stopped both the work on the slope and the traffic on the road, and now it is a game of wait and see. One possibility is that the slope will collapse completely, it may keep creeping or it may stop. Whatever happens the authorities have quite a challenge on their hands to know how to deal with this slope. The pattern of displacement against time is one tool that could be used in this case.

I'll post again as this develops. In the meantime, congratulations to all for their transparency in this case.

Retaining wall collapse, Uganda

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."