The Naches / Nile River landslide and the role of the quarry

Spectacular image of the Naches / Nile River landslide, from the Washington State DNR Flickr site

Hat tip to Andrew Giles for pointing this out to me.

The Seattle Times have run an article today about the role of the quarry in the triggering of the Naches landslide. They quite rightly point out that there is no firm evidence either way as to whether the quarry played a role or not, but the do make some interesting comments about the possible role:

"Washington's Department of Natural Resources warned a Naches, Yakima County, gravel pit four years ago that its operations might be destabilizing a portion of the slope that collapsed onto Highway 410 this week. Records from 2005 show a department geologist noticed a 10-foot-wide fissure between the towering basalt cliffs and a broad talus slope below. The gravel mine appeared to have removed deposits that were buttressing the slope, documents say. "Your surface mining activity may be exacerbating slope instability and, therefore, may be creating a potential hazard to adjacent property and danger to the public health, safety (and) welfare," says a notice issued to the mine owners in September 2005."

The article quotes Prof. David Montgomery from the University of Washington, who is undeniably a world class academic geologist:

"It's definitely premature to rule out the gravel pit as a contributing factor...They were definitely digging at the toe of an active landslide, and that's a recipe for a slide."

The article then goes on to note that the Department of Natural Resources (DNR) wrote a letter dated July 2008 that noted that the agency still had not received a monitoring plan.

The image above clearly shows that this is primarily a slump/rotational failure as outlined in my previous post. This is rather beautifully illustrated by the image below, also from Washington State DNR Flickr site. The back-tilting of the trees are a sure-fire sign that the slope has rotated:

There is undoubtedly some translational movement too, as would be expected in such a large failure.

A strange update on the Nachterstedt landslide

A strange update has appeared in the "The Local" regarding the Nachterstedt landslide. This reports that "Scientists at the Collm Observatory at Leipzig University registered an underground movement in the area measuring 1.0 on the Richter Scale, just six minutes before the first call to the emergency services on July 18. An earthquake can be ruled out, the scientists told Der Spiegel magazine, but they suggested it would be feasible that an old mining tunnel collapsed, causing the landslide. "

For info a magnitude 1.0 earthquake is the equivalent of about 30 kg of TNT. I cannot understand why the scientists believe that this was a tunnel collapsing. To me it is far more likely that the signal was the landslide itself occurring. I must admit that I cannot understand why the collapse of an underground mine working would cause this catastrophic slide anyway.

On another matter, the paper also reports that a further collapse is considered likely:
"Surveyors have been pulled back out of the Auf der Halde community, where the ground has sunk by a further 0.4 millimetres and is expected to also collapse. Preparations are now being made for emergency measures to be taken should the remaining houses also fall into the Concordia Lake, which was created by flooding an old open-pit mine."

Vietnam landslide

There are newswire reports of a nasty landslide in central Vietnam. The reports suggest that this was a rockslide in the morning of 15th December in Tuong Duong district of central Nghe An province triggered by quarrying as part of a hydroelectric powerplant scheme. The number of workers buried is 18. Looking at the picture available here there is little chance of any survivors. The unfortunate victims appear to all have been workers on the powerplant.

Reports on 16th December suggest that the landslide had a volume of about 500,000 cubic metres, burying the victims to a depth of between 30 and 50 m. It is unsurprising then that only three bodies have been recovered to date.

Update (18th December): Thanh Nien reports that eight bodies have now been recovered, but that the rescue operations are proving to be difficult and hazardous. No clear cause of the collapse is evident but "Workers speculated Saturday that an explosion carried out on Friday might have weakened the mountain's structure and caused the landslide, but their theory has yet to be verified or disproved". Certainly poorly planned and/or executed blasting can be the cause of collapses, so it will be interesting to hear whether this is the case here.