Highway 97: test blast

The latest report on the Highway 97 problem in Canada (see earlier posts here, here and here) is as follows:

• The contractor, Arthon, released a statement to the press yesterday, the key points of which stated that "Highway 97 remains closed north of Summerland until further notice due to an unstable rock mass above the highway. The slope continues to move, therefore the highway is not safe to reopen. Safety is our top priority. The ministry is doing everything it can to reopen the highway as quickly as possible, but we won't open it until it's safe. We've built access roads to enable heavy equipment to reach the slope.
  Our first test blast took place this afternoon to help assess the stability of the rock mass.
  The geotechnical experts will now analyze the results of the blast and will have an update tomorrow afternoon. The results of the blasting will help the ministry finalize the work plan."
• The tension crack extended by 8 millimetres overnight Tuesday-Wednesday, giving a displacement of 3.3 cm since Sunday.
• The plan to move material from the crown to the toe of the landslide now appears to be the favoured solution. It is not clear how this will affect the road.

The cautious approach beiung adopted by the authorities is quite correct in my opinion.

Highway 97 latest update

The latest update on the Highway 97 landslide in Canada (see here and here) is as follows:

1. The slope is still moving. Between 19:00 on Monday and 14:00 on Tuesday the mass moved about 15 millimetres (i.e. about 0.8 mm per hour). This is quite high for a large mass. The rate of movement is described as being constant;
   
2. The mobile volume is now estimated to be 200,000 cubic metres;
3. The road remains closed (unsurprisingly!);
4. There are provisional plans to remove material from the top of the unstable slope and to move it to the toe. The aim here is to reduce the mass that is driving the movement and to increase the mass that is supporting it.

I am sure in the background there is an enormous amount of work going on. Analysis of the pattern of movement in time and space, especially by looking at vectors of movement and accelerations and decelerations will give a pretty good idea of what is happening in the slope. This gives a potential for providing a short term warning of an impending collapse, but does not really tell us how likely this is in the medium term.

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.

A follow up to the Sea to Sky Highway landslide

Back on 1st August I posted about a landslide that had blocked the Sea to Sky Highway (Canadian Highway 99), which links Vancouver to Whistler, venue of the 2010 Winter Olympic Games. The road was blocked for a few days, but has now reopened.

Two sets of colleagues in Canada have very kindly provided follow ups to that post. First, Frank Baumann, who is a geotechnical engineer in Squamish has very kindly sent an image (Fig. 1) of the landslide for inclusion on the blog. This very nicely captures both the scale of the problem and the topographic setting of it.

Fig 1: Photograph (copyright Frank Baumann) of the Highway 99 (Sea to Sky Highway / Olympic Corridor) landslide in Canada.

Second, Steve Evans and Keith Delaney from the Landslide Research Programme at the University of Waterloo in Canada have very kindly provided the following explanation of the landslide, including this excellent comparison image:

Fig 1: Illustration (provided by Steve Evans and Keith Delaney from the University of Waterloo) showing the site of the 29th July 2008 landslide. The main image shows the site before the failure, the smaller image the site after. Inset is a stereographic projection plot of the failure surface and joint sets. Click on the image for a better view.

Steve and Keith write:
"The site of the July 29^th 2008 Sea to Sky rockfall/rockslide was examined in 1997. The main photograph in Figure 2 shows the site at that time. The photo at the lower right of Figure 1 was taken soon after the 2008 event by Canadian Press. The slope consists of resistant Coast Plutonic Complex granite but as seen on the 1997 photograph (and on the cover of the book “Rock Slope Engineering” by Hoek and Bray) the rock mass is characterized by more-or-less planar stress relief (sheeting) joints that dip west (downslope) towards Howe Sound (pole concentration is red dot on the stereonet in figure 2). Undercutting of the slope during construction of highway in 1958 resulted in the sheeting joints daylighting in the rock slope (pole concentration is red dot on stereonet). The location of the 2008 rockfall is known as Porteau Bluffs, they run for just under 1 km along Howe Sound (49 33 52.15N; 123 14 01.44W), and the rock slopes along this section of highway exhibit similar rock mass characteristics to those involved in the July 2008 event."

Frank Baumann has kindly also provided a couple of other images of another landslide at the north end on Highway 99 - this time an incipient failure (Figs 3 and 4). It clearly shows that there are other potential problems on this road - although it is also important to stress that, surprising though this might seem, cracks like this do not mean that collapse is imminent or even likely.

Fig 3: Photograph (copyright Frank Baumann) of an ongoing failure on Highway 99 in Canada

Fig 4: Photograph (copyright Frank Baumann) of an ongoing failure on Highway 99 in Canada

Thanks to Frank, Steve and Keith for their contributions to this - I very much appreciate their input.

The problem of cracks in mountains after earthquakes

Xinhua is today reporting the following:

Nearly 9,000 people in a quake zone were evacuated on Tuesday for fear that huge cracks on a mountain could lead to further disasters. Many crevices, measuring up to 1,500 meters long, 250 m high and 50 centimeters wide, have been spotted on the Shiziliang Mountain in the Qingchuan County seat, Guangyuan City, since Sunday, threatening about 50,000 people and quake-relief soldiers. Part of the mountain, distorted in the 8.0-magnitude earthquake and many aftershocks over the past week, has sunk about 1 meter and caused many road works to cave in. A house at the foot of the mountain was damaged by a falling boulder. Any new aftershock or heavy rainfall in the area could trigger serious landslides and cause casualties, according to the quake relief headquarters at the site. The local government on Tuesday launched an emergency evacuation of 9,000 residents near the mountain. The area has been cordoned off and is under round-the-clock monitoring.

Reading this gave me a very strong sense of deja vu. One of the greatest challenges in Pakistan after the 2005 earthquake was trying to interpret the meaning of similar crack networks, which we found all over the hillsides. The following two images show what these looked like.

A cracked slope near to Muzaffarabad in Kashmir after the 2005 earthquake (click for a bigger image). Cracked slopes like this occurred throughout the earthquake zone. To date very few have led to a slope failure, but we remain very concerned by them.

A further cracked slope near to Muzaffarabad in Kashmir after the 2005 earthquake (click for a bigger image). Note that in this case the cracks are very close to a steep cliff. Nonetheless, this slope has still not failed.

In Pakistan we assumed that these were incipient landslides and worked with the authorities to move people off the worst affected areas before the monsoon. We also put instruments to measure four of the most notable sites. However, this was only an assumption. As it turned out, almost none of these slopes have to date failed. We don't know why. Are they in fact not landslides at all - if so, what are they? If they are landslides, are they in fact sufficiently stable that they can only move in a large earthquake? Or are they incipient landslides, but they have not failed yet as the rainfall associated with the two monsoons (2006 and 2007) since the earthquake has not been intense in this area (which is the case)? I must admit that I err on the latter, and worry greatly about what will happen in Pakistan in the next really strong monsoon.

In Sichuan, I would be willing to bet that close inspection will reveal that these cracks also occur very extensively across the landscape. Once again the same conundrum will occur. In the case described by Xinhua above the authorities are absolutely right to be cautious, especially if it is actually moving. Evacuation and monitoring appears to be the correct approach. I suspect that this drama will play out in many other places. In Kashmir essentially no resource was dedicated to finding out what these cracks actually meant. I hope very much that this is not repeated in Sichuan.