29 January 2015
A very interesting slope problem appears to have developed at the site of the new Pakyong Airport in India. The airport project is described well in Wikipedia:
Pakyong Airport is an airport under construction near Gangtok, the state capital of Sikkim, India. The airport, spread over 400 ha (990 acres), is located at Pakyong village about 13 km (8.1 mi) south of Gangtok. It is the first Greenfield airport to be constructed in the Northeastern Region of India and the first airport in the state of Sikkim…The land for the airport was carved from the mountainside using massive geotechnical ‘cut and fill’ engineering works. These state-of-the-art geogrid soil reinforcement and slope stabilisation techniques were employed as traditional retaining structures and embankments were ruled out as being unfeasible. Irish geotechnical company Maccaferri executed the project that envisaged a 550 m (1,800 ft) wide, 1.7 km (1.1 mi) long corridor on which the runway and airport buildings are to be constructed. The company, which completed the project with partners Mott MacDonald and Punj Lloyd, won the ‘International Project of the Year’ award at the Ground Engineering Awards 2012 for its work in constructing 70 m (230 ft) high reinforced soil walls and slopes at the site. The project is among the tallest reinforced soil structures in the world.
The site is impressive – this is a recent Google Earth Image of the location:
Perhaps the most important aspect in terms of this blog is the very extensive slope cutting that has been required to create the bench for the airport site. Unfortunately, since the airport construction project started there have been repeated indications of slope instability on the upslope side of the airport site, although this is often reported in the media as subsidence. An example occurred this week (via the Tibet Earthquake Twitter feed):
The location of the monastery is shown as “temple” in the above image. Meanwhile, last week there were reports of landslides on other parts of the slope:
A key question must be what is occurring on this slope to cause these instability issues, and what are the implications for the airport site? Whilst the link between the airport slope works and the instability is of course not proven (and of course instability occurs naturally on slopes in this area), there must be real cause for concern.
27 January 2015
Tsarap landslide images
The Tsarap landslide crisis in northern India continues, although it is good to see that there has been an appropriate level of response to the hazard from the authorities. This is an interesting contrast to the initial indifference shown by the Pakistani authorities to the Attabad landslide five years ago. Over the weekend, efforts started to airlift people trapped to the north of the landslide – in total 300 people were trapped when the Chadar Trek was suspended. The same report also notes that the expert group formed by NDMA to advise on the management of the landslide dam will visit the site this week, so it will be interesting to hear what they have to say.
Meanwhile, also over the weekend, two sets of satellite images were published of the landslide site. The ever-wonderful NASA Earth Observatory published a beautiful Landsat 8 OLI image of the area, collected on 18th January, that very clearly shows the slide:
This is without doubt the best image of the landslide so far, showing both the source area and the slide itself, together with the impounded water. I remain concerned that the material forming the dam is potentially erodible and that the dam crest is short, both of which suggest that there is the potential for a rapid breach event when the water overtops the barrier. There remains some urgency to get a monitoring system in place for this landslide, and then to start the detailed planning for what will happen when snow melt starts to fill the lake more rapidly.
23 January 2015
This is probably one of the best landslide videos so far. It was posted on Youtube yesterday, with the title: “Terrible Landslide in the Mountains of Dagestan!“
It is a classic and very spectacular example of an earthflow. The people trying to rescue the car were fortunate, and indeed I suspect that with a winch the vehicle would be recovered. The rapid transition from slow creep to fast flow, occurring at the point of the screenshot below, remarkable.
22 January 2015
The Tsarap River Landslide in Zanskar
A new set of much better images of the landslide on the Tsarap River in the Zanskar Valley have been posted on Facebook by Jaskrit Bawa. The origin of the images appears to be the DC Office of LAHDC in Kargil. The most revealing image is this one, taken from a helicopter upstream of the barrier on the Tsarap River:
The landslide barrier appears to be tall but to have quite a narrow crest. Whilst there are many boulders in the debris, from these images there appears to be a great deal of fine-grained material as well. As an aside, the geological structure on the valley above seems to be quite unstable, with slope parallel discontinuities (bedding plans) that can act as detachment surfaces.
A downstream view is also quite interesting:
This image both confirms the fine-grained nature of the landslide materials but also suggests that there may be some seepage through the landslide mass occurring, given that there appears to be water in the Tsarap River channel. That water is not clean, which may suggest that there is some internal erosion occurring.
The National Disaster Management Agency is reported to be forming an expert group to help manage the hazard, which is welcome and wise. This is based on a very sensible assessment of the situation by the local authorities:
“The lake has been created around 90 km from the Padam area of Zanskar and beyond 43 km no one can go by foot. After consulting all local engineers, including Army engineers, we were not able to do anything,” said Muhammad Sadiq Sheikh, Deputy Commissioner, Kargil.
It is hard to know how serious the hazard is given the available information. On the face of it the size of the barrier, the length of the lake, the narrow dam crest and the possibly weak materials would suggest that it is potentially quite risky. But that will depend on the location of people and assets downstream and the nature of the materials that form the dam. The first step must be to put a monitoring system on the dam – in principle this is not difficult, although getting the information out of the valley will be a real challenge – in high mountain areas communications can be quite problematic.
21 January 2015
Zanskar Valley landslide
Colin Stark at Columbia University has used Landsat 8 images from 1st December 2014 and 18th January 2015 to identify the location of the landslide in the Zanskar Valley, and has kindly sent them to me. This is the January 2015 image, showing the landslide scar and deposit:
The landslide scar deposit can be seen blocking the river in the centre of the image, the source of the landslide is on the southern valley wall. Colin has draped this image on a digital elevation model to generate a perspective view:
The landslide source and deposit are indicated on the above image. From this Colin has a location: 33.29N, 77.286E, which is on the Tsarap River. The calculated surface area of the source zone is about 140,000 square metres, and of the deposit is about 90,000 square metres. If we assume an average deposit thickness of 30 m (and note that is a big assumption), we get a ballpark figure of about 2.7 million cubic metres. This is quite a large landslide. So this is the slope that failed, as shown in Google Earth imagery from 290th June 2014:
20 January 2015
Zanskar River landslide
The ever-reliable Tibet Earthquake twitter feed(@aam868) has tracked down some additional information about the valley-blocking landslide on the Zanskar River on northern India. Perhaps most usefully, it includes links to two images of the landslide, the first of which is in a report in the Outdoor Journal:
Whilst the second is from the White Magic twitter feed:
Unfortunately, these images are still not really good enough to be able to properly assess the hazard at this site.
19 January 2015
Zanskar valley landslide
News agencies in India are reporting that a large landslide has blocked the Phuktal River in the Zanskar Valley. The information about this landslide is a little unclear, although most reports suggest that the barrier is about 200 feet (61 m) high. It appears that a helicopter overflight has been undertaken, from which DNA have published this not very high quality, but still interesting, image:
If this is indeed the landslide then a large and already frozen lake appears to have started to develop. The landslide itself is indeed substantial. The material on the left side looks to be quite fine-grained.
Presumably at this time of year the rate of flow along the river is comparatively low, but the landslide undoubtedly poses a significant threat. Reports suggest that four villages are being evacuated and that the Chadar Trek, a 150 km ice hike, has been closed. Kashmir Life suggests that the landslide occurred on 31st December and that the lake is now 5 km long.
Based on the information that is available in the news reports, this is the most likely stretch of river affected by the landslide:
This image suggests that it will be extremely difficult to move sufficient resources to the site to manage the hazard.
6 January 2015
The Scorciavacche Viaduct in Sicily
In Italy there is something of a scandal developing around the newly completed Scorciavacche Viaduct, a new (very impressive) elevated highway not far from Palermo. The road opened a couple of days before Christmas, but has now had to be closed again because of the collapse of a section built on an embankment. The story has attracted considerable attention from the international media, who are generally describing the problem as “subsidence”.
The best images that I can find of the site are on the Repubblica website (text in Italian), which has an excellent portfolio. This includes a good picture of the roadway – it’s not hard to see why the highway has been closed:
The problem appears to be the failure of a set of remarkably steep earthworks alongside one of the bridge abutments (the abutment itself seems to be intact):
A closer view of the problem gives a better indication of the problem:
The failure seems to have affected quite long section of the embankment:
This is clearly an earthworks failure – i.e. a landslide in the embankment. I am not really familiar with the construction technique here – others ate probably better placed to comment. The embankment seems to be founded on gabians. Above this is some sort of slope protection covering a steep earth bank. Close to the abutment (see the second picture) this might be
soil nailed (comments – experts have subsequently suggested that these are not likely to be soil nails, it is not clear what these objects are), but this doesn’t seem to be the case for most of the embankment. The major failure seems to be in the first section without the soil nails, which is showing distress directly above the base of the embankment. Further to the right the failure is more complex and difficult to understand.
This case is now subject to formal investigation by an Italian magistrate, so hopefully the cause will become apparent soon. It will be very interesting to see what emerges. In the meantime, I’d welcome comments from those who know about embankments.
5 January 2015
Erlang Village, Xishui County, Guizhou
The Chinese media is reporting that a large rockslide occurred at Erlang Village in Xishui County within Guizhou Province in SW China. The landslide reportedly occurred at 3:40 pm on Sunday, covering a major highway that links Chengdu and Zunyi City in Guizhou. It is known that at least 3 people have been killed, but search efforts continue in the fear that there may be further vehicles buried beneath the debris.
The best overview of the landslide, which reportedly has a volume of 80,000 cubic metres, that I have found to date is on the Now News website (in Chinese):
Whilst the image is not particularly good, this looks to be about as good an example of a rockslide on a pre-existing discontinuity (bedding plane?) as you are ever likely to see.
CRI English has some rather better images of the aftermath:
The difficulty of recovering the potentially-buried victims is easy to appreciate. My suspicion is that the concrete structures seen in the images are the grid works associated with rockbolts or ground anchors, in place to stabilise the slope. If so then these works have not been successful, so it would be interesting to know why this slope failure has occurred.
19 December 2014
The Ventnor landslide
One of the most interesting landslides in the UK lies under the town of Ventnor, which is located on the southern side of the Isle of Wight in southern England. This is a large, slow moving landslide that nonetheless causes considerable damage, particularly in periods of accelerated movement. Over quite a long period, the Isle of Wight Council worked with Halcrow (now CH2M HILL) to understand the landslide by monitoring its movement patterns. In recent years I have worked with a part time PhD student, Jon Carey (who is now at GNS Science in New Zealand), and Roger Moore at Halcrow, to understand the relationships between the movement of the landslide and pore water pressures. That work is a part of Jon’s PhD thesis (which is available online), and has just been published in the journal Landslides (Carey et al. 2014).
Perhaps the most interesting aspect of this landslide is a large graben structure that is opening up at the crest of the landslide. This is one of the figures from the paper, showing a geomorphological map of the landslide together with, bottom left, the graben structure:
In the paper we examined some very detailed monitoring data that was collected in a study led by our co-author, Roger Moore, between 1998 and 2002. During this time, Halcrow monitored the pore water pressure at depth using piezometers located in deep boreholes and the opening of the graben using crackmeters (to measure horizontal movements) and settlement cells (to measure vertical movements). We found that the movement of landslide can be divided into two key components. In the background is long-term creep of the landslide, at rates of about 5 to 10 mm per year, regardless of the groundwater conditions. This indicates that the landslide is in a condition of marginal stability / instability. However, when the groundwater level rises due to periods of heavy rainfall the landslide moves more rapidly – up to about 35 mm per year – but in a complex manner. This is captured in the diagram below, which is part of Fig. 6 from the paper:
The upper panel of the diagram shows the movement of the landslide as indicated by extension of the graben sides and subsidence of the graben floor. The lower panel shows the same data for the crackmeter, but expressed as a displacement rate, and the measured groundwater level. It is clear that as the groundwater level increases the landslide movement rate goes up, and vice versa. However, across the entire dataset we found that the relationship between the pore water pressure and movement rate was not simple, and in particular that sometimes the movement rate remained elevated even as pore water pressures reduced.
At Ventnor there has long been discussion of the likelihood that the landslide might transition into a more rapid movement event. Our data suggests that because the landslide is occurring on a very well-developed basal shear plane, and because the toe of the landslide is buttressed by large landslide blocks, this is unlikely without some fundamental shift in material behaviour.
Carey, J.M., Moore, R. and Petley, D.N. 2014. Patterns of movement in the Ventnor landslide complex, Isle of Wight, southern England. Landslides. doi: http://dx.doi.org/10.1007/s10346-014-0538-1